Secondary Logo

Journal Logo

EDITORIAL REVIEW

In HIV-1 pathogenesis the die is cast during primary infection

Centlivre, Mireillea; Sala, Monicab; Wain-Hobson, Simonb; Berkhout, Bena

Author Information
doi: 10.1097/QAD.0b013e3280117f7f

Abstract

Introduction

HIV-1 infection is characterized by three phases: (i) the primary infection phase, which is associated with a massive increase in viral load followed by a decrease to a viral load set-point following the initiation of antiviral immune responses; (ii) the asymptomatic or chronic phase, which is associated with a gradual increase in viral load from primary infection set-point concurrent with a gradual, but irreversible decrease in CD4 T-cell numbers, and (iii) the symptomatic phase or AIDS, associated with the terminal failure of the immune system and disease (Fig. 1). Historically, HIV research has been focused on studies of patients during the chronic phase as it was assumed that the mechanisms leading to disease would have their origins in the phase that precedes the symptomatic one. T-cell and viral replication dynamics during primary infection remained largely unknown until recently [1,2]. As demonstrated in the SIV–macaque model and confirmed in HIV-positive patients, the highest rate of CD4 T-cell destruction by the virus occurs during the primary infection before the emergence of a detectable acquired immune response. In this review, we discuss the events taking place during primary infection, particularly in the gut, and their consequences for virus transmission, epidemiology, therapy and vaccine formulation.

Fig. 1
Fig. 1:
Relationship between total/memory CD4 T cells, DNA/RNA based viral load and disease progression during HIV-1/SIV infection of humans/macaques. Curves represent the DNA viral load associated with CD4 T cells (gray lines), RNA viral load in the serum (black lines), % CD4 T cells (black dashes) and % CD4 memory T cells (grey dashes).

Preferential depletion of CD4 memory T cells in mucosa during primary infection

Since 1998, data obtained with SIV infection in the rhesus macaque animal model demonstrated that primary infection is characterized by a dramatic and selective loss of CD4 T cells within mucosa-associated lymphoid tissues that coincides with the massive increase of viral load in the plasma [3–6]. Of all tissues, the gut-associated lymphoid tissue (GALT) is the most profoundly depleted, which is understandable given that around 60% of total CD4 T cells reside there [7,8]. In the SIV-infected rhesus macaque, depletion of GALT CD4 T cells occurs relatively rapidly within 1–2 weeks post-infection and mainly at effector sites, i.e., the lamina propria [9,10]. Recent studies have provided compelling evidence that this massive mucosal CD4 T-cell depletion is not a specific characteristic of the SIV-macaque model, but is also observed in the gut of acutely HIV-infected patients 4–6 weeks post-infection [11–13]. This trait is independent of the route of transmission [5,14–18].

Depletion of the GALT is predominantly limited to CD4 T cells exhibiting a memory phenotype (CD45RA−CD45RO+) [9,10], explained by the preferential replication of HIV-1 in this T-cell subset [19,20]. CD4 memory T-cell depletion is not a consequence of decreased expression of CD4 viral receptor on the cell surface, nor of its internalization, as no increase in the double negative CD3+CD4−CD8− T-cell population is detected in the gut [5]. Moreover, loss of CD4 memory T cells in the periphery is not due to redistribution to tissues, as there is not only depletion of CD4 memory T cells from the GALT, but also from organized lymph nodes and peripheral blood [9,12,21–24]. It is important to note that the proportion of CD4 memory T cells in otherwise normal humans and monkeys is higher in the GALT (more than 95%) than in the blood, spleen, peripheral lymph nodes and other lymphoid tissues. These latter tissues exhibit more heterogeneity in the composition of their CD4 T-cell subsets (50–75% of CD4-naive T cells) than the GALT [25]. As a consequence, CD4 memory T-cell depletion is more rapid, profound and visible in the GALT during HIV-1 primary infection [5]. Since the memory T-cell subset – by their normal intrinsic function – generate faster and more efficient immune responses against pathogenic infections [26], the extent of this initial assault may determine the outcome of the long-term battle between SIV/HIV-1 and the host.

Activated versus resting CD4 memory T cells, CCR5+ versus CCR5−

The CD4 memory T-cell subset is heterogeneous. It was initially believed that HIV infection preferentially targeted activated CD4 memory T cells that express the CCR5 chemokine receptor, thus sustaining high level viral production [10]. Nonetheless, these two issues – the level of activation and CCR5 expression – remain controversial. In macaques and humans, more than 95% of mucosal CD4 T cells exhibit a memory phenotype and 30–75% of these cells express CCR5 [9,27,28]. The expression of this viral co-receptor at their surface facilitates infection of this T-cell subset by R5 tropic viruses, which are preferentially transmitted at least via the sexual route [29]. Since more than 75% of the CCR5+CD4 memory T cells disappear during the first weeks following infection, mucosal T-cell infection and depletion was thought to be restricted to these cells. However, Mattapallil and colleagues [22] found an equivalent proportion of HIV-infected CCR5−CD4+ and CCR5+CD4+ memory T cells. Quantification of CCR5 mRNA levels in these two populations revealed that CCR5+CD4+ memory T cells express 20-fold more mRNA than their CCR5− counterparts, which still contain 20-fold more CCR5 mRNA than CD4-naive T cells [22]. Thus, although CCR5−CD4+ memory T cells appear to be CCR5 negative by flow cytometry, they may express sufficient CCR5 levels at their surface to be susceptible to HIV-1 infection.

The CCR5 expression pattern of CD4 memory T cells was also correlated with their state of activation. The discovery that a subpopulation of CD4 resting memory T cells can be infected in vivo sheds new light on the debate as to whether the infection is restricted to activated T cells or not [30]. Li et al.[15] characterized the activation state of the SIV-producing cells in tissue sections, which predominantly turned out to be immunophenotypically resting lymphocytes (CD69−CD25−Ki67−). In contrast to truly quiescent resting cells, which cannot sustain viral production, these cells support HIV-1 replication, but to a lower extent than activated cells. Hence, mucosal resting T cells are probably better described as ‘recently activated’. Moreover, under healthy conditions, the gut is a poor pro-inflammatory cytokine environment. Nevertheless, it presents constitutive production of interleukin (IL)-2, IL-7 and IL-15. These cytokines regulate the development of lymphocytes, particularly intra-epithelial lymphocytes [31–33], and can promote the infection and propagation of HIV-1 in CD4 resting memory T cells, this process being further enhanced under inflammatory conditions [34–36]. Consequently, even in the absence of complete T-cell receptor (TCR)-mediated antigen-induced activation, the phenotype and the environment of mucosal CD4 memory T cells make them preferred targets for HIV-1.

The model of substrate availability ideally combines all these observations (Fig. 2). Following virus entry into the mucosa [37,38], HIV-1 encounters a large pool of Ki67−CD4+ resting T cells [39,40]. These resting T cells are infected and produce limited amounts of virus, but they constitute a dense network of cells in the mucosa that can propagate the infection from infected resting cells to nearby uninfected cells, through T–T cell contact in the ‘virological synapse’ [41,42]. This cell-to-cell spread allows the maintenance of an unbroken chain of viral transmission. Apart from resting CD4 memory T cells, activated CD4 T cells produce high levels of virus that generates a viral gradient in the mucosal tissue, and therefore amplifies and disseminates the infection over a larger distance [39,40]. In parallel, myeloid dendritic cells (DC) propagate the infection locally in the mucosal tissue through DC–T-cell contact [43,44] and to draining lymph nodes [45,46].

Fig. 2
Fig. 2:
HIV-1 entry in the GALT and dissemination. Different mechanisms allow the virus to cross the epithelial barrier: (1) breach in the epithelium or direct infection of intra-epithelial lymphocytes (IEL), (2) transcytosis through epithelial cells, (3) transcytosis through M cells, (4) HIV-1 transport by DC. After the virus crosses the mucosal epithelium, whereas low levels of virus is generated within infected resting CD4 memory T cells relatively high levels of virus replication occurs within fully activated CD4 memory T cells leading to dissemination of the infection to longer distances. DC drain virus to afferent lymph nodes.

The mechanisms of CD4 memory T-cell depletion during primary infection

Several mechanisms may contribute to CD4 memory T-cell depletion during primary infection. The major mechanism is a direct cytopathic effect of the virus on infected CD4 T cells. Using the technique of quantitative PCR analysis of single-sorted memory T cells, which allows the determination of the number of SIV-Gag DNA copies per cell, Mattapallil et al.[22] showed that 60% of mucosal CD4 memory T cells were indeed infected within 10 days after viral challenge. The majority (80%) of these infected cells disappeared 4 days later, which parallels the plasma viral load decrease (Fig. 1) [22]. These observations suggest that the high rate of infection of these cells is sufficient to account for their loss during acute infection. No bystander mechanism needs to be invoked. However, Li et al. claim that indirect mechanisms may also contribute to T-cell death during acute infection [3,15]. It has been demonstrated that viral infection causes death by apoptosis of not only infected, but also non-infected CD4 T cells [47–49]. Many apoptotic CD4 T cells have been detected in the intestinal mucosa during primary SIV infection [3,15]. The observation that in vitro exposure of CD4 T cells to high concentrations of HIV gp120 induces Fas ligand expression at the cell surface, coupled to the fact that mucosal CD4 memory T cells naturally express the Fas molecule provides one pathway by which Fas–Fas ligand mediated T-cell apoptosis may occur in the gut [50]. Nevertheless, the physiological relevance of this phenomenon can be questioned in regard to the high dose of gp120 used in such ex vivo studies [51].

Besides its major role in sustaining viral replication, the GALT also participates in the initiation of the antiviral immune response. During primary infection, the level of CD4 T-cell depletion inversely correlates with the increase observed in the absolute number of CD8 T cells [3,5], which includes HIV-1-specific CD8 T cells [3]. In SIV-infected rhesus macaques, virus-specific CD8 T cells appear to a comparable level in the mucosa and the blood at 14 days post-infection [52,53]. The extent of virus-specific immune response in the GALT as compared with regional lymph nodes has also been examined by several laboratories. While there is a consensus with regard to the presence of relatively low levels of SIV-specific cellular responses in the mucosa during the acute viraemia period, there is conflicting data concerning the kinetics and levels of virus-specific cellular responses in the regional lymph nodes [54,55]. The reasons for this discrepancy remain to be defined, but could be either due to differences in the techniques employed or to differences in route of infection (intra-rectal/intravenous versus intra-vaginal). In both cases, however, the low levels of virus-specific cellular response during the early acute infection seems unlikely to contribute to CD4 T-cell depletion in the GALT, as the CD8 T-cell response is thought to be ‘too late and too little’ [54]. It should be noted, nonetheless, that since CD8 perforin-positive T cells can be detected in the GALT of SIV-infected macaques after 21 days, such a response may contribute at later time points to the depletion of infected cells sustaining viral replication [56].

Persistent mucosal CD4 T cell depletion during the chronic phase

The massive depletion of CD4 memory T cells in the GALT is a hallmark of primary infection, and persists during the chronic phase. In SIVmac251-infected macaques, the GALT CD4 T-cell numbers remain markedly deleted 5 months post-infection [5], and complete restoration is not observed during the chronic phase [3]. This phenomenon is also observed in HIV-1 infected patients [11]. During the chronic phase of infection, GALT is still a site of active HIV-1-specific CD8 T-cell response [57–59]. In parallel, chronic immune activation takes place, constantly producing new target cells for the virus. Aside from higher T-cell turnover, chronic immune activation leads to increased frequency of activated T and B cells [60–62], and increased serum levels of pro-inflammatory cytokines and chemokines. This chronic immune activation results from: (i) massive depletion of the memory T-cell pool during primary infection, which leads to an increase in T-cell homeostatic proliferation of the remaining cells; and (ii) destruction of the mucosal immune system and consequent spreading of secondary infection from the mucosa to the systemic level [1]. During chronic infection the high rate of T-cell death is a consequence rather than a cause of immune activation [63].

Chronic immune activation differentially affects two components of the CD4 memory T-cell population: the long-lived central memory CD4 T cells (TCM; CD57−CCR7+) and the short-lived effector memory CD4 T cells (TEM; CD57+CCR7−) [64]. Following peak viraemia, the short-lived CD4 TEM cells are profoundly depleted in the mucosal tissue and must be replenished to ensure minimal immune function [65,66]. These CD4 TEM cells are characterized by their intrinsic limited regenerative capacity but are continually produced via proliferation, differentiation and emigration of CD4 TCM cells and/or naive CD4 T cells [26]. Following infection, both of these CD4 T-cell subsets exhibit increased proliferation along with impairment of their own regenerative capacity due to: (i) the diminution of CD4 T cell production through decreased proliferation capacity in the thymus [67]; (ii) the massive differentiation of naive T cells into memory T cells and TCM into TEM cells; and (iii) the activation- and virus-induced cell death and the destruction of lymph node architecture [68]. As a consequence of these phenomena, in chronically advanced HIV-1 infected patients, the long-lived T-cell compartment is reduced in size and exhibits a lower production rate in contrast to short-lived T-cells [64,69]. Therefore, given the inability to maintain production and tissue delivery of short-lived CD4 TEM cells in SIV-infected animals, rapid progression towards disease occurs [66,70]. The resulting biased replication dynamics of CD4 T subsets may explain the prolonged intestinal CD4 memory T-cell depletion (Fig. 3) [60,62,71–73].

Fig. 3
Fig. 3:
HIV-1 induced perturbation of CD4 T-cell pools. CD4 memory T-cell attrition is due to: (1) the massive depletion of this T-cell subset in the GALT during the acute phase of infection and due to repeated immune activation during the chronic infection; (2) increased activation (specific and non-specific) of CD4 naive T cells that will differentiate into activated/memory T cells and serve as targets; (3) altered production of CD4 naive T cells within central lymphoid organs; (4) reduction of the CD4 resting memory T-cell compartment due to chronic stimulation.

Naive CD4 T cells not expressing CCR5 are rarely infected in contrast to the memory CD4 T-cell compartment [74,75]. Furthermore, the CD4 TEM-cell pool contains less viral DNA than CD4 TCM cells [74]. However, short-lived memory CD4 T cells continuously produce viruses, whereas long-lived memory CD4 T cells become latently infected. These results imply that long-lived memory CD4 T cells constitute a persistent viral reservoir in which viral strains can hide and from which they can reemerge later. Moreover, a small number of latently infected cells become activated each day and release virus, which can trigger a viral rebound if antiviral therapy is stopped.

HIV-1 infection not only affects the CD4 T-cell dynamics during primary and chronic infection phase, but also impairs CD4 T-cell function [76]. Indeed, during the T-cell activation process, HIV-1-specific CD4 T cells are preferentially infected by HIV-1, presented by HIV-1 exposed conventional dendritic cells [45,77,78]. Because of their preferential infection, the life span of HIV-1-specific CD4 T cells is reduced [79] and these cells are more prone to apoptosis [80]. As a consequence, this cellular subset, which is essential to coordinate immune response against the virus, notably through secretion of cytokines, is impaired in its fundamental role [81]. Indeed, the capacity of HIV-1-specific CD4 T cells to secrete IL-2 is diminished during infection and preservation of IL-2 secretion correlates with a better control of viraemia [82,83]. Moreover, the absence of IL-2 secreting HIV-1-specific CD4 T cells is associated with the absence of cellular proliferative response also implicated in the control of viraemia [84]. The defective proliferation of HIV-1-specific CD4 T cells might also be a consequence of PD-1 (inhibitory receptor programmed death 1; a regulator of activated T cells) expression at the CD4 T-cell surface. As observed in HIV-1-infected patients, PD-1 expression on CD4 T cells positively correlates with the viral load and negatively with the CD4 T-cell count [85]. In this context, it has been demonstrated that it is possible to rescue the proliferative response of HIV-1-specific CD4 T cells by blocking the interaction of PD-1 with its ligand PD-L1 by using an anti-PD-L1 antibody [85]. In conclusion, persistent negative feedback between HIV-1 and virus-specific CD4 T cells lead to antiviral immune responses unable to control the infection.

In long-term non-progressors, intestinal CD4 T-cell depletion is not visible and the CD4 T-cell numbers are close to normal in the GALT [86]. This could reflect the ability of the host antiviral response to control viral infection and to maintain the integrity of the mucosal immune system, preserving CD4 T cell maturation and their role in coordinating HIV-1-specific immune response [87].

Persistent mucosal CD4 T-cell depletion following antiretroviral treatment

In HIV patients receiving HAART, HIV-1 replication in the GALT is decreased, but recovery of intestinal CD4 T-cell numbers is highly variable among patients, in contrast to recovery in the blood [88]. Guadalupe and colleagues have shown that HIV patients under HAART restore the intestinal CD4 T-cell compartment, but this recovery is dependent on the period when treatment was initiated [86]. Thus, it is generally believed that close to complete restoration of CD4 T cells occurs when patients are treated during primary infection while recovery of CD4 T cells is delayed/incomplete when patients are treated during the chronic phase. Nevertheless, Mehandru and colleagues [13] have shown partial reconstitution of the CD4 T cells in the intestinal mucosa in a cohort of eight patients even when antiretroviral treatment was initiated during primary infection. Neither of the studies, however, analyzed the functionality of the CD4 T cells repopulating the GALT. In SIV-infected monkeys treated with HAART, the capacity of the repopulating CD4 T cells to produce IL-2 upon mitogenic stimulation was reduced [3,89]. Thus, even if there is quantitative repopulation of the gut, this may not reflect functional reconstitution. It is important to note that the early administration of antiretroviral treatment in SIV infected rhesus macaques induces the up-regulation of growth factors and genes involved in repair and regeneration of the intestinal epithelium, leading to better integrity of the mucosal barrier [90]. A new concept has emerged from these studies: the earlier the treatment given after infection, the more beneficial the effects on the preservation of the mucosal immune system. However, not only issues such as drug resistance but also long-term side effects of drugs have to be taken into account and this highlights the urgent need for identifying new therapeutic options which include strategies other than new chemotherapeutic drugs.

GALT: a potential role in differential HIV-1 subtype transmission

The massive depletion of CD4 memory T cells during primary infection may also be relevant for studies that focus on biological differences between the HIV-1 subtypes [91,92]. A major determinant of subtype-specific differences in host cell tropism, and hence pathogenesis, is driven by the long terminal repeat (LTR) promoter of HIV-1 [93,94]. While the HIV-1 minimal promoter is one of the most conserved regions among virus isolates and subtypes, it is polymorphic in terms of transcription factor binding sites (Fig. 4) [95–97]. By means of virus replication studies and competition experiments in vitro, we demonstrated a profound influence of the cellular environment on the subtype-dependent viral fitness [93,94,98]. These findings are in keeping with the intricate interplay between transcription factor pools present in different cell subsets and the availability of transcription factor binding sites in subtype-specific LTR promoters. This phenomenon was addressed in vivo by inserting different subtype-specific HIV-1 minimal promoters in the SIVmac239 proviral genome and following organ specific dissemination of the chimaeric viruses in rhesus macaques [99]. In direct virus competition experiments in SHIV-infected-macaques during primary infection, viral RNA of SHIV chimera bearing the subtype C promoter predominated in faeces and serum, whereas the virus with the subtype B promoter was found predominantly in peripheral blood mononuclear cells [99–101]. Viral RNA in faeces reflects viral production in the GALT and confirmed the predominance of subtype C in this compartment [102].

Fig. 4
Fig. 4:
Schematic organization of the binding sites for transcription factors in A through G subtype-specific HIV-1 minimal promoters.

The different chemokine/interleukin environments of GALT versus blood are likely to differentially influence subtype-specific LTR promoters bearing distinct transcription factor binding sites (Fig. 4). The intestinal environment – enriched for γc-dependent cytokines (IL-2, IL-7, IL-15) [31,32] – could modulate subtype-specific HIV-1 replication during primary infection by promoting the recruitment of transcription factors such as AP-1 and NF-κB [99–101]. Due to its specific cytokine environment, the GALT can preferentially sustain replication of the strain bearing the subtype C promoter and thus plays a key role in viral dissemination during primary infection.

Such differences during acute HIV-1/SIV infection may translate into specificities in disease progression. In this regard, it is of interest to note that slower disease progression for subtype A has been observed [103,104] and significant differences have been described between subtype B and C infections in viral load at peak viraemia shortly after infection and before the set point [105] and between subtypes B and E [106]. In African cohorts, the risk of transmission from mother to child was increased in the case of subtype C infection, notably due to a higher viral load [107,108]. Furthermore, as high viral load correlates with increased chance of virus transmission to uninfected people [109], the probability is higher during primary infection. If so, it can be assumed that the virus bearing the HIV-1 C promoter will be the preferentially transmitted strain. The fact that HIV-1 C clade represents around 50% of the worldwide cases of HIV-1 infection (www.hiv.lanl.org, www.unaids.org) supports this view, although it does not constitute proof (Fig. 5).

Fig. 5
Fig. 5:
Influence of differential HIV-1 subtype-specific replication in the GALT on viral transmission. (a) The selective enhancement of HIV-1 subtype C replication in the GALT leads to a predominance of this strain during primary infection throughout the body. During primary infection, high viral load increases the probability of viral transmission, probably favouring dissemination of subtype C as compared to the other subtypes. (b) HIV-1 subtype C is responsible for approximately 50% of all infections in the global epidemic.

It has been extensively demonstrated that tumour necrosis factor (TNF)α, which leads to activation of the NF-κB pathway, promotes enhanced replication of HIV-1 subtype C as compared to other subtypes (A, B, D, E, F and G) [94,95,110]. It should be noted that the TNFα level is elevated in infected patients, notably in the vagina [111]. These observations correlate with results obtained after intravaginal infection of rhesus macaques with SIV. During acute infection, the induction of the pro-inflammatory cytokines, like TNFα and IFN-γ, was detectable at different mucosal sites, notably within the vagina [112,113]. Hence, HIV-1 infection leads to enhancement of cytokine levels, which will further promote infection (Fig. 5).

Therapeutic implications

Current antiviral therapies may not efficiently target the GALT. New antiviral strategies to complement HAART based on the administration of cytokines have been proposed. This novel immunotherapy intervention aims to improve immune function and thus to control HIV-1 infection [114,115]. In this therapeutic context, the most studied cytokine is IL-2, the first phase I study on exogenous IL-2 administration to patients dates back to 1983. In HIV-infected patients, administration of IL-2 as an immune adjunct to HAART has been shown to lead to a more pronounced increase of CD4 T-cell numbers than with HAART alone, with no or only transient elevation of the plasma viral load [116,117]. Following IL-2 treatment, decreased cell turnover, reduced immune activation and increased survival of these cells are observed, leading to selective expansion of naive and memory CD4 T cells within the periphery [114,115]. In addition, IL-2 induces higher IL-7 production, which may lead to stimulation of neo-thymopoiesis and thus contribute to the accumulation of CD4 naive T cells [118]. Hence, while not inducing massive immune reconstitution, IL-2 plays a major role in limiting the detrimental effects of immune activation in HIV-1 disease progression [119]. These results are promising and will be corroborated in ongoing phase III trials.

Aside from IL-2, the use of IL-7 is now under study. IL-7 is a key cytokine implicated in the development and homeostatic regulation of T lymphocytes [120,121]. In HIV-infected patients, high CD4 T-cell depletion in the blood correlated with a high plasma IL-7 level, probably as a consequence of homeostatic feedback loop on IL-7 production [122–124]. Moreover, a positive correlation exists between high levels of IL-7 and a high viral load [124]. Under HAART, the CD4 T-cell numbers increase while IL-7 plasma levels decrease. In SIV-infected rhesus macaques, IL-7 administration induces renewal and peripheral expansion of T cells, associated with cellular activation, and whereas the CD4 T-cell numbers increase, the RNA viral load does not [125,126]. These results seem promising for the utilization of IL-7 as an immune adjunct to HAART based therapy [114,127]. However, recent ex vivo reports show that IL-7 increases Fas-mediated T-cell death both in CD4 and CD8 T-cell subsets [128] and exogenous IL-7 induces Fas-mediated human neuronal apoptosis [129], suggesting that IL-7 can have a deleterious effect on HIV-1 pathogenesis.

In addition to the above, IL-7 also induces the reactivation of HIV-1 latent reservoirs, particularly of resting CD4 T cells, as shown by the increased cell surface expression of CD25 [130,131]. Interestingly, after activation of viral reservoirs by cytokines, the viral quasispecies differed according to the protocol used for the activation [IL-2, IL-2 + phytohaemagglutinin (PHA), IL-7] [131]. We have shown in vivo that IL-7 differentially activates HIV-1 subtype-specific promoters [99,100]. Thus, IL-7, like IL-2, may have a direct effect on HIV-1 quasispecies evolution. Besides the beneficial effects of IL-2 and/or IL-7 on T-cell homeostasis and the activation of viral reservoirs, these interleukins could contribute to enhanced replication of IL-2/IL-7 sensitive strains, leading to more rapid evolution of drug-resistant strains under HAART. The administration of interleukins therefore needs further experimental evaluation with different HIV-1 subtypes representative of the clades that cause infection in Asia and Africa.

Implications for vaccine development

Our understanding of AIDS pathogenesis has been biased by reliance on examination of peripheral blood, which provides only part of the picture. The recent insights into virus replication in the GALT have important ramifications for the development of vaccines. Mucosal immunity emerges as an essential target for designing an effective AIDS vaccine, which should ideally prevent the massive destruction of the CD4 memory T-cell compartment. Furthermore, countermeasures are now being developed to attack the virus at or shortly after transmission within the cervico-vaginal tissues with antivirals or microbicides [132,133].

Macaque studies have shown that the establishment of an immune response specifically in the mucosa is required for efficient protection against SHIV [134,135]. Immunization by the mucosal route induces mucosal and systemic immunity, whereas systemic immunization does not necessarily induce mucosal immunity [136,137]. This could be explained by the differences documented for T-cell trafficking to different body compartments. Following mucosal immunization, antigen specific T lymphocytes can migrate not only to the mucosal effector sites but also to secondary lymphoid organs, whereas circulating lymphocytes are more limited in their capacity to migrate to mucosal associated lymphoid tissues [138,139]. Moreover, recent studies on early SIV dissemination following intravaginal infection of macaques showed that there is a narrow timeframe of less than 6 days during which the virus is vulnerable such that a preexisting mucosal immune response may control and prevent HIV-1 spreading [16].

Conclusions

The mucosa represents one of the major sites for early HIV-1 replication, amplification and T-cell loss during primary infection. This is particularly true in the GALT, where 60% of total CD4 memory T cells reside. Being the target of early infection, CD4 memory T cells sustain viral amplification and are rapidly depleted during this process. The loss of target cells in the GALT leads to lower viral load in this tissue as well as in the plasma. In spite of the continuous recruitment of new lymphocytes to the GALT, the intestinal CD4 T-cell population is never restored to the original steady-state level. Due to chronic immune activation, newly recruited lymphocytes are perpetually activated and subsequently infected, and thus contribute to viral replication and maintenance of the infection. Intestinal viral replication is accompanied by the emergence of HIV-1-specific mucosal and systemic immune responses, but the magnitude and efficiency of these responses are limited to contain early infection.

Early on in infection, preferential replication of particular HIV-1 subtypes has been observed, which correlates with subtype-specific promoter activity and tissue-specific cytokine expression (e.g., IL-7 in the GALT). The observation that subtype C replication is enhanced in the GALT and that transmission is increased during primary infection may be the basis for the worldwide prevalence of HIV-1 subtype C. Although preliminary results obtained with interleukin-based immunotherapeutic treatment as immune adjuncts to HAART seem promising, IL-2 and/or IL-7 treatment is potentially not neutral from the viral clade point of view.

Acknowledgments

The authors thank Fedde Groot and Nicolas Legrand for useful comments on the manuscript.

Sponsorship: M.C. is supported by an FRM (Fondation Recherche Médicale) fellowship. M.S. and S.W.H. were funded by the Institut Pasteur, the Agence Nationale de Recherches sur le SIDA (ANRS) and the Centre National de la Recherche Scientifique (CNRS) URA 1930. B.B. was supported by the AMC-UvA (Academic Medical Center – University of Amsterdam), ZonMw (Vici grant), NWO (TOP grant), NWO-ALW and Dutch AIDS funds.

References

1. Brenchley JM, Price DA, Douek DC. HIV disease: fallout from a mucosal catastrophe? Nat Immunol 2006; 7:235–239.
2. Mattapallil JJ, Roederer M. Acute HIV infection: it takes more than guts. Curr Opin HIV AIDS 2006; 1:10–15.
3. Smit-McBride Z, Mattapallil JJ, McChesney M, Ferrick D, Dandekar S. Gastrointestinal T lymphocytes retain high potential for cytokine responses but have severe CD4(+) T-cell depletion at all stages of simian immunodeficiency virus infection compared to peripheral lymphocytes. J Virol 1998; 72:6646–6656.
4. Vajdy M, Veazey R, Tham I, deBakker C, Westmoreland S, Neutra M, et al. Early immunologic events in mucosal and systemic lymphoid tissues after intrarectal inoculation with simian immunodeficiency virus. J Infect Dis 2001; 184:1007–1014.
5. Veazey RS, DeMaria M, Chalifoux LV, Shvetz DE, Pauley DR, Knight HL, et al. Gastrointestinal tract as a major site of CD4+ T cell depletion and viral replication in SIV infection. Science 1998; 280:427–431.
6. Veazey RS, Marx PA, Lackner AA. Vaginal CD4+ T cells express high levels of CCR5 and are rapidly depleted in simian immunodeficiency virus infection. J Infect Dis 2003; 187:769–776.
7. Cheroutre H, Madakamutil L. Acquired and natural memory T cells join forces at the mucosal front line. Nat Rev Immunol 2004; 4:290–300.
8. Mowat AM, Viney JL. The anatomical basis of intestinal immunity. Immunol Rev 1997; 156:145–166.
9. Veazey RS, Mansfield KG, Tham IC, Carville AC, Shvetz DE, Forand AE, et al. Dynamics of CCR5 expression by CD4(+) T cells in lymphoid tissues during simian immunodeficiency virus infection. J Virol 2000; 74:11001–11007.
10. Veazey RS, Tham IC, Mansfield KG, DeMaria M, Forand AE, Shvetz DE, et al. Identifying the target cell in primary simian immunodeficiency virus (SIV) infection: highly activated memory CD4(+) T cells are rapidly eliminated in early SIV infection in vivo. J Virol 2000; 74:57–64.
11. Brenchley JM, Schacker TW, Ruff LE, Price DA, Taylor JH, Beilman GJ, et al. CD4+ T cell depletion during all stages of HIV disease occurs predominantly in the gastrointestinal tract. J Exp Med 2004; 200:749–759.
12. Clayton F, Snow G, Reka S, Kotler DP. Selective depletion of rectal lamina propria rather than lymphoid aggregate CD4 lymphocytes in HIV infection. Clin Exp Immunol 1997; 107:288–292.
13. Mehandru S, Poles MA, Tenner-Racz K, Horowitz A, Hurley A, Hogan C, et al. Primary HIV-1 infection is associated with preferential depletion of CD4+ T lymphocytes from effector sites in the gastrointestinal tract. J Exp Med 2004; 200:761–770.
14. Kewenig S, Schneider T, Hohloch K, Lampe-Dreyer K, Ullrich R, Stolte N, et al. Rapid mucosal CD4(+) T-cell depletion and enteropathy in simian immunodeficiency virus-infected rhesus macaques. Gastroenterology 1999; 116:1115–1123.
15. Li Q, Duan L, Estes JD, Ma ZM, Rourke T, Wang Y, et al. Peak SIV replication in resting memory CD4+ T cells depletes gut lamina propria CD4+ T cells. Nature 2005; 434:1148–1152.
16. Miller CJ, Li Q, Abel K, Kim EY, Ma ZM, Wietgrefe S, et al. Propagation and dissemination of infection after vaginal transmission of simian immunodeficiency virus. J Virol 2005; 79:9217–9227.
17. Stahl-Hennig C, Steinman RM, Tenner-Racz K, Pope M, Stolte N, Matz-Rensing K, et al. Rapid infection of oral mucosal-associated lymphoid tissue with simian immunodeficiency virus. Science 1999; 285:1261–1265.
18. Vajdy M, Veazey RS, Knight HK, Lackner AA, Neutra MR. Differential effects of simian immunodeficiency virus infection on immune inductive and effector sites in the rectal mucosa of rhesus macaques. Am J Pathol 2000; 157:485–495.
19. Rosenberg ZF, Fauci AS. Immunopathogenesis of HIV infection. FASEB J 1991; 5:2382–2390.
20. Schnittman SM, Lane HC, Greenhouse J, Justement JS, Baseler M, Fauci AS. Preferential infection of CD4+ memory T cells by human immunodeficiency virus type 1: evidence for a role in the selective T-cell functional defects observed in infected individuals. Proc Natl Acad Sci USA 1990; 87:6058–6062.
21. Lim SG, Condez A, Lee CA, Johnson MA, Elia C, Poulter LW. Loss of mucosal CD4 lymphocytes is an early feature of HIV infection. Clin Exp Immunol 1993; 92:448–454.
22. Mattapallil JJ, Douek DC, Hill B, Nishimura Y, Martin M, Roederer M. Massive infection and loss of memory CD4+ T cells in multiple tissues during acute SIV infection. Nature 2005; 434:1093–1097.
23. Schneider T, Jahn HU, Schmidt W, Riecken EO, Zeitz M, Ullrich R. Loss of CD4 T lymphocytes in patients infected with human immunodeficiency virus type 1 is more pronounced in the duodenal mucosa than in the peripheral blood. Berlin Diarrhea/Wasting Syndrome Study Group. Gut 1995; 37:524–529.
24. Ullrich R, Schneider T, Bergs C, Schmidt W, Jahn HU, Dormann A, et al. Loss of CD4 positive T cells and evidence for impaired differentiation of both CD4 and CD8 positive T cells in the large intestine of patients infected with human immunodeficiency virus (HIV). Berlin Diarrhea/Wasting Syndrome Study Group. Adv Exp Med Biol 1995; 371B:1015–1017.
25. Veazey RS, Marx PA, Lackner AA. The mucosal immune system: primary target for HIV infection and AIDS. Trends Immunol 2001; 22:626–633.
26. Sallusto F, Geginat J, Lanzavecchia A. Central memory and effector memory T cell subsets: function, generation, and maintenance. Annu Rev Immunol 2004; 22:745–763.
27. Meng G, Sellers MT, Mosteller-Barnum M, Rogers TS, Shaw GM, Smith PD. Lamina propria lymphocytes, not macrophages, express CCR5 and CXCR4 and are the likely target cell for human immunodeficiency virus type 1 in the intestinal mucosa. J Infect Dis 2000; 182:785–791.
28. Smith PD, Meng G, Sellers MT, Rogers TS, Shaw GM. Biological parameters of HIV-1 infection in primary intestinal lymphocytes and macrophages. J Leukoc Biol 2000; 68:360–365.
29. Bleul CC, Wu L, Hoxie JA, Springer TA, Mackay CR. The HIV coreceptors CXCR4 and CCR5 are differentially expressed and regulated on human T lymphocytes. Proc Natl Acad Sci USA 1997; 94:1925–1930.
30. Zhang Z, Schuler T, Zupancic M, Wietgrefe S, Staskus KA, Reimann KA, et al. Sexual transmission and propagation of SIV and HIV in resting and activated CD4+ T cells. Science 1999; 286:1353–1357.
31. Guy-Grand D, Vassalli P. Gut intraepithelial lymphocyte development. Curr Opin Immunol 2002; 14:255–259.
32. Porter BO, Malek TR. Thymic and intestinal intraepithelial T lymphocyte development are each regulated by the gammac-dependent cytokines IL-2, IL-7, and IL-15. Semin Immunol 2000; 12:465–474.
33. Watanabe M, Ueno Y, Yajima T, Iwao Y, Tsuchiya M, Ishikawa H, et al. Interleukin 7 is produced by human intestinal epithelial cells and regulates the proliferation of intestinal mucosal lymphocytes. J Clin Invest 1995; 95:2945–2953.
34. Chun TW, Engel D, Mizell SB, Ehler LA, Fauci AS. Induction of HIV-1 replication in latently infected CD4+ T cells using a combination of cytokines. J Exp Med 1998; 188:83–91.
35. Unutmaz D, KewalRamani VN, Marmon S, Littman DR. Cytokine signals are sufficient for HIV-1 infection of resting human T lymphocytes. J Exp Med 1999; 189:1735–1746.
36. Unutmaz D, Pileri P, Abrignani S. Antigen-independent activation of naive and memory resting T cells by a cytokine combination. J Exp Med 1994; 180:1159–1164.
37. Bomsel M, Alfsen A. Entry of viruses through the epithelial barrier: pathogenic trickery. Nat Rev Mol Cell Biol 2003; 4:57–68.
38. Pope M, Haase AT. Transmission, acute HIV-1 infection and the quest for strategies to prevent infection. Nat Med 2003; 9:847–852.
39. Haase AT. Perils at mucosal front lines for HIV and SIV and their hosts. Nat Rev Immunol 2005; 5:783–792.
40. Zhang ZQ, Wietgrefe SW, Li Q, Shore MD, Duan L, Reilly C, et al. Roles of substrate availability and infection of resting and activated CD4+ T cells in transmission and acute simian immunodeficiency virus infection. Proc Natl Acad Sci USA 2004; 101:5640–5645.
41. Jolly C, Kashefi K, Hollinshead M, Sattentau QJ. HIV-1 cell to cell transfer across an Env-induced, actin-dependent synapse. J Exp Med 2004; 199:283–293.
42. Piguet V, Sattentau Q. Dangerous liaisons at the virological synapse. J Clin Invest 2004; 114:605–610.
43. Arrighi JF, Pion M, Garcia E, Escola JM, van Kooyk Y, Geijtenbeek TB, et al. DC-SIGN-mediated infectious synapse formation enhances X4 HIV-1 transmission from dendritic cells to T cells. J Exp Med 2004; 200:1279–1288.
44. Sol-Foulon N, Moris A, Nobile C, Boccaccio C, Engering A, Abastado JP, et al. HIV-1 Nef-induced upregulation of DC-SIGN in dendritic cells promotes lymphocyte clustering and viral spread. Immunity 2002; 16:145–155.
45. Groot F, van Capel TM, Kapsenberg ML, Berkhout B, de Jong EC. Opposing roles of blood myeloid and plasmacytoid dendritic cells in HIV-1 infection of T cells: transmission facilitation versus replication inhibition. Blood 2006; 108:1957–1964.
46. Turville SG, Peretti S, Pope M. Lymphocyte-dendritic cell interactions and mucosal acquisition of SIV/HIV infection. Curr Opin HIV AIDS 2006; 1:3–9.
47. Badley AD, McElhinny JA, Leibson PJ, Lynch DH, Alderson MR, Paya CV. Upregulation of Fas ligand expression by human immunodeficiency virus in human macrophages mediates apoptosis of uninfected T lymphocytes. J Virol 1996; 70:199–206.
48. Finkel TH, Tudor-Williams G, Banda NK, Cotton MF, Curiel T, Monks C, et al. Apoptosis occurs predominantly in bystander cells and not in productively infected cells of HIV- and SIV-infected lymph nodes. Nat Med 1995; 1:129–134.
49. Oyaizu N, Adachi Y, Hashimoto F, McCloskey TW, Hosaka N, Kayagaki N, et al. Monocytes express Fas ligand upon CD4 cross-linking and induce CD4+ T cells apoptosis: a possible mechanism of bystander cell death in HIV infection. J Immunol 1997; 158:2456–2463.
50. Boirivant M, Viora M, Giordani L, Luzzati AL, Pronio AM, Montesani C, et al. HIV-1 gp120 accelerates Fas-mediated activation-induced human lamina propria T cell apoptosis. J Clin Immunol 1998; 18:39–47.
51. Klasse PJ, Moore JP. Is there enough gp120 in the body fluids of HIV-1-infected individuals to have biologically significant effects? Virology 2004; 323:1–8.
52. Veazey RS, Gauduin MC, Mansfield KG, Tham IC, Altman JD, Lifson JD, et al. Emergence and kinetics of simian immunodeficiency virus-specific CD8(+) T cells in the intestines of macaques during primary infection. J Virol 2001; 75:10515–10519.
53. Veazey RS, Lifson JD, Schmitz JE, Kuroda MJ, Piatak M Jr, Pandrea I, et al. Dynamics of Simian immunodeficiency virus-specific cytotoxic T-cell responses in tissues. J Med Primatol 2003; 32:194–200.
54. Reynolds MR, Rakasz E, Skinner PJ, White C, Abel K, Ma ZM, et al. CD8+ T-lymphocyte response to major immunodominant epitopes after vaginal exposure to simian immunodeficiency virus: too late and too little. J Virol 2005; 79:9228–9235.
55. Vingert BC, Le Grand R, Venet A. Heterogeneity of the simian immunodeficiency virus (SIV) specific CD8(+) T-cell response in mucosal tissues during SIV primary infection. Microbes Infect 2003; 5:757–767.
56. Quigley MF, Abel K, Zuber B, Miller CJ, Sandberg JK, Shacklett BL. Perforin expression in the gastrointestinal mucosa is limited to acute simian immunodeficiency virus infection. J Virol 2006; 80:3083–3087.
57. Ibarrondo FJ, Anton PA, Fuerst M, Ng HL, Wong JT, Matud J, et al. Parallel human immunodeficiency virus type 1-specific CD8+ T-lymphocyte responses in blood and mucosa during chronic infection. J Virol 2005; 79:4289–4297.
58. Schmitz JE, Veazey RS, Kuroda MJ, Levy DB, Seth A, Mansfield KG, et al. Simian immunodeficiency virus (SIV)-specific cytotoxic T lymphocytes in gastrointestinal tissues of chronically SIV-infected rhesus monkeys. Blood 2001; 98:3757–3761.
59. Shacklett BL, Cox CA, Sandberg JK, Stollman NH, Jacobson MA, Nixon DF. Trafficking of human immunodeficiency virus type 1-specific CD8+ T cells to gut-associated lymphoid tissue during chronic infection. J Virol 2003; 77:5621–5631.
60. Hellerstein M, Hanley MB, Cesar D, Siler S, Papageorgopoulos C, Wieder E, et al. Directly measured kinetics of circulating T lymphocytes in normal and HIV-1-infected humans. Nat Med 1999; 5:83–89.
61. Kovacs JA, Lempicki RA, Sidorov IA, Adelsberger JW, Herpin B, Metcalf JA, et al. Identification of dynamically distinct subpopulations of T lymphocytes that are differentially affected by HIV. J Exp Med 2001; 194:1731–1741.
62. Mohri H, Bonhoeffer S, Monard S, Perelson AS, Ho DD. Rapid turnover of T lymphocytes in SIV-infected rhesus macaques. Science 1998; 279:1223–1227.
63. Anderson RW, Ascher MS, Sheppard HW. Direct HIV cytopathicity cannot account for CD4 decline in AIDS in the presence of homeostasis: a worst-case dynamic analysis. J Acquir Immune Defic Syndr Hum Retrovirol 1998; 17:245–252.
64. Hellerstein MK, Hoh RA, Hanley MB, Cesar D, Lee D, Neese RA, et al. Subpopulations of long-lived and short-lived T cells in advanced HIV-1 infection. J Clin Invest 2003; 112:956–966.
65. Picker LJ. Immunopathogenesis of acute AIDS virus infection. Curr Opin Immunol 2006; 18:399–405.
66. Picker LJ, Hagen SI, Lum R, Reed-Inderbitzin EF, Daly LM, Sylwester AW, et al. Insufficient production and tissue delivery of CD4+ memory T cells in rapidly progressive simian immunodeficiency virus infection. J Exp Med 2004; 200:1299–1314.
67. Dion ML, Poulin JF, Bordi R, Sylvestre M, Corsini R, Kettaf N, et al. HIV infection rapidly induces and maintains a substantial suppression of thymocyte proliferation. Immunity 2004; 21:757–768.
68. Douek DC, Picker LJ, Koup RA. T cell dynamics in HIV-1 infection. Annu Rev Immunol 2003; 21:265–304.
69. Silvestri G, Feinberg MB. Turnover of lymphocytes and conceptual paradigms in HIV infection. J Clin Invest 2003; 112:821–824.
70. Grossman Z, Meier-Schellersheim M, Paul WE, Picker LJ. Pathogenesis of HIV infection: what the virus spares is as important as what it destroys. Nat Med 2006; 12:289–295.
71. McCune JM, Hanley MB, Cesar D, Halvorsen R, Hoh R, Schmidt D, et al. Factors influencing T-cell turnover in HIV-1-seropositive patients. J Clin Invest 2000; 105:R1–R8.
72. Rosenzweig M, DeMaria MA, Harper DM, Friedrich S, Jain RK, Johnson RP. Increased rates of CD4(+) and CD8(+) T lymphocyte turnover in simian immunodeficiency virus-infected macaques. Proc Natl Acad Sci USA 1998; 95:6388–6393.
73. Sachsenberg N, Perelson AS, Yerly S, Schockmel GA, Leduc D, Hirschel B, et al. Turnover of CD4+ and CD8+ T lymphocytes in HIV-1 infection as measured by Ki-67 antigen. J Exp Med 1998; 187:1295–1303.
74. Brenchley JM, Hill BJ, Ambrozak DR, Price DA, Guenaga FJ, Casazza JP, et al. T-cell subsets that harbor human immunodeficiency virus (HIV) in vivo: implications for HIV pathogenesis. J Virol 2004; 78:1160–1168.
75. Groot F, van Capel TM, Schuitemaker J, Berkhout B, de Jong EC. Differential susceptibility of naive, central memory and effector memory T cells to dendritic cell-mediated HIV-1 transmission. Retrovirology 2006; 3:52–61.
76. Harari A, Cellerai C, Pantaleo G. Role of HIV-1-specific CD4 T cells. Curr Opin HIV AIDS 2006; 1:22–27.
77. Douek DC, Brenchley JM, Betts MR, Ambrozak DR, Hill BJ, Okamoto Y, et al. HIV preferentially infects HIV-specific CD4+ T cells. Nature 2002; 417:95–98.
78. Moris A, Pajot A, Blanchet F, Guivel-Benhassine F, Salcedo M, Schwartz O. Dendritic cells and HIV-specific CD4+ T cells: HIV antigen presentation, T-cell activation, and viral transfer. Blood 2006; 108:1643–1651.
79. Brenchley JM, Ruff LE, Casazza JP, Koup RA, Price DA, Douek DC. Preferential infection shortens the life span of human immunodeficiency virus-specific CD4+ T cells in vivo. J Virol 2006; 80:6801–6809.
80. Yue FY, Kovacs CM, Dimayuga RC, Gu XX, Parks P, Kaul R, et al. Preferential apoptosis of HIV-1-specific CD4+ T cells. J Immunol 2005; 174:2196–2204.
81. Kalams SA, Buchbinder SP, Rosenberg ES, Billingsley JM, Colbert DS, Jones NG, et al. Association between virus-specific cytotoxic T-lymphocyte and helper responses in human immunodeficiency virus type 1 infection. J Virol 1999; 73:6715–6720.
82. Boaz MJ, Waters A, Murad S, Easterbrook PJ, Vyakarnam A. Presence of HIV-1 Gag-specific IFN-gamma+IL-2+ and CD28+IL-2+ CD4 T cell responses is associated with nonprogression in HIV-1 infection. J Immunol 2002; 169:6376–6385.
83. Younes SA, Yassine-Diab B, Dumont AR, Boulassel MR, Grossman Z, Routy JP, et al. HIV-1 viremia prevents the establishment of interleukin 2-producing HIV-specific memory CD4+ T cells endowed with proliferative capacity. J Exp Med 2003; 198:1909–1922.
84. Iyasere C, Tilton JC, Johnson AJ, Younes S, Yassine-Diab B, Sekaly RP, et al. Diminished proliferation of human immunodeficiency virus-specific CD4+ T cells is associated with diminished interleukin-2 (IL-2) production and is recovered by exogenous IL-2. J Virol 2003; 77:10900–10909.
85. Day CL, Kaufmann DE, Kiepiela P, Brown JA, Moodley ES, Reddy S, et al.PD-1 expression on HIV-specific T cells is associated with T-cell exhaustion and disease progression.Nature 2006 (in press).
86. Guadalupe M, Reay E, Sankaran S, Prindiville T, Flamm J, McNeil A, et al. Severe CD4+ T-cell depletion in gut lymphoid tissue during primary human immunodeficiency virus type 1 infection and substantial delay in restoration following highly active antiretroviral therapy. J Virol 2003; 77:11708–11717.
87. Sankaran S, Guadalupe M, Reay E, George MD, Flamm J, Prindiville T, et al. Gut mucosal T cell responses and gene expression correlate with protection against disease in long-term HIV-1-infected nonprogressors. Proc Natl Acad Sci USA 2005; 102:9860–9865.
88. Talal AH, Monard S, Vesanen M, Zheng Z, Hurley A, Cao Y, et al. Virologic and immunologic effect of antiretroviral therapy on HIV-1 in gut-associated lymphoid tissue. J Acquir Immune Defic Syndr 2001; 26:1–7.
89. Mattapallil JJ, Smit-McBride Z, Dailey P, Dandekar S. Activated memory CD4(+) T helper cells repopulate the intestine early following antiretroviral therapy of simian immunodeficiency virus-infected rhesus macaques but exhibit a decreased potential to produce interleukin-2. J Virol 1999; 73:6661–6669.
90. George MD, Reay E, Sankaran S, Dandekar S. Early antiretroviral therapy for simian immunodeficiency virus infection leads to mucosal CD4+ T-cell restoration and enhanced gene expression regulating mucosal repair and regeneration. J Virol 2005; 79:2709–2719.
91. Berkhout B. It's the virus, stupid—part 2. Retrovirology 2005; 2:78.
92. Katzenstein DA. Of molecules, macaques and global AIDS. AIDS 2006; 20:1065–1066.
93. Van Opijnen T, Berkhout B. The host environment drives HIV-1 fitness. Rev Med Virol 2005; 15:219–233.
94. Van Opijnen T, Jeeninga RE, Boerlijst MC, Pollakis GP, Zetterberg V, Salminen M, et al. Human immunodeficiency virus type 1 subtypes have a distinct long terminal repeat that determines the replication rate in a host-cell-specific manner. J Virol 2004; 78:3675–3683.
95. Jeeninga RE, Hoogenkamp M, Armand-Ugon M, de Baar M, Verhoef K, Berkhout B. Functional differences between the long terminal repeat transcriptional promoters of HIV-1 subtypes A through G. J Virol 2000; 74:3740–3751.
96. Van Opijnen T, Kamoschinski J, Jeeninga RE, Berkhout B. The human immunodeficiency virus type 1 promoter contains a CATA box instead of a TATA box for optimal transcription and replication. J Virol 2004; 78:6883–6890.
97. Verhoef K, Sanders RW, Fontaine V, Kitajima S, Berkhout B. Evolution of the HIV-1 LTR promoter by conversion of an NF-kB enhancer element into a GABP binding site. J Virol 1999; 73:1331–1340.
98. Lemieux AM, Pare ME, Audet B, Legault E, Lefort S, Boucher N, et al. T cell activation leads to poor activation of HIV-1 clade E LTR and weak association of NF-kappa B and NFAT to its enhancer region. J Biol Chem 2004; 279:52949–52960.
99. Centlivre M, Sommer P, Michel M, Fang RH, Gofflo S, Valladeau J, et al. HIV-1 clade promoters strongly influence spatial and temporal dynamics of viral replication in vivo. J Clin Invest 2005; 115:348–358.
100. Centlivre M, Sommer P, Michel M, Ho Tsong FR, Gofflo S, Valladeau J, et al. The HIV-1 clade C promoter is particularly well adapted to replication in the gut in primary infection. AIDS 2006; 20:657–666.
101. Sala M, Centlivre M, Wain-Hobson S. Clade-specific differences in active viral replication and compartmentalisation. Curr Opin HIV AIDS 2006; 1:108–114.
102. van der Hoek L, Sol CJ, Snijders F, Bartelsman JF, Boom R, Goudsmit J. Human immunodeficiency virus type 1 RNA populations in faeces with higher homology to intestinal populations than to blood populations. J Gen Virol 1996; 77:2415–2425.
103. Kaleebu P, Ross A, Morgan D, Yirrell D, Oram J, Rutebemberwa A, et al. Relationship between HIV-1 Env subtypes A and D and disease progression in a rural Ugandan cohort. AIDS 2001; 15:293–299.
104. Kanki PJ, Hamel DJ, Sankale JL, Hsieh C, Thior I, BarKKin F, et al. Human immunodeficiency virus type 1 subtypes differ in disease progression. J Infect Dis 1999; 179:68–73.
105. Rinke de Wit TF, Tsegaye A, Wolday D, Hailu B, Aklilu M, Sanders E, et al. Primary HIV-1 subtype C infection in Ethiopia. J Acquir Immune Defic Syndr 2002; 30:463–470.
106. Hu DJ, Vanichseni S, Mastro TD, Raktham S, Young NL, Mock PA, et al. Viral load differences in early infection with two HIV-1 subtypes. AIDS 2001; 15:683–691.
107. Blackard JT, Renjifo B, Fawzi W, Hertzmark E, Msamanga G, Mwakagile D, et al. HIV-1 LTR subtype and perinatal transmission. Virology 2001; 287:261–265.
108. Renjifo B, Fawzi W, Mwakagile D, Hunter D, Msamanga G, Spiegelman D, et al. Differences in perinatal transmission among human immunodeficiency virus type 1 genotypes. J Hum Virol 2001; 4:16–25.
109. Pilcher CD, Tien HC, Eron JJ Jr, Vernazza PL, Leu SY, Stewart PW, et al. Brief but efficient: acute HIV infection and the sexual transmission of HIV. J Infect Dis 2004; 189:1785–1792.
110. Montano MA, Nixon CP, N'dung'u T, Bussmann H, Novitsky VA, Dickman D, et al. Elevated tumor necrosis factor-alpha activation of human immunodeficiency virus type 1 subtype C in Southern Africa is associated with an NF-kB enhancer gain-of-function. J Infect Dis 2000; 181:76–81.
111. Belec L, Gherardi R, Payan C, Prazuck T, Malkin JE, Tevi-Benissan C, et al. Proinflammatory cytokine expression in cervicovaginal secretions of normal and HIV-infected women. Cytokine 1995; 7:568–574.
112. Abel K, Rocke DM, Chohan B, Fritts L, Miller CJ. Temporal and anatomic relationship between virus replication and cytokine gene expression after vaginal simian immunodeficiency virus infection. J Virol 2005; 79:12164–12172.
113. Cheret A, Le Grand R, Caufour P, Dereuddre-Bosquet N, Matheux F, Neildez O, et al. Cytokine mRNA expression in mononuclear cells from different tissues during acute SIVmac251 infection of macaques. AIDS Res Hum Retroviruses 1996; 12:1263–1272.
114. Lévy Y. Cytokine-based modulation of immune function in HIV infection. Curr Opin HIV AIDS 2006; 1:69–73.
115. Marchetti G, Franzetti F, Gori A. Partial immune reconstitution following highly active antiretroviral therapy: can adjuvant interleukin-2 fill the gap? J Antimicrob Chemother 2005; 55:401–409.
116. Pau AK, Tavel JA. Therapeutic use of interleukin-2 in HIV-infected patients. Curr Opin Pharmacol 2002; 2:433–439.
117. Pett SL, Kelleher AD. Cytokine therapies in HIV-1 infection: present and future. Expert Rev Anti Infect Ther 2003; 1:83–96.
118. Marchetti G, Meroni L, Molteni C, Taskaris G, Gazzola L, Galli M, et al. IL-7/IL-7 receptor system regulation following IL-2 immunotherapy in HIV-infected patients. Antivir Ther 2004; 9:447–452.
119. Vento S, Cainelli F, Temesgen Z. Interleukin-2 therapy and CD4+ T cells in HIV-1 infection. Lancet 2006; 367:93–95.
120. Marrack P, Kappler J. Control of T cell viability. Annu Rev Immunol 2004; 22:765–787.
121. Surh CD, Sprent J. Regulation of mature T cell homeostasis. Semin Immunol 2005; 17:183–191.
122. Llano A, Barretina J, Gutierrez A, Blanco J, Cabrera C, Clotet B, et al. Interleukin-7 in plasma correlates with CD4 T-cell depletion and may be associated with emergence of syncytium-inducing variants in human immunodeficiency virus type 1-positive individuals. J Virol 2001; 75:10319–10325.
123. Muthukumar A, Wozniakowski A, Gauduin MC, Paiardini M, McClure HM, Johnson RP, et al. Elevated interleukin-7 levels not sufficient to maintain T-cell homeostasis during simian immunodeficiency virus-induced disease progression. Blood 2004; 103:973–979.
124. Napolitano LA, Grant RM, Deeks SG, Schmidt D, De Rosa SC, Herzenberg LA, et al. Increased production of IL-7 accompanies HIV-1-mediated T-cell depletion: implications for T-cell homeostasis. Nat Med 2001; 7:73–79.
125. Fry TJ, Moniuszko M, Creekmore S, Donohue SJ, Douek DC, Giardina S, et al. IL-7 therapy dramatically alters peripheral T-cell homeostasis in normal and SIV-infected nonhuman primates. Blood 2003; 101:2294–2299.
126. Nugeyre MT, Monceaux V, Beq S, Cumont MC, Ho Tsong FR, Chene L, et al. IL-7 stimulates T cell renewal without increasing viral replication in simian immunodeficiency virus-infected macaques. J Immunol 2003; 171:4447–4453.
127. Nunnari G, Pomerantz RJ. IL-7 as a potential therapy for HIV-1-infected individuals. Expert Opin Biol Ther 2005; 5:1421–1426.
128. Lelievre JD, Petit F, Arnoult D, Ameisen JC. Estaquier J Interleukin 7 increases human immunodeficiency virus type 1 LAI-mediated Fas-induced T-cell death. J Virol 2005; 79:3195–3199.
129. Nunnari G, Xu Y, Acheampong EA, Fang J, Daniel R, Zhang C, et al. Exogenous IL-7 induces Fas-mediated human neuronal apoptosis: potential effects during human immunodeficiency virus type 1 infection. J Neurovirol 2005; 11:319–328.
130. Scripture-Adams DD, Brooks DG, Korin YD, Zack JA. Interleukin-7 induces expression of latent human immunodeficiency virus type 1 with minimal effects on T-cell phenotype. J Virol 2002; 76:13077–13082.
131. Wang FX, Xu Y, Sullivan J, Souder E, Argyris EG, Acheampong EA, et al. IL-7 is a potent and proviral strain-specific inducer of latent HIV-1 cellular reservoirs of infected individuals on virally suppressive HAART. J Clin Invest 2005; 115:128–137.
132. Lederman MM, Veazey RS, Offord R, Mosier DE, Dufour J, Mefford M, et al. Prevention of vaginal SHIV transmission in rhesus macaques through inhibition of CCR5. Science 2004; 306:485–487.
133. Veazey RS, Shattock RJ, Pope M, Kirijan JC, Jones J, Hu Q, et al. Prevention of virus transmission to macaque monkeys by a vaginally applied monoclonal antibody to HIV-1 gp120. Nat Med 2003; 9:343–346.
134. Belyakov IM, Hel Z, Kelsall B, Kuznetsov VA, Ahlers JD, Nacsa J, et al. Mucosal AIDS vaccine reduces disease and viral load in gut reservoir and blood after mucosal infection of macaques. Nat Med 2001; 7:1320–1326.
135. Kuznetsov VA, Stepanov VS, Berzofsky JA, Belyakov IM. Assessment of the relative therapeutic effects of vaccines on virus load and immune responses in small groups at several time points: efficacy of mucosal and subcutaneous polypeptide vaccines in rhesus macaques exposed to SHIV. J Clin Virol 2004; 31(Suppl 1):S69–S82.
136. Belyakov IM, Berzofsky JA. Immunobiology of mucosal HIV infection and the basis for development of a new generation of mucosal AIDS vaccines. Immunity 2004; 20:247–253.
137. Veazey R, Lackner A. The mucosal immune system and HIV-1 infection. AIDS Rev 2003; 5:245–252.
138. Campbell DJ, Butcher EC. Rapid acquisition of tissue-specific homing phenotypes by CD4(+) T cells activated in cutaneous or mucosal lymphoid tissues. J Exp Med 2002; 195:135–141.
139. Cromwell MA, Veazey RS, Altman JD, Mansfield KG, Glickman R, Allen TM, et al. Induction of mucosal homing virus-specific CD8(+) T lymphocytes by attenuated simian immunodeficiency virus. J Virol 2000; 74:8762–8766.
Keywords:

HIV-1 subtypes; GALT; primary infection; CD4 memory T cell; viral replication; interleukin-7; immunotherapy

© 2007 Lippincott Williams & Wilkins, Inc.