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.
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 . 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 . As a consequence, CD4 memory T-cell depletion is more rapid, profound and visible in the GALT during HIV-1 primary infection . Since the memory T-cell subset – by their normal intrinsic function – generate faster and more efficient immune responses against pathogenic infections , 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 . 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 . 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  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 . 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 . Li et al. 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].
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. 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) . 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 . Nevertheless, the physiological relevance of this phenomenon can be questioned in regard to the high dose of gp120 used in such ex vivo studies .
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 . 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’ . 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 .
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 , and complete restoration is not observed during the chronic phase . This phenomenon is also observed in HIV-1 infected patients . 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 . During chronic infection the high rate of T-cell death is a consequence rather than a cause of immune activation .
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−) . 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 . 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 ; (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 . 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].
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 . 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 . 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  and these cells are more prone to apoptosis . 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 . 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 . 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 . 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 . 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 . 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 .
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 . 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 . 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  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 . 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 . 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 .
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  and between subtypes B and E . 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 , 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).
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 . 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).
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 . 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 . 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 . 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  and exogenous IL-7 induces Fas-mediated human neuronal apoptosis , 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] . 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 .
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.
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.
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