Primary HIV infection (PHI) is associated with a burst of viremia, massive systemic dissemination, and generalized immune activation.1-4 Although the impact on absolute CD4 T-cell count in the plasma may be minimal and transient, the impact on gut CD4 T cells and other arms of the immune system is profound and possibly irreversible.5-7 Despite multiple lines of evidence demonstrating that cytotoxic T lymphocytes (CTL) play an important role in viremic control during HIV-1 infection, we still do not have a complete understanding of the protective correlates of immunity or deleterious consequences of generating this response in the setting of acute viremia and immune activation.8,9 CTL have numerous effector functions, most notably polyfunctional cytokine secretion and proliferative capacity, which have been demonstrated in long-term nonprogressors, those patients able to control virus without antiretroviral therapy.10-13 Although these types of responses seem to be deficient in patients with a relative lack of viral control, it remains unclear if polyfunctional and proliferative CTL contribute to the containment of virus or are simply the result of successful viral inhibition by other mechanisms.
Several groups have demonstrated a progressive functional loss of CTL with disease progression. HIV-1-specific CTL lose proliferative capacity before non-HIV-specific CTL, and this correlates with increasing viral load and loss of absolute CD4 T cells.14-16 Cytokine secretion is also compromised in progressive disease with loss of interleukin (IL)-2, TNF-α, and eventually interferon (IFN)-γ.17,18 It remains unclear whether all patients with HIV-1 generate polyfunctional CTL with proliferative capacity that is lost with progressive disease or if only a subset of patients are capable of generating these responses. Furthermore, several laboratories have suggested that IL-2 secretion, from either dual IFN-γ/IL-2-secreting CD8 T cells or CD4 T cells, is necessary to support CD8 T-cell proliferative capacity but whether this occurs at the outset during PHI has not been demonstrated.18,19
Because all patients demonstrate some level of viremic control in PHI and likely have sufficient CD4 T cell help at the time these cells are generated, we hypothesized that polyfunctional HIV-1-specific CD8 T cells capable of viral control are present in most patients in PHI and have a phenotype of IL-2 secretion and proliferative capacity. Here, we demonstrate that HIV-1-specific CTL with proliferative capacity are not uniformly observed in PHI. In addition, the capacity to secrete cytokines after specific HIV-1 peptide stimulation is characterized by significant inter- and intrapatient variability. These defects are only partly explained by the lack of IL-2-producing HIV-1-specific CD4 or CD8 T cells in early infection and do not obviously correlate with polyfunctional T-cell responses. This work adds to our understanding of the correlates of immunity to HIV infection and challenges the paradigm of using surrogate responses such as cytokine secretion to measure vaccine efficacy.
Subjects and HLA Typing
Blood was collected from HIV-infected volunteers (n = 13) at the University of Alabama at Birmingham after obtaining written informed consent, as approved by the Institutional Review Board. HLA typing was performed by the tissue typing center at the University of Alabama at Birmingham Hospital (Micro SSP HLA Typing System, Canoga Park, CA). Volunteers were identified early in infection as part of the National Institutes of Health, sponsored Acute Infection and Early Disease Research Program.20
Quantification of HIV-1 RNA in Plasma and Absolute CD4 T-Cell Count
Plasma HIV-1 RNA levels (copies/mL) were measured using Amplicor Ultra Sensitive HIV-1 Monitor assay (version 1.5; Roche Diagnostics Systems, Indianapolis, IN). Plasma CD4 T-cell counts (cells/mm3) were determined as described previously.21,22
Peripheral blood mononuclear cells (PBMCs) were stimulated using (1) optimized HLA-restricted HIV-1 CD8 T-cell epitopes (9- to 11-mers) (10 μM), as described in the LANL HIV Immunology Database, (2) CMV/EBV/influenza peptides (CEF) pool (10 μM) and (3) HIV-1 pools (Gag, Pol, Nef, Env, accessory) composed of overlapping peptides, 15-to 20-mers, representing the entirety of HIV-1 (2 μg/mL) (the National Institutes of Health AIDS Research and Reference Reagent Program), or (4) staphylococcal enterotoxin B (SEB) (1 μg/mL) (positive control).
Cryopreserved PBMCs were thawed, stained, and visualized with trypan blue to assess viability. They were labeled with 1.25 μM CFSE [5-(and-6)-carboxyfluorescein diacetate-succinimidyl ester] (Molecular Probes, Eugene, OR) for 4 minutes at room temperature. After washing in phosphate-buffered saline, 1 × 106 PBMCs were cultured in complete RPMI with 10% AB serum in 48-well plates at 37°C and 5% CO2. Where stated, PBMCs were depleted of CD4 T cells using Dynal beads (Invitrogen, Carlsbad, CA). Depletion was determined to be >96% by flow cytometry. IL-2 was added to cultures at 50 IU/mL where noted.
PBMCs or CD4 T-cell-depleted populations were stimulated with appropriate antigens. We performed studies using 3, 5, or 7 days for the CFSE proliferation assay. We chose a 7-day assay as we found this to be optimal in our studies. After 7 days, the amount of CFSE per cell was determined by flow cytometry. The criterion for a positive proliferative response was determined to be twice that of media alone (background).
Intracellular Cytokine Staining Assay
Standard intracellular cytokine staining was performed as described.23,24 1 × 106 PBMCs were plated in 48-well plates and stimulated with appropriate antigens for 1 hour followed by incubation with monensin and brefeldin A for 4 more hours.
For proliferation and intracellular cytokine staining (ICCS) assays, surface staining was performed with anti-CD3 (allophycocyanin [APC] or Pacific Blue), anti-CD4 (phycoerythrin [PE]), and anti-CD8 (PerCP or PerCP Cy5.5). Cytokine secretion was detected with anti-IFN-γ(Alexa 700), anti-IL-2 (fluorescein isothiocyanate [FITC]), and tumor necrosis factor (TNF)-α(PE Cy7). A standard lymphocyte gate was selected from forward scatter vs side scatter plots. Next, CD3+ T cells were selected. A minimum of 5 × 105 CD3+ events were collected. From these populations, proliferation by CFSE was determined. For ICCS, CD3+CD8+ or CD3+ CD4+ T cells were gated for cytokine analysis (IFN-γ vs IL-2; IFN-γ vs TNF-α). Events were collected using an LSRII flow cytometer (Becton Dickinson, Franklin Lakes, NJ) and the data analyzed using FlowJo software (v8.1, Ashland, OR), as described.24 T cells were analyzed for polyfunctional cytokine responses using PESTLE (version 1.4) and SPICE (version 4.1) software (courtesy of Mario Roederer, Vaccine Research Center, Bethesda, MD). After the gates for each of the 3 single functions were created, we used the Boolean gate platform to create the 8 possible response patterns. After background correction (media only), the percentage of CD8 T cells producing 1-3 functions was quantified.
Using Prism software (La Jolla, CA), statistical analysis included nonparametric Mann-Whitney U test to compare differences between categorical variables. Spearman rank was used as the nonparametric test to compare the relationship between continuous variables. P values less than 0.05 were considered significant.
HIV-1-Specific CD8 T-Cell Proliferative Capacity in PHI
Despite extensive analysis of CD8 T-cell proliferation in chronic infection, evaluation of this parameter has been limited in PHI. We therefore examined CD8 T cells for proliferative capacity in patients identified and administered antiretroviral therapy within 4 months of seroconversion. Thirteen subjects were analyzed for HIV-1-specific CD8 T-cell proliferative responses. A representative CFSE proliferation plot is shown (Fig. 1). We identified 6 patients with HIV-1-specific CD8 T-cell proliferation (proliferators, defined as CD8 T-cell proliferation to at least 1 tested optimized CTL epitope) and 7 without HIV-1-specific CD8 T-cell proliferation (nonproliferators, lack of proliferative response to any of the tested optimized CTL epitopes). Clinical parameters, including CD4 count and viral load (at entry and assay date), and demographic characteristics were not statistically different between the groups (Table 1). Two of 6 proliferators had protective HLA alleles (B*27, B*51). Despite this, most proliferative responses observed were not restricted by these protective alleles (Figs. 1, 2A, B).
Of the cohort, approximately 75% (10 of 13 subjects) had detectable proliferative responses to non-HIV antigens (CEF). CEF proliferation magnitude was similar between HIV-1 proliferators and nonproliferators (data not shown). Of those with CEF proliferative responses, most (6 of 10 subjects) had HIV-1-specific CD8 T proliferative responses. Across the cohort, we tested a total of 24 HLA-1-restricted HIV-1 epitopes chosen for their ability to elicit strong IFN-γ ELISpot responses (>200 Spot Forming Cells [SFC]/106 cells)22 and 17 of 24 epitopes elicited HIV-1-specific HIV proliferative responses in 1 or more of the 6 “proliferators.”
We next wanted to rescue HIV-specific responses by adding exogenous IL-2. To do this, we further examined 5 patients lacking an HIV-1-specific CD8 T-cell proliferative response to at least 1 tested epitope. In each case, the subjects demonstrated either proliferation to CEF or at least 1 other HIV-1 CD8 T-cell epitope. We were only able to recover 1 of 8 absent proliferative responses, in this case to a nonoptimized 20-mer that elicited CD8 but not CD4 T-cell responses (Table 2). Thus, even in the earliest stages of HIV-1 infection, CTL proliferation is compromised in many patients as evidenced by IFN-γ-secreting CD8 T cells lacking proliferative capacity.
CD4-Independent CD8 T-Cell Proliferation
To better understand components necessary to generate an optimal HIV-1-specific CD8 T-cell response with proliferative capacity, we sought to determine the functional contributions from CD8 and CD4 T cells. Previous studies suggested that the production of cytokines by proliferating CD8 T cells, specifically IL-2, supports proliferative capacity to HIV antigens.19 Because the addition of exogenous IL-2 did not consistently recover lost proliferative responses, we reasoned that some CD8 T-cell proliferative responses in acute disease were primed in the setting of optimal CD4 T cell help and maintained despite the loss of ongoing CD4 help. Furthermore, the majority of the proliferative responses that we detected were to optimized 9-11 amino acid epitopes, expected to elicit CD8 but not CD4 T-cell responses. To prove that CD8 T-cell proliferation can occur independently after in vivo priming, we chose 2 patients (based on sample availability) from our acute cohort and antigenically stimulated PBMCs in the presence (Figs. 1, 2B) or absence (Figs. 2A, B) of CD4 T cells. Even in the absence of CD4 T cells or exogenous IL-2, CD8 T cells were able to maintain proliferative capacity to optimized epitopes. Furthermore, the magnitude of proliferation in response to several epitopes was comparable, regardless of CD4 T cell presence (Figs. 2B). We did observe significant CD8 downregulation on T cells with relatively little evidence of proliferation from this population except to SEB (Fig. 2A). These CD8-low T cells did not costain with CD4.
Antigen-Specific Cytokine Secretion From CD4 and CD8 T Cells in PHI
Several studies have indicated that cytokine secretion, including IFN-γ and IL-2, is associated with viral control.11,19,21,24 These functions, especially IL-2, are lost in chronic progressive disease but are present in controllers and some patients treated during PHI.24 Furthermore, these cytokines may define a CTL phenotype, established early in infection, capable of viral control. Since neither exogenous IL-2 nor coculture of CD4 T cells is necessary to elicit HIV-specific CD8 T-cell proliferation, we sought to determine if functional characteristics of CD8 T cells predict and/or supports their own proliferation. To determine if IL-2 secreted from the CD8 T cells supports proliferation as previously reported,19 we performed ICCS after peptide stimulation, with optimized CD8 T-cell epitopes on 2 subjects demonstrating CD4-independent CD8 T-cell proliferation (Fig. 2B). We found no evidence of dual secretors (IFN-γ/IL-2), when CD8 T cells were stimulated with the optimized epitope only, despite detectable dual secretors from the same sample when stimulated with SEB (Fig. 2C).
To get a more comprehensive assessment of CD4 and CD8 T cell elicited cytokines generated from HIV, we stimulated PBMCs with the entirety of HIV using peptide pools (15-to 20-mers) encompassing Gag, Pol, Env, Nef, and accessory proteins and determined the cytokine secretion of IFN-γ, IL-2, and TNF-α from CD4 T cells and CD8 T cells (Figs. 3A, B). Cumulative cohort data for CD4 (Fig. 3C) and CD8 (Fig. 3D) T-cell responses to peptide pools is shown. We demonstrated Env-dominant IL-2 responses from both CD4 and CD8 T cells, a phenotype previously described.22 Again, we sought to determine if cytokine production correlated with epitope-specific proliferation. Although we demonstrate CD8 T-cell proliferative responses to a variety of optimized epitopes, there was no association between the magnitude of the proliferative response to a given epitope and the magnitude of cytokine secretion from the pool containing the same optimized epitope eliciting the proliferative response.
We next examined polyfunctional cytokine secretion from proliferators and nonproliferators. Surprisingly, we found very few triple functioning cells (IFN-γ, IL-2, and TNF-α) at the earliest time points (data not shown). Rather, the distinguishing characteristic between the 2 groups was the capacity of CD8 (Figs. 4B, D), but not CD4 (Figs. 4A, C), T cells to secrete IL-2 in response to peptide pools. Approximately 50% of total CD8 T-cell cytokine response from proliferators was composed of IL-2 compared with less than 20% in nonproliferators (Fig. 4B). Furthermore, the frequency of IL-2-secreting CD8 T cells was greater in proliferators than in nonproliferators (P = 0.01) (Fig. 4D). From this, it is apparent that IL-2 secretion from CD4 T cells is not associated with CD8 T-cell proliferation as measured by the percent of total response (Fig. 4A) or frequency (Fig. 4C). We also found a significant correlation between the magnitude of IL-2 secreted by CD8 T cells and the total proliferation of all HIV-specific CD8 T cells within a patient (r = 0.6340; P < 0.02). Notably, although total HIV-specific CD8 T-cell proliferative response within a patient correlated with CD8 T-cell IL-2 secretion, it was not generally produced from the same antigen-specific CTL that was capable of proliferating (Fig. 2C). In contrast, no correlation was detected between IL-2 secretion from CD4 T cells and the CD8 T-cell proliferative response in early infection (r = 0.3350; P = 0.26). Neither the total IFN-γ secretion from CD4 or CD8 T cells nor the percentage of multifunctional CD4 or CD8 T cells correlated with proliferation (data not shown).
Understanding the correlates of immune protection in HIV-1 infection is imperative to our efforts in the development and evaluation of candidate vaccines. Many studies focus on responses generated early in PHI and maintained in elite or viremic controllers. Here, we sought to understand the phenotype of HIV-1-specific CD8 T cells in PHI, which are capable of proliferation as these along with their cytolytic capacity are most predictive of viral control.12,13,18,25,26 Despite testing all patients within a few months of infection, only half of our cohort had preserved HIV-1-specific CD8 T-cell proliferative responses. Therefore, we postulated that CD8 T cells were able to support their own proliferation as described for HLA-A*2 SL-9-specific CTL.27 On stimulation with optimized HIV-1 CTL epitopes, cells capable of proliferation did not uniformly have a phenotype of dual cytokine secretion (IFN-γ/IL-2). Instead, we demonstrate that HIV-1 peptide pool-stimulated CD8 T cells from proliferators elicit a heterogeneous population with a greater frequency of IL-2-secreting CD8, but not CD4, T cells when compared with nonproliferators.
The relative lack of proliferative responses in the majority of patients with PHI was unexpected. Many have postulated that CD4 help is necessary in the early priming of CD8 T cells.28 During acute infection, there is a transient decline in absolute CD4 T-cell number in the systemic circulation but profound early depletion at mucosal sites.7,29 Our cohort was comprised of patients presenting with symptomatic infection and may be skewed toward individuals with very high viral loads and immune activation. It is plausible that proliferating CD8 T cells were primed very early during infection with optimal CD4 help. Subsequently, antigen-specific CD4 T cells may have been deleted or became anergic, leaving behind CD8 T cells that function well for a period but rarely persist. In contrast, nonproliferating CD8 T cells initially may have had proliferative potential but eventually became anergic in the highly activated environment of acute HIV infection.30 Alternatively, these suboptimal functioning CD8 T cells may have been primed later during infection with little or no CD4 help, making them compromised from the start. Finally, there may be intrinsic differences in the ability of CD8 T cells to proliferate, which are independent of CD4 help.13,27 The fact that we did not observe a correlation between CD8 T-cell proliferation and time since infection supports the latter mechanism. Notably, all subjects were treated in this study; thus, early treatment does not explain observed differences in proliferative capacity. Furthermore, while 2 of 13 subjects had protective alleles, only a minority of the observed proliferative responses were to epitopes restricted by these alleles.
In this altered setting of acute HIV infection, it is also possible that seemingly paradoxical immune responses are triggered to dampen the cytokine storm by secreting cytokines such as IL-10, which may have a detrimental response on HIV-1-specific T-cell responses. Recent studies demonstrate that blockade of IL-10 in viremic HIV subjects can reverse HIV-1-specific CD8 and CD4 proliferative defects.31 Furthermore, the function of CD4 T cells could be compromised in settings of such high viremia and may have contributed to development of suboptimal CD8 T-cell responses in nonproliferators. This theory is supported by the relative lack of polyfunctional CD4 T cells detected in our cohort, unlike that seen in elite controllers and LTNP.
Controversy exists over the importance of CD8 T-cell phenotype and the interplay of IL-2 and proliferation. Although studies have implicated IL-2 secretion from CD8 T cells as a signature phenotype of proliferators, our findings suggest that it may only be a marker for a broader population of cells with heterogeneous functions. Disparate results have been published regarding the ability to recover defective CD8 T-cell proliferation with the addition of exogenous IL-2 or CD4 T cells.18,32,33 We were unable to recover CD8 proliferative capacity with the addition of IL-2 ex vivo, suggesting that CD4-mediated paracrine signaling does not support long-term CD8 T-cell proliferation. This is supported by recent literature demonstrating that although exogenous IL-2 induces dividing cells to enter more cycles of division, it is unable to rescue cells with defective proliferation.34 Clinical trials utilizing the therapeutic administration of IL-2 validate our data as these studies have consistently failed to show a restoration of CD8 T-cell responses or control of viremia.35 These data suggest that cellular factors other than IL-2 secretion may regulate a CD8 T cell's ability to proliferate. Differences in IL-2 receptor expression or signaling may be a better phenotypic representation of HIV-1-specific CD8 T cells capable of proliferation and would explain the differences seen in recent studies and in the present study, which indicates compromised proliferative capacity in PHI.36
In those subjects capable of CD4-independent CD8 T-cell proliferation, we could not consistently identify dual (IFN-γ/IL-2) secreting HIV-1-specific CD8 T cells after stimulation with an optimized epitope. This suggests that a simple autocrine signaling pathway supporting CD8 proliferation is also unlikely. It is notable that groups describing dual secreting CD8 T cells with proliferative capacity primarily used peptide pools for stimulation, whereas our study focused initially on optimized epitopes. Therefore, we now describe a heterogeneous group of CD8 T cells in subjects with proliferative capacity. Within this heterogeneous groups of cells, some exhibited IL-2 secretion or proliferative capacity and few dual secretors with proliferative capacity as described elsewhere.19 This apparent “dichotomous relationship” in IL-2 secretion and effector function was recently highlighted by the observation that perforin secretion and IL-2 secretion do not occur simultaneously.37,38 This supports our observation that the effector function, proliferation, is not always synonymous with IL-2 secretion from the same cell. Furthermore, the detection of IL-2-secreting cells likely represents a cytokine milieu capable of sustaining effector functions rather than defining a cellular phenotype capable of proliferative capacity. Patients with superior viral control likely have this heterogeneous population as described.
One of the interesting findings of this study was the dominance of IL-2 production from Env-specific CD4 and CD8 T cells. Although the underlying mechanism of this result is not clear, one possible explanation is that B cells may be playing an important role in antigen presentation.39 This may be especially true in the context of HIV-1 infection where monocytes and macrophages are likely functionally skewed early during infection.40,41 Immunoglobulin present on the surface of B cells may be taking up Env and efficiently presenting this antigen to CD4 T cells, inducing a qualitatively different response than those induced by monocyte or macrophage presentation (ie, more IL-2). In this scenario, Env could be presented by B cells more than other HIV-1 proteins because Env is dominantly present at the viral surface, making it more accessible for B cell-immunoglobulin interaction.
Numerous studies have examined functionality of CD8 T cells to better understand the correlates of immunity. Distinguishing the immunological cause of viral control from the deleterious effects of viremia and immune activation remains a challenge. Studies comparing long term non-progressors (LTNP) and successfully treated patients have sought to determine which immune parameters can be recovered by viral control and thus are unlikely to be the correlates of immune control in Elite Controllers [EC] or LTNP. However, our studies are focused on understanding immune control or compromise occurring during the earliest time points of infection, which may determine the level of subsequent viral control. Viral load alone is unlikely to explain differences we observed because all our patients had detectable viral load at the time of sampling. Differences in our studies relative to published literature may be explained in part by differences in cohorts, type of antigenic stimulation, and variability in cell analysis. However, our ability to focus on acute/early events of HIV-1-specific T cells stimulated with optimized epitopes allows this study to generate more precise conclusions about the generation of optimal CD8 T cells during the course of early infection.
Although we continue to define the correlates of immune protection, our focus must remain on early and longitudinal studies of the generation and maintenance of protective immune responses. Furthermore, it is essential to continue to delineate those responses that fluctuate with levels of viremia such as polyfunctionality vs those markers that truly identify unique cellular phenotypes predictive of control. Here, we demonstrate that not all CD8 T-cell responses generated in acute infection have proliferative capacity and that the phenotype of IL-2 secretion, although globally important to support functional CD8 T cells, does not in and of itself define the proliferative phenotype. Rather a more heterogeneous population exists. This is particularly important as we move forward in our evaluations of candidate vaccines, wherein we are searching for those favorable phenotypes both established during acute infection and preserved among elite controllers.
The authors thank the volunteers for their participation, Marion Spell for help with flow cytometric acquisition, and the Center for AIDS Research repository for patient sample processing.
1. Daar ES, Moudgil T, Meyer RD, et al. Transient high levels of viremia in patients with primary human immunodeficiency virus type 1 infection. N Engl J Med
2. Clark SJ, Saag MS, Decker WD, et al. High titers of cytopathic virus in plasma of patients with symptomatic primary HIV-1 infection. N Engl J Med
3. Boasso A, Shearer GM. Chronic innate immune activation as a cause of HIV-1 immunopathogenesis. Clin Immunol
4. Douek D. HIV disease progression: immune activation, microbes, and a leaky gut. Top HIV Med
5. Veazey RS, DeMaria M, Chalifoux LV, et al. Gastrointestinal tract as a major site of CD4+ T cell depletion and viral replication in SIV infection. Science
6. Brenchley JM, Price DA, Schacker TW, et al. Microbial translocation is a cause of systemic immune activation in chronic HIV infection. Nat Med
7. Brenchley JM, Schacker TW, Ruff LE, et al. CD4+ T cell depletion during all stages of HIV disease occurs predominantly in the gastrointestinal tract. J Exp Med
8. Koup RA, Safrit JT, Cao Y, et al. Temporal association of cellular immune responses with the initial control of viremia in primary human immunodeficiency virus type 1 syndrome. J Virol
9. Jin X, Bauer DE, Tuttleton SE, et al. Dramatic rise in plasma viremia after CD8(+) T cell depletion in simian immunodeficiency virus-infected macaques. J Exp Med
10. Almeida JR, Price DA, Papagno L, et al. Superior control of HIV-1 replication by CD8+ T cells is reflected by their avidity, polyfunctionality, and clonal turnover. J Exp Med
11. Betts MR, Nason MC, West SM, et al. HIV nonprogressors preferentially maintain highly functional HIV-specific CD8+ T cells. Blood
12. Migueles SA, Laborico AC, Shupert WL, et al. HIV-specific CD8+ T cell proliferation is coupled to perforin expression and is maintained in nonprogressors. Nat Immunol
13. Horton H, Frank I, Baydo R, et al. Preservation of T cell proliferation restricted by protective HLA alleles is critical for immune control of HIV-1 infection. J Immunol
14. Jagannathan P, Osborne CM, Royce C, et al. Comparisons of CD8+ T cells specific for human immunodeficiency virus, hepatitis C virus, and cytomegalovirus reveal differences in frequency, immunodominance, phenotype, and interleukin-2 responsiveness. J Virol
15. Benito JM, Lopez M, Soriano V. The role of CD8+ T-cell response in HIV infection. AIDS Rev
16. Day CL, Kiepiela P, Leslie AJ, et al. Proliferative capacity of epitope-specific CD8 T-cell responses is inversely related to viral load in chronic human immunodeficiency virus type 1 infection. J Virol
17. Betts MR, Ambrozak DR, Douek DC, et al. Analysis of total human immunodeficiency virus (HIV)-specific CD4(+) and CD8(+) T-cell responses: relationship to viral load in untreated HIV infection. J Virol
18. Lichterfeld M, Kaufmann DE, Yu XG, et al. Loss of HIV-1-specific CD8+ T cell proliferation after acute HIV-1 infection and restoration by vaccine-induced HIV-1-specific CD4+ T cells. J Exp Med
19. Zimmerli SC, Harari A, Cellerai C, et al. HIV-1-specific IFN-gamma/IL-2-secreting CD8 T cells support CD4-independent proliferation of HIV-1-specific CD8 T cells. Proc Natl Acad Sci U S A
20. Kilby JM, Lee HY, Hazelwood JD, et al. Treatment response in acute/early infection versus advanced AIDS: equivalent first and second phases of HIV RNA decline. AIDS
21. Edwards BH, Bansal A, Sabbaj S, et al. Magnitude of functional CD8+ T-cell responses to the gag protein of human immunodeficiency virus type 1 correlates inversely with viral load in plasma. J Virol
22. Bansal A, Gough E, Sabbaj S, et al. CD8 T-cell responses in early HIV-1 infection are skewed towards high entropy peptides. AIDS
23. Fuller MJ, Hildeman DA, Sabbaj S, et al. Cutting edge: emergence of CD127high functionally competent memory T cells is compromised by high viral loads and inadequate T cell help. J Immunol
24. Sabbaj S, Heath SL, Bansal A, et al. Functionally competent antigen-specific CD127(hi) memory CD8+ T cells are preserved only in HIV-infected individuals receiving early treatment. J Infect Dis
25. Saez-Cirion A, Sinet M, Shin SY, et al. Heterogeneity in HIV suppression by CD8 T cells from HIV controllers: association with Gag-specific CD8 T cell responses. J Immunol
26. Calarota SA, Foli A, Maserati R, et al. HIV-1-specific T cell precursors with high proliferative capacity correlate with low viremia and high CD4 counts in untreated individuals. J Immunol
27. Kan-Mitchell J, Bisikirska B, Wong-Staal F, et al. The HIV-1 HLA-A2-SLYNTVATL is a help-independent CTL epitope. J Immunol
28. Sun JC, Bevan MJ. Defective CD8 T cell memory following acute infection without CD4 T cell help. Science
29. Mehandru S, Poles MA, Tenner-Racz K, 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
30. Turnbull EL, Wong M, Wang S, et al. Kinetics of expansion of epitope-specific T cell responses during primary HIV-1 infection. J Immunol
31. Brockman MA, Kwon DS, Tighe DP, et al. IL-10 is up-regulated in multiple cell types during viremic HIV infection and reversibly inhibits virus-specific T cells. Blood
32. Yu J, Chen H, Horton H, et al. Interleukin-2 reconstitutes defective human immunodeficiency virus (HIV), and cytomegalovirus (CMV) specific CD8+ T cell proliferation in HIV infection. J Med Virol
33. Iyasere C, Tilton JC, Johnson AJ, 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
34. Migueles SA, Weeks KA, Nou E, et al. Defective human immunodeficiency virus-specific CD8+ T-cell polyfunctionality, proliferation, and cytotoxicity are not restored by antiretroviral therapy. J Virol
35. Abrams D, Levy Y, Losso MH, et al. Interleukin-2 therapy in patients with HIV infection. N Engl J Med
36. Chopra RK, Raj NB, Scally JP, et al. Relationship between IL-2 receptor expression and proliferative responses in lymphocytes from HIV-1 seropositive homosexual men. Clin Exp Immunol
37. Makedonas G, Hutnick N, Haney D, et al. Perforin and IL-2 upregulation define qualitative differences among highly functional virus-specific human CD8 T cells. PLoS Pathog
38. Hersperger AR, Pereyra F, Nason M, et al. Perforin expression directly ex vivo by HIV-specific CD8 T-cells is a correlate of HIV elite control. PLoS Pathog
39. Crawford A, Macleod M, Schumacher T, et al. Primary T cell expansion and differentiation in vivo requires antigen presentation by B cells. J Immunol
40. Fantuzzi L, Belardelli F, Gessani S. Monocyte/macrophage-derived CC chemokines and their modulation by HIV-1 and cytokines: a complex network of interactions influencing viral replication and AIDS pathogenesis. J Leukoc Biol
41. Estes JD, Harris LD, Klatt NR, et al. Damaged intestinal epithelial integrity linked to microbial translocation in pathogenic simian immunodeficiency virus infections. PLoS Pathog
. 2010;6(8). pii:e1001052.