Mucosal immunity in HIV controllers: the right place at the right time

Comparison of the Biological Basis for Non-HIV Transmission to HIV-Exposed Seronegative Individuals, Disease Non-Progression in HIV Long-Term Non-Progressors and Elite Controllers

HIV-exposed seronegative individuals (HESIs) are a small fraction of persons who are multiply exposed to human immunodeficiency virus (HIV), but do not exhibit serological or clinical evidence of HIV infection. In other words, they are groups of people maintaining an uninfected status for a long time, even after being exposed to HIV several times. The long-term non-progressors (LTNPs), on the other hand, are a group of HIV-infected individuals (approx. 5%) who remain clinically and immunologically stable for an extended number of years without combination antiretroviral therapy (cART). Meanwhile, elite controllers are comprise a much lower number (0.5%) of HIV-infected persons who spontaneously and durably control viremia to below levels of detection for at least 12 months, even when using the most sensitive assays, such as polymerase chain reaction (PCR) in the absence of cART. Despite the fact that there is no universal agreement regarding the mechanisms by which these groups of individuals are able to control HIV infection and/or disease progression, there is a general consensus that the mechanisms of protection are multifaceted and include genetic, immunological as well as viral factors. In this review, we analyze and compare the biological factors responsible for the control of HIV in these unique groups of individuals.

Deciphering the Role of Mucosal Immune Responses and the Cervicovaginal Microbiome in Resistance to HIV Infection in HIV-Exposed Seronegative (HESN) Women

The female genital tract (FGT) is an important site of human immunodeficiency virus (HIV) infection. Discerning the nature of HIV-specific local immune responses is crucial for identifying correlates of protection in HIV-exposed seronegative (HESN) individuals. The present study involved a comprehensive analysis of soluble immune mediators, secretory immunoglobulins (sIg), natural killer (NK) cells, CXCR5⁺ CD8⁺ T cells, T follicular helper (Tfh) cells, and T regulatory cells (Tregs) in the vaginal mucosa as well as the nature and composition of the cervicovaginal microbiome in HESN women. We found significantly elevated antiviral cytokines, soluble immunoglobulins, and increased frequencies of activated NK cells, CXCR5⁺ CD8⁺ T cells, and Tfh cells in HESN females compared to HIV-unexposed healthy (UH) women. Analysis of the genital microbiome of HESN women revealed a greater bacterial diversity and increased abundance of Gardnerella spp. in the mucosa. The findings suggest that the female genital tract of HESN females represents a microenvironment equipped with innate immune factors, antiviral mediators, and critical T cell subsets that protect against HIV infection.

IMPORTANCE The vast majority of human immunodeficiency virus (HIV) infections across the world occur via the sexual route. The genital tract mucosa is thus the primary site of HIV replication, and discerning the nature of HIV-specific immune responses in this compartment is crucial. The role of the innate immune system at the mucosal level in exposed seronegative individuals and other HIV controllers remains largely unexplored. This understanding can provide valuable insights to improve vaccine design. We investigated mucosal T follicular helper (Tfh) cells, CXCR5⁺ CD8⁺ T cells, natural killer (NK) cells subsets, soluble immune markers, and microbiome diversity in HIV-exposed seronegative (HESN) women. We found a significantly higher level of mucosal CXCR5⁺ CD8⁺ T cells, CD4⁺ Tfh cells, activated NK cell subsets, and antiviral immune cell mediators in HESN women. We also found a higher abundance of Gardnerella spp., microbiome dysbiosis, and decreased levels of inflammatory markers to be associated with reduced susceptibility to HIV infection. Our findings indicate that increased distribution of mucosal NK cells, CXCR5⁺ CD8⁺ T cells, Tfh cells, and soluble markers in HIV controllers with a highly diverse cervicovaginal microbiome could contribute effectively to protection against HIV infection. Overall, our findings imply that future vaccine design should emphasize inducing these highly functional cell types at the mucosal sites.

Mucosal IL-4R antagonist HIV vaccination with SOSIP-gp140 booster can induce high-quality cytotoxic CD4+/CD8+ T cells and humoral responses in macaques

Inducing humoral, cellular and mucosal immunity is likely to improve the effectiveness of HIV-1 vaccine strategies. Here, we tested a vaccine regimen in pigtail macaques using an intranasal (i.n.) recombinant Fowl Pox Virus (FPV)-gag pol env-IL-4R antagonist prime, intramuscular (i.m.) recombinant Modified Vaccinia Ankara Virus (MVA)-gag pol-IL-4R antagonist boost followed by an i.m SOSIP-gp140 boost. The viral vector-expressed IL-4R antagonist transiently inhibited IL-4/IL-13 signalling at the vaccination site. The SOSIP booster not only induced gp140-specific IgG, ADCC (antibody-dependent cellular cytotoxicity) and some neutralisation activity, but also bolstered the HIV-specific cellular and humoral responses. Specifically, superior sustained systemic and mucosal HIV Gag-specific poly-functional/cytotoxic CD4⁺ and CD8⁺ T cells were detected with the IL-4R antagonist adjuvanted strategy compared to the unadjuvanted control. In the systemic compartment elevated Granzyme K expression was linked to CD4⁺ T cells, whilst Granzyme B/TIA-1 to CD8⁺ T cells. In contrast, the cytotoxic marker expression by mucosal CD4⁺ and CD8⁺ T cells differed according to the mucosal compartment. This vector-based mucosal IL-4R antagonist/SOSIP booster strategy, which promotes cytotoxic mucosal CD4⁺ T cells at the first line of defence, and cytotoxic CD4⁺ and CD8⁺ T cells plus functional antibodies in the blood, may prove valuable in combating mucosal infection with HIV-1 and warrants further investigation.

Does Mucosal B1 Activation Result in the Accumulation of Peak IgM During Chronic Intrarectal SIVmac239 Exposure to Protect Chinese-Origin Rhesus Macaques From Disease Progression?

Human immunodeficiency virus (HIV) infection is characterized by a dynamic process and highly variable progression. Although extensive comparisons have been reported between the minority of non-progressors (NPGs) and the majority of progressors (PGs), the underlying mechanism is still unclear. One reason for this is that the initial onset of infection is very difficult to track, particularly when men who have sex with men (MSM) are predominantly responsible for the transmission of human HIV. To find potential early protection strategies against later progression during chronic mucosal exposure, 10 Chinese-origin rhesus macaques (ChRhs) that underwent repetitive simian immunodeficiency virus (SIV) intrarectal exposure were longitudinally tracked. The results of the periodic detection of peripheral blood mononuclear cells (PBMCs) and colorectal mucosal lamina propria mononuclear cells (LPMCs) with immunoglobulins in rectal fluid were compared between non-progressive and progressive subgroups, which were classified based on their circulating viral loads. As a result, four NPGs and six PGs were observed after disease onset for 2 months. Upon comparing the mucosal and systemic immune responses, the PBMC response did not differ between the two subgroups. Regarding LPMCs, the increased activation of B1a/B1 cells among B cells and a peak in IgM in rectal fluid was observed approximately 10 days after the first exposure, followed by consistently low viremia in the four non-progressive ChRhs. In the six progressive ChRhs, neither B cell activation nor a peak in IgM was observed, while a robust elevation in IgG was observed, followed by consistently high viremia post exposure. Based on the PBMC-LPMC disparity between the subgroups of monkeys, we hypothesize that early B1 activation in LPMCs that result in an IgM peak might attenuate the entry and acquisition of SIV in the mucosa, resulting in very low dissemination into blood. Our models have suggested that the use of early surveillance both systemically and in the mucosa to comprehensively determine virus–host interactions would be informative for mucosal vaccine development.

Mucosal vaccine efficacy against intrarectal SHIV is independent of anti-Env antibody response

It is widely believed that protection against acquisition of HIV or SIV infection requires anti-envelope (anti-Env) antibodies, and that cellular immunity may affect viral loads but not acquisition, except in special cases. Here we provide evidence to the contrary. Mucosal immunization may enhance HIV vaccine efficacy by eliciting protective responses at portals of exposure. Accordingly, we vaccinated macaques mucosally with HIV/SIV peptides, modified vaccinia Ankara-SIV (MVA-SIV), and HIV-gp120-CD4 fusion protein plus adjuvants, which consistently reduced infection risk against heterologous intrarectal SHIVSF162P4 challenge, both high dose and repeated low dose. Surprisingly, vaccinated animals exhibited no anti-gp120 humoral responses above background and Gag- and Env-specific T cells were induced but failed to correlate with viral acquisition. Instead, vaccine-induced gut microbiome alteration and myeloid cell accumulation in colorectal mucosa correlated with protection. Ex vivo stimulation of the myeloid cell-enriched population with SHIV led to enhanced production of trained immunity markers TNF-α and IL-6, as well as viral coreceptor agonist MIP1α, which correlated with reduced viral Gag expression and in vivo viral acquisition. Overall, our results suggest mechanisms involving trained innate mucosal immunity together with antigen-specific T cells, and also indicate that vaccines can have critical effects on the gut microbiome, which in turn can affect resistance to infection. Strategies to elicit similar responses may be considered for vaccine designs to achieve optimal protective efficacy.

Detection of HIV-1-specific gastrointestinal tissue resident CD8+ T-cells in chronic infection

Tissue-resident memory (TRM) CD8+ T-cells are non-recirculating, long-lived cells housed in tissues that can confer protection against mucosal pathogens. Human immunodeficiency virus-1 (HIV-1) is a mucosal pathogen and the gastrointestinal tract is an important site of viral pathogenesis and transmission. Thus, CD8+ TRM cells may be an important effector subset for controlling HIV-1 in mucosal tissues. This study sought to determine the abundance, phenotype, and functionality of CD8+ TRM cells in the context of chronic HIV-1 infection. We found that the majority of rectosigmoid CD8+ T-cells were CD69+CD103+S1PR1- and T-betLowEomesoderminNeg, indicative of a tissue-residency phenotype similar to that described in murine models. HIV-1-specific CD8+ TRM responses appeared strongest in individuals naturally controlling HIV-1 infection. Two CD8+ TRM subsets, distinguished by CD103 expression intensity, were identified. CD103Low CD8+ TRM primarily displayed a transitional memory phenotype and contained HIV-1-specific cells and cells expressing high levels of Eomesodermin, whereas CD103High CD8+ TRM primarily displayed an effector memory phenotype and were EomesoderminNeg. These findings suggest a large fraction of CD8+ T-cells housed in the human rectosigmoid mucosa are tissue-resident and that TRM contribute to the anti-HIV-1 immune response. Further exploration of CD8+ TRM will inform development of anti-HIV-1 immune-based therapies and vaccines targeted to the mucosa.

Treatment Intensification in HIV-Infected Patients Is Associated With Reduced Frequencies of Regulatory T Cells

In untreated HIV infection, the efficacy of T cell responses decreases over the disease course, resulting in disease progression. The reasons for this development are not completely understood. However, immunosuppressive cells are supposedly crucially involved. Treatment strategies to avoid the induction of these cells preserve immune functions and are therefore the object of intense research efforts. In this study, we assessed the effect of treatment intensification [=5-drug antiretroviral therapy (ART)] on the development of suppressive cell subsets. The New Era (NE) study recruited patients with primary HIV infection (PHI) or chronically HIV-infected patients with conventional ART (CHI) and applied an intensified 5-drug regimen containing maraviroc and raltegravir for several years. We compared the frequencies of the immune suppressive cells, namely, the myeloid-derived suppressor cells (MDSCs), regulatory B cells (Bregs), and regulatory T cells (Tregs), of the treatment intensification patients to the control groups, especially to the patients with conventional 3-drug ART, and analyzed the Gag/Nef-specific CD8 T cell responses. There were no differences between PHI and CHI in the NE population (p > 0.11) for any of the studied cell types. Polymorphonuclear myeloid-derived suppressor cell (PMN-MDSC), monocytic myeloid-derived suppressor cell (M-MDSC), and the Breg frequencies were comparable to those of patients with a 3-drug ART. However, the Treg levels were significantly lower in the NE patients than those in 3ART-treated individuals and other control groups (p ≤ 0.0033). The Gag/Nef-specific CD8 T cell response was broader (p = 0.0134) with a higher magnitude (p = 0.026) in the NE population than that in the patients with conventional ART. However, we did not find a correlation between the frequency of the immune suppressive cells and the interferon-gamma⁺ CD8 T cell response. In the treatment intensification subjects, the frequencies of the immune suppressive cells were comparable or lower than those of the conventional ART-treated subjects, with surprisingly broad HIV-specific CD8 T cell responses, suggesting a preservation of immune function with the applied treatment regimen. Interestingly, these effects were seen in both treatment intensification subpopulations and were not attributed to the start of treatment in primary infection.

Differential Expression of CD8+ T Cell Cytotoxic Effector Molecules in Blood and Gastrointestinal Mucosa in HIV-1 Infection

We previously reported that CD8⁺ T cells in human gastrointestinal mucosa exhibit reduced perforin expression and weak or impaired cytotoxic capacity compared with their counterparts in blood. Nevertheless, these cells degranulate and express cytokines and chemokines in response to cognate Ag. In addition to weak expression of perforin, earlier studies suggested differential regulation of perforin and granzymes (Gzms), with GzmA and B expressed by significantly higher percentages of mucosal CD8⁺ T cells than perforin. However, this topic has not been fully explored. The goal of this study was to elucidate the expression and coexpression patterns of GzmA, B, and K in conjunction with perforin in rectosigmoid CD8⁺ T cells during HIV-1 infection. We found that expression of both perforin and GzmB, but not GzmA or GzmK, was reduced in mucosa compared with blood. A large fraction of rectosigmoid CD8⁺ T cells either did not express Gzms or were single-positive for GzmA. Rectosigmoid CD8⁺ T cells appeared skewed toward cytokine production rather than cytotoxic responses, with cells expressing multiple cytokines and chemokines generally lacking in perforin and Gzm expression. These data support the interpretation that perforin and Gzms are differentially regulated, and display distinct expression patterns in blood and rectosigmoid T cells. These studies may help inform the development of strategies to combat HIV-1 and other mucosal pathogens.

Predominance of weakly cytotoxic, T-betLowEomesNeg CD8+ T-cells in human gastrointestinal mucosa: implications for HIV infection

The gastrointestinal mucosa is an important site of HIV acquisition, viral replication, and pathogenesis. Immune cells in mucosal tissues frequently differ in phenotype and function from their non-mucosal counterparts. Although perforin-mediated cytotoxicity as measured in blood is a recognized correlate of HIV immune control, its role in gastrointestinal tissues is unknown. We sought to elucidate the cytotoxic features of rectal mucosal CD8⁺ T-cells in HIV infected and uninfected subjects. Perforin expression and lytic capacity were significantly reduced in rectal CD8⁺ T-cells compared with their blood counterparts, regardless of HIV clinical status; granzyme B (GrzB) was reduced to a lesser extent. Mucosal perforin and GrzB expression were higher in participants not on antiretroviral therapy compared with those on therapy and controls. Reduction in perforin and GrzB was not explained by differences in memory/effector subsets. Expression of T-bet and Eomesodermin was significantly lower in gut CD8⁺ T-cells compared with blood, and in vitro neutralization of TGF-β partially restored perforin expression in gut CD8⁺ T-cells. These findings suggest that rectal CD8⁺ T-cells are primarily non-cytotoxic, and phenotypically shaped by the tissue microenvironment. Further elucidation of rectal immune responses to HIV will inform the development of vaccines and immunotherapies targeted to mucosal tissues.

Mucosal and Systemic γδ+ T Cells Associated with Control of Simian Immunodeficiency Virus Infection

γδ T cells act as a first line of defense against invading pathogens. However, despite their abundance in mucosal tissue, little information is available about their functionality in this compartment in the context of HIV/SIV infection. In this study, we evaluated the frequency, phenotype, and functionality of Vδ1 and Vδ2 T cells from blood, rectum, and the female reproductive tract (FRT) of rhesus macaques to determine whether these cells contribute to control of SIV infection. No alteration in the peripheral Vδ1/Vδ2 ratio in SIV-infected macaques was observed. However, CD8⁺ and CD4⁺CD8⁺ Vδ1 T cells were expanded along with upregulation of NKG2D, CD107, and granzyme B, suggesting cytotoxic function. In contrast, Vδ2 T cells showed a reduced ability to produce the inflammatory cytokine IFN-γ. In the FRT of SIV⁺ macaques, Vδ1 and Vδ2 showed comparable levels across vaginal, ectocervical, and endocervical tissues; however, endocervical Vδ2 T cells showed higher inflammatory profiles than the two other regions. No sex difference was seen in the rectal Vδ1/Vδ2 ratio. Several peripheral Vδ1 and/or Vδ2 T cell subpopulations expressing IFN-γ and/or NKG2D were positively correlated with decreased plasma viremia. Notably, Vδ2 CD8⁺ T cells of the endocervix were negatively correlated with chronic viremia. Overall, our results suggest that a robust Vδ1 and Vδ2 T cell response in blood and the FRT of SIV-infected macaques contribute to control of viremia.

In silico design of a DNA-based HIV-1 multi-epitope vaccine for Chinese populations

The development of an HIV-1 vaccine that is capable of inducing effective and broadly cross-reactive humoral and cellular immune responses remains a challenging task because of the extensive diversity of HIV-1, the difference of virus subtypes (clades) in different geographical regions, and the polymorphism of human leukocyte antigens (HLA). We performed an in silico design of 3 DNA vaccines, designated pJW4303-MEG1, pJW4303-MEG2 and pJW4303-MEG3, encoding multi-epitopes that are highly conserved within the HIV-1 subtypes most prevalent in China and can be recognized through HLA alleles dominant in China. The pJW4303-MEG1-encoded protein consisted of one Th epitope in Env, and one, 2, and 6 epitopes in Pol, Env, and Gag proteins, respectively, with a GGGS linker sequence between epitopes. The pJW4303-MEG2-encoded protein contained similar epitopes in a different order, but with the same linker as pJW4303-MEG1. The pJW4303-MEG3-encoded protein contained the same epitopes in the same order as that of pJW4303-MEG2, but with a different linker sequence (AAY). To evaluate immunogenicity, mice were immunized intramuscularly with these DNA vaccines. Both pJW4303-MEG1 and pJW4303-MEG2 vaccines induced equally potent humoral and cellular immune responses in the vaccinated mice, while pJW4303-MEG3 did not induce immune responses. These results indicate that both epitope and linker sequences are important in designing effective epitope-based vaccines against HIV-1 and other viruses.

[Key role played by the gut associated lymphoid tissue during human immunodeficiency virus infection]

The gut associated lymphoid tissue (GALT) is the site of numerous immunological disturbances during HIV-1 infection. It constitutes the largest reservoir for HIV, not or very poorly susceptible to antiretroviral therapy (ART), making it a major obstacle to HIV cure. Moreover, the GALT is involved in systemic immune activation in HIV-infected individuals: intestinal damage due to viral replication and severe CD4(+) T cell depletion in the GALT leads to microbial translocation, a key driver of immune activation, and in turn, disease progression. In this review, we describe the role of the GALT in HIV infection and we discuss therapeutic options to decrease the intestinal viral reservoir and to preserve immune function in the gut of HIV-infected people. Achieving these goals is necessary for a long-term infection control after the interruption of ART.

Identification of biomarkers to measure HIV-specific mucosal and systemic CD8(+) T-cell immunity using single cell Fluidigm 48.48 Dynamic arrays

Thirty genes composed of cytokines, chemokines, granzymes, perforin and integrins were evaluated in gut and splenic K(d)Gag197-205-specific single CD8(+) T cells using Fluidigm 48.48 Dynamic arrays, with the aim of identifying biomarkers to predict effective mucosal and systemic vaccine efficacy. The mRNA expression profiles were analyzed in three ways: (i) the "number" of K(d)Gag197-205-specific CD8(+) T cells expressing the biomarker, (ii) "level" of mRNA expression using principal component analysis (PCA) and (iii) poly-functionality in relation to RANTES expression. In total, 21 genes were found to be differentially expressed between the vaccine groups and the immune compartments tested. Overall, the PCA indicated that IL-13Rα2 or IL-4R antagonist adjuvanted vaccines that previously induced high-avidity mucosal/systemic CD8(+) T cells with better protective efficacy, the "level" of mRNA expression, specifically RANTES, MIP-1β, and integrin α4 in gut K(d)Gag197-205-specific single CD8(+) T cells, were significantly elevated compared to unadjuvanted vaccine. Furthermore, significantly elevated granzymes/perforin levels were detected in IL-13(-/-) mice given the unadjuvanted vaccine, indicating that the degree of IL-13 inhibition (total, transient or no inhibition) can considerably alter the level of T-cell activity/poly-functionality. When splenic- and gut-K(d)Gag197-205-specific CD8(+) T cells were compared, PC1 vs. PC2 scores revealed that not only RANTES, MIP-1β, and integrin α4 mRNA, but also perforin, granzymes A/B, and integrins β1 and β2 mRNA were elevated in spleen. Collectively, data suggest that RANTES, MIP-1β, perforin, and integrins α4, β1 and β7 mRNA in single HIV-specific CD8(+) T cells could be used as a measure of effective mucosal and systemic vaccine efficacy.

Different HIV pox viral vector-based vaccines and adjuvants can induce unique antigen presenting cells that modulate CD8 T cell avidity

The lung-derived dendritic cell (LDC) recruitment following intranasal (i.n.) vaccination of different poxviral vector-based vaccines/adjuvants were evaluated to decipher how these factors influenced CD8(+) T cell avidity. Compared to the standard i.n. recombinant fowlpox virus (FPV)-HIV vaccination, the FPV-HIV IL-13Rα2 or IL-4Rα antagonist adjuvanted vaccines that induced higher avidity CD8(+) T cells, also recruited significantly elevated MHCII(+) CD11c(+) CD11b(+) CD103(-) CD64(-) MAR-1(-) conventional DC (cDCs) to the lung mucosae (hierarchy: IL-4R antagonist>IL-13Rα2>unadjuvanted). In contrast, elevated CD11b(-) CD103(+) LDCs were detected in animals that received recombinant HIV vaccinia virus (rVV) or Modified Vaccinia Ankara virus (MVA) vector-based vaccines. Adoptive transfer studies indicated that CD11b(-) CD103(+) LDCs significantly dampened HIV-specific CD8(+) T cell avidity compared to CD11b(+) CD103(-) LDCs. Collectively; our observations revealed that rFPV vector prime and transient inhibition of IL-4/IL-13 at the vaccination site favoured the recruitment of unique LDCs, associated with the induction of high quality immunity.

HIV-1 adenoviral vector vaccines expressing multi-trimeric BAFF and 4-1BBL enhance T cell mediated anti-viral immunity

Adenoviral vectored vaccines have shown considerable promise but could be improved by molecular adjuvants. Ligands in the TNF superfamily (TNFSF) are potential adjuvants for adenoviral vector (Ad5) vaccines based on their central role in adaptive immunity. Many TNFSF ligands require aggregation beyond the trimeric state (multi-trimerization) for optimal biological function. Here we describe Ad5 vaccines for HIV-1 Gag antigen (Ad5-Gag) adjuvanted with the TNFSF ligands 4-1BBL, BAFF, GITRL and CD27L constructed as soluble multi-trimeric proteins via fusion to Surfactant Protein D (SP-D) as a multimerization scaffold. Mice were vaccinated with Ad5-Gag combined with Ad5 expressing one of the SP-D-TNFSF constructs or single-chain IL-12p70 as adjuvant. To evaluate vaccine-induced protection, mice were challenged with vaccinia virus expressing Gag (vaccinia-Gag) which is known to target the female genital tract, a major route of sexually acquired HIV-1 infection. In this system, SP-D-4-1BBL or SP-D-BAFF led to significantly reduced vaccinia-Gag replication when compared to Ad5-Gag alone. In contrast, IL-12p70, SP-D-CD27L and SP-D-GITRL were not protective. Histological examination following vaccinia-Gag challenge showed a dramatic lymphocytic infiltration into the uterus and ovaries of SP-D-4-1BBL and SP-D-BAFF-treated animals. By day 5 post challenge, proinflammatory cytokines in the tissue were reduced, consistent with the enhanced control over viral replication. Splenocytes had no specific immune markers that correlated with protection induced by SP-D-4-1BBL and SP-D-BAFF versus other groups. IL-12p70, despite lack of anti-viral efficacy, increased the total numbers of splenic dextramer positive CD8+ T cells, effector memory T cells, and effector Gag-specific CD8+ T cells, suggesting that these markers are poor predictors of anti-viral immunity in this model. In conclusion, soluble multi-trimeric 4-1BBL and BAFF adjuvants led to strong protection from vaccinia-Gag challenge, but the protection was independent of standard immune markers. Soluble multi-trimeric SP-D-4-1BBL and SP-D-BAFF provide a novel technology to enhance adenoviral vector vaccines against HIV-1.

Immune control of HIV-1 reservoirs

PURPOSE OF REVIEW

To discuss the recent major advances in the understanding of how host immune defenses contribute to HIV reservoir control.

RECENT FINDINGS

Immune control of HIV-1 reservoirs is a two-step process: viral replication activation from latent reservoirs followed by elimination of virus-expressing cells by the host. Environmental factors, such as pro-inflammatory type-I interferon, chemokines or cytokines, can facilitate HIV-1 replication, confer dormancy in CD4 cells or confer resistance to cytopathogenic effects of cytotoxic CD8 T cells. Therefore, they constitute a double-edged sword for immune control of HIV reservoirs. Concomitantly, adaptive immunity takes advantage of CD4 T-cell homeostatic mechanisms and can expose HIV-1 antigen-expressing cells to HIV-specific cytotoxic CD8 T cells, and limit virus spreading. These highly interconnected phenomena can lead to quasi-equilibrium between the HIV-1 reservoirs and host immune control that can serve as a model for the 'shock and kill' immune-based therapeutic strategies in play in the course of finding an HIV cure.

SUMMARY

Immune control of HIV reservoirs in CD4 T cells involves modulation of both HIV-1 latency and the continuous reseeding of the reservoir offering conceptual models that may advance HIV cure strategies.

Probiotic/prebiotic supplementation of antiretrovirals improves gastrointestinal immunity in SIV-infected macaques

HIV infection results in gastrointestinal (GI) tract damage, microbial translocation, and immune activation, which are not completely ameliorated with suppression of viremia by antiretroviral (ARV) therapy. Furthermore, increased morbidity and mortality of ARV-treated HIV-infected individuals is associated with these dysfunctions. Thus, to enhance GI tract physiology, we treated SIV-infected pigtail macaques with ARVs, probiotics, and prebiotics or with ARVs alone. This synbiotic treatment resulted in increased frequency and functionality of GI tract APCs, enhanced reconstitution and functionality of CD4+ T cells, and reduced fibrosis of lymphoid follicles in the colon. Thus, ARV synbiotic supplementation in HIV-infected individuals may improve GI tract immunity and thereby mitigate inflammatory sequelae, ultimately improving prognosis.

Innate immunity in the control of HIV/AIDS: recent advances and open questions

From the publication of the first AIDS issue onwards, major advances have been made in the field of innate immunity during HIV infection. Innate immunity can be defined as the first and unspecific lines of defense constitutively present and ready to be mobilized upon infection. Although a large body of literature adamantly highlights that innate immunity is a critical weapon of defense against HIV and its simian parents (simian immunodeficiency virus, SIV), innate immunity is still underexplored. Focusing on innate immunity may open new paths for the development of innovative therapeutics and vaccine strategies against HIV. Understanding innate immunity may shed light on the natural protection occurring in rare HIV-1-infected individuals who control their infection. This review focuses on innate mechanisms sensing HIV-1 entry and controlling HIV-1 infection, as well as promoting inflammation and shaping adaptive immunity.

Characterization of an effective CTL response against HIV and SIV infections

A vaccine inducing protective immunity in mucosal tissues and secretions may stop or limit HIV infection. Although cytotoxic T lymphocytes (CTLs) are clearly associated with control of viral replication in HIV and simian immunodeficiency virus (SIV) infections, there are examples of uncontrolled viral replication in the face of strong CD8⁺ T-cell responses. The number of functions, breadth, avidity, and magnitude of CTL response are likely to be important factors in the effectiveness of anti-HIV T-cell response, but the location and persistence of effector CD8⁺ T cells are also critical factors. Although the only HIV vaccine clinical trial targeting cellular immunity to prevent HIV infection failed, vaccine strategies using persistent agents against pathogenic mucosal challenge in macaque models are showing unique success. Thus, the key to control the initial focus of viral replication at the portal of entry may rely on the continuous generation of effector CTL responses at mucosal level.

Human immunodeficiency virus (HIV) immunopathogenesis and vaccine development: a review

The development of a safe, effective and globally affordable HIV vaccine offers the best hope for the future control of the HIV-1 pandemic. Since 1987, scores of candidate HIV-1 vaccines have been developed which elicited varying degrees of protective responses in nonhuman primate models, including DNA vaccines, subunit vaccines, live vectored recombinant vaccines and various prime-boost combinations. Four of these candidate vaccines have been tested for efficacy in human volunteers, but, to the exception of the recent RV144 Phase III trial in Thailand, which elicited a modest but statistically significant level of protection against infection, none has shown efficacy in preventing HIV-1 infection or in controlling virus replication and delaying progression of disease in humans. Protection against infection was observed in the RV144 trial, but intensive research is needed to try to understand the protective immune mechanisms at stake. Building-up on the results of the RV144 trial and deciphering what possibly are the immune correlates of protection are the top research priorities of the moment, which will certainly accelerate the development of an highly effective vaccine that could be used in conjunction with other HIV prevention and treatment strategies. This article reviews the state of the art of HIV vaccine development and discusses the formidable scientific challenges met in this endeavor, in the context of a better understanding of the immunopathogenesis of the disease.