HIV type 1 antigen-responsive CD4+ T-lymphocytes in exposed yet HIV Type 1 seronegative Ugandans

Mucosa: Key Interactions Determining Sexual Transmission of the HIV Infection

In the context of HIV sexual transmission at the genital mucosa, initial interactions between the virus and the mucosal immunity determine the outcome of the exposure. Hence, these interactions have been deeply explored in attempts to undercover potential targets for developing preventative strategies. The knowledge gained has led to propose a hypothetical model for mucosal HIV transmission. Subsequent research studies on this topic further revealed new mechanisms and identified new host-HIV interactions. This review aims at integrating these findings to inform better and update the current model of HIV transmission. At the earliest stage of virus exposure, the epithelial integrity and the presence of antiviral factors are critical in preventing viral entry to the submucosa. However, the virus has been shown to enter to the submucosa in the presence of physical abrasion or via epithelial transmigration using paracellular passage or transcytosis mechanisms. The efficiency of these processes is greater with cell-associated viral inoculums and can be influenced by the presence of viral and immune factors, and by the structure of the exposed epithelium. Once the virus reaches the submucosa, dendritic cells and fibroblasts, as recently described, have been shown in vitro of being capable of facilitating the transfer of viral particles to susceptible cells, leading to viral dissemination, most likely in a trans-infection manner. The presence of activated CD4⁺ T cells in submucosa increases the probability of infection, where the predominant microbiota could be implicated through the modulation of an inflammatory microenvironment. Other factors such as genital fluids and hormones could also play an essential role in HIV transmission. Here, we review the most recent evidence described for mucosal HIV-transmission contributing with the understanding of this phenomenon.

Role of Different Subpopulations of CD8+ T Cells during HIV Exposure and Infection

During HIV infection, specific responses exhibited by CD8⁺ T cells are crucial to establish an early, effective, and sustained viral control, preventing severe immune alterations and organ dysfunction. Several CD8⁺ T cells subsets have been identified, exhibiting differences in terms of activation, functional profile, and ability to limit HIV replication. Some of the most important CD8⁺ T cells subsets associated with viral control, production of potent antiviral molecules, and strong polyfunctional responses include Th1-like cytokine pattern and Tc17 cells. In addition, the expression of specific activation markers has been also associated with a more effective response of CD8⁺ T cells, as evidenced in HLA-DR⁺ CD38⁻ cells. CD8⁺ T cells in both, peripheral blood and gut mucosa, are particularly important in individuals with a resistant phenotype, including HIV-exposed seronegative individuals (HESNs), long-term non-progressors (LTNPs) and HIV-controllers. Although the role of CD8⁺ T cells has been extensively explored in the context of an established HIV-1 infection, the presence of HIV-specific cells with effector abilities and a defined functional profile in HESNs, remain poorly understood. Here, we reviewed studies carried out on different subpopulations of CD8⁺ T cells in relation with natural resistance to HIV infection and progression.

Regulatory T-Cell Activity But Not Conventional HIV-Specific T-Cell Responses Are Associated With Protection From HIV-1 Infection

OBJECTIVE

Two distinct hypotheses have been proposed for T-cell involvement in protection from HIV-1 acquisition. First, HIV-1-specific memory T-cell responses generated on HIV-1 exposure could mount an efficient response to HIV-1 and inhibit the establishment of an infection. Second, a lower level of immune activation could reduce the numbers of activated, HIV-1-susceptible CD4 T cells, thereby diminishing the likelihood of infection.

METHODS

To test these hypotheses, we conducted a prospective study among high-risk heterosexual men and women, and tested peripheral blood samples from individuals who subsequently acquired HIV-1 during follow-up (cases) and from a subset of those who remained HIV-1 uninfected (controls).

RESULTS

We found no difference in HIV-1-specific immune responses between cases and controls, but Treg frequency was higher in controls as compared with cases and was negatively associated with frequency of effector memory CD4 T cells.

CONCLUSIONS

Our findings support the hypothesis that low immune activation assists in protection from HIV-1 infection.

Maternal plasma and breastmilk viral loads are associated with HIV-1-specific cellular immune responses among HIV-1-exposed, uninfected infants in Kenya

Infants exposed to maternal HIV-1 provide an opportunity to assess correlates of HIV-1-specific interferon (IFN)-γ responses and may be informative in the development of HIV-1 vaccines. HIV-1-infected women with CD4 counts 200-500 cells/mm(3) were randomized to short-course zidovudine/nevirapine (ZDV/NVP) or highly active anti-retroviral therapy (HAART) between 2003 and 2005. Maternal plasma and breastmilk HIV-1 RNA and DNA were quantified during the first 6-12 months postpartum. HIV-1 gag peptide-stimulated enzyme-linked immunospot (ELISPOT) assays were conducted in HIV-1-exposed, uninfected infants (EU), and correlates were determined using regression and generalized estimating equations. Among 47 EU infants, 21 (45%) had ≥1 positive ELISPOT result during follow-up. Infants had a median response magnitude of 177 HIV-1-specific spot-forming units (SFU)/106 peripheral blood mononuclear cells (PBMC) [interquartile range (IQR)=117-287] directed against 2 (IQR = 1-3) gag peptide pools. The prevalence and magnitude of responses did not differ by maternal anti-retroviral (ARV) randomization arm. Maternal plasma HIV-1 RNA levels during pregnancy (P=0.009) and breastmilk HIV-1 DNA levels at 1 month (P=0.02) were associated with a higher magnitude of infant HIV-1-specific ELISPOT responses at 1 month postpartum. During follow-up, concurrent breastmilk HIV-1 RNA and DNA (cell-free virus and cell-associated virus, respectively) each were associated positively with magnitude of infant HIV-1-specific responses (P=0.01). Our data demonstrate the importance of antigenic exposure on the induction of infant HIV-1-specific cellular immune responses in the absence of infection.

Quantitative and qualitative differences in the T cell response to HIV in uninfected Ugandans exposed or unexposed to HIV-infected partners

HIV-exposed and yet persistently uninfected individuals have been an intriguing, repeated observation in multiple studies, but uncertainty persists on the significance and implications of this in devising protective strategies against HIV. We carried out a cross-sectional analysis of exposed uninfected partners in a Ugandan cohort of heterosexual serodiscordant couples (37.5% antiretroviral therapy naive) comparing their T cell responses to HIV peptides with those of unexposed uninfected individuals. We used an objective definition of exposure and inclusion criteria, blinded ex vivo and cultured gamma interferon (IFN-γ) enzyme-linked immunospot assays, and multiparameter flow cytometry and intracellular cytokine staining to investigate the features of the HIV-specific response in exposed versus unexposed uninfected individuals. A response rate to HIV was detectable in unexposed uninfected (5.7%, 95% confidence interval [CI] = 3.3 to 8.1%) and, at a significantly higher level (12.5%, 95% CI = 9.7 to 15.4%, P = 0.0004), in exposed uninfected individuals. The response rate to Gag was significantly higher in exposed uninfected (10/50 [20.%]) compared to unexposed uninfected (1/35 [2.9%]) individuals (P = 0.0004). The magnitude of responses was also greater in exposed uninfected individuals but not statistically significant. The average number of peptide pools recognized was significantly higher in exposed uninfected subjects than in unexposed uninfected subjects (1.21 versus 0.47; P = 0.0106). The proportion of multifunctional responses was different in the two groups, with a higher proportion of single cytokine responses, mostly IFN-γ, in unexposed uninfected individuals compared to exposed uninfected individuals. Our findings demonstrate both quantitative and qualitative differences in T cell reactivity to HIV between HESN (HIV exposed seronegative) and HUSN (HIV unexposed seronegative) subject groups but do not discriminate as to whether they represent markers of exposure or of protection against HIV infection.

The high frequency of HIV type 1-specific cellular immune responses in seronegative individuals with parenteral and/or heterosexual HIV type 1 exposure

Some individuals remain HIV seronegative despite repeated unprotected exposure to the virus. Recent observations led to a concept that acquired immunity plays a role in protection or at least in altered susceptibility to HIV-1 infection in highly exposed seronegative (ESN) individuals. Our aim was to study HIV-specific cellular immune responses induced in parenterally and/or heterosexually ESN individuals. Nine seronegative injection drug users (IDUs), 10 seronegative individuals, and nine of their HIV-positive sexual and/or IDU partners from the cohort of IDUs were included in the study. The discordant couples had unprotected sex, and some of seronegative partners also had parenteral exposure. Cell-mediated responses were measured in peripheral blood mononuclear cells (PBMCs) by ex vivo interferon (IFN)-γ-ELISpot and ICS combining IFN-γ, tumor necrosis factor (TNF)-α, and interleukin (IL)-2 after stimulation with four consensus peptide pools (Nef, Gag, RT, Env, subtype A-EE). Thirteen out of 19 (68%) seronegative study subjects had strong Nef peptide pool-specific ELISpot responses, three (16%) subjects responded against the Gag peptide pool, and one subject had an RT peptide pool response. Nef peptide pool responses in ESN were as high as in seropositive subjects. The multiple HIV-specific cytokine production in both CD4(+) and CD8(+) T cells was shown for several ESN subjects. The functional profiles of the immune responses were different between seronegative and HIV-positive study groups. Whether the observed cellular responses have any protective role against HIV needs to be further investigated.

HIV-1-specific enzyme-linked immunosorbent spot assay responses in HIV-1-exposed uninfected partners in discordant relationships compared to those in low-risk controls

A number of studies of highly exposed HIV-1-seronegative individuals (HESN) have found HIV-1-specific cellular responses. However, there is limited evidence that responses prevent infection or are linked to HIV-1 exposure. Peripheral blood mononuclear cells (PBMC) were isolated from HESN in HIV-1-discordant relationships and low-risk controls in Nairobi, Kenya. HIV-1-specific responses were detected using gamma interferon (IFN-γ) enzyme-linked immunosorbent spot (ELISpot) assays stimulated by peptide pools spanning the subtype A HIV-1 genome. The HIV-1 incidence in this HESN cohort was 1.5 per 100 person years. Positive ELISpot responses were found in 34 (10%) of 331 HESN and 14 (13%) of 107 low-risk controls (odds ratio [OR] = 0.76; P = 0.476). The median immunodominant response was 18.9 spot-forming units (SFU)/10(6) peripheral blood mononuclear cells (PBMC). Among HESN, increasing age (OR = 1.24 per 5 years; P = 0.026) and longer cohabitation with the HIV-1-infected partner (OR = 5.88 per 5 years; P = 0.003) were associated with responses. These factors were not associated with responses in controls. Other exposure indicators, including the partner's HIV-1 load (OR = 0.99 per log(10) copy/ml; P = 0.974) and CD4 count (OR = 1.09 per 100 cells/μl; P = 0.238), were not associated with responses in HESN. HIV-1-specific cellular responses may be less relevant to resistance to infection among HESN who are using risk reduction strategies that decrease their direct viral exposure.

HIV vaccines: can CD4+ T cells be of help?

Defining immune correlates of protection against the human immunodeficiency virus (HIV) remains a major challenge. While the role of neutralizing antibodies and CD8+ T cell responses has been widely acknowledged and applied in vaccine development, little vaccine candidates have focused on CD4+ T cells. As the main target of HIV, CD4+ T cells play a pivotal role in HIV infection. An HIV vaccine that elicits strong, multi-specific, polyfunctional and persisting CD4+ T cell responses would therefore have the potential of lowering viral set point when HIV infection occurs or reducing viral load in already infected patients. In a combined approach with neutralizing antibodies and CD8+ T cells, CD4+ T cells cannot only enhance the magnitude, quality and durability of the desired antibody response, but will also provide the help needed to induce and maintain effective antiviral CD8+ T cell responses. In addition, the disease-modifying potential of the CD4+ T cell response, by lowering viral set point and/or viral load and thus probability of transmission, may be beneficial both at the individual and public health level.

Differences in HIV-specific T cell responses between HIV-exposed and -unexposed HIV-seronegative individuals

HIV-1-specific T lymphocyte responses in individuals exposed to HIV-1 but who remain persistently seronegative (HESNs) have been reported in some but not all previous studies. This study was designed to resolve unequivocally the question of whether HESNs make HIV-1-specific T cell responses. We performed a blind investigation to measure HIV-1-specific T cell responses in both HIV-1-serodiscordant couples and HIV-1-unexposed seronegative controls (HUSNs). We found low-frequency HIV-1-specific T cells in both HESNs and HUSNs but show that the response rates were higher over time in the former (P = 0.01). Furthermore, the magnitudes of the HIV-1-specific T cell responses were significantly higher among responding HESNs than among HUSNs over time (P = 0.002). In both groups, responses were mediated by CD4 T cells. The responses were mapped to single peptides, which often corresponded to epitopes restricted by multiple HLA-DR types that have previously been detected in HIV-1-infected patients. HIV-1-specific T cell responses in HUSNs and some HESNs likely represent cross-reactivity to self or foreign non-HIV-1 antigens. The significantly greater T cell responses in HESNs, including in two who were homozygous for CCR5Δ32, demonstrates that HIV-1-specific T cell responses can be induced or augmented by exposure to HIV-1 without infection.

Epigenetic control of IRF1 responses in HIV-exposed seronegative versus HIV-susceptible individuals

Not all individuals exposed to HIV become infected. Understanding why these HIV-exposed seronegative individuals remain uninfected will help inform the development of preventative measures against HIV infection. Interferon regulatory factor-1 (IRF1) plays a critical role both in host antiviral immunity and in HIV-1 replication. This study examined IRF1 expression regulation in the ex vivo peripheral blood mononuclear cells of HIV-exposed seronegative commercial sex workers who can be epidemiologically defined as relatively resistant to HIV infection (HIV-R), versus HIV-uninfected, susceptible controls (HIV-S). Whereas HIV-susceptible individuals demonstrated a biphasic, prolonged increase in IRF1 expression after interferon-γ stimulation, HIV-R individuals demonstrated a robust, but transient response. We also found that the IRF1 promoter in HIV-R was primed by increased basal histone deacetylase-2 binding, independently of transcription regulators, STAT1 and nuclear factor-κB/p65, implicating an epigenetic silencing mechanism. Interestingly, the transitory IRF1 response in HIV-R was sufficient in comparable regulation of interleukin-12 and interleukin-4 expression compared with the HIV-susceptible controls. This is the first study characterizing IRF1 responsiveness in individuals who demonstrate altered susceptibility to HIV infection. These data suggest that transitory IRF1 responsiveness in HIV-R may be one of the key contributors to the altered susceptibility to HIV infection during the early stages of primary HIV infection.

Host HLA B*allele-associated multi-clade Gag T-cell recognition correlates with slow HIV-1 disease progression in antiretroviral therapy-naïve Ugandans

BACKGROUND

Some HIV infected individuals remain asymptomatic for protracted periods of time in the absence of antiretroviral therapy (ART). Virological control, CD4 T cell loss and HIV-specific responses are some of the key interrelated determinants of HIV-1 disease progression. In this study, possible interactions between viral load, CD4 T cell slopes, host genetics and HIV-specific IFN-gamma responses were evaluated in chronically HIV-1-infected adults.

METHODOLOGY/PRINCIPAL FINDINGS

Multilevel regression modeling was used to stratify clade A or D HIV-infected individuals into disease progression groups based on CD4 T cell slopes. ELISpot assays were used to quantify the frequency and magnitude of HIV-induced IFN-gamma responses in 7 defined rapid progressors (RPs) and 14 defined slow progressors (SPs) at a single time point. HLA typing was performed using reference strand conformational analysis (RSCA). Although neither the breadth nor the magnitude of the proteome-wide HIV-specific IFN-gamma response correlated with viral load, slow disease progression was associated with over-representation of host immunogenetic protective HLA B* alleles (10 of 14 SPs compared to 0 of 7; p = 0.004, Fisher's Exact) especially B*57 and B*5801, multiclade Gag T-cell targeting (71%, 10 of 14 SPs compared to 14%, 1 of 7 RPs); p = 0.029, Fisher's Exact test and evident virological control (3.65 compared to 5.46 log10 copies/mL in SPs and RPs respectively); p<0.001, unpaired student's t-test

CONCLUSIONS

These data are consistent with others that associated protection from HIV disease with inherent host HLA B allele-mediated ability to induce broader Gag T-cell targeting coupled with apparent virological control. These immunogenetic features of Gag-specific immune response which could influence disease progression may provide useful insight in future HIV vaccine design.

Human susceptibility to viral infection: the search for HIV-protective alleles among Africans by means of genome-wide studies

Human immunodeficiency virus (HIV) infection represents a major global health problem, with HIV now recognized as the fourth leading cause of death on a worldwide basis. One approach to developing effective anti- HIV interventions is to identify and understand the molecular mechanisms by which natural genetic variations provide protection from infection or disease progression. This approach can be used to identify human gene alleles that confer resistance or increased susceptibility to HIV infection. To date, however, this approach has been underutilized in the African population and all HIV-resistance alleles that have been described have been identified by evaluating candidate genes. This limited approach is based upon a researcher's assumption that those genes that will provide the host with a benefit can be predicted, a priori, but it does not provide for a large scale systematic screen of all possible candidate genes. Nonetheless, this method has met with some success in identifying HIV-resistance genes, mostly among the white population. The lack of a comprehensive genetic approach, both in terms of the populations studied and the percentage of the genome investigated, likely explains why all of the HIV-restriction alleles identified to date fall within two gene families, and why no resistance genes have been identified among black Africans. It is likely, as with any complex trait, that most protective alleles will provide only partial HIV resistance. Thus, HIV resistance in most persons likely arises through a QTL (quantitative trait loci) mechanism meaning that protection is a polygenic trait. This feature coupled with interpopulation genetic heterogeneity makes the candidate gene mapping approach a daunting task. A comprehensive genome-wide case-control allelic association study in the African population will maximize our chances of identifying new targets for the development of new therapeutics that have the promise of benefiting all persons infected with HIV.

HIV-specific antibodies but not t-cell responses are associated with protection in seronegative partners of HIV-1-infected individuals in Cambodia

To study biological factors related to protection against HIV-1 infection in Cambodia, we recruited 48 partners of HIV-1-infected patients who remained uninfected (exposed uninfected individuals, EUs) despite unprotected sexual intercourse for more than 1 year and 49 unexposed controls (UCs). HIV-1-specific antibodies (IgA anti-gp41 and IgG anti-CD4-gp120 complex), T-cell responses, and cellular factors that may be involved in protection (peripheral blood mononuclear cell [PBMC] resistance to HIV-1 infection and beta-chemokine production) were evaluated. Anti-HIV-1 antibodies were higher in EUs than those in UCs (P = 0.01 and P = 0.04 for anti-gp41 and anti-CD4-gp120, respectively). We observed a decreased susceptibility to a primary Cambodian isolate, HIV-1KH019, in EU PBMCs as compared with UC PBMCs (P = 0.03). A weak T-cell response to one pool of HIV-1 Gag peptides was found by ELISpot in 1 of 19 EUs. Whereas T-cell specific immunity was not associated to protection, our results suggest that HIV-specific humoral immunity and reduced cell susceptibility to infection may contribute to protection against HIV-1 infection in Cambodian EUs.

Increased IFN-γ production by NK and CD3+/CD56+ cells in sexually HIV-1-exposed but uninfected individuals

The mechanisms involved in controlling the establishment of HIV-1 infection are not fully understood. In particular, the role of innate immunity in natural resistance exhibited by individuals who are continuously exposed to HIV-1 but remain seronegative (ESN) has not been thoroughly evaluated. We determined the frequency and function of peripheral blood innate immune cells (plasmacytoid and myeloid dendritic cells, monocytes, NK cells, CD3+/CD56+ cells and invariant NKT cells) in ESN, chronically HIV-1-infected and low-risk HIV-1 seronegative individuals. ESN demonstrated a similar frequency of innate immune cells in comparison to controls and a higher frequency of dendritic cells, NK and invariant NKT cells compared to HIV-1-infected subjects. Incubation of mononuclear cells with stimulatory CpG ODN induced CD86 and CD69 up-regulation to a similar degree on innate cells from the three study groups. CpG ODN-stimulated secretion of cytokines was also similar between ESN and controls, while secretion of IFN-alpha was significantly decreased in HIV-1+ individuals. Importantly, expression of IFN-gamma by PMA/Ionomycin-activated CD56(bright) NK cells and CD3+/CD56+ cells was significantly higher in ESN when compared with controls. The anti-viral effects of IFN-gamma are well established, and so our results suggest that IFN-gamma production by innate immune cells might be one of the multiple factors involved in controlling the establishment of sexually transmitted HIV-1 infection.

Reduced CD4 T cell activation and in vitro susceptibility to HIV-1 infection in exposed uninfected Central Africans

BACKGROUND

Environmentally driven immune activation was suggested to contribute to high rates of HIV-1 infection in Africa. We report here a study of immune activation markers and susceptibility to HIV-1 infection in vitro of forty-five highly exposed uninfected partners (EUs) of HIV-1 infected individuals in Central African Republic, in comparison with forty-four low-risk blood donors (UCs).

RESULTS

Analysis of T lymphocyte subsets and activation markers in whole blood showed that the absolute values and the percentage of HLA-DR+CD4 T cells and of CCR5+CD4 T cells were lower in the EUs than in the UCs (p = 0.0001). Mutations in the CCR5 coding region were not found in either group. Susceptibility to in vitro infection of unstimulated peripheral blood mononuclear cells, prior of PHA activation, was decreased in EUs compared to UCs, either using a CXCR4-tropic or a CCR5-tropic HIV-1 strain (p = 0.02 and p = 0.05, respectively). Levels of MIP-1beta, but not of MIP-1alpha or RANTES, in the supernatants of PHA-activated PBMC, were higher in the EUs than in the UCs (p = 0.007).

CONCLUSION

We found low levels of CD4 T cell activation and reduced PBMC susceptibility to HIV-1 infection in Central African EUs, indicating that both may contribute to the resistance to HIV-1 infection.

Expression of the common heat-shock protein receptor CD91 is increased on monocytes of exposed yet HIV-1-seronegative subjects

The significantly higher surface expression of the surface heat-shock protein receptor CD91 on monocytes of human immunodeficiency virus type-1 (HIV-1)-infected, long-term nonprogressors suggests that HIV-1 antigen uptake and cross-presentation mediated by CD91 may contribute to host anti-HIV-1 defenses and play a role in protection against HIV-1 infection. To investigate this further, we performed phenotypic analysis to compare CD91 surface expression on CD14(+) monocytes derived from a cohort of HIV-1-exposed seronegative (ESN) subjects, their seropositive (SP) partners, and healthy HIV-1-unexposed seronegative (USN) subjects. The median fluorescent intensity (MFI) of CD91 on CD14(+) monocytes was significantly higher in ESN compared with SP (P = 0.028) or USN (P = 0.007), as well as in SP compared with USN subjects (P = 0.018). CD91 MFI was not normalized in SP subjects on highly active antiretroviral therapy (HAART) despite sustainable, undetectable plasma viraemia. Data in three SP subjects experiencing viral rebounds following interruption of HAART showed low CD91 MFI comparable with levels in USN subjects. There was a significant positive correlation between CD91 MFI and CD8(+) T cell counts in HAART-naïve SP subjects (r = 0.7, P = 0.015). Increased surface expression of CD91 on CD14(+) monocytes is associated with the apparent HIV-1 resistance that is observed in ESN subjects.