Germinal centre CD4+ T cells are an important site of HIV replication in vivo

TLR7 enhancing follicular helper T (Tfh) cells response in C57BL/6 mice infected with Plasmodium yoelii NSM TLR7 mediated Tfh cells in P. yoelii infected mice

Toll-like receptors (TLRs) play an important role in inducing innate and acquired immune responses against infection. However, the effect of Toll-like receptor 7 (TLR7) on follicular helper T (Tfh) cells in mice infected with Plasmodium is still not clear. The results showed that the splenic CD4+ CXCR5+ PD-1+ Tfh cells were accumulated after Plasmodium yoelii NSM infection, the content of splenic Tfh cells was correlated to parasitemia and/or the red blood cells (RBCs) counts in the blood. Moreover, the expression of TLR7 was found higher than TLR2, TLR3 and TLR4 in splenic Tfh cells of the WT mice. TLR7 agonist R848 and the lysate of red blood cells of infected mice (iRBCs) could induce the activation and differentiation of splenic Tfh cells. Knockout of TLR7 leads to a decrease in the proportion of Tfh cells, down-regulated expression of functional molecules CD40L, IFN-γ, IL-21 and IL-10 in Tfh cells; decreased the proportion of plasma cells and antibody production and reduces the expression of STAT3 and Ikzf2 in Tfh cells. Administration of R848 could inhibit parasitemia, enhance splenic Tfh cell activation and increase STAT3 and Ikzf2 expression in Tfh cells. In summary, this study shows that TLR7 could regulate the function of Tfh cells, affecting the immune response in the spleen of Plasmodium yoelii NSM-infected mice.

Human immunodeficiency virus dynamics in secondary lymphoid tissues and the evolution of cytotoxic T lymphocyte escape mutants

In secondary lymphoid tissues, human immunodeficiency virus (HIV) can replicate in both the follicular and extrafollicular compartments. Yet, virus is concentrated in the follicular compartment in the absence of antiretroviral therapy, in part due to the lack of cytotoxic T lymphocyte (CTL)-mediated activity there. CTLs home to the extrafollicular compartment, where they can suppress virus load to relatively low levels. We use mathematical models to show that this compartmentalization can explain seemingly counter-intuitive observations. First, it can explain the observed constancy of the viral decline slope during antiviral therapy in the peripheral blood, irrespective of the presence of CTL in Simian Immunodeficiency Virus (SIV)-infected macaques, under the assumption that CTL-mediated lysis significantly contributes to virus suppression. Second, it can account for the relatively long times it takes for CTL escape mutants to emerge during chronic infection even if CTL-mediated lysis is responsible for virus suppression. The reason is the heterogeneity in CTL activity and the consequent heterogeneity in selection pressure between the follicular and extrafollicular compartments. Hence, to understand HIV dynamics more thoroughly, this analysis highlights the importance of measuring virus populations separately in the extrafollicular and follicular compartments rather than using virus load in peripheral blood as an observable; this hides the heterogeneity between compartments that might be responsible for the particular patterns seen in the dynamics and evolution of the HIV in vivo.

Cytolytic CD8+ T cells infiltrate germinal centers to limit ongoing HIV replication in spontaneous controller lymph nodes

Follicular CD8⁺ T cells (fCD8) mediate surveillance in lymph node (LN) germinal centers against lymphotropic infections and cancers, but the precise mechanisms by which these cells mediate immune control remain incompletely resolved. To address this, we investigated functionality, clonotypic compartmentalization, spatial localization, phenotypic characteristics, and transcriptional profiles of LN-resident virus-specific CD8⁺ T cells in persons who control HIV without medications. Antigen-induced proliferative and cytolytic potential consistently distinguished spontaneous controllers from noncontrollers. T cell receptor analysis revealed complete clonotypic overlap between peripheral and LN-resident HIV-specific CD8⁺ T cells. Transcriptional analysis of LN CD8⁺ T cells revealed gene signatures of inflammatory chemotaxis and antigen-induced effector function. In HIV controllers, the cytotoxic effectors perforin and granzyme B were elevated among virus-specific CXCR5⁺ fCD8s proximate to foci of HIV RNA within germinal centers. These results provide evidence consistent with cytolytic control of lymphotropic infection supported by inflammatory recruitment, antigen-specific proliferation, and cytotoxicity of fCD8s.

Development of an anti-CAR antibody response in SIV-infected rhesus macaques treated with CD4-MBL CAR/CXCR5 T cells

T cells expressing a simian immunodeficiency (SIV)-specific chimeric antigen receptor (CAR) and the follicular homing molecule, CXCR5, were infused into antiretroviral therapy (ART) suppressed, SIV-infected rhesus macaques to assess their ability to localize to the lymphoid follicle and control the virus upon ART interruption. While the cells showed evidence of functionality, they failed to persist in the animals beyond 28 days. Development of anti-CAR antibodies could be responsible for the lack of persistence. Potential antigenic sites on the anti-SIV CAR used in these studies included domains 1 and 2 of CD4, the carbohydrate recognition domain (CRD) of mannose-binding lectin (MBL), and an extracellular domain of the costimulatory molecule, CD28, along with short linker sequences. Using a flow cytometry based assay and target cells expressing the CAR/CXCR5 construct, we examined the serum of the CD4-MBL CAR/CXCR5-T cell treated animals to determine that the animals had developed an anti-CAR antibody response after infusion. Binding sites for the anti-CAR antibodies were identified by using alternative CARs transduced into target cells and by preincubation of the target cells with a CD4 blocking antibody. All of the treated animals developed antibodies in their serum that bound to CD4-MBL CAR/CXCR5 T cells and the majority were capable of inducing an ADCC response. The CD4 antibody-blocking assay suggests that the dominant immunogenic components of this CAR are the CD4 domains with a possible additional site of the CD28 domain with its linker. This study shows that an anti-drug antibody (ADA) response can occur even when using self-proteins, likely due to novel epitopes created by abridged self-proteins and/or the self-domain of the CAR connection to a small non-self linker. While in our study, there was no statistically significant correlation between the ADA response and the persistence of the CD4-MBL CAR/CXCR5-T cells in rhesus macaques, these findings suggest that the development of an ADA response could impact the long-term persistence of self-based CAR immunotherapies.

Transcriptome and TCR Repertoire Measurements of CXCR3+ T Follicular Helper Cells Within HIV-Infected Human Lymph Nodes

Follicular-helper T cells (T_FH) are an essential arm of the adaptive immune system. Although T_FH were first discovered through their ability to contribute to antibody affinity maturation through co-stimulatory interactions with B cells, new light has been shed on their ability to remain a complex and functionally plastic cell type. Due to a lack sample availability, however, many studies have been limited to characterizing T_FH in mice or non-canonical tissue types, such as peripheral blood. Such constraints have resulted in a limited, and sometimes contradictory, understanding of this fundamental cell type. One subset of T_FH receiving attention in chronic infection are CXCR3-expressing T_FH cells (CXCR3⁺T_FH) due to their abnormal accumulation in secondary lymphoid tissues. Their function and clonal relationship with other T_FH subsets in lymphoid tissues during infection, however, remains largely unclear. We thus systematically investigated this and other subsets of T_FH within untreated HIV-infected human lymph nodes using Mass CyTOF and a combination of RNA and TCR repertoire sequencing. We show an inflation of the CXCR3⁺T_FH compartment during HIV infection that correlates with a lower HIV burden. Deeper analysis into this population revealed a functional shift of CXCR3⁺T_FH away from germinal center T_FH (GC-T_FH), including the altered expression of several important transcription factors and cytokines. CXCR3⁺T_FH also upregulated cell migration transcriptional programs and were clonally related to peripheral T_FH populations. In combination, these data suggest that CXCR3⁺T_FH have a greater tendency to enter circulation than their CXCR3^- counterparts, potentially functioning through distinct modalities that may lead to enhanced defense.

CAR/CXCR5-T cell immunotherapy is safe and potentially efficacious in promoting sustained remission of SIV infection

During chronic human immunodeficiency virus (HIV) or simian immunodeficiency virus (SIV) infection prior to AIDS progression, the vast majority of viral replication is concentrated within B cell follicles of secondary lymphoid tissues. We investigated whether infusion of T cells expressing an SIV-specific chimeric antigen receptor (CAR) and the follicular homing receptor, CXCR5, could successfully kill viral-RNA+ cells in targeted lymphoid follicles in SIV-infected rhesus macaques. In this study, CD4 and CD8 T cells from rhesus macaques were genetically modified to express antiviral CAR and CXCR5 moieties (generating CAR/CXCR5-T cells) and autologously infused into a chronically infected animal. At 2 days post-treatment, the CAR/CXCR5-T cells were located primarily in spleen and lymph nodes both inside and outside of lymphoid follicles. Few CAR/CXCR5-T cells were detected in the ileum, rectum, and lung, and no cells were detected in the bone marrow, liver, or brain. Within follicles, CAR/CXCR5-T cells were found in direct contact with SIV-viral RNA+ cells. We next infused CAR/CXCR5-T cells into ART-suppressed SIV-infected rhesus macaques, in which the animals were released from ART at the time of infusion. These CAR/CXCR5-T cells replicated in vivo within both the extrafollicular and follicular regions of lymph nodes and accumulated within lymphoid follicles. CAR/CXR5-T cell concentrations in follicles peaked during the first week post-infusion but declined to undetectable levels after 2 to 4 weeks. Overall, CAR/CXCR5-T cell-treated animals maintained lower viral loads and follicular viral RNA levels than untreated control animals, and no outstanding adverse reactions were noted. These findings indicate that CAR/CXCR5-T cell treatment is safe and holds promise as a future treatment for the durable remission of HIV.

Alterations of the frequency and functions of follicular regulatory T cells and related mechanisms in HIV infection

Human immunodeficiency virus (HIV) infection impairs both cellular and humoral immune system. Follicular regulatory T (Tfr) cells are a recently characterised subset of CD4⁺T cells. Tfr also exerts an immunosuppressive effect on humoral immune system through interaction with follicular helper T (Tfh) cells, but the role of Tfr in HIV infection needs to be further elucidated. 20 treatment-naïve and 20 antiretroviral therapy (ART)-treated HIV-infected individuals were enrolled for cross-sectional study and nine complete responders (CRs) and eight immune non-responders (INRs) after ART were collected for retrospective cohort study. Tfr phenotypes, cytokine secretions, and apoptosis of those subjects were evaluated by flow cytometry. HIV DNA was measured by reverse transcription-quantitative PCR (RT-qPCR). Significantly increased circulating Tfr was observed in chronic HIV⁺ patients and the imbalance between Tfr and Tfh17 was associated with CD4⁺T counts. In addition, an elevated proportion of Tfr was associated with immune reconstruction failure of patients after ART. The IL-10 and CTLA-4 expressions of Tfr cells were up-regulated in treatment-naïve HIV⁺ patients. Ex vivo experiments showed IL-10 and CTLA-4 expressed by Tfr inhibited IL-21 secretion of Tfh. Tfr harboured a comparable HIV-1 DNA level with Tfh in HIV⁺ patients. Compared to Tfr of HCs, Tfr cells of HIV⁺ patients were more insensitive to CD95 and IFN-α induced apoptosis, had a higher proliferation rate, and had more stem-like T cell (Tscm) phenotype. The anti-apoptosis feature, higher proliferation rate, and Tscm-like features of Tfr in HIV⁺ patients, led to the expansion of Tfr which in turn resulted in dysfunction of Tfh. Tfr cells were also involved in immune reconstruction failure and latent infection of HIV. Tfr cells were a novel, and potentially therapeutic, target for the cure of HIV infection, especially for HIV vaccine development and HIV reservoir elimination.

Mapping the Lineage Relationship between CXCR5+ and CXCR5- CD4+ T Cells in HIV-Infected Human Lymph Nodes

CXCR5 is a key marker of follicular helper T (T_FH) cells. Using primary lymph nodes (LNs) from HIV-infected patients, we identified a population of CXCR5⁻ CD4⁺ T cells with T_FH-cell-like features. This CXCR5⁻ subset becomes expanded in severe HIV infection and is characterized by the upregulation of activation markers and high PD-1 and ICOS surface expression. Integrated analyses on the phenotypic heterogeneity, functional capacity, T cell receptor (TCR) repertoire, transcriptional profile, and epigenetic state of CXCR5⁻PD-1⁺ICOS⁺ T cells revealed a shared clonal relationship with T_FH cells. CXCR5⁻PD-1⁺ICOS⁺ T cells retained a poised state for CXCR5 expression and exhibited a migratory transcriptional program. TCR sequence overlap revealed a contribution of LN-derived CXCR5⁻PD-1⁺ICOS⁺ T cells to circulating CXCR5⁻ CD4⁺ T cells with B cell help function. These data link LN pathology to circulating T cells and expand the current understanding on the diversity of T cells that regulate B cell responses during chronic inflammation.

Follicular helper T (T_FH) cells are critical for antibody production. Del Alcazar et al. showed that T_FH cells can lose their characteristic chemokine receptor, giving rise to migratory populations of CXCR5⁻ T cells that retain B cell help function and are poised for CXCR5 expression.

Transforming growth factor-β promotes the function of HIV-specific CXCR5+ CD8 T cells

HIV replication can be inhibited by CXCR5⁺ CD8 T cells (follicular cytotoxic T cell [TFC]) which transfer into B-cell follicles where latent HIV infection persists. However, how cytokines affect TFC remain unclear. Understanding which cytokines show the ability to affect TFC could be a key strategy toward curing HIV. Similar mechanisms could be used for the growth and transfer of TFCs and follicular helper T (TFH) cells; as a result, we hypothesized that cytokines IL-6, IL-21, and transforming growth factor-β (TGF-β), which are necessary for the differentiation of TFH cells, could also dictate the development of TFCs. In this work, lymph node mononuclear cells and peripheral blood mononuclear cells from HIV-infected individuals were cocultured with IL-6, IL-21, and TGF-β. We then carried out T-cell receptor (TCR) repertoire analysis to compare the differences between CXCR5^- and CXCR5⁺ CD8 T cells. Our results showed that the percentage and function of TFC can be enhanced by stimulation with TGF-β. Besides, TGF-β stimulation enhanced the diversity of TCR and complementarity-determining region 3 sequences. HIV DNA showed a negative correlation with TFC. The use of TGF-β to promote the expression of CXCR5⁺ CD8 T cells could become a new treatment approach for curing HIV.

Role of Dendritic Cells in Exposing Latent HIV-1 for the Kill

The development of effective yet nontoxic strategies to target the latent human immunodeficiency virus-1 (HIV-1) reservoir in antiretroviral therapy (ART)-suppressed individuals poses a critical barrier to a functional cure. The ‘kick and kill’ approach to HIV eradication entails proviral reactivation during ART, coupled with generation of cytotoxic T lymphocytes (CTLs) or other immune effectors equipped to eliminate exposed infected cells. Pharmacological latency reversal agents (LRAs) that have produced modest reductions in the latent reservoir ex vivo have not impacted levels of proviral DNA in HIV-infected individuals. An optimal cure strategy incorporates methods that facilitate sufficient antigen exposure on reactivated cells following the induction of proviral gene expression, as well as the elimination of infected targets by either polyfunctional HIV-specific CTLs or other immune-based strategies. Although conventional dendritic cells (DCs) have been used extensively for the purpose of inducing antigen-specific CTL responses in HIV-1 clinical trials, their immunotherapeutic potential as cellular LRAs has been largely ignored. In this review, we discuss the challenges associated with current HIV-1 eradication strategies, as well as the unharnessed potential of ex vivo-programmed DCs for both the ‘kick and kill’ of latent HIV-1.

Detection of Antigen-Specific T Cells Using In Situ MHC Tetramer Staining

The development of in situ major histocompatibility complex (MHC) tetramer (IST) staining to detect antigen (Ag)-specific T cells in tissues has radically revolutionized our knowledge of the local cellular immune response to viral and bacterial infections, cancers, and autoimmunity. IST combined with immunohistochemistry (IHC) enables determination of the location, abundance, and phenotype of T cells, as well as the characterization of Ag-specific T cells in a 3-dimensional space with respect to neighboring cells and specific tissue locations. In this review, we discuss the history of the development of IST combined with IHC. We describe various methods used for IST staining, including direct and indirect IST and IST performed on fresh, lightly fixed, frozen, and fresh then frozen tissue. We also describe current applications for IST in viral and bacterial infections, cancer, and autoimmunity. IST combined with IHC provides a valuable tool for studying and tracking the Ag-specific T cell immune response in tissues.

Low levels of SIV-specific CD8+ T cells in germinal centers characterizes acute SIV infection

CD8+ T cells play an important role in controlling of HIV and SIV infections. However, these cells are largely excluded from B cell follicles where HIV and SIV producing cells concentrate during chronic infection. It is not known, however, if antigen-specific CD8+ T cells are excluded gradually as pathogenesis progresses from early to chronic phase, or this phenomenon occurs from the beginning infection. In this study we determined that SIV-specific CD8+ T cells were largely excluded from follicles during early infection, we also found that within follicles, they were entirely absent in 60% of the germinal centers (GCs) examined. Furthermore, levels of SIV-specific CD8+ T cells in follicular but not extrafollicular areas significantly correlated inversely with levels of viral RNA+ cells. In addition, subsets of follicular SIV-specific CD8+ T cells were activated and proliferating and expressed the cytolytic protein perforin. These studies suggest that a paucity of SIV-specific CD8+ T cells in follicles and complete absence within GCs during early infection may set the stage for the establishment of persistent chronic infection.

Therapeutic HIV-1 vaccine: time for immunomodulation and combinatorial strategies

PURPOSE OF REVIEW

The purpose is to recall some of the key immunological elements that are at the crossroad and need to be combined for developing a potent therapeutic HIV-1 vaccine.

RECENT FINDINGS

Therapeutic vaccines and cytokines have been commonly used to enhance and/or recall preexisting HIV-1 specific cell-mediated immune responses aiming to suppress virus replication. While the vaccine is important to stimulate HIV-1 specific T-cell responses, the cytokine may support the expansion of the stimulated virus-specific T cells. Moreover, the current success of immune checkpoint blockers in cancer therapy render them very attractive to use in HIV-1 infected individuals, with the objective to preserve the function of HIV-specific T cells from exhaustion and target directly HIV-1 cell reservoir. More recently, the development of passive immunotherapy using broad neutralizing HIV antibodies (bNAbs) and their potential capacity to elicit innate or adaptive HIV-cellular responses, beyond their neutralizing activity, offers a new opportunity to improve the efficiency of therapeutic vaccine. These major advances provide the scientific basis for developing potent combinatorial interventions in HIV-1 infected patients.

SUMMARY

Major advances in our immunological understanding resulting from basic science and clinical trials studies have paved the way and established a solid platform to jump over the stumbling blocks that prevent the field from developing a therapeutic HIV-1 vaccine. It is time for immuno-modulation and combinatorial strategies towards HIV-1 eradication.

Molecular Basis of the Differentiation and Function of Virus Specific Follicular Helper CD4+ T Cells

During viral infection, virus-specific follicular helper T cells provide important help to cognate B cells for their survival, consecutive proliferation and mutation and eventual differentiation into memory B cells and antibody-secreting plasma cells. Similar to Tfh cells generated in other conditions, the differentiation of virus-specific Tfh cells can also be characterized as a process involved multiple factors and stages, however, which also exhibits distinct features. Here, we mainly focus on the current understanding of Tfh fate commitment, functional maturation, lineage maintenance and memory transition and formation in the context of viral infection.

The receptor repertoire and functional profile of follicular T cells in HIV-infected lymph nodes

Follicular helper CD4⁺ T cells (T_FH) play an integral role in promoting B cell differentiation and affinity maturation. Whereas T_FH cell frequencies are increased in lymph nodes (LNs) from individuals infected with HIV, humoral immunity remains impaired during chronic HIV infection. Whether HIV inhibits T_FH responses in LNs remains unclear. Advances in this area have been limited by the difficulty of accessing human lymphoid tissues. Here, we combined high-dimensional mass cytometry with T cell receptor repertoire sequencing to interrogate the composition of T_FH cells in primary human LNs. We found evidence for intact antigen-driven clonal expansion of T_FH cells and selective utilization of specific complementarity-determining region 3 (CDR3) motifs during chronic HIV infection, but the resulting T_FH cells acquired an activation-related T_FH cell signature characterized by interleukin-21 (IL-21) dominance. These IL-21⁺ T_FH cells contained an oligoclonal HIV-reactive population that preferentially accumulated in patients with severe HIV infection and was associated with aberrant B cell distribution in the same LN. These data indicate that T_FH cells remain capable of responding to HIV antigens during chronic HIV infection but become functionally skewed and oligoclonally restricted under persistent antigen stimulation.

Therapeutic Targeting of HIV Reservoirs: How to Give T Cells a New Direction

HIV cannot be cured by current antiretroviral therapy (ART) because it persists in a transcriptionally silent form in long-lived CD4+ cells. Leading efforts to develop a functional cure have prioritized latency reversal to expose infected cells to immune surveillance, coupled with enhancement of the natural cytolytic function of immune effectors, or "kick and kill." The most clinically advanced approach to improving the kill is therapeutic immunization, which aims to augment or re-focus HIV-specific cytolytic T cell responses. However, no vaccine strategy has enabled sustained virological control after ART withdrawal. Novel approaches are needed to overcome the limitations of natural adaptive immune responses, which relate to their specificity, potency, durability, and access to tissue reservoirs. Adoptive T cell therapy to treat HIV infection was first attempted over two decades ago, without success. Since then, progress in the field of cancer immunotherapy, together with recognition of the similarities in tumor microenvironments and HIV reservoirs has reignited interest in the application of T cell therapies to HIV eradication. Advances in engineering of chimeric antigen receptor (CAR)-transduced T cells have led to improved potency, persistence and latterly, resistance to HIV infection. Immune retargeting platforms have incorporated non-neutralizing and broadly neutralizing antibodies to generate Bispecific T cell Engagers (BiTEs) and Dual-Affinity Re-Targeting proteins (DARTs). T cell receptor engineering has enabled the development of the first bispecific Immune-mobilizing monoclonal T Cell receptors Against Viruses (ImmTAV) molecules. Here, we review the potential for these agents to provide a better "kill" and the challenges ahead for clinical development.

Insights Into the Molecular Mechanisms of T Follicular Helper-Mediated Immunity and Pathology

T follicular helper (Tfh) cells play key role in providing help to B cells during germinal center (GC) reactions. Generation of protective antibodies against various infections is an important aspect of Tfh-mediated immune responses and the dysregulation of Tfh cell responses has been implicated in various autoimmune disorders, inflammation, and malignancy. Thus, their differentiation and maintenance must be closely regulated to ensure appropriate help to B cells. The generation and function of Tfh cells is regulated by multiple checkpoints including their early priming stage in T zones and throughout the effector stage of differentiation in GCs. Signaling pathways activated downstream of cytokine and costimulatory receptors as well as consequent activation of subset-specific transcriptional factors are essential steps for Tfh cell generation. Thus, understanding the mechanisms underlying Tfh cell-mediated immunity and pathology will bring into spotlight potential targets for novel therapies. In this review, we discuss the recent findings related to the molecular mechanisms of Tfh cell differentiation and their role in normal immune responses and antibody-mediated diseases.

The role of follicular helper CD4 T cells in the development of HIV-1 specific broadly neutralizing antibody responses

The induction of HIV-1-specific antibodies that can neutralize a broad number of isolates is a major goal of HIV-1 vaccination strategies. However, to date no candidate HIV-1 vaccine has successfully elicited broadly neutralizing antibodies of sufficient quality and breadth for protection. In this review, we focus on the role of follicular helper CD4 T-cells (Tfh) in the development of such cross-reactive protective antibodies. We discuss germinal center (GC) formation and the dynamics of Tfh and GC B cells during HIV-1/SIV infection and vaccination. Finally, we consider future directions for the study of Tfh and offer perspective on factors that could be modulated to enhance Tfh function in the context of prophylactic vaccination.

Differentiation and Function of Follicular CD8 T Cells During Human Immunodeficiency Virus Infection

The combination antiretroviral therapeutic (cART) regime effectively suppresses human immunodeficiency virus (HIV) replication and prevents progression to acquired immunodeficiency diseases. However, cART is not a cure, and viral rebound will occur immediately after treatment is interrupted largely due to the long-term presence of an HIV reservoir that is composed of latently infected target cells that maintain a quiescent state or persistently produce infectious viruses. CD4 T cells that reside in B-cell follicles within lymphoid tissues, called follicular helper T cells (TFH), have been identified as a major HIV reservoir. Due to their specialized anatomical structure, HIV-specific CD8 T cells are largely insulated from this TFH reservoir. It is increasingly clear that the elimination of TFH reservoirs is a key step toward a functional cure for HIV infection. Recently, several studies have suggested that a fraction of HIV-specific CD8 T cells can differentiate into a CXCR5-expressing subset, which are able to migrate into B-cell follicles and inhibit viral replication. In this review, we discuss the differentiation and functions of this newly identified CD8 T-cell subset and propose potential strategies for purging TFH HIV reservoirs by utilizing this unique population.

Simian Immunodeficiency Virus (SIV)-Specific Chimeric Antigen Receptor-T Cells Engineered to Target B Cell Follicles and Suppress SIV Replication

There is a need to develop improved methods to treat and potentially cure HIV infection. During chronic HIV infection, replication is concentrated within T follicular helper cells (Tfh) located within B cell follicles, where low levels of virus-specific CTL permit ongoing viral replication. We previously showed that elevated levels of simian immunodeficiency virus (SIV)-specific CTL in B cell follicles are linked to both decreased levels of viral replication in follicles and decreased plasma viral loads. These findings provide the rationale to develop a strategy for targeting follicular viral-producing (Tfh) cells using antiviral chimeric antigen receptor (CAR) T cells co-expressing the follicular homing chemokine receptor CXCR5. We hypothesize that antiviral CAR/CXCR5-expressing T cells, when infused into an SIV-infected animal or an HIV-infected individual, will home to B cell follicles, suppress viral replication, and lead to long-term durable remission of SIV and HIV. To begin to test this hypothesis, we engineered gammaretroviral transduction vectors for co-expression of a bispecific anti-SIV CAR and rhesus macaque CXCR5. Viral suppression by CAR/CXCR5-transduced T cells was measured in vitro, and CXCR5-mediated migration was evaluated using both an in vitro transwell migration assay, as well as a novel ex vivo tissue migration assay. The functionality of the CAR/CXCR5 T cells was demonstrated through their potent suppression of SIV_mac239 and SIV_E660 replication in in vitro and migration to the ligand CXCL13 in vitro, and concentration in B cell follicles in tissues ex vivo. These novel antiviral immunotherapy products have the potential to provide long-term durable remission (functional cure) of HIV and SIV infections.

Expansion of circulating T follicular helper cells is associated with disease progression in HIV-infected individuals

BACKGROUND

T follicular helper (Tfh) cells within germinal centers (GC) of lymphoid tissue play an important role in HIV infection. Recently, circulating Tfh cells have been described, which share phenotypic and functional characteristics with GC Tfh cells. This study aimed to investigate the effect of HIV infection on four circulating Tfh subsets, including CD4+CXCR5+, CD4+CXCR5+ICOS+, CD4+CXCR5+PD-1+, and CD4+CXCR5+ICOS+PD-1+ cells.

PATIENTS AND METHODS

Peripheral blood samples were collected from 33 HIV-infected individuals and 21 healthy controls. The frequency and absolute number of CD3, CD4 and CD8 cells were detected by flow cytometry. The frequency of circulating Tfh cell subsets was also determined by flow cytometry. The correlation between the frequency of Tfh subsets and CD4 T cells counts was assessed by Pearson correlation analysis.

RESULTS

There was no significant difference in the frequency of peripheral CD4+CXCR5+ Tfh cells between HIV-infected individuals and healthy controls. However, the percentages of circulating CD4+CXCR5+ICOS+, CD4+CXCR5+PD-1+, and CD4+CXCR5+ICOS+PD-1+ Tfh cells were significantly higher in individuals with HIV infection than those of healthy controls. Furthermore, the percentage of CD4+CXCR5+PD-1+ Tfh cells showed negative correlation with CD4 T cell counts in HIV-infected individuals.

CONCLUSION

Our results suggested the potential involvement of circulating CD4+CXCR5+PD-1+ Tfh cells during the development of HIV infection.

The Lymph Node in HIV Pathogenesis

Lymph nodes play a central role in the development of adaptive immunity against pathogens and particularly the generation of antigen-specific B cell responses in specialized areas called germinal centers (GCs). Lymph node (LN) pathology was recognized as an important consequence of human immunodeficiency virus (HIV) infection since the beginning of the HIV epidemic. Investigation into the structural and functional alterations induced by HIV and Simian immunodeficiency virus (SIV) has further cemented the central role that lymphoid tissue plays in HIV/SIV pathogenesis. The coexistence of constant local inflammation, altered tissue architecture, and relative exclusion of virus-specific CD8 T cells from the GCs creates a unique environment for the virus evolution and establishment of viral reservoir in specific GC cells, namely T follicular helper CD4 T cells (Tfh). A better understanding of the biology of immune cells in HIV-infected lymph nodes is a prerequisite to attaining the ultimate goal of complete viral eradication.

The B-Cell Follicle in HIV Infection: Barrier to a Cure

The majority of HIV replication occurs in secondary lymphoid organs (SLOs) such as the spleen, lymph nodes, and gut-associated lymphoid tissue. Within SLOs, HIV RNA⁺ cells are concentrated in the B-cell follicle during chronic untreated infection, and emerging data suggest that they are a major source of replication in treated disease as well. The concentration of HIV RNA⁺ cells in the B-cell follicle is mediated by several factors. Follicular CD4⁺ T-cell subsets including T-follicular helper cells and T-follicular regulatory cells are significantly more permissive to HIV than extrafollicular subsets. The B cell follicle also contains a large reservoir of extracellular HIV virions, which accumulate on the surface of follicular dendritic cells (FDCs) in germinal centers. FDC-bound HIV virions remain infectious even in the presence of neutralizing antibodies and can persist for months or even years. Moreover, the B-cell follicle is semi-immune privileged from CTL control. Frequencies of HIV- and SIV-specific CTL are lower in B-cell follicles compared to extrafollicular regions as the majority of CTL do not express the follicular homing receptor CXCR5. Additionally, CTL in the B-cell follicle may be less functional than extrafollicular CTL as many exhibit the recently described CD8 T follicular regulatory phenotype. Other factors may also contribute to the follicular concentration of HIV RNA⁺ cells. Notably, the contribution of NK cells and γδ T cells to control and/or persistence of HIV RNA⁺ cells in secondary lymphoid tissue remains poorly characterized. As HIV research moves increasingly toward the development of cure strategies, a greater understanding of the barriers to control of HIV infection in B-cell follicles is critical. Although no strategy has as of yet proven to be effective, a range of novel therapies to address these barriers are currently being investigated including genetically engineered CTL or chimeric antigen receptor T cells that express the follicular homing molecule CXCR5, treatment with IL-15 or an IL-15 superagonist, use of bispecific antibodies to harness the killing power of the follicular CD8⁺ T cell population, and disruption of the follicle through treatments such as rituximab.

Early initiation of antiretroviral treatment postSIV infection does not resolve lymphoid tissue activation

OBJECTIVE

Germinal center resident follicular helper T (TFH) cells in lymphoid follicles are a potential sanctuary for HIV/simian immunodeficiency virus (SIV) replication. But the dynamics of germinal centers upon early initiation of antiretroviral therapy (ART) and their potential role in the formation of viral sanctuaries post-SIV infection are not fully understood.

DESIGN

Sequential lymph node biopsies (n = 10) were collected from SIVmac239-infected rhesus macaques before infection, at 5 weeks postinfection/pre-ART, 6 and 12 weeks following ART initiation. These tissues and cells were analyzed for frequencies of TFH cells and assignment of germinal center scores.

RESULTS

Modest but significant increases in TFH cells and hyperplastic follicles with large germinal centers were noted during the acute phase of SIV infection (week 5/pre-ART). However, 6 weeks after ART initiation, substantial increases in germinal center TFH cells, germinal center B cells, hyperplastic follicles with large germinal centers, and abundant local IL-21 production were observed, whereas levels of SIV RNA and DNA of lymph nodes had decreased to barely detectable values along with barely detectable levels of SIV antibody-producing cells. An additional 6 weeks of ART did not appreciably decrease germinal center TFH or germinal center scores.

CONCLUSION

Thus, although early ART rapidly controls SIV replication, it does not regulate early lymphoid activation, which may contribute to the seeding and magnitude of viral reservoirs.

Persistent HIV-1 replication during antiretroviral therapy

Purpose of review

The present review will highlight some of the recent findings regarding the capacity of HIV-1 to replicate during antiretroviral therapy (ART).

Recent findings

Although ART is highly effective at inhibiting HIV replication, it is not curative. Several mechanisms contribute to HIV persistence during ART, including HIV latency, immune dysfunction, and perhaps persistent low-level spread of the virus to uninfected cells (replication). The success in curing HIV will depend on efficiently targeting these three aspects. The degree to which HIV replicates during ART remains controversial. Most studies have failed to find any evidence of HIV evolution in blood, even with samples collected over many years, although a recent very intensive study of three individuals suggested that the virus population does shift, at least during the first few months of therapy. Stronger but still not definitive evidence for replication comes from a series of studies in which standard regimens were intensified with an integration inhibitor, resulting in changes in episomal DNA (blood) and cell-associated RNA (tissue). Limited drug penetration within tissues and the presence of immune sanctuaries have been argued as potential mechanisms allowing HIV to spread during ART. Mathematical models suggest that HIV replication and evolution is possible even without the selection of fully drug-resistant variants. As persistent HIV replication could have clinical consequences and might limit the efficacy of curative interventions, determining if HIV replicates during ART and why, should remain a key focus of the HIV research community.

Summary

Residual viral replication likely persists in lymphoid tissues, at least in a subset of individuals. Abnormal levels of immune activation might contribute to sustain virus replication.

Immunologic characteristics of HIV-infected individuals who make broadly neutralizing antibodies

Induction of broadly neutralizing antibodies (bnAbs) capable of inhibiting infection with diverse variants of human immunodeficiency virus type 1 (HIV‐1) is a key, as‐yet‐unachieved goal of prophylactic HIV‐1 vaccine strategies. However, some HIV‐infected individuals develop bnAbs after approximately 2‐4 years of infection, enabling analysis of features of these antibodies and the immunological environment that enables their induction. Distinct subsets of CD4⁺ T cells play opposing roles in the regulation of humoral responses: T follicular helper (Tfh) cells support germinal center formation and provide help for affinity maturation and the development of memory B cells and plasma cells, while regulatory CD4⁺ (Treg) cells including T follicular regulatory (Tfr) cells inhibit the germinal center reaction to limit autoantibody production. BnAbs exhibit high somatic mutation frequencies, long third heavy‐chain complementarity determining regions, and/or autoreactivity, suggesting that bnAb generation is likely to be highly dependent on the activity of CD4⁺ Tfh cells, and may be constrained by host tolerance controls. This review discusses what is known about the immunological environment during HIV‐1 infection, in particular alterations in CD4⁺ Tfh, Treg, and Tfr populations and autoantibody generation, and how this is related to bnAb development, and considers the implications for HIV‐1 vaccine design.

Cytotoxic T Cell Functions Accumulate When CD4 Is Downregulated by CD4+ T Cells in African Green Monkeys

African green monkeys (AGMs) are a natural host of SIV that do not develop simian AIDS. Adult AGMs naturally have low numbers of CD4+ T cells and a large population of MHC class II-restricted CD8αα T cells that are generated through CD4 downregulation in CD4+ T cells. In this article, we study the functional profiles and SIV infection status in vivo of CD4+ T cells, CD8αα T cells, and CD8αβ T cells in lymph nodes, peripheral blood, and bronchoalveolar lavage fluid of AGMs and rhesus macaques (in which CD4 downregulation is not observed). We show that, although CD8αα T cells in AGMs maintain functions associated with CD4+ T cells (including Th follicular functionality in lymphoid tissues and Th2 responses in bronchoalveolar lavage fluid), they also accumulate functions normally attributed to canonical CD8+ T cells. These hyperfunctional CD8αα T cells are found to circulate peripherally, as well as reside within the lymphoid tissue. Due to their unique combination of CD4 and CD8 T cell effector functions, these CD4- CD8αα T cells are likely able to serve as an immunophenotype capable of Th1, follicular Th, and CTL functionalities, yet they are unable to be infected by SIV. These data demonstrate the ambiguity of CD4/CD8 expression in dictating the functional capacities of T cells and suggest that accumulation of hyperfunctional CD8αα T cells in AGMs may lead to tissue-specific antiviral immune responses in lymphoid follicles that limit SIV replication in this particular anatomical niche.

Human Immunodeficiency Virus Playing Hide-and-Seek: Understanding the TFH Cell Reservoir and Proposing Strategies to Overcome the Follicle Sanctuary

Human immunodeficiency virus (HIV) infects millions of people worldwide, and new cases continue to emerge. Once infected, the virus cannot be cleared by the immune system and causes acquired immunodeficiency syndrome. Combination antiretroviral therapeutic regimen effectively suppresses viral replication and halts disease progression. The treatment, however, does not eliminate the virus-infected cells, and interruption of treatment inevitably leads to viral rebound. The rebound virus originates from a group of virus-infected cells referred to as the cellular reservoir of HIV. Identifying and eliminating the HIV reservoir will prevent viral rebound and cure HIV infection. In this review, we focus on a recently discovered HIV reservoir in a subset of CD4⁺ T cells called the follicular helper T (T_FH) cells. We describe the potential mechanisms for the emergence of reservoir in T_FH cells, and the strategies to target and eliminate this viral reservoir.

In Vivo Models of Human Immunodeficiency Virus Persistence and Cure Strategies

Current HIV therapy is not curative regardless of how soon after infection it is initiated or how long it is administered, and therapy interruption almost invariably results in robust viral rebound. Human immunodeficiency virus persistence is therefore the major obstacle to a cure for AIDS. The testing and implementation of novel yet unproven approaches to HIV eradication that could compromise the health status of HIV-infected individuals might not be ethically warranted. Therefore, adequate in vitro and in vivo evidence of efficacy is needed to facilitate the clinical implementation of promising strategies for an HIV cure. Animal models of HIV infection have a strong and well-documented history of bridging the gap between laboratory discoveries and eventual clinical implementation. More recently, animal models have been developed and implemented for the in vivo evaluation of novel HIV cure strategies. In this article, we review the recent progress in this rapidly moving area of research, focusing on the two most promising model systems: humanized mice and nonhuman primates.

The Role of CD4+ T Follicular Helper Cells in HIV Infection: From the Germinal Center to the Periphery

T follicular helper cells (TFh) are key components of the adaptive immune system; they are primarily found in germinal centers (GCs) where their interaction with B cells supports humoral immune responses and efficient antibody production. They are defined by the expression of CXC receptor 5, program death-1, ICOS, and secretion of IL-21. Their differentiation is regulated by B-cell lymphoma 6. The relationship and function of circulating TFh to bona fide TFh resident in the GC is much debated. HIV infection impacts the TFh response with evidence of aberrant TFh function observed in acute and chronic infection. Effective TFh responses are associated with the development of broadly neutralizing antibody responses to HIV and may be important for viral control. In addition, TFh are preferentially infected and act as a key reservoir for latent HIV infection. This review explores recent developments in our understanding of TFh differentiation, regulation, function, and the relationship between cTFh and those in GCs, and the complex interaction between TFh and HIV infection.

Follicular CD8 T cells accumulate in HIV infection and can kill infected cells in vitro via bispecific antibodies

Cytolytic CD8 T cells play a crucial role in the control and elimination of virus-infected cells and are a major focus of HIV cure efforts. However, it has been shown that HIV-specific CD8 T cells are infrequently found within germinal centers (GCs), a predominant site of active and latent HIV infection. We demonstrate that HIV infection induces marked changes in the phenotype, frequency, and localization of CD8 T cells within the lymph node (LN). Significantly increased frequencies of CD8 T cells in the B cell follicles and GCs were found in LNs from treated and untreated HIV-infected individuals. This profile was associated with persistent local immune activation but did not appear to be directly related to local viral replication. Follicular CD8 (fCD8) T cells, despite compromised cytokine polyfunctionality, showed good cytolytic potential characterized by high ex vivo expression of granzyme B and perforin. We used an anti-HIV/anti-CD3 bispecific antibody in a redirected killing assay and found that fCD8 T cells had better killing activity than did non-fCD8 T cells. Our results indicate that CD8 T cells with potent cytolytic activity are recruited to GCs during HIV infection and, if appropriately redirected to kill HIV-infected cells, could be an effective component of an HIV cure strategy.

CD4 T Follicular Helper and Regulatory Cell Dynamics and Function in HIV Infection

T follicular helper cells (T_FH) are a specialized subset of CD4 T cells that reside in B cell follicles and promote B cell maturation into plasma cells and long-lived memory B cells. During chronic infection prior to the development of AIDS, HIV-1 (HIV) replication is largely concentrated in T_FH. Paradoxically, T_FH numbers are increased in early and midstages of disease, thereby promoting HIV replication and disease progression. Despite increased T_FH numbers, numerous defects in humoral immunity are detected in HIV-infected individuals, including dysregulation of B cell maturation, impaired somatic hypermutation, and low quality of antibody production despite hypergammaglobulinemia. Clinically, these defects are manifested by increased vulnerability to bacterial infections and impaired vaccine responses, neither of which is fully reversed by antiretroviral therapy (ART). Deficits in T_FH function, including reduced HIV-specific IL-21 production and low levels of co-stimulatory receptor expression, have been linked to these immune impairments. Impairments in T_FH likely contribute as well to the ability of HIV to persist and evade humoral immunity, particularly the inability to develop broadly neutralizing antibodies. In addition to direct infection of T_FH, other mechanisms that have been linked to T_FH deficits in HIV infection include upregulation of PD-L1 on germinal center B cells and augmented follicular regulatory T cell responses. Challenges to development of strategies to enhance T_FH function in HIV infection include lack of an established phenotype for memory T_FH as well as limited understanding of the relationship between peripheral T_FH and lymphoid tissue T_FH. Interventions to augment T_FH function in HIV-infected individuals could enhance immune reconstitution during ART and potentially augment cure strategies.

Simian Immunodeficiency Virus-Producing Cells in Follicles Are Partially Suppressed by CD8+ Cells In Vivo

Human immunodeficiency virus (HIV)- and simian immunodeficiency virus (SIV)-specific CD8+ T cells are typically largely excluded from lymphoid B cell follicles, where HIV- and SIV-producing cells are most highly concentrated, indicating that B cell follicles are somewhat of an immunoprivileged site. To gain insights into virus-specific follicular CD8+ T cells, we determined the location and phenotype of follicular SIV-specific CD8+ T cells in situ, the local relationship of these cells to Foxp3+ cells, and the effects of CD8 depletion on levels of follicular SIV-producing cells in chronically SIV-infected rhesus macaques. We found that follicular SIV-specific CD8+ T cells were able to migrate throughout follicular areas, including germinal centers. Many expressed PD-1, indicating that they may have been exhausted. A small subset was in direct contact with and likely inhibited by Foxp3+ cells, and a few were themselves Foxp3+ In addition, subsets of follicular SIV-specific CD8+ T cells expressed low to medium levels of perforin, and subsets were activated and proliferating. Importantly, after CD8 depletion, the number of SIV-producing cells increased in B cell follicles and extrafollicular areas, suggesting that follicular and extrafollicular CD8+ T cells have a suppressive effect on SIV replication. Taken together, these results suggest that during chronic SIV infection, despite high levels of exhaustion and likely inhibition by Foxp3+ cells, a subset of follicular SIV-specific CD8+ T cells are functional and suppress viral replication in vivo These findings support HIV cure strategies that augment functional follicular virus-specific CD8+ T cells to enhance viral control.

IMPORTANCE

HIV- and SIV-specific CD8+ T cells are typically largely excluded from lymphoid B cell follicles, where virus-producing cells are most highly concentrated, suggesting that B cell follicles are somewhat of an immunoprivileged site where virus-specific CD8+ T cells are not able to clear all follicular HIV- and SIV-producing cells. To gain insights into follicular CD8+ T cell function, we characterized follicular virus-specific CD8+ T cells in situ by using an SIV-infected rhesus macaque model of HIV. We found that subsets of follicular SIV-specific CD8+ T cells are able to migrate throughout the follicle, are likely inhibited by Foxp3+ cells, and are likely exhausted but that, nonetheless, subsets are likely functional, as they express markers consistent with effector function and show signs of suppressing viral replication in vivo These findings support HIV cure strategies that increase the frequency of functional follicular virus-specific CD8+ T cells.

The Dynamics of T and B Cells in Lymph Node during Chronic HIV Infection: TFH and HIV, Unhappy Dance Partners?

Although the dynamics of germinal center (GC) formation, follicular helper T (TFH) cell recruitment to B cell follicles within lymphoid organs, and changes of lymphoid tissue architecture in HIV/SIV infection have been documented, the underlying immunopathology remains unclear. Here, we summarize what is known regarding the kinetics of TFH cells and GC B cells during the course of infection as well as the potential immunopathological features associated with structural changes in the lymphoid compartment. This review also explores the implications of cell dynamics in the formation and maintenance of viral reservoirs in hyperplastic follicles of secondary lymphoid organs before and after viral suppressive antiretroviral therapy.

B cells in HIV pathogenesis

PURPOSE OF REVIEW

The induction of a virus-clearing humoral immune response in natural HIV infection is impaired. Insights into early events in HIV infection that affect B-cell responses and antibody development are addressed and related to strategies for the design of an HIV vaccine.

RECENT FINDINGS

Broadly neutralizing antibody responses do not develop early in HIV-1 infection, and recent reports highlight the role of preexisting suboptimal B-cell populations that can dominate the early antibody response. Furthermore, from the earliest phases of infection, virus replication is a driving force behind alterations in the B cell and T-follicular helper cell (TFH) compartments. Paradoxically, the factors that drive these abnormalities, such as high virus load, duration of infection, and increased viral diversity, are likely necessary for the development of both TFH and broadly neutralizing antibodies.

SUMMARY

These data provide new insights into prerequisites for an effective HIV vaccine. First, a vaccine should induce specific B-cell lineages so that preexisting cross-reactivity is avoided and, additionally, it must mimic high levels of diverse antigen in the absence of chronic virus replication within immune cells to activate high levels of quality of TFH and stimulate antibody maturation.

Dissecting How CD4 T Cells Are Lost During HIV Infection

Although the replicative life cycle of HIV within CD4 T cells is understood in molecular detail, less is known about how this human retrovirus promotes the loss of CD4 T lymphocytes. It is this cell death process that drives clinical progression to acquired immune deficiency syndrome (AIDS). Recent studies have highlighted how abortive infection of resting and thus nonpermissive CD4 T cells in lymphoid tissues triggers a lethal innate immune response against the incomplete DNA products generated by inefficient viral reverse transcription in these cells. Sensing of these DNA fragments results in pyroptosis, a highly inflammatory form of programmed cell death, that potentially further perpetuates chronic inflammation and immune activation. As discussed here, these studies cast CD4 T cell death during HIV infection in a different light. Further, they identify drug targets that may be exploited to both block CD4 T cell demise and the chronic inflammatory response generated during pyroptosis.

TFH in HIV Latency and as Sources of Replication-Competent Virus

During untreated disease, HIV replication is concentrated within T follicular helper cells (TFH). Heightened permissiveness, the presence of highly infectious virions on follicular dendritic cells (FDCs), low frequencies of virus-specific cytotoxic T lymphocytes (CTLs) in B cell follicles, expansions in TFH, and TFH dysfunction, all likely promote replication in TFH. Limited data suggest that memory TFH play a role in the latent or subclinical reservoir of HIV during antiretroviral therapy (ART), potentially for many of the same reasons. A better understanding of the role of memory TFH and FDC-bound virions in promoting recrudescent viremia in the setting of ART cessation is essential. Studies that target follicular virus reservoirs are needed to determine their role in HIV latency and to suggest successful cure strategies.

Germinal Center T Follicular Helper Cells Are Highly Permissive to HIV-1 and Alter Their Phenotype during Virus Replication

HIV-1 replication is concentrated within CD4(+) T cells in B cell follicles of secondary lymphoid tissues during asymptomatic disease. Limited data suggest that a subset of T follicular helper cells (TFH) within germinal centers (GC) is highly permissive to HIV-1. Whether GC TFH are the major HIV-1 virus-producing cells in vivo has not been established. In this study, we investigated TFH permissivity to HIV-1 ex vivo by spinoculating and culturing tonsil cells with HIV-1 GFP reporter viruses. Using flow cytometry, higher percentages of GC TFH (CXCR5(high)PD-1(high)) and CXCR5(+)programmed cell death-1 (PD-1)(low) cells were GFP(+) than non-GC TFH (CXCR5(+)PD-1(intermediate)) or extrafollicular (EF) (CXCR5(-)) cells. When sorted prior to spinoculation, however, GC TFH were substantially more permissive than CXCR5(+)PD-1(low) or EF cells, suggesting that many GC TFH transition to a CXCR5(+)PD-1(low) phenotype during productive infection. In situ hybridization on inguinal lymph node sections from untreated HIV-1-infected individuals without AIDS revealed higher frequencies of HIV-1 RNA(+) cells in GC than non-GC regions of follicle or EF regions. Superinfection of HIV-1-infected individuals' lymph node cells with GFP reporter virus confirmed the permissivity of follicular cells ex vivo. Lymph node immunostaining revealed 96% of CXCR5(+)CD4(+) cells were located in follicles. Within sorted lymph node cells from four HIV-infected individuals, CXCR5(+) subsets harbored 11-66-fold more HIV-1 RNA than CXCR5(-) subsets, as determined by RT PCR. Thus, GC TFH are highly permissive to HIV-1, but downregulate PD-1 and, to a lesser extent, CXCR5 during HIV-1 replication. These data further implicate GC TFH as the major HIV-1-producing cells in chronic asymptomatic HIV-1 infection.

Innate and Adaptive Immune Regulation During Chronic Viral Infections

Chronic viral infections represent a unique challenge to the infected host. Persistently replicating viruses outcompete or subvert the initial antiviral response, allowing the establishment of chronic infections that result in continuous stimulation of both the innate and adaptive immune compartments. This causes a profound reprogramming of the host immune system, including attenuation and persistent low levels of type I interferons, progressive loss (or exhaustion) of CD8(+) T cell functions, and specialization of CD4(+) T cells to produce interleukin-21 and promote antibody-mediated immunity and immune regulation. Epigenetic, transcriptional, posttranscriptional, and metabolic changes underlie this adaptation or recalibration of immune cells to the emerging new environment in order to strike an often imperfect balance between the host and the infectious pathogen. In this review we discuss the common immunological hallmarks observed across a range of different persistently replicating viruses and host species, the underlying molecular mechanisms, and the biological and clinical implications.

HIV-Infected Spleens Present Altered Follicular Helper T Cell (Tfh) Subsets and Skewed B Cell Maturation

Follicular helper T (Tfh) cells within secondary lymphoid organs control multiple steps of B cell maturation and antibody (Ab) production. HIV-1 infection is associated with an altered B cell differentiation and Tfh isolated from lymph nodes of HIV-infected (HIV⁺) individuals provide inadequate B cell help in vitro. However, the mechanisms underlying this impairment of Tfh function are not fully defined. Using a unique collection of splenocytes, we compared the frequency, phenotype and transcriptome of Tfh subsets in spleens from HIV negative (HIV^-) and HIV⁺ subjects. We observed an increase of CXCR5⁺PD-1^highCD57^-Tfh and germinal center (GC) CD57+ Tfh in HIV⁺ spleens. Both subsets showed a reduced mRNA expression of the transcription factor STAT-3, co-stimulatory, regulatory and signal transduction molecules as compared to HIV^- spleens. Similarly, Foxp3 expressing follicular regulatory T (Tfr) cells were increased, suggesting sustained GC reactions in chronically HIV⁺ spleens. As a consequence, GC B cell populations were expanded, however, complete maturation into memory B cells was reduced in HIV⁺ spleens where we evidenced a compromised production of B cell-activating cytokines such as IL-4 and IL-10. Collectively our data indicate that, although Tfh proliferation and GC reactions seem to be ongoing in HIV-infected spleens, Tfh “differentiation” and expression of costimulatory molecules is skewed with a profound effect on B cell maturation.

The dysfunction of T follicular helper cells

PURPOSE OF REVIEW

T follicular helper (Tfh) cells play a critical role as providers of B-cell help and dysfunction in Tfh/B-cell interactions can lead to autoimmunity or immunodeficiency. These observations have generated a great deal of interest in understanding how these cells are affected during HIV infection and how their functional changes might affect antibody responses.

RECENT FINDINGS

Recent studies have shown that HIV/simian immunodeficiency virus (SIV) infection affects both Tfh-cell frequency and function and suggest that Tfh-cell perturbations might contribute to the relative inefficiency of HIV-infected individuals to generate broadly neutralizing antibodies (bNAbs).

SUMMARY

The present review will highlight these recent findings addressing the role of Tfh cells in HIV infection as well as the impact HIV infection has on Tfh and circulating memory Tfh (cTfh) cell frequency and function.

Distinct patterns of Bcl-2 expression occur in R5- and X4-tropic HIV-1-producing lymphoid tissue cells infected ex vivo

Most HIV-1 replication occurs in secondary lymphoid tissues in T cells within B cell follicles. Mechanisms underlying the accumulation of HIV-1-producing cells at these sites are not understood. Antiapoptotic proteins such as Bcl-2 could promote follicular CD4(+) T cell survival, contributing to sustained virus production. Tonsils obtained from subjects without known HIV infection were disaggregated and analyzed for Bcl-2 expression in follicular (CXCR5(+)) and extrafollicular (CXCR5(-)) CD3(+)CD4(+) cells by flow cytometry. Additional tonsil cells were cultured with phytohemagglutinin (PHA) and interleukin-2 (IL-2) for 2 days, infected with either CCR5(R5) or CXCR4-tropic (X4) GFP reporter viruses, and analyzed for Bcl-2 expression. In freshly disaggregated CD3(+)CD4(+) tonsil cells, mean florescence intensity (MFI) for Bcl-2 was higher in CXCR5(+) (median, 292) compared to CXCR5(-) cells (median, 194; p=0.001). Following in vitro stimulation with PHA and IL-2, Bcl-2 MFI was higher in both CXCR5(+) cells (median, 757; p=0.03) and CXCR5(-) cells (median, 884; p=0.002) in uninfected cultures compared to freshly isolated tonsil cells. Bcl-2 MFI was higher in GFP(+)CD3(+)CD8(-) R5-producing cells (median, 554) than in X4-producing cells (median, 393; p=0.02). Bcl-2 MFI was higher in R5-producing CXCR5(+) cells (median, 840) compared to all other subsets including R5-producing CXCR5(-) cells (median, 524; p=0.04), X4-producing CXCR5(+) cells (median, 401; p=0.02), and X4-producing CXCR5(-) cells (median, 332; p=0.008). Bcl-2 expression is elevated in R5 HIV-1-producing CXCR5(+) T cells in vitro, which may contribute to propagation of R5 virus in B cell follicles in vivo.

CD4 T Cells Mediate Both Positive and Negative Regulation of the Immune Response to HIV Infection: Complex Role of T Follicular Helper Cells and Regulatory T Cells in Pathogenesis

HIV-1 infection results in chronic activation of cells in lymphoid tissue, including T cells, B-cells, and myeloid lineage cells. The resulting characteristic hyperplasia is an amalgam of proliferating host immune cells in the adaptive response, increased concentrations of innate response mediators due to viral and bacterial products, and homeostatic responses to inflammation. While it is generally thought that CD4 T cells are greatly depleted, in fact, two types of CD4 T cells appear to be increased, namely, regulatory T cells (Tregs) and T follicular helper cells (Tfh). These cells have opposing roles, but may both be important in the pathogenic process. Whether Tregs are failing in their role to limit lymphocyte activation is unclear, but there is no doubt now that Tfh are associated with B-cell hyperplasia and increased germinal center activity. Antiretroviral therapy may reduce the lymphocyte activation, but not completely, and therefore, there is a need for interventions that selectively enhance normal CD4 function without exacerbating Tfh, B-cell, or Treg dysfunction.

C peptides as entry inhibitors for gene therapy

Peptides derived from the C-terminal heptad repeat 2 region of the HIV-1 gp41 envelope glycoprotein, so-called C peptides, are very potent HIV-1 fusion inhibitors. Antiviral genes encoding either membrane-anchored (ma) or secreted (iSAVE) C peptides have been engineered and allow direct in vivo production of the therapeutic peptides by genetically modified host cells. Membrane-anchored C peptides expressed in the HIV-1 target cells by T-cell or hematopoietic stem cell gene therapy efficiently prevent virus entry into the modified cells. Such gene-protection confers a selective survival advantage and allows accumulation of the genetically modified cells. Membrane-anchored C peptides have been successfully tested in a nonhuman primate model of AIDS and were found to be safe in a phase I clinical trial in AIDS patients transplanted with autologous gene-modified T-cells. Secreted C peptides have the crucial advantage of not only protecting genetically modified cells from HIV-1 infection, but also neighboring cells, thus suppressing virus replication even if only a small fraction of cells is genetically modified. Accordingly, various cell types can be considered as potential in vivo producer cells for iSAVE-based gene therapeutics, which could even be modified by direct in vivo gene delivery in future. In conclusion, C peptide gene therapeutics may provide a strong benefit to AIDS patients and could present an effective alternative to current antiretroviral drug regimens.

T follicular helper cells and HIV/SIV-specific antibody responses

PURPOSE OF REVIEW

Here, we describe recent data on the characterization of follicular helper CD4 T cells (Tfh) and the dynamics of Tfh-B-cell interactions in HIV and simian immunodeficiency virus (SIV) infection and discuss important aspects of these interactions that need to be addressed in order to design more effective vaccines that elicit broadly neutralizing antibodies.

RECENT FINDINGS

Mouse, nonhuman primate (NHP) and human Tfh cells share phenotypic, functional and molecular programs, which are regulated by local signals and spatiotemporal parameters. Chronic HIV/SIV infection results in accumulation of Tfh, germinal center B cells and circulating virus-specific immunoglobulins in some individuals. However, most HIV/SIV-infected individuals do not mount broadly neutralizing antibodies, pointing to functional defects in Tfh cells in chronic HIV/SIV infection. The susceptibility of particular CD4 T-cell populations to HIV/SIV infection within lymph nodes notably impacts upon the dynamics of Tfh-germinal center B-cell interactions. Some circulating CD4 T cells share certain characteristics with Tfh cells, however, their direct origin from germinal center Tfh cells is not clear.

SUMMARY

There are many ways in which HIV and SIV influence the complex signals and mechanisms regulating the development of Tfh cells and their interactions with germinal center B cells. Understanding the biology of Tfh cells will be necessary to appropriately recruit these cells during vaccination with the goal of stimulating a more broad and potent neutralizing antibody response.

Compartmentalization of simian immunodeficiency virus replication within secondary lymphoid tissues of rhesus macaques is linked to disease stage and inversely related to localization of virus-specific CTL

We previously demonstrated that HIV replication is concentrated in lymph node B cell follicles during chronic infection and that HIV-specific CTL fail to accumulate in large numbers at those sites. It is unknown whether these observations can be generalized to other secondary lymphoid tissues or whether virus compartmentalization occurs in the absence of CTL. We evaluated these questions in SIVmac239-infected rhesus macaques by quantifying SIV RNA(+) cells and SIV-specific CTL in situ in spleen, lymph nodes, and intestinal tissues obtained at several stages of infection. During chronic asymptomatic infection prior to simian AIDS, SIV-producing cells were more concentrated in follicular (F) compared with extrafollicular (EF) regions of secondary lymphoid tissues. At day 14 of infection, when CTL have minimal impact on virus replication, there was no compartmentalization of SIV-producing cells. Virus compartmentalization was diminished in animals with simian AIDS, which often have low-frequency CTL responses. SIV-specific CTL were consistently more concentrated within EF regions of lymph node and spleen in chronically infected animals regardless of epitope specificity. Frequencies of SIV-specific CTL within F and EF compartments predicted SIV RNA(+) cells within these compartments in a mixed model. Few SIV-specific CTL expressed the F homing molecule CXCR5 in the absence of the EF retention molecule CCR7, possibly accounting for the paucity of F CTL. These findings bolster the hypothesis that B cell follicles are immune privileged sites and suggest that strategies to augment CTL in B cell follicles could lead to improved viral control and possibly a functional cure for HIV infection.

Early lymphoid responses and germinal center formation correlate with lower viral load set points and better prognosis of simian immunodeficiency virus infection

We have investigated the dynamics of germinal center (GC) formation in lymphoid tissues following acute SIV infection. SIV induces a marked follicular hyperplasia, associated with an aberrant accumulation of nonproliferating T follicular helper cells within GCs, but with an abundance of cells producing IL-21, demonstrating that the mechanisms involved for these two events appear independent. IL-21-stimulated T follicular helper cells are considered a critical element for GC formation, a physiological process that seems dysregulated and excessive during HIV/SIV infection, contributing to lymphoid pathogenesis. However, the data suggest that the kinetics by which such GCs are formed may be an important predictor of the host-pathogen equilibrium, as early GC hyperplasia was associated with better control of viral replication. In contrast, monkeys undergoing fast disease progression upon infection exhibited an involution of GCs without local IL-21 production in GCs. These results provide important clues regarding GC-related hyperimmune responses in the context of disease progression within various individuals during HIV/SIV infection and may open novel therapeutic avenues to limit lymphoid dysfunction, postinfection.

Immunopathogenesis of simian immunodeficiency virus infection in nonhuman primates

PURPOSE OF REVIEW

Soon after the discovery of HIV-infected humans, rhesus macaques in a colony at the New England Primate Research Center showed similar signs of a progressive immune suppression. The discovery of the simian immunodeficiency virus (SIV)-associated disease opened the door to study an AIDS-like illness in nonhuman primates (NHP). Even after 3 decades, this animal model remains an invaluable tool to provide a greater insight into HIV immunopathogenesis. In this review, recent progress in deciphering pathways of immunopathogenesis in SIV-infected NHP is discussed.

RECENT FINDINGS

The immense diversity of mutations in SIV stocks prepared at different laboratories has recently been realized. The massive expansion of the enteric virome is a key finding in SIV-induced immunopathogenesis. Defining the function of host restriction factors, like the recently discovered SAMHD1, helps to evaluate the impact of the innate immune responses on virus replication. Utilization of pyrosequencing and defining molecular mechanisms of major histocompatibility complex (MHC) class I restriction helps to understand how the virus evades CD8 T-cell responses. The definition of MHC class I molecules in different NHP species provides new animal models to study SIV immunopathogenesis. T follicular helper cells have gained major interest in characterizing humoral immune responses in SIV infection and AIDS vaccine strategies. The ability of natural hosts to remain disease-free despite ongoing replication of SIV is continuing to puzzle the field.

SUMMARY

The HIV research field continues to realize the immense complexity of the host virus interaction. NHP present an invaluable tool to make progress towards an effective AIDS vaccine.

Human T follicular helper (Tfh) cells and disease

The generation of protective antibodies by B cells following natural infection or vaccination requires 'help' from CD4(+) T cells. T follicular helper (Tfh) cells are the specialized CD4(+) T cell subset that has evolved the appropriate mechanisms to induce the activation and differentiation of B cells into immunoglobulin (Ig) secreting cells. As such, appropriate control of Tfh cell generation and function is essential to human health as overactivation is likely to result in autoimmunity, whereas underactivation is often associated with immunodeficiency. Furthermore, an understanding of the regulation of these cells may be invaluable to improved vaccine development strategies. Traditionally Tfh cells have been identified by their anatomical location in secondary lymphoid tissues, which has hindered the study of these cells in humans as access to these tissues is often not feasible. However, recent studies have identified the circulating counterparts to tissue Tfh cells and with this has come a wealth of knowledge gained from the study of these cells in human disease. Here we review some of the recent developments on the role of human Tfh cells in health and disease.