Emerging concepts on T follicular helper cell dynamics in HIV infection

Biology and clinical relevance of follicular cytotoxic T cells

Follicular cytotoxic T (Tfc) cells are a newly identified subset of CD8⁺ T cells enriched in B cell follicles and their surroundings, which integrate multiple functions such as killing, memory, supporting and regulation. Tfc cells share similarities with follicular helper T (Tfh) cells, conventional cytotoxic CD8⁺ T (Tc cells)cells and follicular regulatory T (Tfr) cells, while they express distinct transcription factors, phenotype, and perform different functions. With the participation of cytokines and cell-cell interactions, Tfc cells modulate Tfh cells and B cells and play an essential role in regulating the humoral immunity. Furthermore, Tfc cells have been found to change in their frequencies and functions during the occurrence and progression of chronic infections, immune-mediated diseases and cancers. Strategies targeting Tfc cells are under investigations, bringing novel insights into control of these diseases. We summarize the characteristics of Tfc cells, and introduce the roles and potential targeting modalities of Tfc cells in different diseases.

Interleukin-21 in Viral Infections

Interleukin (IL)-21 is a cytokine that affects the differentiation and function of lymphoid and myeloid cells and regulates both innate and adaptive immune responses. In addition to regulating the immune response to tumor and viral infections, IL-21 also has a profound effect on the development of autoimmune and inflammatory diseases. IL-21 is produced mainly from CD4⁺ T cells-in particular, follicular helper T (Tfh) cells-which have a great influence on the regulation of antibody production. It is also an important cytokine for the activation of CD8⁺ T cells, and its role in recovering the function of CD8⁺ T cells exhausted by chronic microbial infections and cancer has been clarified. Thus, IL-21 plays an extremely important role in viral infections, especially chronic viral infections. In this review, I will introduce the findings to date on how IL-21 is involved in some typical viral infections and the potential of treating viral diseases with IL-21.

SARS-CoV-2-specific circulating T follicular helper cells correlate with neutralizing antibodies and increase during early convalescence

T-cell immunity is likely to play a role in protection against SARS-CoV-2 by helping generate neutralizing antibodies. We longitudinally studied CD4 T-cell responses to the M, N, and S structural proteins of SARS-CoV-2 in 26 convalescent individuals. Within the first two months following symptom onset, a majority of individuals (81%) mounted at least one CD4 T-cell response, and 48% of individuals mounted detectable SARS-CoV-2-specific circulating T follicular helper cells (cTfh, defined as CXCR5⁺PD1⁺ CD4 T cells). SARS-CoV-2-specific cTfh responses across all three protein specificities correlated with antibody neutralization with the strongest correlation observed for S protein-specific responses. When examined over time, cTfh responses, particularly to the M protein, increased in convalescence, and robust cTfh responses with magnitudes greater than 5% were detected at the second convalescent visit, a median of 38 days post-symptom onset. CD4 T-cell responses declined but persisted at low magnitudes three months and six months after symptom onset. These data deepen our understanding of antigen-specific cTfh responses in SARS-CoV-2 infection, suggesting that in addition to S protein, M and N protein-specific cTfh may also assist in the development of neutralizing antibodies and that cTfh response formation may be delayed in SARS-CoV-2 infection.

Since December 2019, the Coronavirus Disease 2019 (COVID-19) pandemic has caused significant morbidity and mortality worldwide. Recently approved vaccines against SARS-CoV-2 are understood to protect against infection by inducing neutralizing antibodies. However, the underlying immune responses necessary for protection remain unclear. It is well established that T follicular helper cells (Tfh), a subset of CD4 T cells, are essential to the development of neutralizing antibodies and that some of these cells, called circulating T follicular helper cells (cTfh), can be studied in the blood. Not much is known about Tfh responses mounted in SARS-CoV-2 infection. Here, we studied cTfh responses to three major structural proteins in individuals recovered from COVID-19. We find that SARS-CoV-2-specific cTfh frequencies correlate with neutralizing antibody responses. We also find that cTfh responses to SARS-CoV-2 increase well into convalescence before contracting. Our results suggest that cTfh responses against proteins other than the spike protein may contribute to the development of neutralizing antibodies and that the formation of cTfh responses in SARS-CoV-2 infection may be delayed.

Production of HIV-1 Env-specific antibodies mediating innate immune functions depends on cognate IL-21- secreting CD4+ T cells

Antibodies with a functional Fc region were previously associated with protection from HIV-1 acquisition and spontaneous suppression of viral replication. Unlike broadly neutralizing antibodies, they are not restricted to neutralizing epitopes and do not require unconventional structural traits to exert their antiviral activity. They, therefore, develop earlier after infection and can be detected in the majority of cases. The conditions under which these antibodies are generated, however, remain largely unknown. Here we demonstrate that the generation of HIV-1 Env-specific antibodies facilitating Fc-dependent innate immune responses, including neutrophil phagocytosis (ADNP), complement deposition (ADCD), and NK cell activation, likely depends on help provided by CD4+ T and peripheral T follicular helper (pTfh) cells secreting IL-21. Other proteins, including CD40L, IFNγ, and IL-4/13, involved in crosstalk between B and T cells were linked to the production of antibodies with functional Fc region but only when co-expressed with IL-21. As a potential source of these antibodies, we identified a subset of Env-specific memory B cells known to be expanded in chronic HIV-1 infection. The frequency and level of Blimp-1 expression in Env-specific tissue-like memory B cells (TLM) correlated with the functional CD4+ T cell subsets associated with robust antibody-dependent innate responses. Thus, our data suggest a mechanism responsible for the generation of antibodies with functional Fc region in chronically HIV-1 infected individuals that is based on CD4+ T cell-induced activation of memory B cells.Importance To develop a vaccine or immunotherapy that would cure the HIV-1 infection it is important to identify helper T cells able to mount an efficient antibody response. Here, we demonstrate that the generation of HIV-1 Env-specific antibodies facilitating antibody-dependent innate immune responses likely depends on Env-specific IL-21-secreting CD4+ T and peripheral T follicular helper cells.

TFH Cells Induced by Vaccination and Following SIV Challenge Support Env-Specific Humoral Immunity in the Rectal-Genital Tract and Circulation of Female Rhesus Macaques

T follicular helper (T_FH) cells are pivotal in lymph node (LN) germinal center (GC) B cell affinity maturation. Circulating CXCR5⁺ CD4⁺ T (cT_FH) cells have supported memory B cell activation and broadly neutralizing antibodies in HIV controllers. We investigated the contribution of LN SIV-specific T_FH and cT_FH cells to Env-specific humoral immunity in female rhesus macaques following a mucosal Ad5hr-SIV recombinant priming and SIV gp120 intramuscular boosting vaccine regimen and following SIV vaginal challenge. T_FH and B cells were characterized by flow cytometry. B cell help was evaluated in T_FH-B cell co-cultures and by real-time PCR. Vaccination induced Env-specific T_FH and Env-specific memory (ESM) B cells in LNs. LN Env-specific T_FH cells post-priming and GC ESM B cells post-boosting correlated with rectal Env-specific IgA titers, and GC B cells at the same timepoints correlated with vaginal Env-specific IgG titers. Vaccination expanded cT_FH cell responses, including CD25⁺ Env-specific cT_FH cells that correlated negatively with vaginal Env-specific IgG titers but positively with rectal Env-specific IgA titers. Although cT_FH cells post-2^nd boost positively correlated with viral-loads following SIV challenge, cT_FH cells of SIV-infected and protected macaques supported maturation of circulating B cells into plasma cells and IgA release in co-culture. Additionally, cT_FH cells of naïve macaques promoted upregulation of genes associated with B cell proliferation, BCR engagement, plasma cell maturation, and antibody production, highlighting the role of cT_FH cells in blood B cell maturation. Vaccine-induced LN T_FH and GC B cells supported anti-viral mucosal immunity while cT_FH cells provided B cell help in the periphery during immunization and after SIV challenge. Induction of T_FH responses in blood and secondary lymphoid organs is likely desirable for protective efficacy of HIV vaccines.

Sharing CD4+ T Cell Loss: When COVID-19 and HIV Collide on Immune System

COVID-19 is a distinctive infection characterized by elevated inter-human transmission and presenting from absence of symptoms to severe cytokine storm that can lead to dismal prognosis. Like for HIV, lymphopenia and drastic reduction of CD4+ T cell counts in COVID-19 patients have been linked with poor clinical outcome. As CD4+ T cells play a critical role in orchestrating responses against viral infections, important lessons can be drawn by comparing T cell response in COVID-19 and in HIV infection and by studying HIV-infected patients who became infected by SARS-CoV-2. We critically reviewed host characteristics and hyper-inflammatory response in these two viral infections to have a better insight on the large difference in clinical outcome in persons being infected by SARS-CoV-2. The better understanding of mechanism of T cell dysfunction will contribute to the development of targeted therapy against severe COVID-19 and will help to rationally design vaccine involving T cell response for the long-term control of viral infection.

SARS-CoV-2-specific peripheral T follicular helper cells correlate with neutralizing antibodies and increase during convalescence

T-cell immunity is likely to play a role in protection against SARS-CoV-2 by helping generate neutralizing antibodies. We longitudinally studied CD4 T-cell responses to the M, N, and S structural proteins of SARS-CoV-2 in 21 convalescent individuals. Within the first two months following symptom onset, a majority of individuals (81%) mount at least one CD4 T-cell response, and 48% of individuals mount detectable SARS-CoV-2-specific peripheral T follicular helper cells (pTfh, defined as CXCR5+PD1+ CD4 T cells). SARS-CoV-2-specific pTfh responses across all three protein specificities correlate with antibody neutralization with the strongest correlation observed for S protein-specific responses. When examined over time, pTfh responses increase in frequency and magnitude in convalescence, and robust responses with magnitudes greater than 5% were detected only at the second convalescent visit, an average of 38 days post-symptom onset. These data deepen our understanding of antigen-specific pTfh responses in SARS-CoV-2 infection, suggesting that M and N protein-specific pTfh may also assist in the development of neutralizing antibodies and that pTfh response formation may be delayed in SARS-CoV-2 infection.

CTLA-4 Blockade, during HIV Virus-Like Particles Immunization, Alters HIV-Specific B-Cell Responses

Studies have shown that blockade of CTLA-4 promoted the expansion of germinal center B-cells in viral infection or immunization with model antigens. Few studies have evaluated the immunological consequences of CTLA-4 blockade during immunization against relevant vaccine candidates. Here, we investigated the effects of CTLA-4 blockade on HIV virus-like particles (VLPs) vaccination in a C57BL/6J mouse model. We found that CTLA-4 blockade during HIV VLP immunization resulted in increased CD4+ T-cell activation, promoted the expansion of HIV envelope (Env)-specific follicular helper T cell (Tfh) cells, and significantly increased HIV Gag- and Env-specific IgG with higher avidity and antibody-dependent cellular cytotoxicity (ADCC) capabilities. Furthermore, after only a single immunization, CTLA-4 blockade accelerated T-cell dependent IgG class switching and the induction of significantly high serum levels of the B-cell survival factor, A proliferation-inducing ligand (APRIL). Although no significant increase in neutralizing antibodies was observed, increased levels of class-switched Env- and Gag-specific IgG are indicative of increased polyclonal B-cell activation, which demonstrated the ability to mediate and enhance ADCC in this study. Altogether, our findings show that CTLA-4 blockade can increase the levels of HIV antigen-specific B-cell and antigen-specific Tfh cell activity and impact humoral immune responses when combined with a clinically relevant HIV VLP-based vaccine.

Critical role of Interleukin 21 and T follicular helper cells in hypertension and vascular dysfunction

T and B cells have been implicated in hypertension, but the mechanisms by which they produce a coordinated response is unknown. T follicular helper (Tfh) cells that produce interleukin 21 (IL21) promote germinal center (GC) B cell responses leading to immunoglobulin (Ig) production. Here we investigate the role of IL21 and Tfh cells in hypertension. In response to angiotensin (Ang) II-induced hypertension, T cell IL21 production is increased, and Il21-/- mice develop blunted hypertension, attenuated vascular end-organ damage, and decreased interleukin 17A (IL17A) and interferon gamma production. Tfh-like cells and GC B cells accumulate in the aorta and plasma IgG1 is increased in hypertensive WT but not Il21-/-mice. Furthermore, Tfh cell deficient mice develop blunted hypertension and vascular hypertrophy in response to Ang II infusion. Importantly, IL21 neutralization reduces blood pressure (BP) and reverses endothelial dysfunction and vascular inflammation. Moreover, recombinant IL21 impairs endothelium-dependent relaxation ex vivo and decreases nitric oxide production from cultured endothelial cells. Finally, we show in humans that peripheral blood T cell production of IL21 correlates with systolic BP and IL17A production. These data suggest that IL21 may be a novel therapeutic target for the treatment of hypertension and its micro- and macrovascular complications.

Early T Follicular Helper Cell Responses and Germinal Center Reactions Are Associated with Viremia Control in Immunized Rhesus Macaques

T follicular helper (T_FH) cells are fundamental in germinal center (GC) maturation and selection of antigen-specific B cells within secondary lymphoid organs. GC-resident T_FH cells have been fully characterized in human immunodeficiency virus (HIV) infection. However, the role of GC T_FH cells in GC B cell responses following various simian immunodeficiency virus (SIV) vaccine regimens in rhesus macaques (RMs) has not been fully investigated. We characterized GC T_FH cells of RMs over the course of a mucosal/systemic vaccination regimen to elucidate GC formation and SIV humoral response generation. Animals were mucosally primed twice with replicating adenovirus type 5 host range mutant (Ad5hr)-SIV recombinants and systemically boosted with ALVAC-SIV_M766Gag/Pro/gp120-TM and SIV_M766&CG7V gD-gp120 proteins formulated in alum hydroxide (ALVAC/Env) or DNA encoding SIVenv/SIVGag/rhesus interleukin 12 (IL-12) plus SIV_M766&CG7V gD-gp120 proteins formulated in alum phosphate (DNA&Env). Lymph nodes were biopsied in macaque subgroups prevaccination and at day 3, 7, or 14 after the 2nd Ad5hr-SIV prime and the 2nd vector/Env boost. Evaluations of GC T_FH and GC B cell dynamics including correlation analyses supported a significant role for early GC T_FH cells in providing B cell help during initial phases of GC formation. GC T_FH responses at day 3 post-mucosal priming were consistent with generation of Env-specific memory B cells in GCs and elicitation of prolonged Env-specific humoral immunity in the rectal mucosa. GC Env-specific memory B cell responses elicited early post-systemic boosting correlated significantly with decreased viremia postinfection. Our results highlight the importance of early GC T_FH cell responses for robust GC maturation and generation of long-lasting SIV-specific humoral responses at mucosal and systemic sites. Further investigation of GC T_FH cell dynamics should facilitate development of an efficacious HIV vaccine.IMPORTANCE The modest HIV protection observed in the human RV144 vaccine trial associated antibody responses with vaccine efficacy. T follicular helper (T_FH) cells are CD4⁺ T cells that select antibody secreting cells with high antigenic affinity in germinal centers (GCs) within secondary lymphoid organs. To evaluate the role of T_FH cells in eliciting prolonged virus-specific humoral responses, we vaccinated rhesus macaques with a combined mucosal prime/systemic boost regimen followed by repeated low-dose intrarectal challenges with SIV, mimicking human exposure to HIV-1. Although the vaccine regimen did not prevent SIV infection, decreased viremia was observed in the immunized macaques. Importantly, vaccine-induced T_FH responses elicited at day 3 postimmunization and robust GC maturation were strongly associated. Further, early T_FH-dependent SIV-specific B cell responses were also correlated with decreased viremia. Our findings highlight the contribution of early vaccine-induced GC T_FH responses to elicitation of SIV-specific humoral immunity and implicate their participation in SIV control.

Modulation of Vaccine-Induced CD4 T Cell Functional Profiles by Changes in Components of HIV Vaccine Regimens in Humans

To date, six vaccine strategies have been evaluated in clinical trials for their efficacy at inducing protective immune responses against HIV infection. However, only the ALVAC-HIV/AIDSVAX B/E vaccine (RV144 trial) has demonstrated protection, albeit modestly (31%; P = 0.03). One potential correlate of protection was a low-frequency HIV-specific CD4 T cell population with diverse functionality. Although CD4 T cells, particularly T follicular helper (Tfh) cells, are critical for effective antibody responses, most studies involving HIV vaccines have focused on humoral immunity or CD8 T cell effector responses, and little is known about the functionality and frequency of vaccine-induced CD4 T cells. We therefore assessed responses from several phase I/II clinical trials and compared them to responses to natural HIV-1 infection. We found that all vaccines induced a lower magnitude of HIV-specific CD4 T cell responses than that observed for chronic infection. Responses differed in functionality, with a CD40 ligand (CD40L)-dominated response and more Tfh cells after vaccination, whereas chronic HIV infection provoked tumor necrosis factor alpha (TNF-α)-dominated responses. The vaccine delivery route further impacted CD4 T cells, showing a stronger Th1 polarization after dendritic cell delivery than after intramuscular vaccination. In prime/boost regimens, the choice of prime and boost influenced the functional profile of CD4 T cells to induce more or less polyfunctionality. In summary, vaccine-induced CD4 T cell responses differ remarkably between vaccination strategies, modes of delivery, and boosts and do not resemble those induced by chronic HIV infection. Understanding the functional profiles of CD4 T cells that best facilitate protective antibody responses will be critical if CD4 T cell responses are to be considered a clinical trial go/no-go criterion.IMPORTANCE Only one HIV-1 candidate vaccine strategy has shown protection, albeit marginally (31%), against HIV-1 acquisition, and correlates of protection suggested that a multifunctional CD4 T cell immune response may be important for this protective effect. Therefore, the functional phenotypes of HIV-specific CD4 T cell responses induced by different phase I and phase II clinical trials were assessed to better show how different vaccine strategies influence the phenotype and function of HIV-specific CD4 T cell immune responses. The significance of this research lies in our comprehensive comparison of the compositions of the T cell immune responses to different HIV vaccine modalities. Specifically, our work allows for the evaluation of vaccination strategies in terms of their success at inducing Tfh cell populations.

A Portrait of CXCR5+ Follicular Cytotoxic CD8+ T cells

CD8⁺ T cells differentiate into multiple effector and memory subsets to carry out immune clearance of infected and cancerous cells and provide long-term protection. Recent research identified a CXCR5⁺Tcf1⁺Tim-3^- subset that localizes in, or proximal to, B cell follicles in secondary lymphoid organs of mice, non-human primates, and humans, hereby termed follicular cytotoxic T (T_FC) cells. With remarkable similarity to follicular helper T (T_FH) cells, T_FC differentiation is dependent on transcription factors E2A, Bcl6, and Tcf1, but inhibited by other regulators, including Blimp1, Id2, and Id3. This review summarizes the phenotype, function, and differentiation of this new subset. Owing to its follicular location and self-renewal capability, we propose immunotherapeutic strategies to target T_FC cells to potentially treat certain cancers and chronic infections such as HIV-1.

Characterising antibody avidity in individuals of varied Mycobacterium tuberculosis infection status using surface plasmon resonance

There is increasing evidence supporting a role for antibodies in protection against tuberculosis (TB), with functional antibodies being described in the latent state of TB infection. Antibody avidity is an important determinant of antibody-mediated protection. This study characterised the avidity of antibodies against Ag85A, an immunodominant Mycobacterium tuberculosis (M.tb) antigen and constituent of several anti-TB vaccine candidates, in individuals of varied M.tb infection status. Avidity of Ag85A specific antibodies was measured in 30 uninfected controls, 34 individuals with latent TB infection (LTBI) and 75 active pulmonary TB (APTB) cases, employing the more commonly used chaotrope-based dissociation assays, and surface plasmon resonance (SPR). Chaotrope-based assays indicated that APTB was associated with a higher antibody avidity index compared to uninfected controls [adjusted geometric mean ratio (GMR): 1.641, 95% confidence interval (CI): 1.153, 2.337, p = 0.006, q = 0.018] and to individuals with LTBI [adjusted GMR: 1.604, 95% CI: 1.282, 2.006, p < 0.001, q <0.001]. SPR assays showed that APTB was associated with slower dissociation rates, an indication of higher avidity, compared to uninfected controls (adjusted GMR: 0.796, 95% CI: 0.681, 0.932, p = 0.004, q = 0.012) and there was also weak evidence of more avid antibodies in the LTBI compared to the uninfected controls (adjusted GMR: 0.871, 95% CI: 0.763, 0.994, p = 0.041, q = 0.123). We found no statistically significant differences in anti-Ag85A antibody avidity between the APTB and LTBI groups. This study shows that antibodies of increased avidity are generated against a principle vaccine antigen in M.tb infected individuals. It would be important to determine whether TB vaccines are able to elicit a similar response. Additionally, more research is needed to determine whether antibody avidity is important in protection against infection and disease.

K. V, Cheedarla N, Nesakumar M, Kathirvel S, Goyal R, Singla N, Mukherjee J, Bergin P, T. Kopycinski J, Gilmour J, Prasad Tripathy S, Luke HE. Induction of circulating T follicular helper cells and regulatory T cells correlating with HIV-1 gp120 variable loop antibodies by a subtype C prophylactic vaccine tested in a Phase I trial in India

A Phase I HIV-1 vaccine trial sponsored by the International AIDS Vaccine Initiative (IAVI) was conducted in India in 2009 to test a subtype C prophylactic vaccine in a prime-boost regimen comprising of a DNA prime (ADVAX) and MVA (TBC-M4) boost. The trial demonstrated that the regimen was safe and well tolerated and resulted in enhancement of HIV-specific immune responses. Preliminary observations on vaccine-induced immune responses were limited to analysis of neutralizing antibodies and IFN-γ ELISPOT response. The present study involves a more detailed analysis of the nature of the vaccine-induced humoral immune response using specimens that were archived from the volunteers at the time of the trial. Interestingly, we found vaccine induced production of V1/V2 and V3 region-specific antibodies in a significant proportion of vaccinees. Variable region antibody levels correlated directly with the frequency of circulating T follicular helper cells (Tfh) and regulatory T cells (Treg). Our findings provide encouraging evidence to demonstrate the immunogenicity of the tested vaccine. Better insights into vaccine-induced immune responses can aid in informing future design of a successfulHIV-1 vaccine.

Distinct biomarker signatures in HIV acute infection associate with viral dynamics and reservoir size

Estimating the size of the viral reservoir is critical for HIV cure strategies. Biomarkers in peripheral circulation may give insights into the establishment of the viral reservoir in compartments not easily accessible. We therefore measured systemic levels of 84 soluble biomarkers belonging to a broad array of immune pathways in acute HIV infection in both antiretroviral therapy-naive (ART-naive) individuals as well as individuals who began ART upon early detection of HIV infection. These biomarkers were measured longitudinally during acute and chronic infection and their relationship to viral reservoir establishment and persistence was assessed. We observed several distinct biomarker pathways induced following HIV infection such as IFN-γ-signaled chemokines, proinflammatory markers, and TNF-α-family members. Levels of several of these factors directly correlated with contemporaneous viral loads and/or frequency of peripheral blood mononuclear cells harboring HIV DNA during acute HIV infection. MCP-1, MIP-3β, sTNFR-II, and IL-10 levels prior to ART associated with HIV DNA levels after 96 weeks of treatment, suggesting a link between early immune signaling events and the establishment and persistence of the viral reservoir during ART. Furthermore, they offer potentially novel tools for gaining insight into relative reservoir size in acutely infected individuals and the potential of associated risks of treatment interruption.

What Are the Primary Limitations in B-Cell Affinity Maturation, and How Much Affinity Maturation Can We Drive with Vaccination? Lessons from the Antibody Response to HIV-1

Most broadly neutralizing antibodies to HIV-1 have in common an extreme degree of somatic hypermutation (SHM), which correlates with their ability to neutralize multiple viral strains. However, achieving such extreme SHM by immunization remains a challenge. Here, we discuss how antigenic variation during HIV-1 infection may work to exacerbate SHM by permitting multiple iterative cycles of affinity maturation in germinal centers, and speculate on how this could be recapitulated through vaccination.

Barriers to HIV Cure

Since the beginning of the epidemic, more than 70 million people have been infected with human immunodeficiency virus (HIV) and about 38 million have died from acquired immune deficiency syndrome (AIDS)-related illnesses. While the discovery of highly active antiretroviral therapy (HAART) in the mid 90's has saved millions of lives, a complete eradication of HIV is still not possible as HIV can persist for decades in a small reservoir of latently infected cells. Once reactivated, these latently infected cells can actively produce viral particles. Recent studies suggest that several sanctuaries exist within infected individuals where HIV can remain undetected by the immune system. These cellular, anatomical and microanatomical viral reservoirs represent a major obstacle for the eradication of HIV. Here we review recent findings on potential sanctuaries of HIV and address potential avenues to overcome these immunological barriers.

Peripheral T follicular helper Cells Make a Difference in HIV Reservoir Size between Elite Controllers and Patients on Successful cART

HIV latency is the main barrier to HIV eradication. Peripheral T follicular helper (pTfh) cells have a prominent role in HIV persistence. Herein, we analyzed the HIV reservoir size within memory CD4+ T-cell subsets in patients with HIV replication control. Twenty HIV-infected patients with suppressed HIV replication were included, with 10 elite controllers (EC) and 10 treated (TX) individuals. The HIV reservoir size was analyzed in resting memory CD4+ T-cells (Trm), pTfh, and non-pTfh cells using an ultrasensitive digital-droplet-PCR assay. Inter-group and intra-group differences were tested using non-parametric tests. Compared with the TX patients, the EC patients had smaller HIV reservoir not only in Trm but also in pTfh and non-pTfh subsets of memory CD4+ T-cells. The largest differences were observed in pTfh cells (p = 0.025). The pTfh and non-pTfh cells harbored similar levels of HIV-DNA in the EC (p = 0.60) and TX patients (p = 0.17); however, the contribution to HIV-DNA levels in memory CD4+ T-cells varied among the pTfh and non-pTfh subsets in both groups of patients. The EC patients showed smaller HIV reservoir in memory CD4+ cells, especially in the pTfh subset, a population of cells with a pivotal role in the antiviral immune response, suggesting a potential link between low levels of infection in pTfh cells and the ability of the EC patients to spontaneously control HIV replication.

Differential Inhibitory Receptor Expression on T Cells Delineates Functional Capacities in Chronic Viral Infection

Inhibitory receptors have been extensively described for their importance in regulating immune responses in chronic infections and cancers. Blocking the function of inhibitory receptors such as PD-1, CTLA-4, 2B4, Tim-3, and LAG-3 has shown promise for augmenting CD8 T cell activity and boosting pathogen-specific immunity. However, the prevalence of inhibitory receptors on CD4 T cells and their relative influence on CD4 T cell functionality in chronic HIV infection remains poorly described. We therefore determined and compared inhibitory receptor expression patterns of 2B4, CTLA-4, LAG-3, PD-1, and Tim-3 on virus-specific CD4 and CD8 T cells in relation to their functional T cell profile. In chronic HIV infection, inhibitory receptor distribution differed markedly between cytokine-producing T cell subsets with, gamma interferon (IFN-γ)- and tumor necrosis factor alpha (TNF-α)-producing cells displaying the highest and lowest prevalence of inhibitory receptors, respectively. Blockade of inhibitory receptors differentially affected cytokine production by cells in response to staphylococcal enterotoxin B stimulation. CTLA-4 blockade increased IFN-γ and CD40L production, while PD-1 blockade strongly augmented IFN-γ, interleukin-2 (IL-2), and TNF-α production. In a Friend retrovirus infection model, CTLA-4 blockade in particular was able to improve control of viral replication. Together, these results show that inhibitory receptor distribution on HIV-specific CD4 T cells varies markedly with respect to the functional subset of CD4 T cells being analyzed. Furthermore, the differential effects of receptor blockade suggest novel methods of immune response modulation, which could be important in the context of HIV vaccination or therapeutic strategies.IMPORTANCE Inhibitory receptors are important for limiting damage by the immune system during acute infections. In chronic infections, however, their expression limits immune system responsiveness. Studies have shown that blocking inhibitory receptors augments CD8 T cell functionality in HIV infection, but their influence on CD4 T cells remains unclear. We assessed the expression of inhibitory receptors on HIV-specific CD4 T cells and their relationship with T cell functionality. We uncovered differences in inhibitory receptor expression depending on the CD4 T cell function. We also found differences in functionality of CD4 T cells following blocking of different inhibitory receptors, and we confirmed our results in a Friend virus retroviral model of infection in mice. Our results show that inhibitory receptor expression on CD4 T cells is linked to CD4 T cell functionality and could be sculpted by blockade of specific inhibitory receptors. These data reveal exciting possibilities for the development of novel treatments and immunotherapeutics.

Designing optimal HIV-vaccine T-cell responses

PURPOSE OF REVIEW

T cells can efficaciously control HIV replication, and it has been hypothesized that inducing those responses before exposure occurs may prevent HIV infection. However, conventional attempts to generate protective CD8 T-cell responses against HIV have generally failed. Based on current knowledge from chronic HIV infection and previous vaccine trials, this review details optimal CD8 and CD4 T-cell response design that may confer protection from HIV infection.

RECENT FINDINGS

The failure of two vaccines geared toward inducing T-cell response (STEP trial and HVTN505/Phambili) as well as the modest protection of the RV144 that mainly demonstrated nonneutralizing antibodies to be a correlate of protection have rattled the idea that a pure T-cell-based vaccine may induce protection. Moreover, in the recent years, CD4 T cells, and in particular the T follicular helper cell subset, received attention as a critical component for T-cell-inducing and antibody-inducing vaccines.

SUMMARY

It is apparent that all vaccines depend for their efficacy on a cellular component either to directly kill virally infected cells or to provide important helper signals for the development of efficacious B-cell responses. Recent vaccine trials have had a major impact on the field and are guiding new approaches for HIV vaccine design.

Extrafollicular activities: perspectives on HIV infection, germinal center-independent maturation pathways, and KSHV-mediated lymphoproliferation

Early events in the pathogenesis of KSHV-associated lymphoproliferations in the context of HIV disease remain poorly understood. Recent research indicates that latent HIV infection causes persistent immune dysfunction in B cell follicles. Simultaneously, lack of T cell immune surveillance in the lymph nodes dysregulates the biology of EBV. In sum, these defects bias B lymphocyte maturation away from traditional T cell-dependent germinal center-mediated pathways and towards extrafollicular pathways. Recent advances in B lymphocyte immunology suggest that extrafollicular maturation pathways for antibody secreting cells are more flexible and robust than previously believed. These responses are now understood to be both durable and antigen-specific, and even canonically germinal center-restricted events such as class switch recombination and somatic hypermutation have now been demonstrated in an extrafollicular context. As a lymphotrophic pathogen which causes disease primarily in the context of HIV and EBV co-infection, future studies examining the interactions of KSHV biology with extrafollicular B cell maturation pathways will be critical to uncovering key aspects of KSHV-mediated immune pathology.

Preservation of Peripheral T Follicular Helper Cell Function in HIV Controllers

The maturation process of high-affinity antibodies is a result of intricate interactions between B cells and follicular helper T (Tfh) cells occurring in lymphoid germinal centers. HIV infection induces significant chronic immune activation, phenotypic skewing, and inflammation driven by years of continuous viral replication. High levels of viremia as well as immune activation and dysfunction have been demonstrated to have a perturbing impact on the B cell memory compartment and contribute to B cell exhaustion. Counterintuitively, the factors associated with perturbation of the B cell compartment seem to be favorable for the generation of highly affinity-matured Env-specific antibodies in a minority of HIV-infected individuals. Thus, the impact of HIV antigenemia on B cells and Tfh cell interactions warrants further exploration. We therefore studied immunophenotypes of HIV-specific B cells in individuals with differing levels of viral control using HIV Env gp120 probes and characterized the functionality of matched T cells in peripheral blood. While CXCR5⁺ CD4⁺ T cells were significantly diminished in HIV progressors, we found that a small subset of gp120-specific interleukin-21 (IL-21)-secreting CXCR5⁺ CD4⁺ T cells were significantly associated with gp120-specific B cell frequencies. In contrast, neither bulk CXCR5⁺ CD4⁺ T cells nor other HIV antigen specificities were associated with gp120-specific B cell levels. HIV-specific B cells derived from elite controllers displayed greater amounts of gp120-specific B cells in the resting memory subset, whereas HIV-specific B cells in progressors accumulated in tissue-like and activated memory subsets. Furthermore, CXCR5⁺ CD4⁺ T cells from elite controllers showed a stronger ex vivo capacity to induce B cell maturation and immunoglobulin class switching than cells from HIV progressors.IMPORTANCE Dissecting the factors that are involved in B cell maturation and antibody development is important for HIV vaccine design. In this study, we found that HIV Env-specific CXCR5⁺ CD4⁺ T cells that secrete interleukin-21 are strongly associated with B cell memory phenotypes and function. Moreover, we found that the immune responses of HIV controllers showed intrinsically better helper activity than those of HIV progressors.

CXCR5-Dependent Entry of CD8 T Cells into Rhesus Macaque B-Cell Follicles Achieved through T-Cell Engineering

Follicular helper CD4 T cells, TFH, residing in B-cell follicles within secondary lymphoid tissues, are readily infected by AIDS viruses and are a major source of persistent virus despite relative control of viral replication. This persistence is due at least in part to a relative exclusion of effective antiviral CD8 T cells from B-cell follicles. To determine whether CD8 T cells could be engineered to enter B-cell follicles, we genetically modified unselected CD8 T cells to express CXC chemokine receptor 5 (CXCR5), the chemokine receptor implicated in cellular entry into B-cell follicles. Engineered CD8 T cells expressing human CXCR5 (CD8hCXCR5) exhibited ligand-specific signaling and chemotaxis in vitro Six infected rhesus macaques were infused with differentially fluorescent dye-labeled autologous CD8hCXCR5 and untransduced CD8 T cells and necropsied 48 h later. Flow cytometry of both spleen and lymph node samples revealed higher frequencies of CD8hCXCR5 than untransduced cells, consistent with preferential trafficking to B-cell follicle-containing tissues. Confocal fluorescence microscopy of thin-sectioned lymphoid tissues demonstrated strong preferential localization of CD8hCXCR5 T cells within B-cell follicles with only rare cells in extrafollicular locations. CD8hCXCR5 T cells were present throughout the follicles with some observed near infected TFH In contrast, untransduced CD8 T cells were found in the extrafollicular T-cell zone. Our ability to direct localization of unselected CD8 T cells into B-cell follicles using CXCR5 expression provides a strategy to place highly effective virus-specific CD8 T cells into these AIDS virus sanctuaries and potentially suppress residual viral replication.IMPORTANCE AIDS virus persistence in individuals under effective drug therapy or those who spontaneously control viremia remains an obstacle to definitive treatment. Infected follicular helper CD4 T cells, TFH, present inside B-cell follicles represent a major source of this residual virus. While effective CD8 T-cell responses can control viral replication in conjunction with drug therapy or in rare cases spontaneously, most antiviral CD8 T cells do not enter B-cell follicles, and those that do fail to robustly control viral replication in the TFH population. Thus, these sites are a sanctuary and a reservoir for replicating AIDS viruses. Here, we demonstrate that engineering unselected CD8 T cells to express CXCR5, a chemokine receptor on TFH associated with B-cell follicle localization, redirects them into B-cell follicles. These proof of principle results open a pathway for directing engineered antiviral T cells into these viral sanctuaries to help eliminate this source of persistent virus.

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.

Induction of Th1-Biased T Follicular Helper (Tfh) Cells in Lymphoid Tissues during Chronic Simian Immunodeficiency Virus Infection Defines Functionally Distinct Germinal Center Tfh Cells

Chronic HIV infection is associated with accumulation of germinal center (GC) T follicular helper (Tfh) cells in the lymphoid tissue. The GC Tfh cells can be heterogeneous based on the expression of chemokine receptors associated with T helper lineages, such as CXCR3 (Th1), CCR4 (Th2), and CCR6 (Th17). However, the heterogeneous nature of GC Tfh cells in the lymphoid tissue and its association with viral persistence and Ab production during chronic SIV/HIV infection are not known. To address this, we characterized the expression of CXCR3, CCR4, and CCR6 on GC Tfh cells in lymph nodes following SIVmac251 infection in rhesus macaques. In SIV-naive rhesus macaques, only a small fraction of GC Tfh cells expressed CXCR3, CCR4, and CCR6. However, during chronic SIV infection, the majority of GC Tfh cells expressed CXCR3, whereas the proportion of CCR4(+) cells did not change, and CCR6(+) cells decreased. CXCR3(+), but not CXCR3(-), GC Tfh cells produced IFN-γ (Th1 cytokine) and IL-21 (Tfh cytokine), whereas both subsets expressed CD40L following stimulation. Immunohistochemistry analysis demonstrated an accumulation of CD4(+)IFN-γ(+) T cells within the hyperplastic follicles during chronic SIV infection. CXCR3(+) GC Tfh cells also expressed higher levels of ICOS, CCR5, and α4β7 and contained more copies of SIV DNA compared with CXCR3(-) GC Tfh cells. However, CXCR3(+) and CXCR3(-) GC Tfh cells delivered help to B cells in vitro for production of IgG. These data demonstrate that chronic SIV infection promotes expansion of Th1-biased GC Tfh cells, which are phenotypically and functionally distinct from conventional GC Tfh cells and contribute to hypergammaglobulinemia and viral reservoirs.

CD4+CXCR5+ T cells activate CD27+IgG+ B cells via IL-21 in patients with hepatitis C virus infection

BACKGROUND

Chronic hepatitis C virus (HCV) infection causes the skewing and activation of B cell subsets, but the characteristics of IgG+ B cells in patients with chronic hepatitis C (CHC) infection have not been thoroughly elucidated. CD4+CXCR5+ follicular helper T (Tfh) cells, via interleukin (IL)-21 secretion, activate B cells. However, the role of CD4+CXCR5+ T cells in the activation of IgG+ B cells in CHC patients is not clear.

METHODS

The frequency of IgG+ B cells, including CD27-IgG+ B and CD27+IgG+ B cells, the expression of the activation markers (CD86 and CD95) in IgG+ B cells, and the percentage of circulating CD4+CXCR5+ T cells were detected by flow cytometry in CHC patients (n=70) and healthy controls (n=25). The concentrations of serum IL-21 were analyzed using ELISA. The role of CD4+CXCR5+ T cells in the activation of IgG+ B cells was investigated using a co-culture system.

RESULTS

A significantly lower proportion of CD27+IgG+ B cells with increased expression of CD86 and CD95 was observed in CHC patients. The expression of CD95 was negatively correlated with the percentage of CD27+IgG+ B cells, and it contributed to CD27+IgG+ B cell apoptosis. Circulating CD4+CXCR5+ T cells and serum IL-21 were significantly increased in CHC patients. Moreover, circulating CD4+CXCR5+ T cells from CHC patients induced higher expressions of CD86 and CD95 in CD27+IgG+ B cells in a co-culture system; the blockade of the IL-21 decreased the expression levels of CD86 and CD95 in CD27+IgG+ B cells.

CONCLUSIONS

HCV infection increased the frequency of CD4+CXCR5+ T cells and decreased the frequency of CD27+IgG+ B cells. CD4+CXCR5+ T cells activated CD27+IgG+ B cells via the secretion of IL-21.

Decreased T Follicular Regulatory Cell/T Follicular Helper Cell (TFH) in Simian Immunodeficiency Virus-Infected Rhesus Macaques May Contribute to Accumulation of TFH in Chronic Infection

T follicular helper cells (TFH) are critical for the development and maintenance of germinal center (GC) and humoral immune responses. During chronic HIV/SIV infection, TFH accumulate, possibly as a result of Ag persistence. The HIV/SIV-associated TFH expansion may also reflect lack of regulation by suppressive follicular regulatory CD4(+) T cells (TFR). TFR are natural regulatory T cells (TREG) that migrate into the follicle and, similar to TFH, upregulate CXCR5, Bcl-6, and PD1. In this study, we identified TFR as CD4(+)CD25(+)FOXP3(+)CXCR5(+)PD1(hi)Bcl-6(+) within lymph nodes of rhesus macaques (RM) and confirmed their localization within the GC by immunohistochemistry. RNA sequencing showed that TFR exhibit a distinct transcriptional profile with shared features of both TFH and TREG, including intermediate expression of FOXP3, Bcl-6, PRDM1, IL-10, and IL-21. In healthy, SIV-uninfected RM, we observed a negative correlation between frequencies of TFR and both TFH and GC B cells, as well as levels of CD4(+) T cell proliferation. Post SIV infection, the TFR/TFH ratio was reduced with no change in the frequency of TREG or TFR within the total CD4(+) T cell pool. Finally, we examined whether higher levels of direct virus infection of TFR were responsible for their relative depletion post SIV infection. We found that TFH, TFR, and TREG sorted from SIV-infected RM harbor comparable levels of cell-associated viral DNA. Our data suggest that TFR may contribute to the regulation and proliferation of TFH and GC B cells in vivo and that a decreased TFR/TFH ratio in chronic SIV infection may lead to unchecked expansion of both TFH and GC B cells.

TFH cells accumulate in mucosal tissues of humanized-DRAG mice and are highly permissive to HIV-1

CD4⁺ T follicular helper cells (T_FH) in germinal centers are required for maturation of B-cells. While the role of T_FH-cells has been studied in blood and lymph nodes of HIV-1 infected individuals, its role in the mucosal tissues has not been investigated. We show that the gut and female reproductive tract (FRT) of humanized DRAG mice have a high level of human lymphocytes and a high frequency of T_FH (CXCR5⁺PD-1⁺⁺) and precursor-T_FH (CXCR5⁺PD-1⁺) cells. The majority of T_FH-cells expressed CCR5 and CXCR3 and are the most permissive to HIV-1 infection. A single low-dose intravaginal HIV-1 challenge of humanized DRAG mice results in 100% infectivity with accumulation of T_FH-cells mainly in the Peyer’s patches and FRT. The novel finding of T_FH-cells in the FRT may contribute to the high susceptibility of DRAG mice to HIV-1 infection. This mouse model thus provides new opportunities to study T_FH-cells and to evaluate HIV-1 vaccines.

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.

Alterations of circulating follicular helper T cells and interleukin 21 in diffuse large B-cell lymphoma

CD4+ CXCR5+ T cell in peripheral blood is known as circulating follicular helper T cell (Tfh), which can produce interleukin 21 (IL-21). In the current study, we investigated changes of circulating Tfh and its correlation with IL-21 in diffuse large B-cell lymphoma (DLBCL). Circulating Tfh and its subtypes were detected by flow cytometry in the peripheral blood of 32 healthy donors and 62 DLBCL cases. Data demonstrated that percentage of circulating Tfh in CD4+ T cells was significantly increased in DLBCL (11.3 %) than in controls (8.5 %) (p = 0.001). Studying the subtypes of Tfh revealed that the upregulation of circulating Tfh was contributed by Tfh-Th2 subtype and Tfh-Th17 subtype. Also, we identified that prevalence of Tfh was significantly elevated in cases with advanced stages (stages III and IV). Interestingly, the elevation of circulating Tfh was negatively correlated with serum IL-21 in DLBCL patients. In addition, a positive correlation between circulating Tfh and IL-21 receptor on CD + 8 T cells was observed in patients. This study suggests involvement of circulating Tfh and IL-21 in the pathogenesis and progression of DLBCL and provides a potential target for treating this disease.