CD4 T follicular helper (TFH) cells interact with and stimulate the generation of antigen-specific B cells. TFH cell interaction with B cells correlates with production of SIV-specific immunoglobulins. However, the fate of TFH cells and their participation in SIV-induced antibody production is not well understood. We investigated the phenotype, function, location, and molecular signature of TFH cells in rhesus macaques. Similar to their human counterparts, TFH cells in rhesus macaques represented a heterogeneous population with respect to cytokine function. In a highly differentiated subpopulation of TFH cells, characterized by CD150^lo expression, production of Th1 cytokines was compromised while IL-4 production was augmented, and cells exhibited decreased survival, cycling, and trafficking capacity. TFH cells exhibited a distinct gene profile that was markedly altered by SIV infection. TFH cells were infected by SIV; yet, in some animals, these cells actually accumulated during chronic SIV infection. Generalized immune activation and increased IL-6 production helped drive TFH differentiation during SIV infection. Accumulation of TFH cells was associated with increased frequency of activated germinal center B cells and SIV-specific antibodies. Therefore, chronic SIV does not disturb the ability of TFH cells to help B cell maturation and production of SIV-specific immunoglobulins.

The interaction between follicular T helper cells (T_FH) and B cells in the lymph nodes and spleen has a major impact on the development of antigen-specific B cell responses during infection or vaccination. Recent studies described a functional equivalent of these cells among circulating CD4 T cells, referred to as peripheral T_FH cells. Here, we characterize the phenotype and in vitro B cell helper activity of peripheral T_FH populations, as well as the effect of HIV infection on these populations. In co-culture experiments we confirmed CXCR5+ cells from HIV-uninfected donors provide help to B cells and more specifically, we identified a CCR7^highCXCR5^highCCR6^highPD-1^high CD4 T cell population that secretes IL-21 and enhances isotype-switched immunoglobulin production. This population is significantly decreased in treatment-naïve, HIV-infected individuals and can be recovered after anti-retroviral therapy. We found impaired immunoglobulin production in co-cultures from HIV-infected individuals and found no correlation between the frequency of peripheral T_FH cells and memory B cells, or with neutralization activity in untreated HIV infection in our cohort. Furthermore, we found that within the peripheral T_FH population, the expression level of T_FH-associated genes more closely resembles a memory, non-T_FH population, as opposed to a T_FH population. Overall, our data identify a heterogeneous population of circulating CD4 T cells that provides in vitro help to B cells, and challenges the origin of these cells as memory T_FH cells. Follicular T helper cells (T_FH) interact with B cells within germinal centers of lymphoid tissue to promote the survival, isotype switching and generation of high affinity memory B cells and plasma cells. Recently, a population of circulating CD4 T cells that shares phenotypic and functional characteristics with T_FH cells, named peripheral T_FH cells, has been identified. The relationship between peripheral T_FH cells in the blood and T_FH cells within the lymphoid tissue remains unclear, and whether or not peripheral T_FH cells can provide insight into T cell and B cell dynamics within lymphoid tissue during infection or vaccination is not understood. Here we characterize peripheral T_FH cells and show that unlike T_FH cells, peripheral T_FH cells secrete a diverse array of cytokines and decrease, rather than increase, during chronic HIV infection. Furthermore, we did not observe a relationship between peripheral T_FH cells and memory B cells, or with the production of neutralizing antibodies to HIV. Overall, our data indicate that while peripheral T_FH cells share some characteristics with T_FH cells, they may not represent a good surrogate to study T cell and B cell dynamics within lymphoid tissue.

Programmed death-1 (PD-1) is a critical mediator of virus-specific CD8⁺ T-cell exhaustion. Here, we examined the expression of PD-1 on simian immunodeficiency virus (SIV)-specific CD8⁺ T cells and its possible involvement in regulation of cytokine production, proliferation, and survival of these cells. The majority of SIV-specific CD8⁺ T cells expressed a PD-1^high phenotype, independent of their differentiation status, in all tissues tested. PD-1 expression gradually declined on CD8⁺ T cells specific for SIV-derived epitopes that had undergone mutational escape, indicating that antigen-specific TCR stimulation is the primary determinant of PD-1 expression. SIV-specific PD-1^highCD8⁺ T cells produced IFN-γ, TNF-α, and IL-2 under cognate peptide stimulation. While CD8⁺ T cells that proliferated in response to antigen had a PD-1^high phenotype, it was determined that there was a reduced proliferative capacity of PD-1^high compared with PD-1^low SIV-specific CD8⁺ T cells. PD-1^high SIV-specific CD8⁺ T cells were highly susceptible to cell death leading to loss of such cells after in vitro stimulation. Thus, PD-1 is a negative regulator of SIV-specific CD8⁺ T cells, operating predominantly through the induction of cell death. Manipulation of the interaction of PD-1 with its ligands could thus potentially restore the CD8⁺ T-cell responses in SIV infection.

Current paradigms of CD8 ⁺ T cell–mediated protection in HIV infection center almost exclusively on studies of peripheral blood, which is thought to provide a window into immune activity at the predominant sites of viral replication in lymphoid tissues (LTs). Through extensive comparison of blood, thoracic duct lymph (TDL), and LTs in different species, we show that many LT memory CD8 ⁺ T cells bear phenotypic, transcriptional, and epigenetic signatures of resident memory T cells (T _RMs ). Unlike their circulating counterparts in blood or TDL, most of the total and follicular HIV-specific CD8 ⁺ T cells in LTs also resemble T _RMs . Moreover, high frequencies of HIV-specific CD8 ⁺ T _RMs with skewed clonotypic profiles relative to matched blood samples are present in LTs of individuals who spontaneously control HIV replication in the absence of antiretroviral therapy (elite controllers). Single-cell RNA sequencing analysis confirmed that HIV-specific T _RMs are enriched for effector-related immune genes and signatures compared with HIV-specific non-T _RMs in elite controllers. Together, these data indicate that previous studies in blood have largely failed to capture the major component of HIV-specific CD8 ⁺ T cell responses resident within LTs.

LN inflammation in chronic HIV is associated with accumulation of follicular CD8 T cells that could be redirected to kill infected cells.

During viremia, the HIV-specific memory response in lymph nodes is mediated by T-bet ^hi B cells that inefficiently participate in affinity selection.