B cell follicles, which may not be accessible to ART or antiviral CD8+ T cells, are considered an immune privileged site for HIV/SIV replication. However, recent evidence suggests follicular cytotoxic T cells (Tfc) can locate to B cell follicles of secondary lymphoid tissue and might contribute to viremia control. Here we aimed to clarify the role of viral-specific Tfc cells in lymph nodes (LNs) over the course of SIV infection in rhesus macaques.
LN biopsy specimens were collected from naïve and acutely and chronically (low viral load, LVL; and high viral load, HVL) SIV-infected rhesus macaques. Assays performed to clarify the interaction of Tfc cells with other LN follicular T cells and assess their impact on disease course included: immunohistochemistry to localize Tfc and Tfh cells; flowcytometry to determine the frequency of Tfc, Tfh and T follicular regulatory (Tfreg) cells and SIV-specific cytokine production; ELISPOT using sorted cells to quantify SIV-specific Tfc; a flowcytometry based killing assay to evaluate the functionality of sorted Tfc cells.
Tfc and Tfh cells were localized in LN B cell follicles. Positive correlations between percentages of these cells were observed in acute (r=0.55, p=0.022) and LVL (r=0.60, p=0.0073) but not HVL animals. SIV-specific Tfc cell frequencies were comparable between HVL and LVL animals, however, LVL animals tended to express more Gag-specific granB, exhibited greater killing than HVL animals (p=0.008), and their Tfc cell frequencies negatively correlated with viremia (r=-0.63, p=0.04). In LVL but not HVL animals, Tfh and Tfc cells correlated directly (r=0.63, p=0.04). Env- and Gag-specific IL-21+Tfh of LVL but not HVL macaques negatively correlated with viral load (r=-0.65, p=0.036; r=-0.67, p=0.028, respectively), suggesting better provision of T cell help to Tfc. In LVL animals Tfreg and Tfc cells were positively correlated (r=0.74, p=0.013) and negatively correlated with viremia (r=-0.81, p=0.0033), perhaps due to suppression of chronic inflammation. In contrast, Tfreg and Tfc cell frequencies of HVL macaques tended to negatively correlate (r=-0.57, p=0.071). A positive correlation was seen between Tfreg cell number in HVL macaques and viremia (r=0.65, p=0.034), suggesting dysfunction and suppression of an effective Tfc immune response.
Our results indicate that control of virus-infected cells in B cell follicles not only depends on Tfc cell cytotoxicity but also on Tfc cell interaction with Tfh and Tfreg.