It has been demonstrated that the lymphoid B cell follicle is an important HIV reservoir capable of harboring high levels of virus even when HIV is undetectable in the periphery. It is believed that the B cell follicle is an immune privileged site which blocks HIV-specific CTLs from entering, while allowing T follicular helper cells to pass freely in and out of the follicle. Here, we describe a population of HIV-specific cytolytic CD4+ T cells that have the ability to enter the B cell follicle and may be harnessed to reduce the follicular viral burden.
We analyzed PBMCs from chronic HIV+ ART-naïve, HIV+ ART-treated and HIV- individuals, as well as lymphatic tissue from tonsils and lymph nodes of HIV+ and HIV- individuals. We used multicolor flow cytometry and immunofluorescence microscopy to determine the phenotype, frequency and functional properties of these cells. Furthermore, we assessed the plasticity of these cells in an in vitro culture and determined their ability to inhibit viral replication.
In the peripheral blood, we identified a CD4+ CXCR5+ T cells population expressing granzyme B and perforin that showed the ability to degranulate (CD107a) after stimulation. This cellular subset was significantly expanded in chronic HIV infection and contracted upon antigen reduction by ART. In response to Gag stimulation we found that a substantial portion of these cytolytic CD4+ T cells with follicular homing properties (TFC) were HIV-specific and further secreting interferon-γ. We next determined their frequency in lymph nodes and also found a substantial frequency of CD4+ CXCR5+ effector T cells in samples from HIV+ individuals. Importantly, the expansion of these cells after SEB stimulation was elevated in comparison to TFCs in lymph nodes and tonsils from healthy controls or blood samples from chronic HIV+ individuals. Using immunofluorescence microscopy we determined an enrichment of TFCs in the B cell follicle confirming their follicular homing. We next determined their ability to kill virally infected cells and found that similar to regular CTL CD4+ T cells, these cells showed enhanced ability to recognize and kill infected cells.
In conclusion, we demonstrate a novel subset of cytolytic CD4+ T cells with follicular homing properties that can be found in the B cell follicle and shows enhanced viral inhibitory activity. Harnessing this cellular subset through therapeutic intervention may provide a novel strategy for HIV eradication attempts.