Follicular Regulatory T Cells Are Highly Permissive to R5-Tropic HIV-1

Follicular regulatory T (TFR) cells are a subset of CD4(+) T cells in secondary lymphoid follicles. TFR cells were previously included in the follicular helper T (TFH) cell subset, which consists of cells that are highly permissive to HIV-1. The permissivity of TFR cells to HIV-1 is unknown. We find that TFR cells are more permissive than TFH cells to R5-tropic HIV-1 ex vivo. TFR cells expressed more CCR5 and CD4 and supported higher frequencies of viral fusion. Differences in Ki67 expression correlated with HIV-1 replication. Inhibiting cellular proliferation reduced Ki67 expression and HIV-1 replication. Lymph node cells from untreated HIV-infected individuals revealed that TFR cells harbored the highest concentrations of HIV-1 RNA and highest levels of Ki67 expression. These data demonstrate that TFR cells are highly permissive to R5-tropic HIV-1 both ex vivo and in vivo. This is likely related to elevated CCR5 levels combined with a heightened proliferative state and suggests that TFR cells contribute to persistent R5-tropic HIV-1 replication in vivo. IMPORTANCE In chronic, untreated HIV-1 infection, viral replication is concentrated in secondary lymphoid follicles. Within secondary lymphoid follicles, follicular helper T (TFH) cells have previously been shown to be highly permissive to HIV-1. Recently, another subset of T cells in secondary lymphoid follicles was described, follicular regulatory T (TFR) cells. These cells share some phenotypic characteristics with TFH cells, and studies that showed that TFH cells are highly permissive to HIV-1 included TFR cells in their definition of TFH cells. The permissivity of TFR cells to HIV-1 has not previously been described. Here, we show that TFR cells are highly permissive to HIV-1 both ex vivo and in vivo. The expression of Ki67, a marker of proliferative capacity, is predictive of expression of viral proteins, and downregulating Ki67 leads to concurrent decreases in expression of viral proteins. Our study provides new insight into HIV-1 replication and a potential new cell type to target for future treatment.