Background:  A T-cell subset, defined as CD4⁺CD25⁺ (Treg cells), was recently shown to suppress T-cell activation. Because Treg cells express CD4, they are potential targets of HIV in vivo. However, the role of Treg cells during HIV infection remains unknown. Here we tested the susceptibility of, both naturally occurring and in vitro genetically programmed, Treg cells to HIV infection.

Methods:  Human Treg cells were highly purified from PBMC of healthy donors based on expression of CD4 and CD25. The phenotypic and suppressive characteristics of freshly isolated Treg cells were determined. The susceptibility of human Treg cells to HIV infection was assessed by replication competent HIV and HIV-derived vectors using flow cytometry and replication assays. Because Treg cells are present in very few numbers and are difficult to expand in vitro, we also genetically modified conventional human naïve T cells to generate Treg cells in vitro by ectopic expression of FoxP3, a transcription factor associated with reprogramming T cells into a Treg subset.

Results:  We demonstrate that human Treg cells isolated from healthy donors express the HIV-co-receptor CCR5 and are highly susceptible to HIV infection and replication. Overexpression of FoxP3 in naïve human CD4⁺ T cells recapitulated the hyporesponsiveness and suppressive function of naturally occurring Treg cells. However, FoxP3 was less efficient in reprogramming memory T cell subset into regulatory T cells.  Remarkably, we found that genetic reprogramming of conventional T cells into regulatory cells greatly enhanced their susceptibility to HIV infection.

Conclusions:  Our findings may have profound implications in understanding the chronic activated state of T cells during HIV infection. Targeting and disruption of the T-cell regulatory system by HIV may contribute to hyperactivation of conventional T cells, a characteristic of HIV disease progression. Moreover, the ability to reprogram naive human T cells into Treg cells in vitro will greatly aid in decoding their mechanism of suppression, enhanced susceptibility to HIV infection and unique markers expressed by this important T-cell subset.

Keywords: Regulatory T cells; HIV; FoxP3