| Popis: |
Expression of the IL-7 receptor alpha-chain (CD127) is decreased on CD8 T-cells in HIV infected patients and recovers in those receiving antiretroviral therapy with sustained viral suppression. Here I demonstrate that the HIV Tat protein, acting in a paracrine fashion, specifically down regulates cell surface expression of CD127 on human CD8 T-cells. The effect of Tat on CD127 is dose- and time-dependent, specific, and can be blocked with anti-Tat monoclonal antibodies or by pre-incubating Tat with heparin. Pre-incubation of purified CD8 T-cells with Tat protein inhibited CD8 T-cell proliferation and perforin synthesis following stimulation with IL-7. I also show that low CD127 expression on CD8 T-cells from HIV+ patients recovers to normal levels when CD8 T-cells are isolated and cultured in vitro. Recovery ofCD127 was completely inhibited by the addition of HIV Tat protein to the culture media. This suggests that soluble factor(s) are responsible for low CD127 expression on circulating CD8 T-cells in HIV+ individuals and further supports the role of Tat in this process. I have elucidated the mechanism by which Tat removes CD127 from the cell surface. Soluble Tat protein is initially taken up by CD8 T-cells via endocytosis and exits the endosome through a process dependent on endosomal acidification. Once in the cytoplasm, Tat localizes to the inner leaflet of the cell membrane where it binds to the cytoplasmic tail of CD127 and induces receptor clustering and internalization. Through its association with Tat, CD127 is then directed to the proteasome for degradation. While Tat and IL-7 both independently down regulate surface expression of CD127, I have demonstrated that at near physiologic concentrations these viral and host factors act synergistically to suppress CD127. Inhibition of JAK kinase completely blocks IL-7's ability to down regulate CD127 on the surface of CD8 T-cells and also abolishes synergy with Tat. Interestingly, while Tat acts synergistically with IL-7 to reduce CD127 expression, it antagonizes IL-7 induced cell proliferation and Ki-67 expression. Since IL-7 signalling is essential for optimal CD8 T-cell proliferation and function, the down regulation of CD127 and apparent inhibition of cytotoxic activity by Tat may play an important role in HIV induced immune dysregulation and impaired cell mediated immunity. |
