Platelet Derived CD154 Induced by Tat Contributes to HIV-1 Associated Autoimmune Thrombocytopenia.

Abstract 2669

Poster Board II-645

Enhanced platelet activation was reported in HIV-1 infected patients and strongly correlated with plasma viral load. The HIV-1 Tat protein is able to serve as a ligand of the integrin αvβ3 and several chemokine receptors (CCR2 and CCR3) and to induce downstream signaling in cells express those receptors but the effect of Tat on platelet activation has not been fully investigated yet. Activated platelets are known to express and release a variety of proteins that can modulate the immune system, and platelet derived CD154 is reportedly involved in the development of autoimmune thrombocytopenia. However, the full mechanism underlying HIV-1 induced platelet activation and the biological consequences are not completely understood. In this study, we demonstrate that Tat is able to interact with platelets and β3 integrin by S³⁵ label Tat-platelet binding assay and GST-Tat protein pull-down. We then show that Tat is able to induce platelet activation, up-regulates both CD62P and CD154 in both mouse and human platelets, and results in micro-particle release (as demonstrated by electron microscopy). Tat induces greatly diminished activation in integrin β3 knock out platelets or in platelets pretreated with CCR3 or calcium flux inhibitors, suggesting the requirement of chemokine receptor CCR3, integrin β3 and calcium flux for Tat induced platelet activation. The effect of Tat induced platelet activation on the immune response was studied both in vitro and in vivo. An enhanced immunoglobulin class switch was found in mouse spleen B cells co-cultured with platelets treated with Tat in vitro. In addition, an early antibody response against adenovirus was found in Tat injected mouse immunized with adenovirus suggesting an enhanced immune response in vivo. Thus, we have described a new mechanism in which Tat is able to induce platelet activation and have generated a model in which platelet activation can contribute to the development of HIV-1 associated thrombocytopenia.