Primitive Hematopoietic Cells Resist HIV-1 Infection Via P21Waf1/Cip1/Sdi1.

Hematopoietic stem cells are resistant to HIV-1 infection. We have identified a novel mechanism by which the cyclin-dependent kinase inhibitor, p21Waf1/Cip1/Sdi1 (p21), known for its regulation of stem cell pool size (1,2), restricts HIV-1 infection of primitive hematopoietic cells in a non-cell cycle dependent manner. Knocking down p21 by siRNA increased HIV-1 infection and induction of p21 expression by phorbol ester (TPA) blocked HIV-1 replication. P21 did not affect the overall levels of cDNA synthesis, but significantly blocked viral integration and resulted in marked increase in 2-LTR circles, a surrogate marker of abortive integration. Consistent with these observations, p21 coimmunoprecipitated with viral integrase and both were detected in the preintegration complex (PIC). Furthermore, silencing p27Kip1 and p18INK4C, cyclin dependent kinase inhibitors related to p21 that affect cell cycle, revealed no impact on viral DNA integration. A closely related dual-tropic lentivirus with a distinct integrase, SIVmac-251 and the other cell-intrinsic inhibitors of HIV-1, Trim5a, PML, Murr1, and IFN-a were unaffected by p21. These results indicate a new function for p21, participating in prevention of HIV integration into the cellular genome. Therefore p21 is an endogenous cellular component in stem cells that provides a unique molecular barrier to HIV-1 infection and may explain the basis for these cells being an uninfected ‘sanctuary’ in HIV disease.