Summary/Abstract The envelope membrane of HIV-1 particles contains not only viral Env glycoprotein but also host plasma membrane proteins, some of which have positive or negative effects on virion infectivity. We previously found that a subset of adhesion molecules expressed in CD4+ T cells, i.e., CD43, CD44, and PSGL-1, specifically cocluster with HIV-1 structural protein Gag at virus assembly sites on the plasma membrane and are incorporated into nascent virus particles more efficiently than other cell surface adhesion molecules. In migrating polarized T cells, these proteins localize to the rear-end protrusion known as the uropod where Gag also accumulates; however, the uropod protrusion itself is not necessary for the coclustering with Gag at the cell surface or the efficient incorporation into virions. We recently discovered that when incorporated into virions, CD44 promotes capture of the virions by fibroblastic reticular cells (FRCs), which are stromal cells of lymph node T cell zones, and thereby enables trans-infection of T cells that come into contact with the FRCs. In contrast to the pro-viral effect of CD44, our preliminary results indicate that virion-incorporated CD43 and PSGL-1 impair the infectivity of the virions. Furthermore, we found that when excess CD44 is incorporated, the advantage for virus conferred by trans-infection is diminished due to a defect in virus entry. Finally, our preliminary study also showed that HIV-1 infection in T cells reduces cell surface expression of CD43 and PSGL-1 but not CD44. Altogether, these data support the possibility that HIV-1 balances the incorporation of these proteins for its optimal entry into the next target cells. The overall objective of this application is to elucidate how the uropod-directed proteins inhibit infection of HIV-1 and how such inhibitory effects can be altered. Our central hypothesis is that the uropod-directed proteins incorporated into virions modulate the organization of the virus surface, thereby inhibiting an early step(s) of virus infection. To test this hypothesis as well as to gain initial mechanistic insights into viral counteractions, in this exploratory grant application, we plan i) to identify the molecular determinants for the inhibitory effects imposed by virion-incorporated CD43, PSGL- 1, and CD44; ii) to elucidate the infection-dependent mechanism that reduces CD43 and PSGL-1; and iii) to examine the organization of virus particles to which excess CD44 is incorporated.