Enhancement of HIV-1-Induced Syncytium Formation in T Cells by the Tyrosyl Kinase p56lck

Abstract

The CD4 glycoprotein is the primary cellular receptor for human immunodeficiency virus type 1 (HIV-1) and has also been reported to be physically associated with p56lck, a tyrosyl protein kinase. p56lckis a member of thesrcfamily of nonreceptor protein-tyrosine kinases and is expressed predominantly in T lymphocytes. Our objective was to study the effect of p56lckon the biology of HIV-1. For this purpose, we have stably transfected two human p56lck-negative T cell lines (C8166-45 and MT-2) with plasmids encoding for this cellular protein. Following coculture with HIV-1-infected cells or infection with cell-free virus, p56lck-expressing cell lines showed a greater propensity for virus-mediated syncytium formation than parental p56lck-negative cells. The enhancement of HIV-1-induced syncytium formation was not associated with the kinase activity of p56lck, as demonstrated by experiments using a kinase-deficient mutant. However, the physical interaction between CD4 and p56lckwas shown to be necessary to obtain the enhancement of syncytium formation since a mutated version of p56lck, which is deficient in its capacity to associate with CD4, did not lead to an increase in virus-mediated cell-to-cell fusion events. Finally, we determined that cells transfected with wild-type and kinase-negative mutant p56lckshowed a reduced rate of CD4 endocytosis compared to parental p56lck-negative cells. Together, these results suggest that p56lckcan be seen as an accessory molecule facilitating HIV-1-mediated syncytium formation in T cells by a mechanism involving the stabilization of the CD4 molecule at the cell surface.