Regulation of the cell-surface expression of an HTLV-I binding protein in human T cells during immune activation

Little is known about the requirements for human T-cell leukemia virus type I (HTLV-I) entry, including the identity of the cellular receptor(s). Recently, we have generated an HTLV-I surface glycoprotein (SU) immunoadhesin, HTSU-IgG, which binds specifically to cell-surface protein(s) critical for HTLV-I-mediated entry in cell lines. Here, expression of the HTLV-I SU binding protein on primary cells of the immune system was examined. The immunoadhesin specifically bound to adult T cells, B cells, NK cells, and macrophages. Cell stimulation dramatically increased the amount of binding, with the highest levels of binding on CD4(+) and CD8(+) T cells. Naive (CD45RA(high), CD62L(high)) CD4(+) T cells derived from cord blood cells, in contrast to other primary cells and all cell lines examined, bound no detectable HTLV-I SU. However, following stimulation, the level of HTSU-IgG binding was rapidly induced (fewer than 6 hours), reaching the level of binding seen on adult CD4(+) T cells by 72 hours. In contrast to HTLV-I virions, the soluble HTSU-IgG did not effect T-cell activation or proliferation. When incubated with human peripheral blood mononuclear cells in a mixed leukocyte reaction, HTSU-IgG inhibited proliferation at less than 1 ng/mL. These results indicate that cell-surface expression of the HTLV SU binding protein is up-regulated during in vitro activation and suggest a role for the HTLV-I SU binding proteins in the immunobiology of CD4(+) T cells.