Close Colocalization of CD4 and a Serine Esterase Tryptase TL2 on the Cell Surface of Human Monocytoid and CD4⁺ Lymphoid Cells

Tryptase TL2, a serine esterase in the membrane of human monocytoid and CD4⁺ lymphoid cells, specifically binds to the V3 domain of HIV-1 gp120. Here we report that monoclonal antibodies against CD4 that recognize the epitope interacting with gp120 specifically blocked the immunostaining of cell-surface tryptase TL2, although the antibody does not crossreact with tryptase TL2. Down-regulation of cell-surface CD4 induced by HIV-1 Nef prevented this blocking effect. These data suggest that CD4 is closely co-localized with tryptase TL2 on the cell surface and that regulation of the expression of tryptase TL2 is not associated with that of CD4.