Cross-linking of CD4 molecules upregulates Fas antigen expression in lymphocytes by inducing interferon-γ and tumor necrosis factor-α secretion

We have recently shown that, in unfractioned peripheral blood mononuclear cells (PBMCs), the cross-linking of CD4 molecules (CD4XL) is sufficient to induce T-cell apoptosis. However, the underlying mechanism for the CD4XL- mediated T-cell apoptosis is largely unknown. Several recent studies have shown that Fas antigen (Ag), a cell-surface molecule, mediates apoptosis- triggering signals. We show here that cross-linking of CD4 molecules, induced either by anti-CD4 monoclonal antibody (MoAb) Leu3a or by human immunodeficiency virus-1 (HIV-1) envelope protein gp160, upregulates Fas Ag expression as well as Fas mRNA in normal lymphocytes. Addition of the tyrosine protein kinase inhibitor genistein or of the immunosuppressive agent cyclosporin A abrogated these effects. The upregulation of Fas Ag closely correlated with apoptotic cell death, as determined by flow cytometry. In addition, CD4XL resulted in the induction of interferon-γ (INF-γ) and tumor necrosis factor-α (TNF-α) in the absence of interleukin-2 (IL-2) and IL-4 secretion in PBMCs. Both INF-γ and TNF-α were found to contribute to Fas Ag upregulation and both anti-INF-γ and anti-TNF-α antibodies blocked CD4XL- induced Fas Ag upregulation and lymphocyte apoptosis. These findings strongly suggest that aberrant cytokine secretion induced by CD4XL and consequent upregulation of Fas Ag expression might play a critical role in triggering peripheral T-cell apoptosis and thereby contribute to HIV disease pathogenesis.