SUMMARYIn the present study, we evaluated the NK cell cytotoxic activity in a group of HCV-infected individuals. Although the number of NK cells present in the peripheral blood of the HCV-infected patients was comparable to non-infected individuals, spontaneous NK cytotoxicity was four-fold lower (P < 0 . 001) than in normal donors. This functional impairment was not overcome by depletion of adherent or B cells, and it was partially restored by short-term (18 h) stimulation with IL-2. However, long-term stimulation (72 h) with this lymphokine induced activated killer cell (LAK) activity comparable to normal controls. The reduction in NK cytotoxic response does not seem to be due to soluble suppressive factors, since incubation of normal peripheral blood mononuclear cells (PBMC) with infectious HCV serums for a 4-h period does not affect NK spontaneous cytotoxic activity. Successful in vitro infection of PBMC with HCV infectious serum also resulted in an impairment of NK cytotoxicity, suggesting that altered NK function is associated with HCV infection and may be responsible, at least in part, for the chronicity of the infection.

We report the consequences of low expression of the T cell receptor (TcR)/CD3 complex by T lymphocytes from a 4-year-old boy with a mild immunodeficiency. TcR/CD3 expression was found to be deficient on both resting and activated T cells, using both anti-CD3 and anti-TcR alpha/beta monoclonal antibodies. As shown by immunofluorescence and immunoprecipitation studies, residual expression (corresponding to about 10% of normal) was detectable on resting and activated TcR alpha/beta+ T cells. Other T cell membrane receptors were normally expressed. The functional consequences of this TcR/CD3 expression deficiency included an absence of T cell proliferation, interleukin 2 receptor expression and calcium flux following anti-CD3 and anti-CD2 antibody-triggered T cell activation. Antigen (tetanus toxoid, Candida and allogeneic cell)-induced proliferation was detectable. In contrast, cytotoxic T cell activity towards allogeneic cells was deficient. These findings shed light on the function of the TcR/CD3 complex and indicate that the expression of a limited number of TcR/CD3 receptors may be sufficient to trigger antigen-specific T cell activation (and, possibly, differentiation) and that anti-CD3 antibody-induced T cell activation differs somewhat from antigen/major histocompatibility complex molecule-induced activation. These results also confirm that the CD2 pathway of T cell activation is CD3 dependent.

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