Antibody-Induced CD3–CD4 Coligation Inhibits TCR/CD3 Activation in the Absence of Costimulatory Signals in Normal Mouse CD4+ T Lymphocytes

The effect of CD3–CD4 coligation on CD3-mediated activation of normal mouse CD4+ T lymphocytes has been analyzed in the absence of exogenous lymphokines. If anti-CD3 and anti-CD4 antibodies are adsorbed to culture wells by means of previously adsorbed anti-Ig antibodies (indirect binding), CD3–CD4 coligation inhibits activation measured as cell proliferation or as secretion of IL-2, IL-4, and IFN-γ. Addition of IL-2, anti-CD28 antibodies, or phorbol esters, but not IL-1, IL-4, or ionomycin, blocked CD4-mediated inhibition and restored the response to levels equal or higher than those of cultures activated by anti-CD3 alone. In contrast, CD3–CD4 coligation by antibodies directly adsorbed to culture wells potentiated anti-CD3-induced activation, either in the absence or in the presence of exogenous costimuli. Similar results were observed when CD4+ T cells of naive phenotype (CD44low, CD45RBhigh) were used in the experiments. The analysis of early tyrosine phosphorylation in CD4+ T cells shows that phosphorylation of many cell substrates is clearly enhanced upon CD3–CD4 coligation using indirectly or directly bound antibodies, yet certain substrates are mainly phosphorylated under inhibitory conditions. Although CD28 ligation does not produce any clear change in the tyrosine phosphorylation pattern in lysates from cells activated by indirectly bound anti-CD3 plus anti-CD4 antibodies, the analysis of active forms of the MAP kinase ERK suggests that downstream signaling pathways involved in IL-2 gene activation can be differentially activated depending on the direct or indirect CD3–CD4 adsorption and CD28 ligation.