Ag-dependent activation of human T cells results in high level expression of class II MHC molecules. As part of this process, Ag recognition by TCR generates a series of second signals including protein kinase C, tyrosine kinase, and Ca2+ mobilization. To investigate the role of these second messengers in class II MHC expression, purified T cells were first stimulated by PMA, ionomycin, OKT3 accompanied by IL-2, or the mitogenic anti-CD2 antibodies T112 and T113 and were then stained with FITC-conjugated anti-class I and -class II MHC antibodies for analysis by flow cytometry. OKT3 and IL-2 induced optimal expression of HLA-DR (DR) on 70% of T cells with high density. Despite their high mitogenicity, induction of class II MHC expression by PMA, even with co-stimulation by ionomycin, was reduced to less than 20% of T cells, with an intensity 50-fold lower than in OKT3/IL-2-stimulated T cells. Furthermore, PMA inhibited class II MHC expression by OKT3/IL-2-stimulated T cells in a dose-dependent manner and additional stimuli, such as IL-1, IL-4, IFN-gamma, TCR cross-linkers, or monocytes, did not restore class II MHC expression by PMA-activated T cells. DR beta mRNA analysis showed that the low induction of class II molecules by PMA extends to the transcriptional level. Interestingly, anti-T112 and anti-T113 induced not only proliferation of T cells but also DR expression on more than 90% of T cells. These results indicate that transduction of a specific signal, probably selective phosphorylation of the CD3 molecule, contributes to class II MHC induction in the process of T cell activation.

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