Expression of HLA-DR mRNA in T cells following activation is early and can precede DNA synthesis

The stimulation of T cells is accompanied by the appearance of a number of activation antigens not found on resting T cells. We have studied the expression of one group of such antigens, the HLA class II antigens (DR, DQ and DP), following stimulation with PHA and PMA. Immunofluorescence studies using monoclonal antibodies indicated that cell surface class II determinants were detectable as little as 30 minutes following stimulation, and that their levels remained constant for about 24 hours, after which they began to increase. Studies on the molecule level demonstrated an increase in the steady state levels of mRNA for DR beta by 15 minutes. Immunoblot analysis of resting T cell lysates using a monoclonal antibody reactive with DR detected a polypeptide of approximately 28 KDa, which agrees with the known molecular mass of the light chain of class II molecules on SDS-PAGE, suggesting that the rapid surface expression might have been due to the presence of pre-formed class II polypeptides. These results demonstrate 1) that T cells defined as resting by conventional criteria contain class II polypeptides and 2) that increases in steady state levels of class II mRNA is a very early event which precedes the initiation of DNA synthesis.