Alloantigens derived from stimulator cells and bound onto MLC-activated rat T lymphoblasts

Rat T lymphocytes were activated in MLC (mixed leucocyte culture) against allogeneic stimulator cells. A few percent of such T blasts could be shown to carry IgM on their surface when a mixture of T and B lymphocytes was used as responder cells. The immunoglobulin was absent when purified T lymphocytes served as responders. A large proportion of MLC T lymphoblasts could be shown to carry stimulator alloantigens on their surface. These antigens were demonstrated (a) by indirect immunofluorescence using anti-stimulator alloantibody and FITC (fluorescein isothiocyanate Isomer I)-labelled F(ab)2 fragments of rabbit anti-rat Ig; (b) by autoradiography after internal labelling of stimulator cells, which showed the transfer of label from stimulator to responder cells. For biochemical analysis T blasts were restimulated with internally labelled allogeneic or syngeneic stimulator cells, and bound radiolabelled stimulator alloantigens were analysed on SDS-PAGE (sodium lauryl sulphate polyacrylamide gel electrophoresis). The results suggest that intact SD antigens with a molecular weight of 45,000 daltons and LD determinants with a molecular weight of 34,000 daltons derived from stimulator cells are found on MLC T blasts. No evidence of binding of the 27,000-dalton Ia chain to T blasts was found. Possible arrangements of these molecules on the responder blasts is discussed.