LIGATION OF CD45 ON B-CELLS CAN FACILITATE PRODUCTION OF SECONDARY IGISOTYPES

The possibility that isotype switching in B cells may be affected by e ngagement of the CD45 molecule on B cells has been investigated in mic rocultures containing limiting numbers of B cells and nonlimiting numb ers of both alloreactive Th cells and purified dendritic cells (DC). A ddition of Abs to the B cell-specific isoform, B220, to the microcultu res leads to an increase in the proportion of B cell clones that secre te secondary Ig isotypes. In the presence of anti-CD45 Ab, microcultur e wells show a 39% frequency of secondary isotypes (560/1440) compared with a 11% frequency in control microcultures (89/780). Cross-linking appears to enhance this effect. Even in cultures of B cells and DC wi thout T cells, addition of anti-B220 induces isotype switching in a si gnificant number of microwells. Cross-linking and capping B220 molecul es results in co-capping of surface Ig and MHC class II molecules. The results suggest that signal transduction through the CD45 molecule ma y affect pathways involved in isotype switching.