CD45 expression by B cells. Expression of different CD45 isoforms by subpopulations of activated B cells

To determine the effect of distinct activation stimuli on CD45 expression by B cells, we have examined the expression of CD45 molecules on murine B cells stimulated with LPS or the Th cell cytokine IL-5. Analysis of CD45 by flow cytometry revealed that unstimulated and stimulated B cells expressed homogeneous amounts of total CD45 but that stimulation with IL-5 resulted in a CD44hi, hyaluronate-adherent subpopulation of activated B cells that expressed a markedly altered pattern of expression of exon-specific CD45R or B220 determinants. The predominant CD45 immunoprecipitated from either unstimulated or LPS-stimulated B cells was of the high molecular mass form (approximately 220 kDa) usually associated with B cells. In contrast, the CD45 proteins immunoprecipitated from the hyaluronate-adherent subpopulation of IL-5-activated B cells were predominantly lower m.w. forms. PCR analysis of amplified CD45 cDNA also showed distinct expression profiles characteristic of each B cell population. The highest molecular size PCR product, corresponding to expression of all three variably expressed CD45 exons (A, B, and C) was prominent in resting B cells and in LPS-activated B cells but was selectively reduced in hyaluronate-adherent IL-5-activated B cells, where lower molecular size PCR products predominated, corresponding to expression of one or two of the variable exons. In contrast, LPS-activated B cells expressed reduced levels of these one- or two-exon forms. In addition, all B cell populations expressed a lower m.w. PCR product corresponding in size to the product expected when exons A, B, and C are spliced out of CD45 mRNA. Thus, analysis of alternative splicing of CD45 mRNA, as well as cell surface expression of CD45 provides a novel parameter for analysis of B cell activation by different stimuli.