Direct demonstration of MHC class II surface expression on murine pre-B cells

Using multi-parameter flow cytometry, MHC class II (Ia) antigens were found to be expressed at readily detectable levels on the majority of surface IgM^- B lineage cells in the bone marrow of adult mice. Approximately 90% of the pre-B cells (B220⁺IgM^-) in BALB/c bone marrow (BM) were MCH class II positive, while in C57BL/6 this figure was somewhat less, ̃60%. Analysis of adult BM for the correlated expression of Ia both with heat stable antigen (HSA) and BP-1 on B lineage cells revealed in both cases a small fraction of cells expressing either the HSA or BP-1 but lacking Ia. This result suggests that the onset of expression of both HSA and BP-1 precede that of Ia in B cell development. When the B220¹⁰CD43⁺ pro-B cell compartment from BALB/c BM was analysed for the expression of Ia very few, if any, cells were found to be positive. Of the remaining B220⁺ cells, >90% express detectable Ia. Thus the onset of Ia expression would appear to be soon after the pro-B cell compartment. The results presented here resolve the previous discrepancy between human and murine pre-B cell differentiation of when Ia is first expressed, and additionally, by identifying a new pre-B cell antigen, allow for the refinement of existing schemes of murine B cell development.