Absence of MHC class II expression distinguishes fetal from adult B lymphopoiesis in mice.

Early B-lineage progenitor cells (Pro-B) isolated from murine fetal liver and adult bone marrow can differentiate to the immature B cell stage in a stromal cell-dependent culture system and to mature B cells upon transfer into immunodeficient SCID mice. By using immunofluorescence analysis, we found that progenitor cells from day 16 fetus differentiating in culture lacked MHC class II expression during the Pre-B and immature B cell stages, whereas such expression was readily apparent on the surface of corresponding adult-derived populations. RT-PCR analysis of RNA message levels for the four class II genes (A alpha, A beta, E alpha, E beta) yielded completely concordant results. B cells of fetal progeny did eventually express class II upon further maturation in vivo. Thus, the onset of class II expression is uniquely delayed during fetal B cell differentiation. This result explains an apparent paradox, i.e., that class II expression is absent from B cells in neonatal spleen but present as early as the Pre-B cell stage in adult bone marrow. Furthermore, we suggest that such distinct programs of class II expression during fetal and adult lymphopoiesis could result in differences in susceptibility to tolerance.