MHC EXPRESSION IN NONLYMPHOID TISSUES OF THE DEVELOPING EMBRYO - STRONGEST CLASS-I OR CLASS-II EXPRESSION IN SEPARATE POPULATIONS OF POTENTIAL ANTIGEN-PRESENTING CELLS IN THE SKIN, LUNG, GUT, AND INTER-ORGAN CONNECTIVE-TISSUE

We define expression of major histocompatibility complex (MHC) antigen s in the nonlymphoid tissues of the developing rat. Antibodies to clas s I heavy and light chains (b2-m), and to class II MHC proteins were u sed. Strongest MHC expression was by individual cells in the skin, lun g, gut, and inter-organ connective tissue. The class I+ and class IIcells were distinct populations, differing in morphology, distribution , and expression of macrophage-associated antigens. A nonimmunologic r ole for MHC proteins in development has been proposed. Yet the distrib utions and antigenic profiles lead us to emphasize immunologic functio ns that may be served by the early presence of MHC(+) cells outside th e forming lymphoid organs. Potential contributions to establishment of extrathymic or maternal/fetal tolerance are discussed. Localization o f strongest expression to individual connective cells of the developin g organs, rather than parenchymal cells, is of clinical relevance to t ransplantation of fetal tissue.