During embryogenesis, stem cells migrate from the bloodstream into the thymic rudiment where they proliferate and differentiate into T lymphocytes. The epithelial cells of the thymic stroma may influence these processes by providing hormonal and/or contact stimuli to the developing lymphoblasts 1,2. Recently, it has been shown that T cells 'learn' to recognise the major histocompatability complex (MHC) antigens during thymic lymphopoiesis and become MHC-restricted. Their subsequent response to other antigens can only occur in the context of MHC antigens of the haplotype encountered in the thymus3. Little is known, however, of antigen expression on the thymic stroma which may provide the reference framework on which this MHC restriction is based. In this study we use monoclonal antibodies to show that antigens of the K and I regions of the MHC are detectable on cells of the embryonic mouse thymic stroma from around the 14th day of gestation, just when lymphocyte differentiation is commencing. Furthermore, I-region antigen (Ia antigen) expression is probably limited to thymic epithelium at this stage of gestation and we have not detected Ia on other epithelial tissues of the pharyngeal complex. This pattern of expression is consistent with a role for the thymic stroma in MHC restriction, perhaps by the selection of lymphoid cells for survival on the basis of their recognition of stromal MHC determinants.
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