The expression and distribution of antigens coded by the K and I regions of the major histocompatibility complex in the developing thymus of normal and nude mice has been investigated using monoclonal antibodies. Both immunohistological studies of intact rudiments and in vitro labeling of cultures derived from microdissected rudiments indicate that, while K region antigens are present on epithelial and mesenchymal elements, I region antigens are only detectable on the epithelium. This view is also substantiated by the selective absence of I region antigens in the abnormal nude thymic rudiment where the defect is considered to be epithelial in nature. The findings are considered in relation to the role of the thymus in providing an environment for the differentiation and selection of developing T cells, and it is proposed that the Ia-expressing epithelial elements play a central role in these functions.

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