Cells bearing Ia antigens in the murine thymus. An ultrastructural study.

Light- and electron-microscopic immunohistochemical examination of the murine thymus revealed that epithelial cells throughout the thymus expressed Ia antigens, as did interdigitating cells, monocytes, and macrophages located in perivascular "cuffs" associated with corticomedullary and medullary blood vessels. Thymic lymphocytes also expressed Ia antigens, but only at areas of contact with Ia-positive nonlymphoid cells. We observed evidence for synthesis of Ia antigens by epithelial cells, but not by thymic lymphocytes. These data indicate that several morphologically distinct populations of cells in the thymus express Ia antigens and thus may be involved in cellular interactions regulating I-region-restricted T-lymphocyte differentiation.