Ia positive cells in the medulla of rat thymus are bone marrow derived.

Abstract

The thymus is of key importance for the production of T-cells and the development of their ability to recognize foreign antigens in association with their own MHC (major histocompatibility complex) antigens (1,2). The Ia antigens are glycoprotein products of the MHC and probably the products of the Ir genes that control the level of immune responses (3,4). la antigens are present on B-cells and antigen presenting cells; localization studies on human and murine thymus have shown that la antigens were present on epithelial cells in the cortex and less well defined cells in the medulla (4–9). This paper describes a similar localization of la antigen in rat thymus. In addition the origin of the la antigen was studied by distinguishing la antigens of donor bone marrow origin and host origin using radiation chimeras. The finding that the cortex retained la antigen of host origin whilst the medulla was repopulated with la positive cells of donor bone marrow origin is discussed with respect to the role of these two cell populations and the use of similar chimeras to study T-cell restriction to MHC antigens. Some of these findings have been previously described (10).