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The various cell types expressing Ia antigens in the chick and quail thymus have been studied by means of monoclonal antibodies (mAb) prepared by using chick and quail thymic adherent cells (macrophages and dendritic cells) as immunogens. Three reagents were selected by the following criteria: (a) they react with a surface determinant carried by thymic adherent cells and bursal lymphocytes, (b) they can be used to immunoprecipitate from spleen cell membrane extracts molecular entities of an apparent molecular weight close to 55,000, which can be fractionated into monomers at approximately 30,000 mol wt in dissociating conditions. Among these three reagents, two are strictly species specific, i.e., they recognize either chick (TaPl) or quail (TaCl) Ia determinants, whereas the third, TaC2, recognizes both chick and quail Ia molecules. Chimeric thymuses in which the epithelioconnective stroma is derived from the quail thymic primordium and the whole hemopoietic cell population (lymphocytes and accessory cells) are of chick origin were constructed as previously described by our group (20). The different mAb were applied on normal (quail and chick) and chimeric thymuses. It appears that the thymus is divided into two compartments in terms of the nature of cells expressing Ia: the cortex, in which class II antigens are exclusively expressed by endodermal epithelial cells, and the medulla, where the majority of nonlymphoid cells are Ia-positive cells of hemopoietic origin. A few epithelial cells only are present in the thymic medulla. They are closely intricated with the sessile Ia-positive cells, whose precursors penetrate the thymus along with the lymphocyte progenitors and which are renewed in the course of thymic development. In contrast, the epithelial reticulum, expressing Ia both in the cortex and medulla, contributes a stable thymic component. During early thymic ontogeny, the hemopoietic cells expressed detectable levels of Ia antigen before the epithelial cell network.

Cells expressing Ia antigens in the avian thymus.