The ontogeny of thymic macrophages: thymic macrophages express Ia from 15 days gestation onwards in the mouse.

This study reports the isolation and enrichment of murine foetal thymic macrophages. The majority of enriched adherent cells of thymus from 15 days gestation onwards were classical macrophages as judged by phagocytosis as well as expression of F4.80 and Fc receptors. In addition, a proportion of foetal thymic macrophages was shown to be Ia+ by double labelling with latex particles and indirect immunofluorescence for surface Ia antigens, and using monoclonal antibodies, it was shown that the Ia+ cells expressed both I-A- and I-E-region-coded determinants. The function of Ia+ macrophages in the developing thymus is discussed.