Thymic nurse cells—Ia-bearing epithelium involved in T-lymphocyte differentiation?

Abstract

The vertebrate thymus has a crucial role in the development and organisation of the immune system. Physiologically, maturation of primitive incompetent precursor cells to immunocompetent T lymphocytes takes place in the thymus¹. Recently, evidence has been presented suggesting that the capacity to recognise determinants of the self major histocompatibility complex (MHC), which underlies MHC restriction of T-lymphocyte recognition, is also acquired within the thymus². Pathologically, T-cell lymphomas are known to arise within the thymus³. In all three cases, it is probable that at least some developmental events require close spatial interactions, if not direct contact, between migrating lymphoid cells and sessile stromal thymus elements. We describe here a state of maximal contact between both thymic cell partners, which might represent certain contact-dependent stages of T-cell development. We found that in all normal mouse and rat strains tested so far, the thymuses contain a sizeable number of huge cells of epithelial origin, which engulf and release up to 50 thymic lymphocytes. As the epithelial cells do not degrade the invading lymphocytes, but may rather provide microenvironmental requirements necessary for lymphocyte proliferation and differentiation, we call them ‘thymic nurse cells’ (TNCs). These TNCs express all the MHC determinants required by a cell involved in the establishment of T-lymphocyte MHC restriction. They express high doses of K/D antigens of the H-2 complex, and equally high doses of I–A region determinants. However, TNCs lack surface immunoglobulin and T-lymphocyte-specific Thy-1 antigen.