Macrophage ontogeny: Implications for host defence, T-lymphocyte differentiation, and the acquisition of self-tolerance

It has become increasingly more apparent that Ia-bearing accessory cells are important in regulating the function of mature T lymphocytes as well as the maturation of immature T lymphocytes in the thymus. The experiments reviewed here have focused on the ontogeny of Ia-bearing macrophages in the peritoneal cavity, spleen, and thymus. In the former two sites, Ia-bearing macrophages appear late in ontogeny. This makes the neonate and fetus vulnerable to infection, but may also offer the immune system a critical mechanism for inducing self-tolerance. The delayed ontogenesis of Ia-bearing macrophages at these two sites is regulated by high concentrations of PGE₂ as well as alpha-fetoprotein. On the other hand, Ia-bearing thymic macrophages are present early in ontogeny and may contribute to the expansion and maturation of appropriate T lymphocyte clones early in development.