Maturation of bone marrow lymphocytes. IV. Kinetics of maturation and renewal of lymphocytes expressing Ia and H-2K antigens.

As part of a series of studies on bone marrow lymphocyte differentiation, I region-associated (Ia) and H-2K antigens have been quantified on small lymphocytes in mouse bone marrow and spleen, and correlated with cell renewal. Ia antigens were revealed radioautographically on some marrow small lymphocytes, the antigen density being lower than on spleen cells. Kinetic studies, combining surface rosetting and [3H]-thymidine labelling, showed that most Ia antigen-bearing lymphocytes were newly formed cells, rapidly renewed in the marrow. Ia antigens were first expressed after a post-mitotic lag period and increased in density with time. A minority of Ia antigen-bearing small lymphocytes in the marrow and a large majority of those in the spleen remained unlabelled by [3H]-thymidine, infused for 3-5 days. Some putative progenitor cells in the marrow showed surface Ia antigens of medium density. In neonatal mice the incidence and density of Ia antigens were low in the marrow and spleen, reaching adult values by 4 and 10 weeks of age, respectively. In contrast, H-2K antigens were expressed in high density on all lymphoid cells in both marrow and spleen from early postnatal life onwards. Thus, Ia antigens, but not H-2K antigens, behave as differentiation markers in the maturation of small lymphocytes produced in the bone marrow. In addition, some Ia antigen-bearing small lymphocytes in adult bone marrow are slowly renewed cells, putative long-lived immigrants from the recirculating pool.