Quantitative analysis of the chimeric state in mice. IV. Cytological examination of mice rendered tolerant by neonatal inoculation of F1 hybrid spleen cells.

Newborn CBA mice were injected intraperitoneally with adult F hybrid spleen cells chromosomally distinguishable from host cells (T6). Two donor—host combinations were studied: (A×CBA)F→CBA and (C57×CBA)F→CBA. Chimeric analyses of host lymphoid organs were performed between days 8 and 260 post-natal in the former and between days 5 and 567 in the latter donor—host combination. Mice injected with (A×CBA)F hybrid cells yielded uniformly low frequencies of donor cells in the spleen, thymus, bone marrow and caecum lymphoid tissue throughout the duration of the experiment; relatively high proportions of donor cells were identified during the first 3 weeks of life in the lymph nodes and Peyer's patches, but there was a subsequent slow but steady fall to low values. A similar phenomenon was noted also in the (C57×CBA)F→CBA combination, so that after the 18th month of life few donor cells were found in all lymphoid organs examined. Despite marked fall in the degree of chimerism specific tolerance, as tested by repeated skin grafting, was fully maintained. Comparison of the results of chimeric analysis of hybrid→parent combinations with the results obtained in mice neonatally injected with similar numbers of either syngeneic or allogeneic spleen cells gave some insight into the mechanism of induction of `acquired tolerance', the speed by which tolerance is induced and the degree of tolerance attained.