The concentrations of different lymphocyte subsets in the blood of lambs which had been thymectomized (Tx) in utero between days 67‐75 of fetal gestation were measured at birth and at various intervals during the first year of life. Compared to thymus‐intact (Ti) controls, Tx lambs were severely depleted of both α/β and γ/δ T cells at birth (< 10% of control levels). The majority of the residual α/β T cells present in Tx lambs at birth were CD4+CD8−. As the Tx lambs aged, the concentration of α/β T cells in blood increased steadily to reach levels around 50% of control values. In contrast, the circulating γ/δ T cells did not expand in Tx animals and remained barely detectable throughout the observation period, although these cells accounted for 30%‐60% of the T cells in the blood of Ti lambs. The expansion of α/β but not γ/δ T cells was also reflected in changes in the cellular composition of solid lymphoid organs in Tx lambs. B cell numbers were similar in both groups at birth but Tx lambs were persistently B lymphopenic from 3 weeks of age onwards. The α/β T cells that had expanded in Tx lambs responded to stimulation with bacterial antigens in a way that was qualitatively similar to the response in Ti lambs. By contrast, the few γ/δ T cells in Tx lambs responded abnormally. Our results show that although sheep α/δ and γ/δ T cells are equally thymus dependent during ontogeny, the early fetal thymic emigrants which establish the two T cell lineages in the periphery have strikingly different antigen reactivities and capacities for self‐renewal and expansion. Copyright © 1990 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim
