T lymphocytes emigrating from the thymus to the spleen during postpartum regulate serum immunoglobulin levels in mice.

We noted an increase of a splenic T-cell population in early postpartum normal mice, but not in thymectomized (Tx) postpartum mice. The T-cell population consisted mainly of Lyt-1+2- cells and exerted suppressive activity in in vitro assays. To determine the in vivo function of these emigrant T lymphocytes during postpartum, we investigated the effect on immunoglobulin levels in the sera of mice, from pregnancy through the postpartum stages. A reduction in serum IgG1 levels in the late postpartum period was subsequent to the transitory elevation of IgG1 levels in early postpartum. However, this reduction of IgG1 was not seen in Tx mice at the late postpartum stage. To confirm the relationship between emigrant T cells and the immunoglobulin level, an adoptive transfer experiment was done. The level of IgG1 was reduced in the sera of recipients to which enriched splenic T cells from 3-day postpartum mice had been transferred. The data from the adoptive transfer experiment provided evidence that the increased T-cell population in mice in the early postpartum period regulates the IgG1 level in the serum. It was also recognized that T lymphocytes with the Lyt-1+2-phenotype, which emigrated from the thymus to the spleen after delivery, were able to suppress the IgG1, IgG2a,IgG2b levels in the serum. In addition, the appearance of the suppressive function of the Lyt-1+2- cells in these IgG subclasses required co-operation with the Lyt-1-2+ cells. Therefore, the thymus-dependent emigrant cells in the periphery in early postpartum mice may play a significant role in controlling immunoglobulin levels in the serum.