The circulating life span, immunocompetence and 3H-uridine uptake of small lymphocytes from thymus-grafted, neonatally thymectomized rats.

By 7 weeks post-grafting, the number of small lymphocytes in the thoracic duct lymph (TDL) and blood of the thymus-grafted neonatally thymectomized adult rats had increased to 60% of the number of cells in sham controls, or 2-1/2 times thymectomized control values. This increasing consisted almost exclusively of long-lived, recirculating small lymphocytes and corresponded to a 60% recovery of cellular immunocompetence as measured by the mixed lymphocyte reaction (MLR). Associated with the return of cellular immunocompetence was an increased incorporation of 3H-uridine by the small lymphocytes. Cells from thymectomized animals grafted with lymph node fragments demonstrated no significant increase in lymphocyte numbers nor was there a return of immunocompetence as compared to thymectomized controls.