Immunocompetence of spleen cells from neonatally thymectomized mice conferred in vitro by a syngeneic thymus extract.

Impaired immunological competence of spleen cells from neonatally thymectomized C57B1/6 young adult mice was apparent when these cells were tested in an in vitro graft-versus-host assay. Spleen cell inocula prepared from thymectomized mice did not induce enlargement of (C3H/eb x C57BI/6)F(1) newborn spleen explants, whereas the same number of cells from intact donors consistently initiated splenomegaly. Spleen enlargement was observed, however, when the explants were challenged by cells from thymectomized donors in the presence of syngeneic thymus extract, indicating that the spleen cells in suspension attained immunological competence under the influence of a non-cellular component of the thymus. Immunocompetence was also evident when the cells from thymectomized donors were first incubated with thymus extract for 1 hr and subsequently tested for reactivity. Cells from the same thymectomized donor mice exposed in parallel to extracts from syngeneic spleen or mesenteric lymph node at an equivalent protein concentration did not initiate a graft-versus-host response. These experiments demonstrate that immune reactivity in the graft-versus-host response involves activation of lymphoid cells by a humoral factor of the thymus acting directly upon these cells.