CONTRIBUTION OF A THYMIC HUMORAL FACTOR TO THE DEVELOPMENT OF AN IMMUNOLOGICALLY COMPETENT POPULATION FROM CELLS OF MOUSE BONE MARROW

The hypothesis that cells located in mouse bone marrow can acquire immunological competence by a process that involves interaction with a noncellular component of the thymus was tested using an in vitro assay of graft-versus-host reactivity as a criterion of cell competence. When suspensions of C57BL bone marrow cells were incubated in thymus extract and injected into mice incapable of inducing a response in the graft-versus-host assay as a result of neonatal thymectomy, or adult thymectomy plus irradiation, or because of genetic similarity with the (C3H x C57BL)F1 tissue used for challenge in the assay, competent cells were recovered from the spleens of the injected mice. The reactive cells were shown to be of bone marrow origin since immune reactivity was related to the genetic makeup of the bone marrow cells rather than that of the intermediate recipients. A thymic factor was involved in the process leading to immune reactivity by these cells, as bone marrow cells incubated in xenogeneic or syngeneic thymic extracts induced a graft-versus-host response after passage through nonresponsive mice, whereas incubation of bone marrow cells in xenogeneic lymph node or spleen extracts or in culture medium only did not lead to subsequent reactivity. Participation of peripheral lymphoid tissue seemed essential in this process since bone marrow cells tested directly after exposure to thymic extract failed to induce a graft-versus-host response. C57BL bone marrow cells exposed to thymus extract and cultured together with fragments of (C3H x C57BL)F1 spleen tissue in vitro were competent to induce a graft-versus-host response; thus, these components would seem to be sufficient as well as necessary for the immunodifferentiation process leading to graft-versus-host activity. It is concluded that one step in the process by which bone marrow cells acquire competence vis-a-vis the graft-versus-host response depends upon a thymic agent that is noncellular and extractable, and that another stage in this process is under the influence of components found within the peripheral lymphoid tissue environment. It is suggested that differentiation of precursor cells to competence could occur by progressive development of the cells in separate compartments of the lymphoid system

STUDIES ON SOME PHYSICOCHEMICAL PROPERTIES OF A THYMUS HUMORAL FACTOR CONFERRING IMMUNOCOMPETENCE ON LYMPHOID CELLS

By means of an assay of graft-versus-host activity some properties of the thymic humoral factor which confers immunocompetence upon lymphoid cells in vitro have been studied. Allogeneic and xenogeneic thymic preparations were found to activate lymphoid cells from neonatally thymectomized mice, enabling initiation of a graft-versus-host response. Thus, this thymus factor is apparently neither strain nor species specific. The active principle of calf thymus extracts was found to be in the supernatant after prolonged ultracentrifugation. When exhaustive dialysis and ultrafiltration through Diaflo membranes were performed, the active thymus agent was found to pass through both the dialysis sac and Diaflo UM-2 membranes. The molecule which confers immunocompetence upon lymphoid cells thus seems to be of molecular weight of an order of magnitude of 1000 or less. Dialyzed thymus preparations injected into neonatally thymectomized mice also restored the capacity of spleen cells of these mice to induce graft-versus-host activity. When injected into intact mice, thymus extract also increased the proportion of competent cells in the spleens of these animals, probably by activation of target cells originating outside the spleen