Studies on lymph humoral factor. Evidence for a lymphocytopoietic factor in rat thoracic duct lymph.

Normal rats were injected with the partially purified material extracted from the thoracic duct lymph which was collected from normal syngeneic rats. The cellular changes in the peripheral lymphoid tissues and thymus were examined cytologically and histologically. The intravenous injection of the lymph extract into normal syngeneic rats resulted in increase in weight of lymphoid tissues. Histologically, a massive proliferation of large pyroninophilic lymphoid cells and an increase in mitotic index was detected predominantly in the thymus-dependent areas and thymic cortex. In spleens from rats injected with lymph extract, the marginal zone bridging channel was shown as one route for the translocation or mobilization of newly borne lymphoid cells to the venous circulation. Similar lymphocytopoietic activity, but to a lesser extent than lymph extract, was also detected in lymph plasma, serum and serum extract. The lymph extract was shown to be non-immunogenic in syngeneic rats. It is suggested that the effects of the lymph extract on lymphoid cell proliferation are due to the presence of a lymphocytopoietic factor in body fluid, particularly in the lymph.