Studies on the cellular basis of the immunological defect following thymectomy in the mouse

© 1969 Dr. Graham Frank MitchellAnswers have been sought for two key questions relevant to the proposed mode of action of the thymus as a “central” lymphoid organ. Does the peripheral lymphocyte population contain a large number of thymus-derived cells and do these cells respond to antigens by producing a progeny of antibody-forming cells? The number of small lymphocytes emerging from a thoracic duct fistula in young adult neonatally-thymectomized CBA mice was approximately 2% of that in intact mice of the same age and body weight. Multiple injections of chromosomally-marked thymus cells increased the size of the lymphocyte pool and the majority of thoracic duct cells, stimulated into division, carried the chromosome marker of the thymus cell donor. Thymectomy in adult life was not followed by a significant decrease in the output of thoracic duct lymphocytes until many months after the operation. Whole body x-irradiation resulted in a dramatic reduction in the number of cells drained from a thoracic duct fistula and, in mice protected from the lethal effects of haemopoietic failure, the reestablishment of the lymphocyte population was dependent upon the presence of the thymus. The data supports an increasing bulk of indirect evidence which, when taken in toto, strongly suggests that a large proportion of recirculating lymphocytes are thymus -derived cells or their descendants. The number of sheep erythrocyte antigen-reactive cells (ARC) in the thoracic duct lymphocyte population of neonatally-thymectomized mice was markedly reduced when compared with the number in the population from normal mice. The bone marrow did not contain ARC but was a potent source of ARC precursors. Neonatally-thymectomized mice did not lack precursor cells in the bone marrow but apparently lacked the thymus influence necessary for the differentiation of these precursors into ARC. Further studies on the thymus-dependent development of ARC hinted at the possibility that the entity known as "an ARC" required the presence of both thymus- and bone marrow-derived cells to express itself in terms of haemolysin production. Neonatally-thymectomized CBA mice failed to respond in normal fashion to a primary injection of sheep erythrocytes (SRBC) and the peak number of haemolysin plaque-forming cells (PFC) in the spleen was reduced by a factor of 1 log 10. The PFC response was increased by injections of either thymus or thoracic duct lymphocytes from CBA, (CBA x C57BL)F 1 hybrid, and C57BL donor mice. In thymectomized mice reconstituted with semiallogeneic and allogeneic cellular inocula, the PFC carried the immunogenetic characteristics of the host and not those of the inoculated cells. Hence, thymus and thoracic duct cells were not reconstitutive simply by virtue of the ability to transform into PFC. Thymus and thoracic duct cell inocula contained "reactor cells" which responded to SRBC antigens in irradiated mice by undergoing a burst of mitosis. Thoracic duct lymphocytes, unlike thymus and bone marrow cells, were able to produce PFC when injected together with SRBC into irradiated mice. However, the PFC response in irradiated recipients of thoracic duct lymphocytes was increased substantially by a simultaneous injection of bone marrow cells. Combinations of cells from semiallogeneic mice did not interact upon transfer to irradiated recipients but clear evidence of synergism in PFC production was apparent in adult-thymectomized irradiated mice protected with CBA bone marrow cells and injected two weeks later with (CBA x C57BL)F 1 thoracic duct cells. In this case, the vast majority of PFC were derived from the bone marrow inoculum. The results suggest that the bone marrow contains only PFC precursors, the thymus only "reactor cells", but that the thoracic duct lymph contains both cell types. It seems that the normal 19S haemolysin response to SRBC in the CBA mouse requires the collaboration of bone marrow-derived PFC precursors and thymus-derived "reactor cells". The neonatally-thymectomized mouse contains adequate numbers of PFC precursors but, after challenge with SRBC, few are recruited into 19S haemolysin production because of the severe deficiency in thymus-derived ''reactor cells"