MOUSE ISOANTIGENS: SEPARATION OF SOLUBLE TL (THYMUS- LEUKEMIA) ANTIGEN FROM SOLUBLE H-2 HISTOCOMPATI- BILITY ANTIGEN BY COLUMN CHROMATOGRAPHY* Bx

Serological study of cell membrane antigens in the mouse has revealed several isoanfigenic systems demonstrable by cytotoxic isoantisera. Four of these systems of antigens are exclusively or predominantly represented on thymocytes. One of these, TL (thymus-leukemia) (1), is exclusively represented on thymocytes: two (Ly-A and Ly-B) have predominant representation on thymocytes and are present also on lymphocytes, but not on any other cell type (reference 2 and unpublished data1); another, designated 0 (3), also is predominantly represented on thymocytes and in lower concentration on lymphocytes, but is found in at least one other tissue---adult brain. The major histocompatibility isoantigen of the mouse, H-2, also is demonstrable on thymocytes by the cytotoxic test, but in contrast to the first four systems (TL, Ly-A, Ly-B, and 0), H-2 is represented poorly on thymocytes in comparison with lymphocytes and some other tissues. Of these five systems, TL has particular importance in tumor immunology because of its appearance in leukemias of strains that do not have the antigen on their normal thymocytes. In normal mice TL has the properties of an organspecific isoantigen. Mice of T L + strains cannot form TL antibody but mice of T L strains can do so. T L + leukemias, however, occur not only in T L + strains but also in T L strains. The conclusion is that all strains have a structural gene for TL synthesis, but that phenotypic expression on normal thymocytes is restricted to T L + strains. In relation to TL there evidently are two consequences of leukemogenesis. First, in T L strains the mechanism repressing TL synthesis is modified so that the TL character is now expressed. Secondly, in