After injection of isografted mammary tumor M M 2 sensitized with rabbit antiserum, resistance was induced in C3HIHe mice after repeated challenges with MM2. The serum taken from these mice was found to be cytotoxic against M M 2 cells. The serum also inhibited the outgrowth of transplant of primary tissue culture cells of spontaneous mammary tumor of C3HIHe mice. A series of transplanted mammary tumors recently converted into ascitic form in our laboratory ( M M 3 , MM4-t, MM4-2, MM4-3, MM6, MM8, MM9, M M I I , M M I 2 and MM13), and Ehrlich ascites tumor were found to be susceptible to this serum, as tested by trypan blue uptake in vitro. Outgrowth of these tumors was also inhibited when tumor premixed with this serum was injected.
No cytotoxic efect was observed against normal mouse mammary gland cells of a C3HIHe mouse.
Sera obtained from hyperimmunized syngeneic C3HIHe mice were able to fix complement with M M 2 tumors. They were partially inactivated by heating at 56" C and by treatment with 2-mercaptoethanol. Afrer gel filtration through Sephadex G-200, complement fixing and cytotoxic activity were found in both 19s and 7s fractions. The
7 s fractions, afrer DEAE cellulose column chromatography, gave a precipititin line at the IgG position in immunoelectrophoresis.
From the above evidence, it is concluded that the target cells of the cytotoxic factors of this serum are primary cultured cells or isografred cells of mammary tumors of C3HIHe mice. The cytotoxic factors present in the serum are considered to be antibodies against isografts of mammary tumors in C3HIHe mice.
Histocompatible syngeneic or autochthonous hosts have been employed to detect immune reactions of tumor cells. In this way, evidence of immune reactions has been accumulated through observation of phenomena such as induction of heightened resistance or enhancement of tumor grafts. Immunochemical characterization of antibodies and antigens involved in this system is required for further progress.