Growth of normal bone marrow and L2C leukemia cell grafts was studied in lethally irradiated strain 2 and strain 13 guinea pigs by measuring 125IUdR uptake in the spleen and bone marrow and by histologic examination of these tissues. Allogeneic bone marrow cells proliferated as well as syngeneic cells in both strain 2 and 13 animals. Since strains 2 and 13 guinea pigs differ only at the I region of the major histocompatibility complex, this observation indicates that Ia disparities are not relevant to marrow graft rejection in the guinea pig.

Both Ia positive and Ia negative L2C leukemia cells of strain 2 origin grew well in the spleen of irradiated strain 2 animals. However, irradiated strain 13 animals showed considerable resistance to the growth of both leukemia cell lines. F1 hybrids (2 × 13) also showed resistance to the growth of the leukemia cells, although resistance was less than in strain 13 animals. These observations suggest the existence of an effector system capable of mediating natural resistance to L2C cells in unimmunized strain 13 and F1 guinea pigs. The nature of antigens recognized by these radiation resistant effector cells are not entirely clear. However, Ia antigens, or tumor-associated antigens dependent upon Ia antigens for immunogenicity, do not seem to be the primary targets in this phenomenon.

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This work was supported by National Institutes of Health Grants CA-19393, CA-15369, CA-21401, HL-18646, and by a grant from Organon, Inc., West Orange, New Jersey.

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