Incidence of spontaneous ileocecal immunocytomas in hybrids of LOU/C rats and rat strains with low spontaneous tumor incidence.

Monoclonal immunoglobulin-secreting tumors (immunocytomas or plasmacytomas) appear in many species, but they occur at a low incidence and usually originate in lymphoid tissues. However, in the rat, the incidence of malignant spontaneous immunocytomas (or plasmacytomas) was high and the tumors consistently arose in the ileocecal lymph nodes. In inbred LOU/C/Wsl rats, these immunocytomas developed in twice as many males (31%) as females (16%). The susceptibility of the rats to immunocytoma was under genetic control; e.g., LOU/C/Wsl rats had a dominant locus (or loci) of susceptibility that could induce immunocytoma in inbred AUG/Wsl or inbred A X C9935/Wsl rats. However, inbred Okamoto/Wsl rats had at least one dominant locus of resistance that did not exist in LOU/C/Wsl, LOU/M/Wsl, AUG/Wsl, or A X C9935/Wsl rats.