Inbred rat strains, Fischer 344 (F-344) and Lewis (LEW), share the serological Ag-Bl allele and react very weakly in mixed lymphocyte culture (MLC). Despite this apparent identity at Ag-B, these strains differ markedly in their immune responses to an Ag-B disparate third strain Marshall 520 (M-520) (Ag-B6). F-344 recipients allowed M-520 heart grafts an extended survival, whereas LEW recipients rejected them rapidly. F-344 and M-520 showed a weak response in MLC in contrast to a strong response for LEW and M-520. F-344 produced antisera in response to injection of M-520 cells that had a relatively high antibody titer but low cytotoxic activity. F-344 responded to another strain, Buffalo (BUF) (also Ag-B6), in a similar fashion. F-344 apparently can produce a strong allogeneic response, as it was able to rapidly reject heart grafts from (LEW x Brown-Norway) F1 donors (LBN) (Ag-B 1/3). The low response of F-344 to M-520 probably was not due to shared antigens between the two strains because M-520 heart grafts underwent rapid rejection in LEW hosts highly tolerant to F-344. To explain the contrasting response of F-344 and LEW to the Ag-B6 disparity, we propose that it is controlled by an immune-response gene(s); that F-344 has a low-responding allele and LEW has a high-responding allele. The data do not reveal a location for this proposed gene. The high-responding allele appears to be dominant, as M-520 hearts were rejected rapidly by (F-344 x LEW) F1 recipients.
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