Individual rats belonging to two different F2 crosses were serotyped for HI (AgB) determined antigens and tested in mixed lymphocyte culture (MLC) against parental strain rats. In agreement with the general contention that MLC reactivity is controlled solely by the major histocompatibility locus of the species, all HI different mixtures were found to stimulate, while HI identical mixtures failed to stimulate. The lack of MLC stimulation by minor loci antigens may be due to low frequency of antigen sensitive cells against these antigens. Immunization may be expected to increase their frequency and indeed in some combinations low but clear‐cut stimulation was produced by non HI antigens after specific immunization. In the HI (different AS‐AS2 combination an unexpected discrepancy was found between the relative antigenic strength as measured by MLC on one side and by the local graft‐versus‐host assay (12, 13) and by kidney graft survival on the other side. Possible reasons for this discrepancy are discussed.
Kidney‐, heart‐ and skin‐grafting were performed from individual donor rats belonging to segregating F2 hybrid populations to recipients of one of the parental strains in two combinations: (AS × BN) F2 → AS and (Fischer × BN) F2 → Fischer. Before grafting all donors were tested for presence or absence of the Ag‐B allele from BN by serotyping and by mixed lymphocyte culture reaction. According to the one locus hypothesis one would expect that organs from F2 donors, which carried the BN allele would be rejected acutely, whereas organs from negative F2 donors would survive for a long time concurrently with acute or subacute rejection of skin grafts. In both combinations, acute or subacute rejection of the skin grafts occurred, whether or not the donor carried the foreign Ag‐B allele from BN, and acute or subacute rejection occurred of all kidneys and hearts from BN positive donors except for a single kidney which survived for 197 days. A very distinctly prolonged survival for both kidney and heart grafts was noticed in both combinations when the donor lacked the Ag‐B allele from BN. However, one exception occurred, viz. a grafted heart which was rejected after 62 days. But in general the prediction came true, and it may be concluded that rejection of kidney and heart grafts is the common rule, when incompatibility at the Ag‐B locus exists, while compatibility at this locus leads to permanent acceptance of organ grafts independent of incompatibilities at the weaker loci.
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