A.BY, B10.LPa, and B10.129(5M) mice were presensitized in vivo against B10.A(5R) cells and then restimulated in vitro by the same cells in the standard CML assay. The effector cells thus generated lysed not only B10.A(5R), but also C57BL/6 targets, indicating that, in addition to anti-H-2Dd response [measured on the B10.A(5R) targets], response to minor histocompatibility (H) antigens (measured on the C57BL/6 targets) also occurred. The latter response was directed against multiple minor H antigens in the case of the A.BY effectors, and against H-1 and H-3 antigens in the case of B10.129(5M) and B10.LPa effectors, respectively. The sensitization against minor H antigens occurred in the context of H-2Kb H-2Dd antigens, but by testing the response on C57BL/6 targets, only cells reacting with minor H antigens in the context of H-2Kb were assayed. The same effector cells were then tested against H-2b mutant strains, in which the H-2Kb allele was replaced by a mutant one. All three effector types [A.BY, B10.LPa, and B10.129(5M)] behaved in a similar way: they all reacted with the H-2bg1 mutant to the same degree as with H-2b, they did not react at all or reacted only weakly with the H-2bd and H-2bh mutants, and they reacted moderately or strongly with the H-2ba mutant. The degree of crossreactivity with the mutants reflects, with one exception, the degree of relatedness of these mutants to H-2b, as established by other methods. The one exception is the H-2ba mutant, which is the most unrelated to H-2b, and yet it crossreacted strongly. Further testing, however, suggested that in this instance the crossreactivity was probably directed against H-2 antigens: the anti-H-2Dd effectors apparently crossreacted with the H-2Kba antigens. This finding is an example of cell-mediated crossreactivity between the products of two different H-2 genes (H-2K and H-2D). It is also an example of an H-2 mutation generating an antigenic determinant known to be present in another strain.
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