Minor histocompatibility (H) antigens are typically identified as barriers to transplantation in graft exchange experiments between major histocompatibility complex (H2)-matched hosts. Alner et al. have recently shown that a barrier to histocompatibility is encoded on the X Chromosome in strain B mice (X^B) that strain C’s X chromosome (X^C) has lost. Alner also showed that this H barrier is comprised of a minimum of two X^B-encoded antigens, since two different antigen-presenting molecules, H2^B and H2^C, must be carried by the host for graft rejection to occur.

To restrict the locations(s) of these H-hix^c-encoded, loss-of-antigen alleles, a trihybrid cross, (C x B.C-H2^c)F1 x B was performed. The F1 females carried X^B and X^C, and were homozygous for H2^c (which is the only C-derived locus carried by the “congenic” strain, B.C-H2^c). When these F1 females were crossed to strain B males, all the “N2-like” progeny must be H2^b/c heterozygotes, giving these males the potential to be good rejecters of H-hix^b-bearing grafts—if they have also inherited the complete H-hix^c haplotype from their F1 mother. All N2-like males were grafted with (B x B.C)F1, H-hix^b-bearing skin to determine which carried the H-hix^c component(s) necessary for graft rejection. DNA was isolated by tail-tip biopsy from all rejecter N2-like males and typed using the polymerase chain reaction (PCR) for 12 DNA markers on the X chromosome to limit the possible location(s) of C-derived sequences that might include H-hix^c.

At the same time, female N2-like mice were also grafted with (B x B.C)F1 skin, to control for the purity of our inbred mouse strains. Unfortunately, some of these females aggressively rejected their control grafts, suggesting that our starting strains were not pure. This unexpected H barrier segregated within only certain families in our N2-like panel, and we have now hypothesized that the B.C-H2^c strain may no longer be identical with the B strain, due to random genetic change (i.e., a spontaneous minor histocompatibility mutation) since the original derivation of the B and B.C congenic lines.

We are responding to this problem in two ways:

1) We have eliminated from our N2-like panel of rejecter males any with a sister sib that rejected a control graft (since such rejecter males might carry either the requisite H-hix^c haplotype on their X or because they (or their graft) may have carried the “new” spontaneous mutation). This restricted data set may allow us to draw at least tentative conclusions about the possible location(s) of <>H-hix^c.