To verify that the MHC haplotypic combinations that we chose from the literature indeed showed opposite (or similar) susceptibility profiles, we evaluated these profiles in both single infections and coinfections. The results of single-infection experiments with the pathogen Salmonella
or TMEV are indicated in Fig. . MHC had a significant effect on resistance, as predicted from the published literature (34
). MHC haplotypic combinations (b
, and d
) showed opposite susceptibility profiles, whereas the MHC haplotypic combination (f
) showed similar susceptibility profiles when comparing these two pathogens. Resistance was usually dominant (11 out of 16 comparisons), with some exceptions. These haplotypic combinations were then coinfected, and pathogen loads were determined 4 weeks postinfection.
Figure illustrates the pathogen loads for MHC genotypes of coinfected mice showing either opposite (Fig. ) or similar (Fig. ) patterns of susceptibility. Female data are depicted in Fig. , and male data are shown in Fig. . As expected, the MHC b/b and q/q genotypes show opposite patterns of susceptibility to Salmonella and TMEV, as do the MHC d/d and q/q genotypes (Fig. ). Also as expected, the MHC f/f and k/k genotypes show similar patterns of susceptibility to these two pathogens. However, the MHC genotypes d/d and k/k (Fig. ) showed similar patterns of susceptibility to Salmonella and TMEV, the reverse of what we found in the single-infection experiments (Fig. ).
FIG. 3. Salmonella and TMEV loads for coinfected mice of either opposite or similar susceptibility profiles. Pathogen loads are given for coinfected female (a) and male (c) animals of different MHC heterozygote combinations showing opposite susceptibility profiles. (more ...)
This apparent reversal of susceptibility patterns between single infections and coinfections suggests there is an interaction between these two pathogens during coinfection and these specific MHC genotypes, as seen in other coinfections such as HIV and many parasitic diseases (19
). When we tested this hypothesis, there was not a significant difference in the overall pattern of susceptibility (opposite or similar) between single infections and coinfections (data not shown). However, there was no significant difference between the d
homozygotes for either the single infections (Fig. ) or coinfections (Fig. ). Thus, for this genotypic combination, we cannot determine whether the TMEV loads show an opposite or similar pattern of susceptibility, and so we excluded them from further analysis. All patterns seen in single infections and coinfections (Fig. and ) are similar to those seen in previous studies (34
), except the d
Figure compares the results of eight coinfection groups of MHC genotypic combinations showing opposite (reciprocal) patterns of susceptibility. These eight experiments represent comparisons of two haplotype combinations (b with q plus d with q), both sexes, and two independent experiments. The pattern of heterozygote superiority was observed for standardized combined pathogen loads in seven of the eight groups (P = 0.0024 by binomial distribution, where P = 1/3). Even if we conservatively excluded the two groups where the heterozygote load was similar to the best homozygote (Fig. ), the pattern of heterozygote superiority is still significant (five out of six groups, P = 0.01 by binomial distribution, where P = 1/3). Analyzing all eight groups, the standardized combined pathogen load in heterozygotes was 41% lower than the load in homozygotes (P = 0.01, by the Wilcoxon rank-sum test).
To estimate the relative strength that each genotypic combination contributed to the overall significant pattern of heterozygote superiority (Fig. ), we pooled the standardized data across sex and experiments for each genotype. Figure illustrates the standardized combined pathogen loads for the genotypic combinations showing opposite (Fig. ) and similar (Fig. ) susceptibility profiles. For opposite susceptibility profiles, the b with q haplotypic combinations showed the largest effect, with pathogen loads of the b/q heterozygotes approximately 71% lower than those of both the b/b and q/q homozygotes (P < 0.0001 and P = 0.04, respectively, by the Wilcoxon rank-sum test) (Fig. ). The d and q genotypic combination (Fig. ) showed a nonsignificant pattern of heterozygote superiority, and the relative strength was smaller than that of b and q, with pathogen loads of the d/q heterozygote being 9% lower than those of d/d homozygotes and 20% lower than those of q/q homozygotes. When the MHC genotypic combination showing similar patterns of susceptibility was examined, MHC f/k heterozygotes were worse than both homozygotes (Fig. ). This nonsignificant pattern may be due to the susceptibility being dominant for the Salmonella infection (Fig. ).
We used pathogen load as a measure of host immunocompetence due to its strong inverse correlation with overall host fitness (e.g., HIV) (41
). We tested the effects of MHC genotype on death and weight, but no significant differences were found (data not shown). These infections were generally benign, with these young, growing animals gaining weight during the infection, although at a slower rate than their uninfected siblings (data not shown).