Hereditary haemochromatosis (HH), which affects some 1 in 400 and has an estimated carrier frequency of 1 in 10 individuals of Northern European descent, results in multi-organ dysfunction caused by increased iron deposition, and is treatable if detected early. Using linkage-disequilibrium and full haplotype analysis, we have identified a 250-kilobase region more than 3 megabases telomeric of the major histocompatibility complex (MHC) that is identical-by-descent in 85% of patient chromosomes. Within this region, we have identified a gene related to the MHC class I family, termed HLA-H, containing two missense alterations. One of these is predicted to inactivate this class of proteins and was found homozygous in 83% of 178 patients. A role of this gene in haemochromatosis is supported by the frequency and nature of the major mutation and prior studies implicating MHC class I-like proteins in iron metabolism.

Genes of the major histocompatibility complex (MHC) play a critical role in immune recognition, and many alleles confer susceptibility to infectious and autoimmune diseases. How these deleterious alleles persist in populations is controversial. One hypothesis postulates that MHC heterozygote superiority emerges over multiple infections because MHC-mediated resistance is generally dominant and many allele-specific susceptibilities to pathogens will be masked by the resistant allele in heterozygotes. We tested this hypothesis by using experimental coinfections with Salmonella enterica (serovar Typhimurium C5TS) and Theiler's murine encephalomyelitis virus (TMEV) in MHC-congenic mouse strains where one haplotype was resistant to Salmonella and the other was resistant to TMEV. MHC heterozygotes were superior to both homozygotes in 7 out of 8 comparisons (P ‫؍‬ 0.0024), and the mean standardized pathogen load of heterozygotes was reduced by 41% over that of homozygotes (P ‫؍‬ 0.01). In contrast, no heterozygote superiority was observed when the MHC haplotype combinations had similar susceptibility profiles to the two pathogens. This is the first experimental evidence for MHC heterozygote superiority against multiple pathogens, a mechanism that would contribute to the evolution of MHC diversity and explain the persistence of alleles conferring susceptibility to disease.

There are many instances where males and females differ in the susceptibility to infections. The reason for these differences in susceptibility is multifactorial. The primary cause is thought to be due to differences induced by sex hormones and their effects on gene expression as well as the immune system, but may also be due to innate physiological differences between males and females. This review summarizes gender specific differences seen in infections caused by bacteria, fungi, parasites and viruses. Ultimately, gender specific differences appear to be dependent on the microbe causing the infection, as not every infection with a specific microbial type results in increased susceptibility of one gender over the other. This suggests that there is an interaction between gender specific immune differences and the specific immune response to individual microbes.

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Cryptococcus neoformans (Cn) is a pathogenic yeast and the cause of cryptococcal meningitis. Prevalence of disease between males and females is skewed, with males having an increased incidence of disease. Based on the reported gender susceptibility differences to Cn in the literature, we used clinical isolates from Botswanan HIV-infected patients to test the hypothesis that different gender environments exerted different selective pressures on Cn. When we examined this data set, we found that men had significantly higher risk of death despite having significantly higher CD4+ T lymphocyte counts upon admittance to the hospital. These observations suggested that Cn strains are uniquely adapted to different host gender environments and that the male immune response may be less efficient in controlling Cn infection. To discriminate between these possibilities, we tested whether there were phenotypic differences between strains isolated from males and females and whether there was an interaction between Cn and the host immune response. Virulence phenotypes showed that Cn isolates from females had longer doubling times and released more capsular glucoronoxylomannan (GXM). The presence of testosterone but not 17-β estradiol was associated with higher levels of GXM release for a laboratory strain and 28 clinical isolates. We also measured phagocytic efficiency, survival of Cn, and amount of killing of human macrophages by Cn after incubation with four isolates. While macrophages from females phagocytosed more Cn than macrophages from males, male macrophages had a higher fungal burden and showed increased killing by Cn. These data are consistent with the hypothesis that differential interaction between Cn and macrophages within different gender environments contribute to the increased prevalence of cryptococcosis in males. This could be related to differential expression of cryptococcal virulence genes and capsule metabolism, changes in Cn phagocytosis and increased death of Cn-infected macrophages.