Multiple sclerosis (MS) is a complex trait in which alleles at or near the class II loci HLA-DRB1 and HLA-DQB1 contribute significantly to genetic risk. HLA-DRB1*15 and HLA-DRB1*17-bearing haplotypes and interactions at the HLA-DRB1 locus increase risk of MS but it has taken large samples to identify resistance HLA-DRB1 alleles. In this investigation of 7,093 individuals from 1,432 MS families, we have assessed the validity, mode of inheritance, associated genotypes, and the interactions of HLA-DRB1 resistance alleles. HLA-DRB1*14-, HLA-DRB1*11-, HLA-DRB1*01-, and HLA-DRB1*10-bearing haplotypes are protective overall but they appear to operate by different mechanisms. The first type of resistance allele is characterised by HLA-DRB1*14 and HLA-DRB1*11. Each shows a multiplicative mode of inheritance indicating a broadly acting suppression of risk, but a different degree of protection. In contrast, a second type is exemplified by HLA-DRB1*10 and HLA-DRB1*01. These alleles are significantly protective when they interact specifically in trans with HLA-DRB1*15-bearing haplotypes. HLA-DRB1*01 and HLA-DRB1*10 do not interact with HLA-DRB1*17, implying that several mechanisms may be operative in major histocompatibility complex–associated MS susceptibility, perhaps analogous to the resistance alleles. There are major practical implications for risk and for the exploration of mechanisms in animal models. Restriction of antigen presentation by HLA-DRB1*15 seems an improbably simple mechanism of major histocompatibility complex–associated susceptibility.
Multiple sclerosis (MS) is a complex neurological disease with a strong genetic component. With the possible exception of a weak association at Chromosome 5p, the major histocompatibility complex is the only locus consistently linked to MS. Because of this the major histocompatibility complex has recently undergone renewed attention. A region at or near the gene HLA-DRB1 influences the risk of MS. HLA-DRB1 comes in over 400 different forms (or alleles). A common form in Europe, named 1501, increases risk of MS by 3-fold. In this paper, to our knowledge the largest-ever analysis of this region in MS, we examine the inheritance of newly discovered HLA-DRB1 MS resistance alleles, namely HLA-DRB1*14, HLA-DRB1*11, *10, and HLA-DRB1*01. We show that HLA-DRB1*14 and HLA-DRB1*11 are dominantly protective; e.g., HLA-DRB1*14 significantly reduces the risk associated with HLA-DRB1*15 when they are inherited together. This may explain, in part, why MS is rare in Asia; there, the HLA-DRB1*14 allele is frequent. HLA-DRB1*01 and HLA-DRB1*10 are protective only in the presence of HLA-DRB1*15. HLA-DRB1*14 and HLA-DRB1*11 haplotypes and HLA-DRB1*01 and HLA-DRB1*10 haplotypes share common ancestral origins and this may be why the alleles can be grouped in terms of their protective nature. Discovery of the mechanism of protection against MS may lead to the discovery of new treatments to make a palpable difference in the lives of those who have been affected by this devastating disease.