Evaluation of IgA deficiency in Sardinians indicates a susceptibility gene is encoded within the HLA class III region

IgA deficiency (IgA‐D) has been associated with the HLA region, in particular with the North European haplotype HLA‐A1, ‐B8, ‐DR3, but the exact location of the susceptibility gene(s) is unknown. Some reports suggest that a susceptibility gene is encoded in the class II region, while others implicate the class III region. We exploited differences between the common Sardinian and North European HLA‐DR3 haplotypes to help localize the IgA‐D susceptibility gene(s). With the knowledge that approximately 13% of HLA‐DR3 homozygous individuals of North European origin are IgA‐D, we examined 43 HLA‐DR3 homozygous Sardinians to find that all had normal serum IgA, IgG and IgM levels. A detailed analysis of their MHC haplotypes indicated a common Sardinian HLA‐DR3 haplotype TAP1A , TAP2A , HLA‐DQB1*0201 , ‐DQA1*0501 , ‐DRB1*0301 , LH1‐(Z  +  2) , D3A‐(Z  +  2) , C4B‐0 , C4A‐L , G11‐15 , Bf‐0.4 , C2‐a , HSP70‐7.5 , 9N3‐(Z  +  10) , 82I‐(Z  −  2) , TNFα‐9 , 62‐(Z  −  20) , HLA‐B18 , ‐Cw5 , ‐A30 which diverges from the common North European HLA‐DR3 haplotype telomeric to the HLA‐DR region. In parallel studies of five Sardinians with IgA‐D, two of the 10 HLA haplotypes (20%) contained HLA‐DR3, a frequency similar to that observed in the background population. One of these was the HLA‐DR3‐ B8 North European haplotype, which occurs rarely in Sardinia. Our data favour the hypothesis that a class III region allele, present on the common North European but not on the Sardinian HLA‐DR3 haplotype, confers susceptibility to IgA‐D.