HLA-DQB1 alleles confer susceptibility and resistance to insulin-dependent diabetes mellitus (IDDM). We investigated whether the susceptibility alleles DQB1*0302 and DQB1*0201 affect progression to diabetes among islet cell antibody–positive (ICA+) first-degree relatives of IDDM patients and whether the protective allele DQB1*0602 can be found and is still protective among such relatives. We human leukocyte antigen–typed and periodically tested β-cell function (first-phase insulin release [FPIR] during the intravenous glucose tolerance test) in 72 ICA+ relatives, of whom 30 became diabetic on follow-up (longest follow-up 12 years); 54 (75%) relatives carried DQB1*0302 and/or DQB1*0201. The frequency of DQB1*0302 and DQB1*0201 and of the high-risk genotype DQB1*0302/DQB1*0201 did not differ significantly between diabetic relatives and those remaining nondiabetic. On follow-up, progression to IDDM was not statistically different for relatives with or without the DQB1*0302/DQB1*0201 genotype. However, those relatives with the DQB1*0302/DQB1*0201 genotype had a tendency to develop diabetes at an earlier age (log-rank P = 0.02). We found DQB1*0602 in 8 of 72 (11.1%) ICA+ relatives. Relatives with DQB1*0602 did not develop diabetes or show any decline of FPIR versus 28 of 64 DQB1*0602 relatives who developed IDDM (log-rank P = 0.006; Wilcoxon's P = 0.02). The protective allele DQB1*0602 is found in ICA+ relatives who have minimal risk of progression to IDDM. Therefore, DQB1*0602 is associated with protection from IDDM both in population studies and among relatives with evidence of autoimmunity who should not enter prevention trials.

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