Objective: To investigate the contributions of the major histocompatibility complex (MHC) and C4 alleles to systemic sclerosis (SSc), and to pulmonary fibrosis and autoantibody expression in SSc, by analysis at the DNA level.
Methods: One hundred fifteen patients with SSc were tested serologically for alleles of the class I MHC loci, and were tested for class II alleles (DRB, DQA, and DPB) by a combination of restriction fragment length polymorphism (RFLP) analysis and oligonucleotide probes with polymerase chain reaction amplification. C4 was studied by protein phenotyping and RFLP analysis in 80 patients. Correlations were made between disease status, pulmonary fibrosis, and expression of anticentromere antibodies (ACA) and anti-Scl-70.
Results: The C4A-null phenotype was found to provide the strongest disease association factor of the MHC region (P = 0.000064, relative risk [RR] = 2.8, etiologic fraction [EF] = 32.1). The primary MHC susceptibility allele was found to be DQA2 (Pcorr = 0.0009, RR = 2.5, EF = 35.6), which is in linkage disequilibrium with both DR3 and DR11. DR2 was protective, but only for female patients (P = 0.0021, RR = 0.42, protective fraction = 19.4). DR52a was the primary MHC allele associated with pulmonary fibrosis in SSc patients. Expression of ACA was associated with the presence of either DR1 or DR4 (P = 0.0015, RR = 6.7, EF = 78.0). Anti-Scl-70 expression correlated with an acidic residue of DP beta (DPB1:69:E) (P = 0.0063, RR = 4.6, EF = 53.1).
Conclusion: Of all the potential markers of disease susceptibility analyzed, the C4A locus was the strongest. C4AQ0 and DQA2 are independent susceptibility factors for SSc. The development of pulmonary fibrosis in SSc patients can be predicted using combined MHC and autoantibody analysis. The MHC alleles associated with the expression of disease-specific autoantibodies are not markers for disease susceptibility.