Blood samples from 65 patients with primary Sjögren's syndrome were evaluated for the presence of antiphospholipid antibodies. Increased levels of antiphospholipid antibodies were found in 13 of 65 (20%) of patients. These antiphospholipid antibodies were predominantly of the IgA isotype, in contrast with the IgG isotype antiphospholipid antibodies found in patients with systemic lupus erythematosus (SLE). The presence of IgA antiphospholipid antibodies in the patients with primary Sjögren's syndrome was not significantly associated with arterial or vascular thrombosis, nor peripheral or central nervous system vasculitis. There was no association with laboratory determined features such as lupus anticoagulant or false positive results of the Venereal Disease Research Laboratory (VDRL) test. Oligonucleotide specific DNA amplification and hybridisation with allele specific probes was used to examine the HLA-D antigens occurring in this group of patients with primary Sjögren's syndrome. Of 13 patients with antiphospholipid antibodies, seven had the genotype HLA-DR2/DR3. However, compared with the whole group of 65 patients with Sjögren's syndrome, no increased occurrence of haplotype DR2 or DR3 was noted. These results suggest that gene interaction between DR2 and DR3 may play a part in the production of antiphospholipid antibodies in patients with Sjögren's syndrome. In contrast with patients with SLE, the IgA antiphospholipid antibodies in patients with Sjögren's syndrome are not risk factors for thrombosis or vasculitis. The presence of IgA antiphospholipid antibodies in patients with Sjögren's syndrome probably reflects its production at mucosal sites of inflammation and the absence of vasculopathy may be due to the inability of IgA antibodies to activate complement.
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