CARBOHYDRATE SPECIFICITY OF IGM AUTOANTIBODIES TO CD45 IN SYSTEMIC LUPUS-ERYTHEMATOSUS

Patients with SLE develop IgM autoantibodies to different isoforms of CD45, the major surface membrane protein tyrosine phosphatase on lymph ocytes and other nucleated hemopoietic cells. Because such autoantibod ies could have a potential role in the development of immune dysfuncti on in this disorder, we performed a series of experiments to character ize their antigenic specificity further. Blots of recombinant E. coli fusion proteins encoded by exons 3-7 of the p220 and p180 isoforms wer e uniformly non-reactive with SLE IgM, suggesting that anti-CD45 autoa ntibodies in SLE are directed against conformational and/or carbohydra te epitopes, rather than linear polypeptide epitopes. This issue was e xamined further using chemically and enzymatically modified CD45 purif ied from T cells by lectin affinity chromatography as substrates. Trea tment of CD45 with 25 mM sodium-m-periodate, sufficient to abrogate bi nding to various lectins, abolished the reactivity with SLE anti-CD45 autoantibodies. On the other hand, digestion of CD45 with neuraminidas e enhanced the binding of anti-CD45 autoantibodies from some of the SL E sera. This result probably reflects decreased steric hindrance or ch arge repulsion because the binding of mouse monoclonal antibodies dire cted against linear polypeptide epitopes of CD45 was similarly enhance d. Digestion of CD45 with N-glycosidase F had no effect on autoantibod y staining. Taken together, these data suggest that IgM anti-CD45 auto antibodies in SLE recognize non-sialylated carbohydrate determinants i n the highly O-glycosylated polymorphic domains of CD45.