An antibody reactive with soluble saline extracts from calf or rabbit thymus, termed extractable nuclear antigen (ENA), includes a ribonuclease-sensitive fraction, known as RNP, and a ribonuclease-resistant fraction known as Sm (57). Anti-Sm antibodies recognize Sm core proteins B'/B, D, E, F, and G, shared by U1, U2, U4-6, and U5 small nuclear RNP(snRNP), while antibodies against the nuclear RNP (nRNP) recognize the U1 RNP-specific 70K, A, and C proteins (58). The posttranslational modifications of Sm proteins seem to play a role in the etiology of SLE (59). A recent study represents the first evidence that anti-Sm (as well as anti-La) autoantibodies are capable of entering living cells. The phenomenon of intracellular autoantibodies may have a larger scope than previously reported and are consistent with a potential pathogenic role for ANA (60).
High titers of antibodies against the U1nRNP complex, at levels of 1:10,000 or higher are observed in patients with MCTD (61). Long-term follow up of patients with MCTD has shown that many of these patients go on to develop RA, scleroderma or other rheumatic diseases (62). Thus, U1nRNP antibodies are clinically not as specific as previously suggested (63). In contrast, antibodies to Sm are highly specific for SLE and are predominantly targeted to the Sm-B/B' and -D polypeptides (64, 65), Unfortunately, these antibodies are seen in only about 30% of SLE cases. With disease evolution anti-Sm might appear (66). |
(ANCA)
