[Mixed connective tissue disease--etiology, pathogenesis, clinical significance, treatment].

Some patients have features of more than one rheumatic disease and thus do not fit into traditional classification. Patients with combination of clinical finding similar to those of systemic lupus erythematosus (SLE), progressive systemic sclerosis (PSS), polymyositis, rheumatoid arthritis (RA) and with unusually high titers of circulating antinuclear antibody with specificity for nuclear ribonucleoprotein (RNP) are considered to have mixed connective tissue disease (MCTD). The overlap was described by Sharp and colleagues in 1972. During the post 20 years many studies exposed the clinical correlates of this antibody system (now called anti U1RNP). Controversy arose about whether MCTD was a distinct entity or would be better defined as subset of SLE. Anti RNP antibodies precipitate three proteins uniquely associated with U1RNP. Clinical correlates considered to be distinctive of MCTD are associated with 68 kD antigen specificity. Its to be expected that T cells receptors and HLA molecules are involved in the generation of these antibodies. Several observations have indicated, that 68 kD anti U1RNP antibody response in associated with HLA DR 4 and DR2 phenotype. Several studies have pointed a role of viruses initiating an antibody response against URNPs. Initial observations of MCTD suggested infrequent renal disease, a good response to corticosteroids and favourable prognosis. Future study has shown that some patients may require aggressive and prolonged pharmacologic therapy and that pulmonary involvement is common. Pulmonary hypertension associated proliferative vascular lesions may be serious complication with not always favourable prognosis.