Mixed connective tissue disease (MCTD) was originally defined as a connective tissue disorder characterized by the presence of high titers of a distinct autoantibody in combination with clinical features commonly seen in systemic lupus erythematosus (SLE), scleroderma, and polymyositis (referred to as overlap syndrome) . The antigen recognized by these antibodies, originally called a ribonuclease-sensitive extractable nuclear antigen (RNAse sensitive ENA), is known to be a U1 ribonucleoprotein (RNP) complex. The presence of antibodies to U1 RNP remains a sine qua non for the diagnosis of this disorder; indeed, their emergence often precedes the onset of clinical disease [2,3].
The immunobiology of anti-U1 RNP antibodies in mixed connective tissue disease will be reviewed here. The clinical manifestations, diagnosis, prognosis, and treatment of this disorder, as well as a general discussion of anti-RNP autoantibodies, are discussed separately. (See "Clinical manifestations of mixed connective tissue disease" and "Definition and diagnosis of mixed connective tissue disease" and "Prognosis and treatment of mixed connective tissue disease" and "Antibodies to double-stranded (ds)DNA, Sm, and U1 RNP", section on 'Anti-Sm and anti-U1 RNP antibodies'.)
GENERATION OF U1 RNP AUTOIMMUNITY
The antibody response in patients with mixed connective tissue disease (MCTD) is very vigorous and is characterized by a hypergammaglobulinemia that can amount to 30 percent of the total serum immunoglobulins (Ig) . In the laboratory, this antibody response is first evident as a very high titer speckled antinuclear antibody (ANA) pattern . The titer is often greater than 1:1000 and is sometimes greater than 1:10,000; this finding should prompt the measurement of antibodies to U1 ribonucleoprotein (RNP), Sm, Ro, and La.
The specificity of the antibodies most closely related to the clinical features of MCTD is for a 70 kD epitope on U1 RNP molecules . This epitope has been further characterized as the RNA-binding domain on the peptide component of U1 RNP that spans residues 92 to 202 . Anti-U1 RNP antibodies display Ig class switching from IgM to IgG, as evidenced by variable region mutations, a feature that is typical of a T-cell dependent B-cell maturation response .
The interaction of T-cell receptors and peptides presented by human leukocyte antigen (HLA) molecules is a critical event in the generation of autoimmunity. The 70 kD anti-U1 RNP antibody response is associated with CD4/Th1 T cells expressing a HLA-DR4 or -DR2 phenotype . DNA sequencing of HLA-DB genes has revealed that DR2 and DR4-positive patients share a common set of amino acids in the beta chain at positions 26, 28, 30, 31, 32, 70, and 73 . Such amino acids form a pocket for antigen binding (figure 1).