Detection of antibodies to a recombinant gag protein derived from human endogenous retrovirus clone 4-1 in autoimmune diseases.

To investigate whether human endogenous retroviruses (HERV) contribute to autoimmune diseases, we prepared a recombinant p30gag protein derived from clone 4-1 of the HERV family, using a baculovirus-vector system. This p30gag protein (CA41B) was approximately 30 kDa, as expected, and reacted with antibodies for p30gag purified from both murine and feline leukemia virus. This result suggested that the antigenic determinant for p30gag was well conserved in CA41B. Analysis of serum antibodies to p30gag in patients with autoimmune diseases was done by Western blotting. CA41B detected anti-p30gag antibodies in 48.3% of systemic lupus erythematosus (SLE) patients, 35.0% of Sjögren's syndrome (SS) patients, and 33.3% of mixed connective tissue disease (MCTD) patients, whereas no anti-p30gag antibodies were found in healthy subjects. This suggested that HERV p30gag or other retroviral p30gag proteins possessing the same antigenic determinant as CA41B may play a role in these diseases. Although detection of antibodies to HERV p30gag in autoimmune diseases is indirect evidence that HERV proteins are involved, this study showed that patients with autoimmune diseases have antibodies to HERV p30gag using a recombinant HERV protein rather than synthetic peptides based on HERV or retroviral proteins of other species.