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 p30(gag) protein derived from clone 4-1 of the HERV family, using a baculovirus-vector system. This p30(gag) protein (CA41B) was approximately 30 kDa, as expected, and reacted with antibodies for p30(gag) purified from both murine and feline leukemia virus. This result suggested that the antigenic determinant for p30(gag) was well conserved in CA41B. Analysis of serum antibodies to p30(gag) in patients with autoimmune diseases was done by Western blotting. CA41B detected anti-pg30(gag) antibodies in 48.3% of systemic lupus erythematosus (SLE) patients, 35.0% of Sjogren's syndrome (SS) patients, and 33.3% of mixed connective tissue disease (MCTD) patients, whereas no anti-p30(gag) antibodies were found in healthy subjects. This suggested that HERV p30(gag) or other retroviral p30(gag) proteins possessing the same antigenic determinant as CA41B may play a role in these diseases. Although detection of antibodies to HERV p30(gag) in autoimmune diseases is indirect evidence that HERV proteins are involved, this study showed that patients with autoimmune diseases have antibodies to HERV p30(gag) using a recombinant HERV protein rather than synthetic peptides based on HERV or retroviral proteins of other species.