Retroviral gp70 and anti-gp70 antibodies were isolated from circulating immune complexes (IC) of 7-10-mo-old (NZB X NZW)F1 mice, after which the nature and origin of this gp70 (IC-gp70) and the immunologic characteristics of these anti-gp70 antibodies (IC-anti-gp70) were investigated. Immunochemical and structural analyses of IC-gp70 demonstrated that among multiple immunologically related gp70 expressed in all mice, the IC-gp70 had characteristics similar to those of NZB xenotropic viral gp70 (NZB-X1 gp70) that is commonly present in sera of virtually all strains of mice. The study of binding by IC-anti-gp70 antibodies to retroviral gp70 from various sources showed that the IC-anti-gp70 were primarily directed to NZB-X1 gp70 as well as serum gp70. These data strongly suggest that the abnormality of murine strains with systemic lupus erythematosus causing them to produce antibodies to their own xenotropic viral gp70 and to form IC with serum gp70 is not based on their expression of an unusual type of gp70, but rather their ability to make an antibody to NZB-X1 gp70, probably as a result of their immunologic dysfunction.

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