Naturally occurring masked antibodies in murine sera recognize a component of the mitotic spindle apparatus

Antibody activities previously masked in autoimmune MRL and normal Balb c mice were expressed by briefly subjecting their sera to acidic (pH 2.0) or alkaline (pH 12.0) environments. An enzyme-linked immunosorbent assay (ELISA) revealed that these pH-expressable immunoglobulins reacted with specific nucleotide-BSA antigens (primarily 5′-AMP, -GMP, -TMP) but not with single (ss)- or double (ds)-stranded DNAs or with unconjugated BSA. ELISA analysis of pH-expressed antibodies purified via GMP-BSA/Sepharose indicated that they bound not only to the homologous hapten (GMP) but to AMP and TMP as well, i.e., anti-pAGT antibodies. Further, indirect immunofluorescent assays (IIF) with fixed HEp-2 cells demonstrated that purified anti-pAGT antibodies recognized an epitope within the mitotic spindle apparatus. These results document the existence of a previously undefined masked antibody population in murine sera with specificities directed toward certain nucleotides and a component of the mitotic spindle apparatus. Last, these anti-pAGT (or anti-mitotic spindle) antibodies are not restricted to murine systems inasmuch as they have been detected in every human serum (>100 samples) examined thus far.