Autoantibodies against nuclear proteins like RNA polymerase I (RNA pol I) are produced in a number of rheumatic autoimmune diseases. Production of antibodies specific for the 190-kD subunit of RNA pol I appears to be characteristic in the patients with systemic sclerosis. Previous investigations have shown that the tight skin (TSK) mouse is an experimental model for systemic sclerosis. In the present study we show that the TSK mice produce high titers of anti-RNA pol I antibodies, both of IgM and IgG classes. To characterize the immunochemical properties of these antibodies we obtained a large panel of hybridomas from these mice. Analysis of these hybridomas revealed that clonal frequency of autoreactive B cells specific for RNA pol I are higher in the TSK mice that in the controls. mAbs obtained from the TSK mice were specific for the 190-kD subunit and cross-reacted with Escherichia coli and phage T7 RNA polymerases (155-, 150-, and 107-kD polypeptides). We have also demonstrated that these antibodies bind better to the phosphorylated enzymes. The anti-RNA pol I mAbs were divided into three groups in terms of their functional property. The first group of antibodies increased the catalytic activity of the enzyme whereas the antibodies of the second group inhibited the enzymatic activity. Competitive inhibition RIAs showed that these two groups of antibodies bound to distinct epitopes. The third group of antibodies was neutral and had no activity on the enzyme function. These results suggest that TSK mouse anti-RNA pol I antibodies recognize three or more conserved epitopes. To understand the molecular basis of the generation of such autoreactive antibodies we analyzed their V gene repertoire. Northern analysis of RNAs of 14 TSK hybridomas showed that the VH genes encoding these antibodies were mainly from VH J558 family. It is possible that these genes were derived from a single germline gene or from a set of related genes of a single subgroup.
S Shibata, T Muryoi, Y Saitoh, T D Brumeanu, C A Bona, K N Kasturi