Measurement of antibody to Jo-1 by ELISA and comparison to enzyme inhibitory activity.

Antibody to the Jo-1 antigen (histidyl-tRNA synthetase) is found almost exclusively in myositis patients, usually those with adult PM, but has been found in only 30% of that group by immunodiffusion or other techniques thus far reported. We have reexamined the prevalence of antibody to Jo-1 in sera from 130 patients and 82 controls by using the sensitive ELISA technique. The ELISA used affinity-purified, enzymatically active bovine Jo-1 antigen. A wide range of antibody level by ELISA was found among 24 immunodiffusion positive sera. Six myositis and two control sera had apparent specific antibody detectable only by ELISA. Overall, however, the antibody continued to show high myositis specificity with predominance in adult PM (35.8% in that group). Because the antibody inhibits enzymatic activity of the synthetase antigen, we also studied the quantitative inhibitory activity of these sera to compare with the antibody activity as determined by ELISA. Twenty-four immunodiffusion-positive sera, 29 immunodiffusion-negative sera, and 15 normal sera were tested at 1/50 dilution in the reaction mixture. There was background inhibition by all normal sera tested that averaged 30.5%. All but one immunodiffusion negative myositis sera (a high binder by ELISA) inhibited less than 50% of the average with normal serum. Twenty-three of 24 immunodiffusion positive sera inhibited greater than 80% of this normal average; the other inhibited 66%. The serum dilution giving 50% inhibition was highly correlated (R = 0.83) with the ELISA activity. Thus, inhibition of histidyl-tRNA synthetase activity is a relatively accurate measure of Jo-1 antibody. This method should be applicable to measuring antibody to other aminoacyl-tRNA synthetases.