Smooth muscle antibodies in Mycoplasma pneumoniae infection.

Paired sera from forty-five cases of Mycoplasma pneumoniae (MP) infection associated with acute lower respiratory tract illness were examined by immunofluorescence for antibodies to smooth muscle. Twenty-five (56%) of these cases had smooth muscle antibodies (SMA) of IgM class. A significant (greater than or equal to 4-fold) increase in titre of these antibodies was demonstrated in fifteen of thirty-five patients with a significant rise in titre of MP antibodies. SMA of IgG class occurred in eleven of forty-five cases (24%), but a 4-fold rise in antibody titre was found only in two cases. Three of forty-five sera (7%) from healthy donors contained SMA of IgM class and eight sera (18%) SMA of IgG class. MP antigen did not absorb SMA. Liver tests were performed in twenty-nine patients. In eighteen patients SGPT values were moderately or slightly elevated. There was no correlation between the occurrence of increased levels of transaminases and the presence of SMA in serum. In a patient with active chronic hepatitis, who had had a high titre of SMA exclusively of IgG class for 2 years, SMA of IgM class appeared transiently in association with an acute respiratory illness due to MP.