Serological studies of antigenic similarity between Japanese spotted fever rickettsiae and Weil-Felix test antigens.

Acute and convalescent-phase sera obtained from 10 patients infected with a Japanese strain of spotted fever group (SFG) rickettsia were tested by the indirect immunoperoxidase test, the Weil-Felix test, an enzyme-linked immunosorbent assay (ELISA), and immunoblotting. By the Weil-Felix test, the reactivity of these sera to the OX2 antigen was higher than those to the OX19 antigen, as is the case with sera from persons infected with other SFG rickettsiae. By ELISA, the titers of immunoglobulin M (IgM) antibodies against OX2 corresponded to the Weil-Felix test titers of these sera against OX2 but not to the titers obtained with IgG antibodies. The reactivity of the patient sera with the OX2 antigen in the Weil-Felix test was probably due to IgM antibodies against antigens which OX2 and SFG rickettsiae have in common. By immunoblotting tests, both IgG and IgM antibodies from the patient sera reacted with lipopolysaccharides from SFG rickettsiae and Proteus strain OX2. These results may show that these lipopolysaccharides contain similar epitopes.