Competitive enzyme immunoassay for antibodies to a 43,000-molecular-weight Francisella tularensis outer membrane protein for the diagnosis of tularemia.

Antibodies against a 43,000-molecular-weight Francisella tularensis outer membrane (OM) protein (43K protein) were measured in paired serum specimens from 23 patients with tularemia and matched against antibodies in sera from 25 patients with nontularemic infectious diseases and from 25 blood donors. Antibodies were measured by a competition enzyme-linked immunosorbent assay which tested the ability of human serum to compete with rabbit anti-43K protein antibodies for its binding to the F. tularensis OM coat. The sera from nontularemic patients and from blood donors, in dilutions of 1:16 and 1:64, respectively, showed no or very low levels of antibodies. All of the tularemia patients showed positive tests with the first, the second, or both of the serum specimens examined. For instance, with serum diluted 1:64, each of the serum specimens showed a sensitivity of 95.7% and a specificity of 96%. When used for antibody competition in Western blotting (immunoblotting), the rabbit anti-43K selectivity blocked the binding of human serum antibodies to the 43,000-molecular-weight protein. This protein was immunoaccessible in Formalin-killed F. tularensis. These data indicate an important role of the 43,000-molecular-weight OM protein in the immunobiology of tularemia and emphasize its potential usefulness as an antigen in serodiagnostic tests.