Immunodominant Nocardia asteroides antigens: isolation and characterisation by enzyme-linked immunoelectrotransfer blotting, and the value of immunoblot strips.

Concentrated cell-free filtrates (nocardins) were prepared from Nocardia asteroides cultures grown on Sauton's synthetic broth. Nocardins from 10 strains of six N. asteroides serotypes were produced and the proteins separated by isoelectric focusing. N. asteroides antigens among these proteins were tested for specificity using rabbit antisera and monoclonal antibodies (MAbs) against N. asteroides and Mycobacterium tuberculosis by the enzyme-linked immunoelectrotransfer blot test. At least 15 protein antigens were identified from each of the 10 nocardins. The immunodominant antigens were one serotype-specific N. asteroides protein with an isoelectric point (pI) of 4.0 (factor 1) and two group antigens with pIs of 4.43 (factor 6) and 4.68 (factor 8). The nitrocellulose strips prepared with these antigens did not react with antibodies to M. tuberculosis, nor with normal sera from humans, rabbits, or mice, but reacted specifically with anti-N. asteroides MAbs and polyclonal antibodies. Four purified protein derivatives of tuberculin were tested and did not cross-react with the three anti-N. asteroides MAbs. These reactions suggest that the antigens identified as factors 1, 6 and 7 are specific to N. asteroides and that factor 1 is specific for serotype 2, while factors 6 and 8 are species-specific.