Combinatorial use of antibodies to secreted mycobacterial proteins in a host immune system-independent test for tuberculosis

Laboratory diagnosis of tuberculosis is often difficult, Immunodetection of circulating Mycobacterium tuberculosis proteins shed during active infecti on would not depend on an intact host immune response and could take advant age of the speed and low costs afforded by antibody-based assays, We previo usly showed that patients with active tuberculosis had increased levels of circulating antigen 85 (Ag85) proteins independent of their tuberculin skin test status (S, I. Bentley-Hibbert, X, Quan, T, Newman, Ii. Huygen, and H, P, Godfrey, Infect, Immun, 67:581-588, 1999), To extend these observations to a Mycobacterium bovis BCC-vaccinated population and to another secreted mycobacterial protein, Ag85 and PstS-1 (protein antigen B, p38 antigen) we re quantified in sera from 97 Chilean tuberculosis patients and healthy con trols (many of whom had received BCG as children) using dot immunobinding, mouse monoclonal anti-BCG Ag85 complex antibody, and chicken antipeptide an tibodies reactive with M. tuberculosis Ag85B and PstS-1, The latter antibod ies had been raised to peptide-derived immunogens expressed on a novel prop rietary protein carrier in Escherichia coli, Median serum Ag85 levels measu red by using either anti-Ag85 antibody were significantly higher in patient s with active tuberculosis than in healthy controls (P, <0.001 to 0.01); th e median serum PstS-1 levels were similar in patients and controls, The sen sitivity of significantly elevated circulating Ag85 levels in patients with pulmonary tuberculosis measured by anti-Ag85 complex or anti-Ag85B antibod ies was 60 and 55%, respectively, but increased to 77% when results obtaine d with both anti-Ag85 antibodies were considered jointly (P < 0.02), The co rresponding specificities for individual and joint consideration were 95, 8 5, and 80%, respectively, These results indicate that elevated Ag85 levels can be detected in patients with active tuberculosis even after BCG vaccina tion and suggest that combinatorial use of antibodies directed at different epitopes of this protein could provide a viable strategy for developing ne w host immune response-independent diagnostic tests for tuberculosis.