Expression of a Mycobacterium tuberculosis arabinomannan antigen in vitro and in vivo

The outermost layer of Mycobacterium tuberculosis contains two major polysa ccharides, arabinomannan (AM) and glucan (GC). We studied the in vitro and in vivo expression of an M. tuberculosis AM antigen using monoclonal antibo dy (MAb) 9d8 (2a), an isotype-switched variant of the immunoglobulin G3 (Ig G3) MAb 9d8. MAb 9d8 had been previously shown to bind M. tuberculosis AM a nd the M. tuberculosis surface. Our in vitro experiments showed that MAb 9d 8(2a) hound strongly to whole-cell M. tuberculosis Erdman but not to the CD C 1551 strain grown in medium for an extended period. However, AM antigen w as detected in the culture supernatant of both strains, and its concentrati on increased in a time-dependent manner. The detection of AM antigen from b oth strains was decreased in the presence of Tween 80. In mice infected wit h M. tuberculosis Erdman, AM antigen accumulated in organ homogenates conco mitant to an increase in bacterial organ burden and an increase in IgG and IgM titer to AM. These results (i) indicate that the surface expression of AM during in vitro growth changes with culture age, is strain dependent, an d is affected by the presence of Tween 80 in the culture media; (ii) show t hat AM is produced by bacteria growth in vivo; and (iii) demonstrate that t he amount of in vivo-detected AM can be dependent on the number of bacteria in the infected organ.