Serum antibodies of the three major isotypes to two CPs of Mtb in HIV-negative adults stratified according to their tuberculosis status were measured by ELISA with purified antigens. Antibodies to both polysaccharides were detectable in all groups. Importantly, the Ig A anti-Glu levels were significantly higher in the active and treated TB groups compared to unexposed subjects who were PPD-negative. Antibodies against AM demonstrated a similar pattern, with the exception that IgG anti-AM was higher in subjects with active TB or previously documented active TB, and the IgA anti-AM was higher in subjects with previously documented active TB when compared with unexposed, PPD-negative subjects. Serum IgG anti-Glu levels were higher in groups with active TB or previously documented active TB than in a group of unexposed, PPD-negative subjects, but the differences were not significant. Thus, with this small number of subjects, groups of patients who had active or treated disease could be identified by the level of antibodies to these two CPs. These data suggest that assays based on antibody responses to the CPs of Mtb may be of value for the serodiagnosis of TB.
What could be the origin of CP antibodies in unexposed, PPD-negative people? Similar to other capsulated pathogens, cross-reactive anti-polysaccharide antibodies were probably generated by exposure to environmental mycobacteria or non-pathogenic enteric or pulmonary bacteria [13–15]. In fact, we have previously shown that the development of low level humoral immune responses to Mycobacterium avium sonic extracts and mycobacterial LAM in children correlates with increasing age, and by 18 years of age many individuals were seroreactive to mycobacterial antigens . This could explain the levels observed in the adults with no history or evidence of TB or of exposure to Mtb. The presence of pre-existing anti-CP antibodies could confound use of this method in TB diagnostics in areas with high exposure to environmental mycobacteria or in countries where BCG vaccination is routine.
The role of antibodies in the pathogenesis of tuberculosis and protection against Mtb infections is controversial. Further, the impact of specific antibody isotypes on TB infections is uncertain. The role of serum anti-CPs is under study, but there are accumulating data that this immune component may be protective [4, 17–22]. The capsule of Mtb has been shown to limit the association of the organism with macrophages . The antiphagocytic nature of the capsule is similar to that of other encapsulated organisms. Antibody to the CPs of Mtb may therefore enhance the uptake of organisms. A protein-conjugate vaccine containing AM has been shown to reduce the mycobacterial burden in vaccinated mice . Likewise, administration of a monoclonal antibody to the AM moiety of lipoarabinomannan to mice resulted in a dose-dependent reduction in splenic and lung bacterial loads and promoted survival . Interestingly, most of the protective antibodies have an IgG isotype. Our data regarding the serum antibody responses of healthy and Mtb-infected adults to the two Mtb CPs provide information that may be useful in evaluating the effect of CP-based vaccines. Clearly, the pre-existing anti-CP antibody titers should be assessed in volunteers prior to immunization with TB CP vaccines. In initial studies of the safety and immunogenicity of these vaccines, volunteers should have low anti-CP antibody titers to preclude pre-existing antibody interference with the analysis of vaccine activity. Subsequent studies could evaluate whether volunteers with high pre-existing anti-CP antibody levels could be boosted by vaccination.