Specificity and diversity of antibodies to Mycobacterium tuberculosis arabinomannan

Abstract

Arabinomannan (AM) is a polysaccharide antigen of the mycobacterial capsule. However, it is uncertain whether AM constitutes an immunologically distinct fraction of Mycobacterium tuberculosis. In this study, we analyzed the repertoire and specificity of antibodies to AM by using AM-binding murine monoclonal antibodies (MAbs) and human serum samples. Murine MAbs were found to be diverse in their specificity to AM and cross-reactivity with other arabinose-containing mycobacterial polysaccharides, with MAb 9d8 binding exclusively to AM. Human antibodies to AM were detected in serum samples from patients with pulmonary tuberculosis (TB), as well as in those from healthy, purified protein derivative-negative controls, with significantly higher titers among patients. The binding of human antibodies to AM was inhibited by MAb 9d8 in three patients with TB but not in controls. MAb 5c11, which recognizes other mycobacterial arabinose-containing carbohydrates in addition to AM, inhibited the binding of serum samples from 75% of patients and 76% of controls. Analysis of human antibodies with murine MAbs to human V(H) determinants demonstrated diversity among antibodies to AM with qualitative and quantitative differences compared with antibodies to lipoarabinomannan. In summary, our study suggests that antibodies to AM are diverse and heterogeneous with respect to antigen recognition and V(H) determinant expression, with human serum samples containing different subsets of antibodies to AM with the specificities of AM-binding murine MAbs. One MAb and a subset of human antibodies bind AM specifically, suggesting that this polysaccharide is antigenically distinct and is expressed in human infection.

FIG. 1.

Competition ELISA between human antibodies from three individuals with TB and MAb 9d8. The ELISA configuration demonstrates the binding of TB patients' serum IgG to AM in the presence or absence of MAb 9d8 (A, patient B; B, patient C; C, patient E). The graph shows decreased binding of human antibodies to AM in the presence of MAb 9d8, and arrows indicate points of maximal inhibition.

FIG. 2.

Detection of V_H determinant expression among antibodies to AM (black areas) and LAM (grey areas) recognized by specific mouse anti-human MAbs to V_H determinants in human serum samples of patients with pulmonary TB and healthy, purified protein derivative (PPD)-negative individuals.