Specificity and Diversity of Antibodies to Mycobacterium tuberculosis Arabinomannan

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 arabinosecontaining 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 VH 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 VH 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. Tuberculosis (TB) is a leading cause of mortality worldwide, and it is estimated that 1.86 billion people are infected with Mycobacterium tuberculosis (13). Contributing factors to this severe health problem are the human immunodeficiency virus epidemic, the long and complicated course of treatment, drug resistance, and a lack of efficient diagnostic and preventive modalities. In addition, we lack a full understanding of the immunopathogenesis of TB. Our group is interested in the M. tuberculosis carbohydrate arabinomannan (AM), which is thought to be a component of the mycobacterial capsule. This interest arose from the generation of a monoclonal antibody (MAb), 9d8, that prolonged the survival of mice infected with M. tuberculosis (36) and binds AM. These findings suggested