The role of IgA and IgG in Mycobacterium tuberculosis infection: A cross-sectional study in Ethiopia.

Introduction: Despite the high global prevalence of Mycobacterium tuberculosis (Mtb) infection in humans, most infected individuals achieve a stable immunological equilibrium, without showing clinical signs and symptoms of tuberculosis (TB). Although the role of antibodies in TB is assumed to be relatively small compared to cell-mediated immunity, their role in TB has been documented in a few recent studies.

Methods: In this cross-sectional study, we quantitated antibody responses to Mtb antigens, lipoarabinomannan (LAM), and heparin-binding hemagglutinin adhesin (HBHA) by determining antigen-specific immunoglobulin A(IgA) and G(IgG) secretion levels using enzyme-linked immunosorbent assay (ELISA) in serum and saliva of pulmonary TB patients (PTB), their household contacts (HHC), and community controls (CC) (determined by QuantiFERON TB Gold assay QFT- test result).

Results: The HBHA-specific IgA levels were significantly higher in both saliva and serum in HHC groups compared to PTB patients (P=0.013, P=0.023). Exposed contacts, who were QFT-negative had higher serum HBHA-specific IgA responses compared to PTB patients (P=0.04). QFT-negative HHC and QFT-positive CC showed higher HBHA and LAM-specific IgG responses (P=0.006, P=0.002, P=0.0009, P=0.006, respectively) than PTB patients. Generally, LAM and HBHA-specific IgA levels were significantly higher in saliva compared to serum (P<0.0001) in all study groups.

Conclusion: Overall, the observed higher levels of IgA and IgG in controls, and exposed but QFT-negative contacts suggest a correlation with, and perhaps a role for these antibodies in preventing the development of active TB. The findings highlighted the potential involvement of saliva IgA in the immune response to Mtb, underscoring the relevance of mucosal immunity in TB infection.