Does IgA play a role in protection against pulmonary tuberculosis?

Abstract: More than a century after the identification of the tubercle bacillus and the first attempts at vaccination, tuberculosis (TB) still remains one of the world’s most serious infectious diseases. TB is typically a disease of the lung, which serves both as port of entry and as the major site of disease manifestation. The currently used vaccine, Mycobacterium bovis bacillus Calmette-Guérin (BCG), is administered parentally and induces a systemic immune response. However, it fails to protect against pulmonary TB, thereby raising the question whether vaccination targeting the mucosal immunity in the lungs could be favourable.The respiratory mucosal surfaces represent the first line of defence against a multitude of pathogens. Secretory IgA (sIgA) in mucosal secretions has an important function by blocking entrance of pathogenic organisms and preventing infections. Yet, another role for IgA in protection against intracellular pathogens has lately been appreciated, when sIgA was demonstrated to neutralize viruses intracellulary. We aimed to investigate the relevance of sIgA in protection against mycobacterial infections using mice deficient for IgA and the polymeric Ig receptor. Mice were immunized intranasally with a mycobacterial antigen which elicited, in wild-type mice, a strong IgA response in mucosal secretions in the respiratory tract. Gene-targeted mice failed to induce the same response and more importantly, were more susceptible to mycobacterial infections in the respiratory tract, as demonstrated by higher bacterial loads in the lungs than wild-type mice. Analysis of immune responses after infection revealed reduced production of proinflammatory, and protective, factors such as IFN-? and TNF-? in the lungs of deficient mice, which was in concordance with the higher bacterial burden seen in the lungs of these mice. The mechanisms explaining the defective proinflammatory responses in the lungs of deficient mice are not clear but might involve impaired signalling through Fc? receptors, or homologous receptors, which could lead to inadequate activation of pulmonary macrophages. This could subsequently result in suboptimal induction and production of cytokines and chemokines important for attraction and migration of cells to sites of infection in the lungs.Our results demonstrate a role for IgA in protection against mycobacterial infection in the respiratory tract by blocking the entrance of the mycobacterium into the lungs, and/or by modulating the locally induced proinflammatory immune responses.