**5. Mechanisms of action**

nisms involved as well as for using this knowledge for the future development of new

Mouse models with deficiency in antibody production can provide useful information for understanding certain roles of antibodies in protection against mycobacterial infections. Rodríguez and colleagues reported that after immunization of IgA deficient (IgA-/-) and wild type mice by the intranasal route with the mycobacterial surface antigen PstS-1, IgA-/- mice were more susceptible to BCG infection than IgA+/+ mice [34]. Cytokine response analysis demonstrated reduction in the IFN-γ and TNF-α production in the lungs of IgA-/- as compared with IgA+/+ mice, suggesting that IgA may play a role in protection against mycobacterial infections in the respiratory tract. Furthermore, these authors demonstrated that immunized pIgR-/- mice were more susceptible to BCG infection than immunized wild-

In an attempt to elucidate whether antibody-mediated immunity has a special role in the defense against TB, different experiments with B cell knockout mice were performed by several authors. In 11016, Vordermeier and colleagues developed an infection model of TB in μ chain knockout Ig- mice. Organs from *M. tuberculosis* infected IgG- mice had three to eight fold elevated counts of viable bacilli compared with those from normal mice. This result suggested that B cells play a role in the containment of murine tuberculous infection [35]. In another study B cell KO mice and controls were infected by aerosol with *M. tuberculosis*. They were subse‐ quently given chemotherapy to destroy remaining bacilli and then re-challenged by aerosol exposure. There were not differences in the ability of animals to control this second infection, indicating that, in this low dose pulmonary infection model, any local production of antibodies

neither impeded nor enhanced the expression of specific acquired resistance [36].

bacterial burden in the lungs but not in the spleen or liver [38].

In another series of experiments the role of B cells was evaluated during early immune responses to infection with a clinical strain of *M. tuberculosis* (CDC 1561). In this study, despite comparable bacillary loads in the lungs, B cell KO mice had a less severe pulmonary granuloma formation and delayed dissemination of bacteria from lungs to peripheral organs. Additional analysis of lung cells demonstrated higher numbers of lymphocytes, particular‐ ly CD8+ T cells, macrophages, and neutrophils in wild-type and reconstituted mice as compared with B cell KO mice. These results demonstrate that less severe granuloma formation and delayed dissemination of mycobacteria found in B cell KO mice were dependent on B cells, (not antibodies, at least in this study) and were associated with modification of cellular infiltrate in the lungs [37]. This latter result differs from a study carried out by Maglione and colleagues in which B cell-/- mice demonstrated exacerbated immunopathology corresponding with enhanced pulmonary recruitment of neutrophils following aerosol challenge with *M. tuberculosis* Erdman strain [38]. Infected B cell-/- mice demonstrated increased production of IL-10 in the lungs, while IFN- γ, TNF-α, and IL-10R were not significantly different from those of wild type mice [38].. B cell-/- mice demonstrat‐ ed enhanced susceptibility to aerosol infection of 300 CFU of *M. tuberculosis* with elevated

immunotherapies for TB [71].

64 Tuberculosis - Current Issues in Diagnosis and Management

*4.2.5. Transgenic mice*

type mice [34].

The various effects of antibodies demonstrated in the studies analyzed, suggest that different mechanisms of action are involved in the effect of monoclonal and polyclonal antibodies on *M. tuberculosis*. Secretions found on mucosal surfaces contain significant levels of Igs, partic‐ ularly, IgA. IgA has both direct and indirect functional roles for combating infectious agents such as viruses and bacteria that cross the mucosal barrier [72]. Moreover, experimental evidence suggests that IgA associated with the pIgR may neutralize pathogens and antigens intracellularly during their transport from the basolateral to the apical zone of epithelial cells [73,74]. In addition, IgA may interact with Gal-3 (an intracellular binding β-galactosidase lectin), and interfere with the interaction of mycobacteria with the phagosomal membrane, resulting in the decrease of bacterial survival and replication in the phagosome [75].

Antibodies may be critical, during the extracellular phases of intracellular facultative patho‐ gens. They may act by interfering with adhesion, by neutralizing toxins and by activating complement. Moreover, antibodies may be able to penetrate recently infected cells, bind internalised pathogens, and enhance antigen processing (76). Antibodies may also play a crucial role in modulating the immune response by activating faster secretion of selected cytokines that in turn, contribute to more efficient and rapid Th1 response [76,77], increasing the efficacy of co-stimulatory signals, enhancing Antibody Dependent Cellular Cytotoxicity (ADCC) and the homing of immune cells to the lungs after the respiratory infection [10,78, 79, 80, 81, 82, 83].

Examples of relevant potential action mechanisms of antibodies against *M. tuberculosis* were discussed by Glatman-Freedman [40].
