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The mechanism of synergy between cell‐mediated and humoral immune responses in protection against acute respiratory pathogens

To maintain the sterility of the lungs, cell‐mediated and humoral responses (CMI and HI) co‐operate to rapidly eliminate pathogens. The importance of interactions between the two arms of immune responses for protection against intracellular pathogens is well appreciated but not understood. Therefore, the objective of this study was to elucidate the mechanism of synergy between CMI and HI in enhancing microbicidal activities of phagocytes. Using a mouse model of respiratory tularemia, we observed that specific antibodies in concert with IFN‐γ rapidly eliminated intracellular Francisella tularensis. Similarly, IFN‐γ‐activated, but not resting alveolar macrophages (AMs) mediated rapid killing of opsonized bacteria. Further analysis revealed that both FcγR and IFN‐γR dependent signaling events were necessary and led to enhanced RNS and ROS generation, which were also essential for bacterial killing. In addition, differential trafficking of opsonized bacteria within AMs was found to be necessary. Studies to determine the compartment where opsonized bacteria are killed by activated AMs and the cellular events that result in the formation of these compartments are currently in progress. These results establish a potential mechanism to explain the synergy between CMI and HI and provide an opportunity for intervention to augment the efficacy of antibodies and vaccines.