<p><b>Background</b></p> <p>Diabetes mellitus is associated with increased tuberculosis risk and severity. We previously reported that tuberculosis susceptibility in diabetic mice results from a delay in innate immune response to inhaled <i>Mycobacterium tuberculosis</i>, leading to delayed adaptive immune priming and, consequently, a higher plateau lung bacterial burden and greater immune pathology.</p> <p><b>Methods</b></p> <p>We tested the capacity of alveolar macrophages from diabetic mice to phagocytose <i>M. tuberculosis</i> ex vivo and promote T-cell activation in vivo.</p> <p><b>Results</b></p> <p>Alveolar macrophages from diabetic mice had reduced expression of CD14 and macrophage receptor with collagenous structure (MARCO), which recognize the bacterial cell wall component trehalose 6,6′-dimycolate (TDM). Diabetic alveolar macrophages exhibited reduced phagocytosis of <i>M. tuberculosis</i> or TDM-coated latex beads. This alveolar macrophage phenotype was absent in peritoneal and bone marrow–derived macrophages. Transfer of infected alveolar macrophages from diabetic mice into nondiabetic recipients confirmed an intrinsic alveolar macrophage defect that hindered T-cell priming. The diabetic alveolar macrophage phenotype depended in part on expression of the receptor for advanced glycation end products.</p> <p><b>Conclusions</b></p> <p>Reduced MARCO and CD14 expression contributes to defective sentinel function of alveolar macrophages, promoting tuberculosis susceptibility in diabetic hosts at a critical early step in the immune response to aerosol infection.</p>