Diabetes impairs IFNγ-dependent antibacterial defense in the lungs.

Diabetes mellitus is associated with an increased risk of pneumonia, often caused by so-called typical and atypical pathogens including Streptoccocus pneumoniae and Legionella pneumophila, respectively. Here, we employed a variety of mouse models to investigate how diabetes influences pulmonary antibacterial immunity. Following intranasal infection with S. pneumoniae or L. pneumophila, type 2 diabetic and prediabetic mice exhibited higher bacterial loads in their lungs compared to control animals. Single cell RNA sequencing, flow cytometry, and functional analyses revealed a compromised IFNγ production by natural killer cells in diabetic and prediabetic mice, which was associated with reduced IL-12 production by CD103 dendritic cells. Blocking IFNγ enhanced susceptibility of non-diabetic mice to L. pneumophila, while IFNγ treatment restored defense against this intracellular pathogen in diabetic animals. In contrast, IFNγ treatment did not increase resistance of diabetic mice to S. pneumoniae, suggesting that impaired IFNγ production is not the sole mechanism underlying the heightened susceptibility of these animals to pneumococcal infection. Thus, our findings uncover a mechanism that could help to explain how type 2 diabetes predisposes to pneumonia. We establish proof of concept for host-directed treatment strategies to reinforce compromised IFNγ-mediated antibacterial defense against atypical lung pathogens.