Non-hematopoietic tryptophan metabolism is a driver of ineffective T cell responses during secondary pulmonary bacterial infection.

Pulmonary infections often fail to produce long-lived immune memory and the underlying mechanism(s) for this are unclear. Given the complex interactions between cells within the lung, we predicted intrinsic and extrinsic factors contribute to development of poor memory immune responses. To identify these factors, we used a multiomics approach to determine host-driven responses that undermine or support development of effective immune responses in two mouse models of pulmonary bacterial infections. Single cell RNA analysis and spatial imaging of the lung revealed that, in contrast to Bordetella pertussis driven immunity, subpar responses following Francisella tularensis infection were associated with the inability of T cells to readily proliferate upon re-challenge and absence of formation of iBALT. Further, we also identified that these features were partially a consequence of IFN-γ driven reprogramming of endothelial cells resulting in expression of IDO1 and dysregulated tryptophan metabolism. Interestingly, IDO1 expression and imbalanced tryptophan persisted even after clearance of the primary infection. The importance of expression of IDO1 was confirmed using IDO1 knock out mice. Specifically, these animals could withstand higher doses of the initial infection and developed significantly larger pools of functional T cells compared to wild type controls. Together, these results demonstrate critical crosstalk among cells in the lung that influences spatial organization of immune cells which affects the ability to develop effective memory immune responses against secondary bacterial infection. Our data also underscores the challenge of utilizing a live vaccine strategy against tularemia and the necessity for identifying novel, acellular vaccine candidates.