Absence of c-Maf and IL-10 enables type I IFN enhancement of innate responses to LPS in alveolar macrophages

Alveolar macrophages (AMs) are lung-resident myeloid cells and airway sentinels for inhaled pathogens and environmental particles. While AMs can be highly inflammatory in response to respiratory viruses, they do not mount proinflammatory responses to all airborne pathogens. For example, we previously showed that AMs fail to mount a robust proinflammatory response to Mycobacterium tuberculosis. Here, we address this discrepancy by investigating the capacity of murine AMs for direct innate immune sensing, using LPS as a model. Use of LPS-coated fluorescent beads enabled us to distinguish between directly exposed and bystander cells to measure transcriptional responses, by RNA-sequencing after cell sorting, and cytokine responses, by flow cytometry. We find that AMs have decreased proinflammatory responses to low-dose LPS compared to other macrophage types (bone marrow-derived macrophages, peritoneal macrophages), as measured by TNF, IL-6, Ifnb, and Ifit3. The reduced response to low-dose LPS correlates with minimal TLR4 and CD14 surface expression, despite sufficient internal expression of TLR4. We also find that AMs do not produce IL-10 in response to a variety of stimuli due to low expression of the transcription factor c-Maf, while exogenous c-Maf expression restores IL-10 production in AMs. Lastly, we show that lack of IL-10 enables type I IFN enhancement of AM responses to LPS. Overall, we demonstrate AMs have a cell-intrinsic hyporesponsiveness to LPS, which makes them uniquely tolerant to low-dose exposure. Regulation of AM innate responses by distinct CD14, c-Maf, and IL-10 expression patterns has important implications for both respiratory infections and environmental airborne exposures.