Innate Immune Memory in Lung Alveolar Macrophages Promotes Resilience and a Pro-Resolving Phenotype

Abstract Immune memory has been well-established in adaptive immunity but recent studies suggest a key for memory in innate immunity. However, the precise mechanisms and phenotypes of innate immune memory in macrophages remain poorly understood. Here, we studied the memory responses of lung alveolar macrophages (AMs) exposed consecutively (2 exposures, one week apart) intratracheally to the bacterial endotoxin lipopolysaccharide (LPS). We observed a 60%–80% loss of AMs following the initial LPS exposure, consistent with the known cell death of resident AMs during severe inflammatory injury followed by gradual replenishment. Importantly, AM loss following the second LPS injury was markedly attenuated. Lineage tracing using a CX3CR1-monocyte reporter showed no increase in monocyte-derived AMs, thus suggesting that the higher AM numbers after the second LPS exposure were due to greater resilience of AMs and not due to rapid replenishment by circulating monocytes. RNA-Seq analysis of showed higher expression of pro-survival genes and anti-inflammatory cytokines such as IL-10 in memory AMs (post 1 week LPS) than in naïve AMs. Ex vivo and in vivo functional assay showed that these memory AMs demonstrated enhanced efferocytosis of cellular debris, consistent with a pro-resolving phenotype. We also found upregulation of the transcription factor KLF4 which promotes cell survival as well as pro-resolving macrophage polarization. Our findings suggest that alveolar macrophages are reprogrammed by an initial inflammatory exposure to increase their resilience to pro-apoptotic stimuli and to promote resolution of inflammation. Such a memory mechanism could be leveraged therapeutically in severe inflammatory diseases.