Abstract
Alveolar macrophages (AMs) play a crucial role in protecting the lungs from pathogens by inducing immunogenic cell death (ICD). However, the type of cell death that effectively induces protective immunity remains to be fully understood. In our investigation of the mechanisms regulating AM activation and lung immune responses, we found that AMs are highly susceptible to necroptosis, a form of ICD. Treatment with pan-caspase inhibitors, such as emricasan or benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethyl ketone (ZVAD-fmk), directly induced cell death in AMs, resulting in the release of interleukin (IL)-1α in the lungs. This phenomenon was not observed in mouse lung fibroblasts or bone marrow-derived macrophages, indicating a cell type-specific sensitivity. The process was mediated by receptor-interacting protein kinase (RIPK)1 and RIPK3, and notably occurred without any additional necroptosis triggers such as tumor necrosis factor (TNF) or second mitochondria-derived activator of caspase (SMAC) mimetics. Moreover, activation of the RIPK1-RIPK3 signaling pathway not only triggered necroptosis but also promoted IL-1α production and release. These responses were absent in AMs lacking functional RIPK1 kinase activity or deficient in RIPK3. Importantly, RIPK1/RIPK3-mediated cell death and IL-1α release were sufficient to trigger lung immune responses, as shown by increased antigen-specific IgG and IgA production, which was significantly decreased in mice deficient in necroptosis or lacking the IL-1 receptor. Taken together, these findings demonstrate that the pan-caspase inhibitor emricasan induces necroptotic cell death in AMs and may act as a promising AM-targeted adjuvant to enhance lung-specific acquired immunity.