Single-cell spatiotemporal analysis of the lungs reveals Slamf9+ macrophages involved in viral clearance and inflammation resolution

How the lung achieves immune homeostasis after a pulmonary infection is not fully understood. Here, we analyzed the spatiotemporal changes in the lungs over a 2-week natural recovery from severe pneumonia in a Syrian hamster model of SARS-CoV-2 infection. We find that SARS-CoV-2 infects multiple cell types and causes massive cell death at the early stage, including alveolar macrophages. We identify a group of monocyte-derived Slamf9(+) macrophages, which are induced after SARS-CoV-2 infection and resistant to impairment caused by SARS-CoV-2. Slamf9(+) macrophages contain SARS-CoV-2, recruit and interact with Isg12(+)Cst7(+) neutrophils to clear the viruses. After viral clearance, Slamf9(+) macrophages differentiate into Trem2(+) and Fbp1(+) macrophages, contributing to inflammation resolution at the late stage, and finally replenish alveolar macrophages. These findings are validated in a SARS-CoV-2-infected hACE2 mouse model and confirmed with publicly available human autopsy single-cell RNA-seq data, demonstrating the potential role of Slamf9+ macrophages and their coordination with neutrophils in post-injury tissue repair and inflammation resolution.