Novel regulator Fpr2 controls immune homeostasis via CCL4-dependent neutrophils

Polymorphonuclear leukocytes (PMNs) in Fpr2-deficient mice have significantly impaired bactericidal activity, yet the precise mechanism remains elusive. Our research unveiled the cellular heterogeneity of peripheral blood PMNs through single-cell RNA-sequencing (scRNA-seq) and identified two new neutrophil clusters related to bacterial infection, including a sterilization cluster named Neu_CSTA and a regulation cluster named Neu_CCL4. The lack of Fpr2 leads to an immune regulation imbalance of Neu_CCL4 on other clusters in uninfected conditions, resulting in persistent instability of PMNs in peripheral blood, elevated inflammatory factors, and increased NOX2 expression. Furthermore, our findings corroborate that the regulatory function of Neu_CCL4 on PMNs immune balance is closely tied to its high expression of the anti-inflammatory ligand Anxa1. Regulon analysis inferred by scRNA-seq and ChIP-seq data suggested that a specific transcription factor Elf4, could modulate the expression of Anxa1, thereby facilitating its regulatory impact on other clusters. In the infection state, there was a noted increase in intercellular interactions across different clusters, leading to a general improvement in the bactericidal activity of PMNs. In summary, our research revealed distinct functional cellular sub-populations of PMNs, discovered the potential regulatory mechanisms, and elucidated the essential roles of Fpr2 in their ability to resist infection.