Phenotypically distinct neutrophils patrol uninfected human and mouse lymph nodes

Significance The classical view of neutrophils is as circulating phagocytes that are recruited to tissues following infection or injury. Here we show that neutrophils were present in mouse and human lymph nodes in the absence of perturbation. Lymph node neutrophils were phenotypically distinct, with increased expression of major histocompatibility complex II, and predominantly localized to the interfollicular zone, where CD4 T lymphocytes are activated. Neutrophils trafficked into lymph nodes via blood and lymphatic vessels, and were capable of rapidly carrying systemically acquired, IgG antibody-opsonized cargo to lymph nodes. These data support a novel role for neutrophils in homeostatic immune surveillance, sampling circulating antigens and delivering them to lymph nodes, with the potential to activate adaptive immunity. Neutrophils play a key role in innate immunity. As the dominant circulating phagocyte, they are rapidly recruited from the bloodstream to sites of infection or injury to internalize and destroy microbes. More recently, neutrophils have been identified in uninfected organs, challenging the classical view of their function. Here we show that neutrophils were present in lymph nodes (LNs) in homeostasis. Using flow cytometry and confocal imaging, we identified neutrophils within LNs in naive, unchallenged mice, including LNs draining the skin, lungs, and gastrointestinal tract. Neutrophils were enriched within specific anatomical regions, in the interfollicular zone, a site of T cell activation. Intravital two-photon microscopy demonstrated that LN neutrophils were motile, trafficked into LNs from both blood and tissues via high endothelial venules and afferent lymphatics, respectively, and formed interactions with dendritic cells in LNs. Murine and human LN neutrophils had a distinct phenotype compared with circulating neutrophils, with higher major histocompatibility complex II (MHCII) expression, suggesting a potential role in CD4 T cell activation. Upon ex vivo stimulation with IgG immune complex (IC), neutrophils up-regulated expression of MHCII and costimulatory molecules and increased T cell activation. In vivo, neutrophils were capable of delivering circulating IC to LNs, suggesting a broader functional remit. Overall, our data challenge the perception that neutrophil patrol is limited to the circulation in homeostasis, adding LNs to their routine surveillance territory.