Steady-state immune surveillance of the nasal mucosa by neutrophils

Introduction/Rationale: The nasal cavity serves as a critical barrier, filtering airborne pathogens to prevent them from reaching the lungs. Immune cells in the nasal mucosa neutralize these pathogens, protecting against local infection. However, the composition, origins, and functions of these immune populations remain largely unexplored. Our research identified a prominent population of extravascular neutrophils (EVNs) in the nasal mucosa during homeostasis.

Methods: The nasal mucosa was accessed through a burr hole in the skull and imaged by two-photon excitation to visualize EVN dynamics under steady-state conditions. Neutrophil subsets were identified by fluorescent surface labeling. Parabiosis determined tissue residency, and single-cell and bulk RNA sequencing identified transcriptional differences between subsets.

Results: EVNs were categorized into three subsets: nN1, nN2, and nN3. We found that nN1 neutrophils reside in bone marrow adjacent to the nasal cavity and likely migrate directly into the surrounding mucosa via bone conduits. By contrast, most nN2 neutrophils access the nasal mucosa from the blood stream and then differentiate into nN3 neutrophils within 5–6 days. Functionally, nN2 neutrophils express genes characteristic of conventional neutrophils and are the primary subset responsible for phagocytosis of bacterial and fungal pathogens. Meanwhile, nN3 neutrophils progressively downregulate neutrophil specific genes and upregulate genes associated with antigen-presenting cells. Indeed, nN3 neutrophils constantly acquire material from surrounding cells under steady-state conditions, and effectively cross-present exogenous antigens to CD8+ T cells in vitro.

Conclusion: The nasal mucosa harbors a diverse population of EVNs with distinct origins, phenotypes, and specialized functions. These findings highlight the complexity of the nasal immune landscape and shed light on its essential and unique role in host defense.