Olfactomedin 4 marks a subset of neutrophils in mice.

Neutrophils are the most abundant immune cell of the innate immune system and participate in essential immune functions. Heterogeneity within neutrophils has been documented, but it is difficult to distinguish if these are altered activation states of a single population or separate subpopulations of neutrophils determined at the time of differentiation. Several groups have identified a subset of human neutrophils that express olfactomedin 4 (OLFM4) and increased OLFM4+ neutrophils during sepsis is correlated with worse outcome, suggesting these neutrophils or the OLFM4 they secrete may be pathogenic. We tested if mice could be used as a model to study OLFM4+ neutrophils. We found the OLFM4 expressing subset of neutrophils is conserved in mice. Depending on the strain, 7-35% of murine neutrophils express OLFM4 and expression is determined early in neutrophil differentiation. OLFM4+ neutrophils phagocytose and transmigrate with similar efficiency as OLFM4- neutrophils. Here we show that within neutrophil extracellular traps (NETs) OLFM4+ and OLFM4- neutrophils undergo NETosis and OLFM4 colocalizes. Finally, we generated an OLFM4 null mouse and show that these mice are protected from death when challenged with sepsis, providing further evidence that the OLFM4 expressing subpopulation of neutrophils, or the OLFM4 they secrete, may be pathogenic during overwhelming infection.