Human neutrophils in the saga of cellular heterogeneity: insights and open questions

Recent findings have uncovered novel fascinating aspects of the biology of neutrophils, which ultimately attribute to these cells a broader role in inflammation and immunity. One aspect that is currently under intensive investigation is the notion of neutrophil 'heterogeneity'. Studies examining neutrophils in a variety of acute and chronic inflammatory conditions report, in fact, the recovery of CD66b(+) cells displaying neutrophil-like morphology at different degrees of maturation/activation, able to exert either immunosuppressive or proinflammatory properties. These heterogeneous populations of mature and immature neutrophils are indicated with a variety of names, including 'low density neutrophils (LDNs)', 'low density granulocytes (LDGs)', 'granulocytic-myeloid derived suppressor cells (G-MDSCs)', and immunosuppressive neutrophils. However, due to the lack of discrete markers that can unequivocally allow their specific identification and isolation, the precise phenotype and function of all these presumably novel, neutrophil-like, populations have not been correctly defined yet. Aim of this article is to summarize current knowledge on the mature and immature neutrophil populations described to date, featuring immunosuppressive or proinflammatory properties, often defined as 'subsets', as well as to critically discuss unresolved issues in the field.