Neutrophil Diversity in Health and Disease

Abstract

New evidence has challenged the outdated dogma that neutrophils are a homogeneous population of short-lived cells. Although neutrophil subpopulations with distinct functions have been reported under homeostatic and pathological conditions, a full understanding of neutrophil heterogeneity and plasticity is currently lacking. We review here current knowledge of neutrophil heterogeneity and diversity, highlighting the need for deep genomic, phenotypic, and functional profiling of the identified neutrophil subpopulations to determine whether these cells truly represent bona fide novel neutrophil subsets. We suggest that progress in understanding neutrophil heterogeneity will allow the identification of clinically relevant neutrophil subpopulations that may be used in the diagnosis of specific diseases and lead to the development of new therapeutic approaches.

Keywords: immune regulation; neutrophils; subpopulations.

Figure 1

Neutrophil Populations in Homeostatic Conditions. Humans: neutrophils mature in the bone marrow (BM) from committed myeloid precursors that, through subsequent differentiation stages (here defined as ‘immature neutrophils’), differentiate into segmented mature neutrophils . Under resting conditions, only terminally differentiated neutrophils are released into the circulation and recovered as normal-density neutrophils (NDNs) (Box 1) . Current evidence suggests that different neutrophil subpopulations, defined by the indicated markers, are present in human blood under homeostatic conditions , , , , , , . Under resting conditions, neutrophils are also marginated in pools within the lungs, spleen, and liver, but the phenotypes and functions of these cells remain poorly defined , , . A unique population of human neutrophils was found in the marginal zone (MZ) of the spleen, and these were termed B cell helper neutrophils (N_BH cells) owing to their robust B cell-activating properties . Mice: mouse neutrophil development within the BM comprises three distinct subpopulations, namely preNeu (proliferative neutrophil precursor), immature, and mature neutrophils . As observed in humans, based on differential surface marker expression, circulating mouse neutrophils exhibit heterogeneity that associates with distinct effector activities , , , , , . In mice as in humans, neutrophils not only marginate in the spleen, liver, and lung vasculature but also transit and reside in other organs such as muscle, skin, lymph nodes, and intestine . In the mouse spleen, three neutrophil subpopulations localized in the red pulp or MZ participate in emergency granulopoiesis (Ly6G^int) and pneumococcal clearance (Ly6G^high) , and can regulate antibody production in MZ B cells (NB_H, as observed in humans) .

Figure I

Low-Density Neutrophils (LDNs). Graphical representation of the main LDN populations reported so far: (i) immunosuppressive polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs); (ii) proinflammatory low-density granulocytes (LDGs). Whether autologous normal-density neutrophils (NDNs) from the same diseased individual also display either pro- or anti-inflammatory properties remains poorly defined.

Figure 2

Neutrophil Populations in Infections/Inflammation. Humans: density gradient centrifugation of blood from patients with acute and chronic inflammatory conditions, including sepsis or infection, leads to the detection of heterogeneous populations of low-density neutrophils (LDNs) within the mononuclear cell fraction, in addition to normal-density neutrophils (NDNs) (Box 1) . Depending on the study, some of these heterogeneous LDNs have been shown to display immunosuppressive properties – termed LDNs/polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) – whereas other LDNs have been shown to display either proinflammatory or so far undefined functions , . In addition, populations of mature activated suppressive neutrophils can be present in both the LDN and NDN fractions , , or can be obtained by red blood cell lysis without performing density gradient centrifugation . Mice: as observed in humans, pathogen infections of different types may induce the appearance of immature PMN-MDSCs with immunosuppressive functions in the circulation (LDNs/PMN-MDSCs) and infected tissues (PMN-MDSCs) , , , , . In Staphylococcus aureus or Candida albicans models of infection, two subpopulations of neutrophils with opposite functions (proinflammatory, CD11b⁻CD49d⁺IL-12⁺; anti-inflammatory, CD11b⁺CD49d⁻IL-10⁺) have been identified . Neutrophils with antigen-presenting activities (APC-like neutrophils) are generated during infection. After encountering pathogens in the tissue, a subpopulation of CD11b^highCD62L^lowC-X-C chemokine receptor type 2 (CXCR2)^low neutrophils can migrate to the lymph node to promote adaptive immunity . Populations of APC-like neutrophil–DC hybrids have also been identified in different mouse models of infection , . Moreover, after infection or sterile injury, CD54⁺ neutrophils can retro-transmigrate from the affected tissue to the bloodstream, exerting deleterious actions on secondary organs but also exhibiting an enhanced ability to fight pathogens . Abbreviations: APC, antigen-presenting cell; DC, dendritic cell.

Figure 3

Neutrophil Populations in Autoimmune Diseases. Humans: proinflammatory low-density neutrophils (LDNs) or ‘low-density granulocytes’ (LDGs) (Box 1) have been found in patients with autoimmune diseases, including systemic lupus erythematosus (SLE), psoriasis, chronic granulomatous disease, anti-neutrophil cytoplasmic autoantibody (ANCA)-associated vasculitis, rheumatoid arthritis (RA), juvenile idiopathic arthritis, and pyogenic arthritis-pyoderma gangrenosum-acne syndrome , , , , . However, not all studies tested/reported the proinflammatory properties of LDNs from autoimmune patients. Furthermore, whether and to what degree normal-density neutrophils (NDNs) from autoimmune patients display proinflammatory properties similar to those described for LDNs (e.g., by generating neutrophil extracellular traps, NETosis) is still unclear.

Figure 4

Neutrophil Populations in Cancer. Humans: suppressive low-density neutrophils (LDNs)/polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) (Box 1) are the main neutrophil subpopulations in the blood of cancer patients , , . However, in some studies LDNs have been defined as PMN-MDSCs without testing their inhibitory actions on T cell functions, or have not even been reported to be suppressive , . Furthermore, whether and to what degree circulating normal-density neutrophils (NDNs) from cancer patients also display suppressive properties remains poorly defined. Depending on the study, human tumor-associated neutrophils (TANs) have been reported to either promote , or inhibit , T cell functions. A population of human leukocyte antigen DR isotype (HLA-DR)⁺ antigen-presenting cell (APC)-like hybrid TANs supporting antitumoral T cell responses has been isolated from the tumor tissue of early-stage lung cancer patients . Mice: populations of immature neutrophils displaying immunosuppressive properties have been reported in the blood (LDNs/PMN-MDSCs), spleen, bone marrow (BM) and tumor tissue of tumor-bearing mice (Box 2) , . In addition, two subtypes of mature circulating neutrophils have been proposed to be present in tumor-bearing mice, namely NDNs (displaying antitumor properties) and LDNs (displaying immunosuppressive and protumor properties) . Similarly, N1 TANs displaying antitumor properties (e.g., promotion of antitumor T cell responses and direct tumor cytotoxicity) or N2 TANs (both mature and immature cells) displaying protumor properties (e.g., immunosuppression, promotion of tumor cell proliferation/survival, promotion of angiogenesis and tumor cell extravasation into distant metastatic tissue) have also been reported within tumor tissues .