Lost in neutrophil heterogeneity? CD10!

In this issue of Blood, Marini and coworkers identified CD10 as a cell surface marker that distinguishes T-cell suppressive from T-cell stimulatory neutrophils in the peripheral blood,¹  a finding that has major implications for our understanding of how neutrophils modulate the adaptive immune system in cancer, autoimmune diseases, and chronic inflammation in general.

Although traditionally regarded as a short-lived, uniform, and functionally-restricted immune cell population, emerging evidence indicates that neutrophils feature a plasticity and heterogeneity that enables them to adapt and respond to disease situations in a flexible and differential manner.^2,3  Neutrophils isolated from the peripheral blood can be classified based on their density after density-gradient centrifugations into high-density neutrophils (HDN) and low-density neutrophils (LDN), with HDN corresponding to conventional and LDN to a distinct less defined phenotype. LDN express CD66b and CD15 at their cell surface and accumulate in the peripheral blood from patients with different pathologic and nonpathologic settings, particularly cancer, autoimmune and autoinflammatory diseases, infections, lung diseases, graft-versus-host disease, pregnancy, and in individuals receiving granulocyte colony-stimulating factor (G-CSF) for stem cell mobilization. Because LDN were found to suppress T-cell proliferation, these cells were referred to as granulocytic/neutrophilic myeloid-derived suppressor cells (MDSCs). Unlike conventional neutrophils, MDSCs are believed to be a product of disease-induced altered myelopoiesis, which results in myeloexpansion and pathologic activation of these cells.⁴  However, the phenotypic characterization and definition of LDN as T-cell–suppressive neutrophilic MDSCs remains complex and challenging, because (1) LDN also showed proinflammatory characteristics in patients with certain autoimmune conditions, and (2) immunosuppressive neutrophil activities were also observed in the HDN fraction and with nonseparated leukocytes.⁵  Thus, the precise identification of neutrophilic MDSCs within the LDN population is difficult and controversial,⁶  making it imperative to define phenotypic or functional markers to discriminate these cell populations more precisely. Recently, recommendations for MDSC nomenclature and characterization standards were published, highlighting that the capacity to suppress T-cell function should become a key and defining feature of these cells.⁷  This led to the study by Marini and coworkers, who observed that neutrophil populations in G-CSF–mobilized donors are heterogeneously composed of immature and mature neutrophils, and that this distinction might be related to their potential to inhibit T-cell proliferation.

Marini et al analyzed the peripheral blood of stem cell donors receiving recombinant G-CSF. After density gradient centrifugation, the authors identified LDN composed of both immature and mature neutrophils, with the latter showing patient-specific degrees of activation. This observation is reminiscent of CD66b⁺ neutrophilic MDSCs, which have been identified in the peripheral blood of patients with cancer and similarly contain a mixture of immature and mature cells.^8,9  Importantly, Marini et al identified CD10 as a surface marker that distinguished mature from immature neutrophils in both CD66b⁺ LDN and HDN. Of note, CD11b and CD16 are also commonly used to determine the maturation status of neutrophils, but need to be used in conjunction with analysis of nuclear morphology because these surface receptors can be downregulated and shed upon neutrophil activation. In the study of Marini and coworkers, CD10 expression was found to be rather stable and reflected the T-cell–suppressive potential of neutrophils, whereas CD10^– LDN promoted T-cell activities. Mechanistically, CD10-expressing neutrophils inhibited T-cell proliferation via a CD18-mediated contact-dependent release of arginase 1. Finally, CD10 was found to serve as a useful surface marker to distinguish immature from mature neutrophils also in patients with cancer or systemic lupus erythematosus.

Overall, the work of Marini and coworkers sheds light on the emerging concept of neutrophil heterogeneity. However, because this is the first comprehensive study proposing CD10 as a stratification marker for maturation state and suppressive potential of neutrophils, further investigations in independent patient cohorts are required to validate and extend these findings—especially in cancer, where a pathophysiologic role for suppressive neutrophils has been proposed.^8,9  The findings of Marini and coworkers are also of particular importance because they may help to explain previous findings on T-cell–suppressive effects in neutrophils defined by surface markers and segmentation status instead of by density.⁵ 

Several aspects arising from this study require further investigation: (1) beyond G-CSF–treated patients, it remains to be defined how useful CD10 positivity is to characterize immunosuppressive neutrophils in different malignant and nonmalignant pathologies; (2) Marini and coworkers studied peripheral blood–derived neutrophils only and, accordingly, it remains to be defined whether CD10 also distinguishes neutrophil subtypes within tissue; (3) in contrast to the study by Marini et al, where mature neutrophils were T-cell–suppressive and immature counterparts T-cell–stimulatory, Solito et al reported previously that an immature promyelocytic-like population was responsible for the immune suppression mediated by MDSCs.¹⁰  The reasons underlying this apparent discrepancy might be a result of the cellular systems used and remain to be dissected in future investigations.

Taken together, the phenotype of suppressive neutrophils seems to be more complex than previously thought and more closely related to the maturation state of these cells. Cell density by itself seems insufficient to define suppressive neutrophils, pointing to CD10 as a novel surface marker, which could be useful in this context. Because neutrophils rapidly change their surface marker profile and effector functions according to the surrounding microenvironment, it will be a challenge to define the value of CD10 as a biomarker for suppressive neutrophils across different diseases, tissues, and treatments, to acquire a better understanding of the modulatory potential of neutrophils in the complex interplay of innate and adaptive immunity.