The journey of neutropoiesis: how complex landscapes in bone marrow guide continuous neutrophil lineage determination

The importance of morphological changes in neutrophils in the diagnosis of bacterial infections in dogs with confirmed urinary tract infections in a Veterinary Care Service, Rio de Janeiro, Brazil

Neutrophils (PMNs) are cellular markers used for diagnosing inflammation and/or infections. In this study, the objective was to highlight the importance of recording the toxic morphological alterations of the PMNs as markers of infection in 10 cases, positive bacterial isolation by culture due to dysuria, hematuria and/or fetid urine, as manifestations of urinary tract disease. Smear observations were performed by immersion for counting and morphological evaluations of 3,000 leukocytes in smears and in leukocyte concentrate. One (10.0%) of the dogs had leukocytosis, and two (20.0%) dogs had leukopenia. All animals showed toxic PMNs with positive bacterial culture. None of the cases in the study showed any quantitative alterations in PMNs such as: neutropenia or neutrophilia, where 100% had nuclear displacement of the regenerative type of PMNs to the left. 100% cases had toxic morphological changes: 90.0% had PMNs with toxic granulations, 80.0% had giant rod neutrophils, 70.0% had target PMNs, in 50.0% of those with vacuolation in the cytoplasm, in 40.0% of the animals, the presence of giant PMNs, 10.0% with Döhle bodies, and another animal 10.0% with karyorrhexis. All case studies had at least one association of two types of toxic changes. Toxic morphological alterations observed in PMNs through cystoscopy proved to be more reliable and sensitive in evidencing the diagnosis of infections than the quantitative alterations of absolute values of total leukocytes; therefore, they were essential in the laboratory diagnosis by blood count in the course of infections in dogs.

Inflammatory abrasion of hematopoietic stem cells: a candidate clue for the post-CAR-T hematotoxicity?

Chimeric antigen receptor T-cell (CAR-T) therapy has shown remarkable effects in treating various hematological malignancies. However, hematotoxicity, specifically neutropenia, thrombocytopenia, and anemia, poses a serious threat to patient prognosis and remains a less focused adverse effect of CAR-T therapy. The mechanism underlying lasting or recurring late-phase hematotoxicity, long after the influence of lymphodepletion therapy and cytokine release syndrome (CRS), remains elusive. In this review, we summarize the current clinical studies on CAR-T late hematotoxicity to clarify its definition, incidence, characteristics, risk factors, and interventions. Owing to the effectiveness of transfusing hematopoietic stem cells (HSCs) in rescuing severe CAR-T late hematotoxicity and the unignorable role of inflammation in CAR-T therapy, this review also discusses possible mechanisms of the harmful influence of inflammation on HSCs, including inflammatory abrasion of the number and the function of HSCs. We also discuss chronic and acute inflammation. Cytokines, cellular immunity, and niche factors likely to be disturbed in CAR-T therapy are highlighted factors with possible contributions to post-CAR-T hematotoxicity.

Current knowledge on the early stages of human neutropoiesis

Polymorphonuclear neutrophils are no longer considered as a homogeneous population of terminally differentiated and short-lived cells that belong to the innate immune system only. In fact, data from the past decades have uncovered that neutrophils exhibit large phenotypic heterogeneity and functional versatility that render them more plastic than previously thought. Hence, their precise role as effector cells in inflammation, in immune response and in other pathophysiological processes, including tumors, needs to be better evaluated. In such a complex scenario, common knowledge of the differentiation of neutrophils in bone marrow refers to lineage precursors, starting from the still poorly defined myeloblasts, and proceeding sequentially to promyelocytes, myelocytes, metamyelocytes, band cells, segmented neutrophils, and mature neutrophils, with each progenitor stage being more mature and better characterized. Thanks to the development and utilization of cutting-edge technologies, novel information about neutrophil precursors at stages earlier than the promyelocytes, hence closer to the hematopoietic stem cells, is emerging. Accordingly, this review discusses the main findings related to the very early precursors of human neutrophils and provides our perspectives on human neutropoiesis.

Visualization of the inflammatory response to injury by neutrophil phenotype categories

Purpose

The risk of infectious complications after trauma is determined by the amount of injury-related tissue damage and the resulting inflammatory response. Recently, it became possible to measure the neutrophil phenotype in a point-of-care setting. The primary goal of this study was to investigate if immunophenotype categories based on visual recognition of neutrophil subsets are applicable to interpret the inflammatory response to trauma. The secondary goal was to correlate these immunophenotype categories with patient characteristics, injury severity and risk of complications.

Methods

A cohort study was conducted with patients presented at a level 1 trauma center with injuries of any severity, who routinely underwent neutrophil phenotyping. Data generated by automated point-of-care flow cytometry were prospectively gathered. Neutrophil phenotypes categories were defined by visual assessment of two-dimensional CD16/CD62L dot plots. All patients were categorized in one of the immunophenotype categories. Thereafter, the categories were validated by multidimensional analysis of neutrophil populations, using FlowSOM. All clinical parameters and endpoints were extracted from the trauma registry.

Results

The study population consisted of 380 patients. Seven distinct immunophenotype Categories (0–6) were defined, that consisted of different neutrophil populations as validated by FlowSOM. Injury severity scores and risk of infectious complications increased with ascending immunophenotype Categories 3–6. Injury severity was similarly low in Categories 0–2.

Conclusion

The distribution of neutrophil subsets that were described in phenotype categories is easily recognizable for clinicians at the bedside. Even more, multidimensional analysis demonstrated these categories to be distinct subsets of neutrophils. Identification of trauma patients at risk for infectious complications by monitoring the immunophenotype category is a further improvement of personalized and point-of-care decision-making in trauma care.

Human neutrophil kinetics: a call to revisit old evidence

The half-life of human neutrophils is still controversial, with estimates ranging from 7-9 h to 3.75 days. This debate should be settled to understand neutrophil production in the bone marrow (BM) and the potential and limitations of emergency neutropoiesis following infection or trauma. Furthermore, cellular lifespan greatly influences the potential effect(s) neutrophils have on the adaptive immune response. We posit that blood neutrophils are in exchange with different tissues, but particularly the BM, as it contains the largest pool of mature neutrophils. Furthermore, we propose that the oldest neutrophils are the first to die following a so-called conveyor belt model. These guiding principles shed new light on our interpretation of existing neutrophil lifespan data and offer recommendations for future research.

Longitudinal assessment of the inflammatory response: The next step in personalized medicine after severe trauma

Infections in trauma patients are an increasing and substantial cause of morbidity, contributing to a mortality rate of 5-8% after trauma. With increased early survival rates, up to 30-50% of multitrauma patients develop an infectious complication. Trauma leads to a complex inflammatory cascade, in which neutrophils play a key role. Understanding the functions and characteristics of these cells is important for the understanding of their involvement in the development of infectious complications. Recently, analysis of neutrophil phenotype and function as complex biomarkers, has become accessible for point-of-care decision making after trauma. There is an intriguing relation between the neutrophil functional phenotype on admission, and the clinical course (e.g., infectious complications) of trauma patients. Potential neutrophil based cellular diagnostics include subsets based on neutrophil receptor expression, responsiveness of neutrophils to formyl-peptides and FcγRI (CD64) expression representing the infectious state of a patient. It is now possible to recognize patients at risk for infectious complications when presented at the trauma bay. These patients display increased numbers of neutrophil subsets, decreased responsiveness to fMLF and/or increased CD64 expression. The next step is to measure these biomarkers over time in trauma patients at risk for infectious complications, to guide decision making regarding timing and extent of surgery and administration of (preventive) antibiotics.