Innate Immune Training of Granulopoiesis Promotes Anti-tumor Activity.

Trained innate immunity, induced via modulation of mature myeloid cells or their bone marrow progenitors, mediates sustained increased responsiveness to secondary challenges. Here, we investigated whether anti-tumor immunity can be enhanced through induction of trained immunity. Pre-treatment of mice with ß-glucan, a fungal-derived prototypical agonist of trained immunity, resulted in diminished tumor growth. The anti-tumor effect of ß-glucan-induced trained immunity was associated with transcriptomic and epigenetic rewiring of granulopoiesis and neutrophil reprogramming toward an anti-tumor phenotype; this process required type I interferon signaling irrespective of adaptive immunity in the host. Adoptive transfer of neutrophils from ß-glucan-trained mice to naive recipients suppressed tumor growth in the latter in a ROS-dependent manner. Moreover, the anti-tumor effect of ß-glucan-induced trained granulopoiesis was transmissible by bone marrow transplantation to recipient naive mice. Our findings identify a novel and therapeutically relevant anti-tumor facet of trained immunity involving appropriate rewiring of granulopoiesis.

Prolonged activation of nasal immune cell populations and development of tissue-resident SARS-CoV-2-specific CD8+ T cell responses following COVID-19.

Systemic immune cell dynamics during coronavirus disease 2019 (COVID-19) are extensively documented, but these are less well studied in the (upper) respiratory tract, where severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) replicates1-6. Here, we characterized nasal and systemic immune cells in individuals with COVID-19 who were hospitalized or convalescent and compared the immune cells to those seen in healthy donors. We observed increased nasal granulocytes, monocytes, CD11c+ natural killer (NK) cells and CD4+ T effector cells during acute COVID-19. The mucosal proinflammatory populations positively associated with peripheral blood human leukocyte antigen (HLA)-DRlow monocytes, CD38+PD1+CD4+ T effector (Teff) cells and plasmablasts. However, there was no general lymphopenia in nasal mucosa, unlike in peripheral blood. Moreover, nasal neutrophils negatively associated with oxygen saturation levels in blood. Following convalescence, nasal immune cells mostly normalized, except for CD127+ granulocytes and CD38+CD8+ tissue-resident memory T cells (TRM). SARS-CoV-2-specific CD8+ T cells persisted at least 2 months after viral clearance in the nasal mucosa, indicating that COVID-19 has both transient and long-term effects on upper respiratory tract immune responses.

Fate Mapping via Ms4a3-Expression History Traces Monocyte-Derived Cells.

Most tissue-resident macrophage (RTM) populations are seeded by waves of embryonic hematopoiesis and are self-maintained independently of a bone marrow contribution during adulthood. A proportion of RTMs, however, is constantly replaced by blood monocytes, and their functions compared to embryonic RTMs remain unclear. The kinetics and extent of the contribution of circulating monocytes to RTM replacement during homeostasis, inflammation, and disease are highly debated. Here, we identified Ms4a3 as a specific gene expressed by granulocyte-monocyte progenitors (GMPs) and subsequently generated Ms4a3TdT reporter, Ms4a3Cre, and Ms4a3CreERT2 fate-mapping models. These models traced efficiently monocytes and granulocytes, but no lymphocytes or tissue dendritic cells. Using these models, we precisely quantified the contribution of monocytes to the RTM pool during homeostasis and inflammation. The unambiguous identification of monocyte-derived cells will permit future studies of their function under any condition.

Megakaryocyte production is sustained by direct differentiation from erythromyeloid progenitors in the yolk sac until midgestation.

The extra-embryonic yolk sac contains the first definitive multipotent hematopoietic cells, denominated erythromyeloid progenitors. They originate in situ prior to the emergence of hematopoietic stem cells and give rise to erythroid, monocytes, granulocytes, mast cells and macrophages, the latter in a Myb transcription factor-independent manner. We uncovered here the heterogeneity of yolk sac erythromyeloid progenitors, at the single cell level, and discriminated multipotent from committed progenitors, prior to fetal liver colonization. We identified two temporally distinct megakaryocyte differentiation pathways. The first occurs in the yolk sac, bypasses intermediate bipotent megakaryocyte-erythroid progenitors and, similar to the differentiation of macrophages, is Myb-independent. By contrast, the second originates later, from Myb-dependent bipotent progenitors expressing Csf2rb and colonize the fetal liver, where they give rise to megakaryocytes and to large numbers of erythrocytes. Understanding megakaryocyte development is crucial as they play key functions during vascular development, in particular in separating blood and lymphatic networks.

Orchestrated intron retention regulates normal granulocyte differentiation.

Intron retention (IR) is widely recognized as a consequence of mis-splicing that leads to failed excision of intronic sequences from pre-messenger RNAs. Our bioinformatic analyses of transcriptomic and proteomic data of normal white blood cell differentiation reveal IR as a physiological mechanism of gene expression control. IR regulates the expression of 86 functionally related genes, including those that determine the nuclear shape that is unique to granulocytes. Retention of introns in specific genes is associated with downregulation of splicing factors and higher GC content. IR, conserved between human and mouse, led to reduced mRNA and protein levels by triggering the nonsense-mediated decay (NMD) pathway. In contrast to the prevalent view that NMD is limited to mRNAs encoding aberrant proteins, our data establish that IR coupled with NMD is a conserved mechanism in normal granulopoiesis. Physiological IR may provide an energetically favorable level of dynamic gene expression control prior to sustained gene translation.

Hematopoietic stem cells undergo a lymphoid to myeloid switch in early stages of emergency granulopoiesis.

Emergency granulopoiesis is the enhanced and accelerated production of granulocytes that occurs during acute infection. The contribution of hematopoietic stem cells (HSCs) to this process was reported; however, how HSCs participate in emergency granulopoiesis remains elusive. Here, using a mouse model of emergency granulopoiesis we observe transcriptional changes in HSCs as early as 4 h after lipopolysaccharide (LPS) administration. We observe that the HSC identity is changed towards a myeloid-biased HSC and show that CD201 is enriched in lymphoid-biased HSCs. While CD201 expression under steady-state conditions reveals a lymphoid bias, under emergency granulopoiesis loss of CD201 marks the lymphoid-to-myeloid transcriptional switch. Mechanistically, we determine that lymphoid-biased CD201+ HSCs act as a first response during emergency granulopoiesis due to direct sensing of LPS by TLR4 and downstream activation of NF-κΒ signaling. The myeloid-biased CD201- HSC population responds indirectly during an acute infection by sensing G-CSF, increasing STAT3 phosphorylation, and upregulating LAP/LAP* C/EBPß isoforms. In conclusion, HSC subpopulations support early phases of emergency granulopoiesis due to their transcriptional rewiring from a lymphoid-biased to myeloid-biased population and thus establishing alternative paths to supply elevated numbers of granulocytes.

Somatic mutation landscapes at single-molecule resolution.

Somatic mutations drive the development of cancer and may contribute to ageing and other diseases1,2. Despite their importance, the difficulty of detecting mutations that are only present in single cells or small clones has limited our knowledge of somatic mutagenesis to a minority of tissues. Here, to overcome these limitations, we developed nanorate sequencing (NanoSeq), a duplex sequencing protocol with error rates of less than five errors per billion base pairs in single DNA molecules from cell populations. This rate is two orders of magnitude lower than typical somatic mutation loads, enabling the study of somatic mutations in any tissue independently of clonality. We used this single-molecule sensitivity to study somatic mutations in non-dividing cells across several tissues, comparing stem cells to differentiated cells and studying mutagenesis in the absence of cell division. Differentiated cells in blood and colon displayed remarkably similar mutation loads and signatures to their corresponding stem cells, despite mature blood cells having undergone considerably more divisions. We then characterized the mutational landscape of post-mitotic neurons and polyclonal smooth muscle, confirming that neurons accumulate somatic mutations at a constant rate throughout life without cell division, with similar rates to mitotically active tissues. Together, our results suggest that mutational processes that are independent of cell division are important contributors to somatic mutagenesis. We anticipate that the ability to reliably detect mutations in single DNA molecules could transform our understanding of somatic mutagenesis and enable non-invasive studies on large-scale cohorts.

Blood and immune development in human fetal bone marrow and Down syndrome.

Haematopoiesis in the bone marrow (BM) maintains blood and immune cell production throughout postnatal life. Haematopoiesis first emerges in human BM at 11-12 weeks after conception1,2, yet almost nothing is known about how fetal BM (FBM) evolves to meet the highly specialized needs of the fetus and newborn. Here we detail the development of FBM, including stroma, using multi-omic assessment of mRNA and multiplexed protein epitope expression. We find that the full blood and immune cell repertoire is established in FBM in a short time window of 6-7 weeks early in the second trimester. FBM promotes rapid and extensive diversification of myeloid cells, with granulocytes, eosinophils and dendritic cell subsets emerging for the first time. The substantial expansion of B lymphocytes in FBM contrasts with fetal liver at the same gestational age. Haematopoietic progenitors from fetal liver, FBM and cord blood exhibit transcriptional and functional differences that contribute to tissue-specific identity and cellular diversification. Endothelial cell types form distinct vascular structures that we show are regionally compartmentalized within FBM. Finally, we reveal selective disruption of B lymphocyte, erythroid and myeloid development owing to a cell-intrinsic differentiation bias as well as extrinsic regulation through an altered microenvironment in Down syndrome (trisomy 21).

In situ mapping identifies distinct vascular niches for myelopoiesis.

In contrast to nearly all other tissues, the anatomy of cell differentiation in the bone marrow remains unknown. This is owing to a lack of strategies for examining myelopoiesis-the differentiation of myeloid progenitors into a large variety of innate immune cells-in situ in the bone marrow. Such strategies are required to understand differentiation and lineage-commitment decisions, and to define how spatial organizing cues inform tissue function. Here we develop approaches for imaging myelopoiesis in mice, and generate atlases showing the differentiation of granulocytes, monocytes and dendritic cells. The generation of granulocytes and dendritic cells-monocytes localizes to different blood-vessel structures known as sinusoids, and displays lineage-specific spatial and clonal architectures. Acute systemic infection with Listeria monocytogenes induces lineage-specific progenitor clusters to undergo increased self-renewal of progenitors, but the different lineages remain spatially separated. Monocyte-dendritic cell progenitors (MDPs) map with nonclassical monocytes and conventional dendritic cells; these localize to a subset of blood vessels expressing a major regulator of myelopoiesis, colony-stimulating factor 1 (CSF1, also known as M-CSF)1. Specific deletion of Csf1 in endothelium disrupts the architecture around MDPs and their localization to sinusoids. Subsequently, there are fewer MDPs and their ability to differentiate is reduced, leading to a loss of nonclassical monocytes and dendritic cells during both homeostasis and infection. These data indicate that local cues produced by distinct blood vessels are responsible for the spatial organization of definitive blood cell differentiation.

CXCR2 inhibition in G-MDSCs enhances CD47 blockade for melanoma tumor cell clearance.

The use of colony-stimulating factor-1 receptor (CSF1R) inhibitors has been widely explored as a strategy for cancer immunotherapy due to their robust depletion of tumor-associated macrophages (TAMs). While CSF1R blockade effectively eliminates TAMs from the solid tumor microenvironment, its clinical efficacy is limited. Here, we use an inducible CSF1R knockout model to investigate the persistence of tumor progression in the absence of TAMs. We find increased frequencies of granulocytic myeloid-derived suppressor cells (G-MDSCs) in the bone marrow, throughout circulation, and in the tumor following CSF1R deletion and loss of TAMs. We find that G-MDSCs are capable of suppressing macrophage phagocytosis, and the elimination of G-MDSCs through CXCR2 inhibition increases macrophage capacity for tumor cell clearance. Further, we find that combination therapy of CXCR2 inhibition and CD47 blockade synergize to elicit a significant anti-tumor response. These findings reveal G-MDSCs as key drivers of tumor immunosuppression and demonstrate their inhibition as a potent strategy to increase macrophage phagocytosis and enhance the anti-tumor efficacy of CD47 blockade in B16-F10 melanoma.

Low-density granulocytes in systemic autoimmunity and autoinflammation.

A body of evidence has re-energized the interest on the role neutrophils in inflammatory and autoimmune conditions. For decades, neutrophils have been considered a homogenous population. Nevertheless, accumulating evidence suggests that neutrophils are more versatile and heterogeneous than initially considered. The notion of neutrophil heterogeneity has been supported by the identification of low-density granulocytes (LDGs) in systemic lupus erythematosus (SLE) and other systemic autoimmune and autoinflammatory conditions. Transcriptomic, epigenetic, proteomic, and functional analyses support that LDGs are a distinct subset of proinflammatory neutrophils implicated in the pathogenesis of SLE and other autoimmune diseases. Importantly, it remains incompletely characterized whether LDGs detected in other inflammatory/autoimmune conditions display the same phenotype that those present in SLE. A shared feature of LDGs across diseases is their association with vascular damage, an important contributor to morbidity and mortality in chronic inflammatory conditions. Additionally, the lack of specific markers to identify LDGs in circulation or in tissue, makes it a challenge to elucidate their role in the pathogenesis of inflammatory and autoimmune conditions. In this review, we aim to examine the evidence on the biology and the putative pathogenic role of LDGs in systemic autoimmune diseases.

The miR-221/222 cluster regulates hematopoietic stem cell quiescence and multipotency by suppressing both Fos/AP-1/IEG pathway activation and stress-like differentiation to granulocytes.

Throughout life, hematopoietic stem cells (HSCs), residing in bone marrow (BM), continuously regenerate erythroid/megakaryocytic, myeloid, and lymphoid cell lineages. This steady-state hematopoiesis from HSC and multipotent progenitors (MPPs) in BM can be perturbed by stress. The molecular controls of how stress can impact hematopoietic output remain poorly understood. MicroRNAs (miRNAs) as posttranscriptional regulators of gene expression have been found to control various functions in hematopoiesis. We find that the miR-221/222 cluster, which is expressed in HSC and in MPPs differentiating from them, perturbs steady-state hematopoiesis in ways comparable to stress. We compare pool sizes and single-cell transcriptomes of HSC and MPPs in unperturbed or stress-perturbed, miR-221/222-proficient or miR-221/222-deficient states. MiR-221/222 deficiency in hematopoietic cells was induced in C57BL/6J mice by conditional vav-cre-mediated deletion of the floxed miR-221/222 gene cluster. Social stress as well as miR-221/222 deficiency, alone or in combination, reduced HSC pools 3-fold and increased MPPs 1.5-fold. It also enhanced granulopoisis in the spleen. Furthermore, combined stress and miR-221/222 deficiency increased the erythroid/myeloid/granulocytic precursor pools in BM. Differential expression analyses of single-cell RNAseq transcriptomes of unperturbed and stressed, proficient HSC and MPPs detected more than 80 genes, selectively up-regulated in stressed cells, among them immediate early genes (IEGs). The same differential single-cell transcriptome analyses of unperturbed, miR-221/222-proficient with deficient HSC and MPPs identified Fos, Jun, JunB, Klf6, Nr4a1, Ier2, Zfp36-all IEGs-as well as CD74 and Ly6a as potential miRNA targets. Three of them, Klf6, Nr4a1, and Zfp36, have previously been found to influence myelogranulopoiesis. Together with increased levels of Jun, Fos forms increased amounts of the heterodimeric activator protein-1 (AP-1), which is known to control the expression of the selectively up-regulated expression of the IEGs. The comparisons of single-cell mRNA-deep sequencing analyses of socially stressed with miR-221/222-deficient HSC identify 5 of the 7 Fos/AP-1-controlled IEGs, Ier2, Jun, Junb, Klf6, and Zfp36, as common activators of HSC from quiescence. Combined with stress, miR-221/222 deficiency enhanced the Fos/AP-1/IEG pathway, extended it to MPPs, and increased the number of granulocyte precursors in BM, inducing selective up-regulation of genes encoding heat shock proteins Hspa5 and Hspa8, tubulin-cytoskeleton-organizing proteins Tuba1b, Tubb 4b and 5, and chromatin remodeling proteins H3f3b, H2afx, H2afz, and Hmgb2. Up-regulated in HSC, MPP1, and/or MPP2, they appear as potential regulators of stress-induced, miR-221/222-dependent increased granulocyte differentiation. Finally, stress by serial transplantations of miR-221/222-deficient HSC selectively exhausted their lymphoid differentiation capacities, while retaining their ability to home to BM and to differentiate to granulocytes. Thus, miR-221/222 maintains HSC quiescence and multipotency by suppressing Fos/AP-1/IEG-mediated activation and by suppressing enhanced stress-like differentiation to granulocytes. Since miR-221/222 is also expressed in human HSC, controlled induction of miR-221/222 in HSC should improve BM transplantations.

Autosomal recessive hyposegmentation of granulocytes in Australian Shepherd Dogs indicates a role for LMBR1L in myeloid leukocytes.

Pelger-Huët anomaly (PHA) in humans is an autosomal dominant hematological phenotype without major clinical consequences. PHA involves a characteristic hyposegmentation of granulocytes (HG). Human PHA is caused by heterozygous loss of function variants in the LBR gene encoding lamin receptor B. Bi-allelic variants and complete deficiency of LBR cause the much more severe Greenberg skeletal dysplasia which is lethal in utero and characterized by massive skeletal malformation and gross fetal hydrops. HG phenotypes have also been described in domestic animals and homology to human PHA has been claimed in the literature. We studied a litter of Australian Shepherd Dogs with four stillborn puppies in which both parents had an HG phenotype. Linkage analysis excluded LBR as responsible gene for the stillborn puppies. We then investigated the HG phenotype in Australian Shepherd Dogs independently of the prenatal lethality. Genome-wide association mapped the HG locus to chromosome 27 and established an autosomal recessive mode of inheritance. Whole genome sequencing identified a splice site variant in LMBR1L, c.191+1G>A, as most likely causal variant for the HG phenotype. The mutant allele abrogates the expression of the longer X2 isoform but does not affect transcripts encoding the shorter X1 isoform of the LMBR1L protein. The homozygous mutant LMBR1L genotype associated with HG is common in Australian Shepherd Dogs and was found in 39 of 300 genotyped dogs (13%). Our results point to a previously unsuspected function of LMBR1L in the myeloid lineage of leukocytes.

Limited plasticity of monocyte fate and function associated with epigenetic scripting at the level of progenitors.

Myeloid cell heterogeneity is known, but whether it is cell-intrinsic or environmentally-directed remains unclear. Here, an inducible/reversible system pausing myeloid differentiation allowed the definition of clone-specific functions that clustered monocytes into subsets with distinctive molecular features. These subsets were orthogonal to the classical/nonclassical categorization and had inherent, restricted characteristics that did not shift under homeostasis, after irradiation, or with infectious stress. Rather, their functional fate was constrained by chromatin accessibility established at or before the granulocyte-monocyte or monocyte-dendritic progenitor level. Subsets of primary monocytes had differential ability to control distinct infectious agents in vivo. Therefore, monocytes are a heterogeneous population of functionally restricted subtypes defined by the epigenome of their progenitors that are differentially selected by physiologic challenges with limited plasticity to transition from one subset to another.

Activated granulocytes and inflammatory cytokine signaling drive T-cell lymphoma progression and disease symptoms.

Peripheral T-cell lymphomas (PTCLs), especially angioimmunoblastic and follicular TCLs, have a dismal prognosis because of the lack of efficient therapies, and patients' symptoms are often dominated by an inflammatory phenotype, including fever, night sweats, weight loss, and skin rash. In this study, we investigated the role of inflammatory granulocytes and activated cytokine signaling on T-cell follicular helper-type PTCL (TFH-PTCL) disease progression and symptoms. We showed that ITK-SYK-driven murine PTCLs and primary human TFH-PTCL xenografts both induced inflammation in mice, including murine neutrophil expansion and massive cytokine release. Granulocyte/lymphoma interactions were mediated by positive autoregulatory cytokine loops involving interferon gamma (CD4+ malignant T cells) and interleukin 6 (IL-6; activated granulocytes), ultimately inducing broad JAK activation (JAK1/2/3 and TYK2) in both cell types. Inflammatory granulocyte depletion via antibodies (Ly6G), genetic granulocyte depletion (LyzM-Cre/MCL1flox/flox), or IL-6 deletion within microenvironmental cells blocked inflammatory symptoms, reduced lymphoma infiltration, and enhanced mouse survival. Furthermore, unselective JAK inhibitors (ruxolitinib) inhibited both TCL progression and granulocyte activation in various PTCL mouse models. Our results support the important role of granulocyte-driven inflammation, cytokine-induced granulocyte/CD4+ TCL interactions, and an intact JAK/STAT signaling pathway for TFH-PTCL development and also support broad JAK inhibition as an effective treatment strategy in early disease stages.

Ferroptosis contributes to airway epithelial E-cadherin disruption in a mixed granulocytic asthma mouse model.

Aberrant expression of airway epithelial E-cadherin is a key feature of asthma, yet the underlying mechanisms are largely unknown. Ferroptosis is a novel form of regulated cell death involved in asthma pathogenesis. This study was aimed to evaluate the role of ferroptosis and to investigate whether ferroptosis mediates E-cadherin disruption in mixed granulocyte asthma (MGA). Two murine models of MGA were established using toluene diisocyanate (TDI) or ovalbumin with Complete Freund's Adjuvant (OVA/CFA). Specific antagonists of ferroptosis, including Liproxstatin-1 (Lip-1) and Ferrostatin-1 (Fer-1) were given to the mice. The allergen-exposed mice displayed markedly shrunk mitochondria in the airway epithelia, with decreased volume and denser staining accompanied by down-regulated GPX4 as well as up-regulated FTH1 and malondialdehyde, which are markers of ferroptosis. Decreased pulmonary expression of E-cadherin was also observed, with profound loss of membrane E-cadherin in the airway epithelia, as well as increased secretion of sE-cadherin. Treatment with Lip-1 not only showed potent protective effects against the allergen-induced airway hyperresponsiveness and inflammatory responses, but also rescued airway epithelial E-cadherin expression and inhibited the release of sE-cadherin. Taken together, our data demonstrated that ferroptosis mediates airway epithelial E-cadherin dysfunction in MGA.

Targeting GM-CSF in inflammatory and autoimmune disorders.

Granulocyte macrophage-colony stimulating factor (GM-CSF) was originally identified as a growth factor for its ability to promote the proliferation and differentiation in vitro of bone marrow progenitor cells into granulocytes and macrophages. Many preclinical studies, using GM-CSF deletion or depletion approaches, have demonstrated that GM-CSF has a wide range of biological functions, including the mediation of inflammation and pain, indicating that it can be a potential target in many inflammatory and autoimmune conditions. This review provides a brief overview of GM-CSF biology and signaling, and summarizes the findings from preclinical models of a range of inflammatory and autoimmune disorders and the latest clinical trials targeting GM-CSF or its receptor in these disorders.

A novel syndrome of silent rhinovirus-associated bronchoalveolitis in children with recurrent wheeze.

BACKGROUND: Rhinovirus (RV) infections trigger wheeze episodes in children. Thus, understanding of the lung inflammatory response to RV in children with wheeze is important. OBJECTIVES: This study sought to examine the associations of RV on bronchoalveolar lavage (BAL) granulocyte patterns and biomarkers of inflammation with age in children with treatment-refractory, recurrent wheeze (n = 616). METHODS: Children underwent BAL to examine viral nucleic acid sequences, bacterial cultures, granulocyte counts, and phlebotomy for both general and type-2 inflammatory markers. RESULTS: Despite the absence of cold symptoms, RV was the most common pathogen detected (30%), and when present, was accompanied by BAL granulocytosis in 75% of children. Compared to children with no BAL pathogens (n = 341), those with RV alone (n = 127) had greater (P < .05) isolated neutrophilia (43% vs 16%), mixed eosinophils and neutrophils (26% vs 11%), and less pauci-granulocytic (27% vs 61%) BAL. Children with RV alone furthermore had biomarkers of active infection with higher total blood neutrophils and serum C-reactive protein, but no differences in blood eosinophils or total IgE. With advancing age, the log odds of BAL RV alone were lower, 0.82 (5th-95th percentile CI: 0.76-0.88; P < .001), but higher, 1.58 (5th-95th percentile CI: 1.01-2.51; P = .04), with high-dose daily corticosteroid treatment. CONCLUSIONS: Children with severe recurrent wheeze often (22%) have a silent syndrome of lung RV infection with granulocytic bronchoalveolitis and elevated systemic markers of inflammation. The syndrome is less prevalent by school age and is not informed by markers of type-2 inflammation. The investigators speculate that dysregulated mucosal innate antiviral immunity is a responsible mechanism.

Granulocyte pro-myeloperoxidase is redundantly processed by proprotein convertase furin and PC7 in HL-60 cells.

Neutrophil myeloperoxidase/H2O2/chloride system is a key mechanism to control pathogen infection. This enzyme, myeloperoxidase, plays a pivotal role in the arsenal of azurophilic granules that are released through degranulation upon neutrophil activation, which trigger local hypochlorous acid production. Myeloperoxidase gene encodes a protein precursor named promyeloperoxidase that arbors a propeptide that gets cleaved later during secretory routing in post-endoplasmic reticulum compartments. Although evidence suggested that this processing event was performed by one or different enzymes from the proprotein convertases family, the identity of this enzyme was never investigated. In this work, the naturally producing myeloperoxidase promyelocytic cell line HL-60 was used to investigate promyeloperoxidase cleavage during granulocytic differentiation in response to proprotein convertase inhibitors decanoyl-RVKR-chloromethylketone and hexa-d-arginine. Stable PC knockdown of endogenously expressed proprotein convertases, furin and PC7, was achieved using lentiviral delivery of shRNAs. None of the knockdown cell line could reproduce the effect of the pan-proprotein convertases inhibitor decanoyl-RVKR-chloromethylketone that accumulated intracellular promyeloperoxidase stores in HL-60 cells, therefore illustrating that both furin and PC7 redundantly process this proprotein.