Preparation of secretory vesicle-free plasma membranes by isopycnic sucrose gradient fractionation of neutrophils purified by the gelatin method

Impact of endoplasmic reticulum aminopeptidases 1 (ERAP1) and 2 (ERAP2) on neutrophil cellular functions

Introduction

Endoplasmic reticulum aminopeptidases 1 (ERAP1) and 2 (ERAP2) modulate a plethora of physiological processes for the maintenance of homeostasis in different cellular subsets at both intra and extracellular level.

Materials and methods

In this frame, the extracellular supplementation of recombinant human (rh) ERAP1 and ERAP2 (300 ng/ml) was used to mimic the effect of stressor-induced secretion of ERAPs on neutrophils isolated from 5 healthy subjects. In these cells following 3 h or 24 h rhERAP stimulation by Western Blot, RT-qPCR, Elisa, Confocal microscopy, transwell migration assay, Oxygraphy and Flow Cytometry we assessed: i) rhERAP internalization; ii) activation; iii) migration; iv) oxygen consumption rate; v) reactive oxygen species (ROS) accumulation; granule release; vi) phagocytosis; and vii) autophagy.

Results

We observed that following stimulation rhERAPs: i) were internalized by neutrophils; ii) triggered their activation as witnessed by increased percentage of MAC-1+CD66b+ expressing neutrophils, cytokine expression/release (IL-1β, IL-8, CCL2, TNFα, IFNγ, MIP-1β) and granule enzyme secretion (myeloperoxidase, Elastase); iii) increased neutrophil migration capacity; iv) increased autophagy and phagocytosis activity; v) reduced ROS accumulation and did not influence oxygen consumption rate.

Conclusion

Our study provides novel insights into the biological role of ERAPs, and indicates that extracellular ERAPs, contribute to shaping neutrophil homeostasis by promoting survival and tolerance in response to stress-related inflammation. This information could contribute to a better understanding of the biological bases governing immune responses, and to designing ERAP-based therapeutic protocols to control neutrophil-associated human diseases.

Priming of the neutrophil respiratory burst: role in host defense and inflammation

Neutrophils are the major circulating white blood cells in humans. They play an essential role in host defense against pathogens. In healthy individuals, circulating neutrophils are in a dormant state with very low efficiency of capture and arrest on the quiescent endothelium. Upon infection and subsequent release of pro-inflammatory mediators, the vascular endothelium signals to circulating neutrophils to roll, adhere, and cross the endothelial barrier. Neutrophils migrate toward the infection site along a gradient of chemo-attractants, then recognize and engulf the pathogen. To kill this pathogen entrapped inside the vacuole, neutrophils produce and release high quantities of antibacterial peptides, proteases, and reactive oxygen species (ROS). The robust ROS production is also called 'the respiratory burst', and the NADPH oxidase or NOX2 is the enzyme responsible for the production of superoxide anion, leading to other ROS. In vitro, several soluble and particulate agonists induce neutrophil ROS production. This process can be enhanced by prior neutrophil treatment with 'priming' agents, which alone do not induce a respiratory burst. In this review, we will describe the priming process and discuss the beneficial role of controlled neutrophil priming in host defense and the detrimental effect of excessive neutrophil priming in inflammatory diseases.

C-Terminal Tail Phosphorylation of N-Formyl Peptide Receptor: Differential Recognition of Two Neutrophil Chemoattractant Receptors by Monoclonal Antibodies NFPR1 and NFPR2

The N-formyl peptide receptor (FPR), a G protein-coupled receptor that binds proinflammatory chemoattractant peptides, serves as a model receptor for leukocyte chemotaxis. Recombinant histidine-tagged FPR (rHis-FPR) was purified in lysophosphatidyl glycerol (LPG) by Ni(2+)-NTA agarose chromatography to >95% purity with high yield. MALDI-TOF mass analysis (>36% sequence coverage) and immunoblotting confirmed the identity as FPR. The rHis-FPR served as an immunogen for the production of 2 mAbs, NFPR1 and NFPR2, that epitope map to the FPR C-terminal tail sequences, 305-GQDFRERLI-313 and 337-NSTLPSAEVE-346, respectively. Both mAbs specifically immunoblotted rHis-FPR and recombinant FPR (rFPR) expressed in Chinese hamster ovary cells. NFPR1 also recognized recombinant FPRL1, specifically expressed in mouse L fibroblasts. In human neutrophil membranes, both Abs labeled a 45-75 kDa species (peak M(r) approximately 60 kDa) localized primarily in the plasma membrane with a minor component in the lactoferrin-enriched intracellular fractions, consistent with FPR size and localization. NFPR1 also recognized a band of M(r) approximately 40 kDa localized, in equal proportions to the plasma membrane and lactoferrin-enriched fractions, consistent with FPRL1 size and localization. Only NFPR2 was capable of immunoprecipitation of rFPR in detergent extracts. The recognition of rFPR by NFPR2 is lost after exposure of cellular rFPR to f-Met-Leu-Phe (fMLF) and regained after alkaline phosphatase treatment of rFPR-bearing membranes. In neutrophils, NFPR2 immunofluorescence was lost upon fMLF stimulation. Immunoblotting approximately 60 kDa species, after phosphatase treatment of fMLF-stimulated neutrophil membranes, was also enhanced. We conclude that the region 337-346 of FPR becomes phosphorylated after fMLF activation of rFPR-expressing Chinese hamster ovary cells and neutrophils.

The two neutrophil plasma membrane markers alkaline phosphatase and HLA class I antigen localize differently in granule-deficient cytoplasts. An ideal plasma membrane marker in human neutrophils is still lacking

Neutrophil function relies largely on the ability of the cell to mobilize its different granules and vesicles to the cell surface and thereby expose and/or release effector molecules to the surrounding tissue. To properly identify these subcellular compartments is thus a prerequisite for studies of neutrophil physiology. A range of specific markers for the classical granules is available, but finding optimal markers for the secretory vesicles and plasma membrane has historically been more challenging. Latent and non-latent alkaline phosphatase activities are often used to distinguish these two light membrane structures, but the outcome using this technique depends on the level of cellular activation. Therefore, HLA-I was introduced some years ago as a specific, stimulation-independent marker for the plasma membrane. In this study we however report that detailed fractionation studies of neutrophil cytoplasts, lacking secretory vesicles, granules and other dense organelles, reveal that the HLA-I antigen is not only co-localizing with the plasma membrane marker ALP, but is also present in other, more dense organelles. Further, we found the mixed enzyme-linked immunosorbent assay (MELISA), detecting the beta(2)-microglobulin/HLA-I complex, to be negatively influenced by uncomplexed beta(2)-microglobulin present in the specific granules and secretory vesicles, making it difficult to use HLA-I as a plasma membrane marker during maturation of for example phagolysosomes.

Reorganization of the human neutrophil plasma membrane is associated with functional priming: implications for neutrophil preparations

Changes in the functional and plasma membrane organizational states of human neutrophils were examined using two isolation procedures, which may simulate altered physiological states in vivo. A gelatin-based method of blood-neutrophil isolation was used to model in vivo priming, and neutrophils isolated by this method were compared with control populations prepared by a pyrogen-free, dextran-based method. Gelatin-prepared neutrophils were functionally primed for adherence and agonist-stimulated superoxide generation relative to unprimed, control neutrophils. The organizational state of the membrane cortex was examined by mapping the subcellular distribution of select cortical and transmembrane proteins by several methods, including subcellular fractionation, indirect immunofluorescence, and compositional analysis of Triton X-100-insoluble membrane skeleton preparations. Filamentous actin, fodrin, and the fodrin anchor, CD45, were largely cytoplasmic in unprimed neutrophils but translocated to plasma membranes upon priming, whereas CD43 and ezrin were exclusively surface-associated in both populations. Isopycnic sucrose density gradient analysis of N(2)-cavitated neutrophils revealed a major shift in the distribution of surface-associated transmembrane and membrane cortical components relative to the plasma membrane marker alkaline phosphatase in primed but not unprimed neutrophils. Similar results were obtained after neutrophil stimulation with known priming agents, LPS, TNF-alpha, or GM-CSF. Together, these results may suggest that priming of suspended, circulating neutrophils is associated with a large-scale reorganization of the plasma membrane and associated membrane cortex in a process that is independent of cellular adhesion and gross morphologic polarization.