REVIEW published: 18 February 2021 doi: 10.3389/fimmu.2020.619925 b2 Integrin Signaling Cascade in Neutrophils: More Than a Single Function † † Panagiota Bouti 1* , Steven D. S. Webbers 1,2* , Susanna C. Fagerholm 3, Ronen Alon 4, Markus Moser 5, Hanke L. Matlung 1 and Taco W. Kuijpers 1,2 1 Sanquin Research and Landsteiner Laboratory, Department of Blood Cell Research, Amsterdam University Medical Center, Edited by: University of Amsterdam, Amsterdam, Netherlands, 2 Department of Pediatric Immunology, Rheumatology and Infectious Vicky L. Morrison, Disease, Amsterdam University Medical Center (AUMC), Emma Children’s Hospital, University of Amsterdam, Amsterdam, University of Glasgow, Netherlands, 3 Research Program of Molecular and Integrative Biosciences, Faculty of Biological and Environmental United Kingdom Sciences, University of Helsinki, Helsinki, Finland, 4 Department of Immunology, The Weizmann Institute of Science, Rehovot, Reviewed by: Israel, 5 Institute of Experimental Hematology, School of Medicine, Technical University of Munich, Munich, Germany Vineet Gupta, Rush University Medical Center, United States Neutrophils are the most prevalent leukocytes in the human body. They have a pivotal role Keehoon Jung, in the innate immune response against invading bacterial and fungal pathogens, while Seoul National University, South Korea recent emerging evidence also demonstrates their role in cancer progression and anti- *Correspondence: Panagiota Bouti tumor responses. The efficient execution of many neutrophil effector responses requires [email protected] the presence of b2 integrins, in particular CD11a/CD18 or CD11b/CD18 heterodimers. Steven D. S. Webbers [email protected] Although extensively studied at the molecular level, the exact signaling cascades †These authors have contributed downstream of b2 integrins still remain to be fully elucidated. In this review, we focus equally to this work mainly on inside-out and outside-in signaling of these two b2 integrin members expressed on neutrophils and describe differences between various neutrophil stimuli with respect to Specialty section: integrin activation, integrin ligand binding, and the pertinent differences between mouse This article was submitted to Molecular Innate Immunity, and human studies. Last, we discuss how integrin signaling studies could be used to a section of the journal explore the therapeutic potential of targeting b2 integrins and the intracellular signaling Frontiers in Immunology cascade in neutrophils in several, among other, inflammatory conditions in which Received: 21 October 2020 Accepted: 31 December 2020 neutrophil activity should be dampened to mitigate disease. Published: 18 February 2021 Keywords: CD11b/CD18 integrin, b2 integrin signaling, neutrophils, neutrophil function, therapeutic targets Citation: Bouti P, Webbers SDS, Abbreviations: ADCC, Antibody-dependent cellular cytotoxicity; ADCP, Antibody-dependent cellular phagocytosis; EGF, Fagerholm SC, Alon R, Moser M, Epidermal growth factor; ECM, Extracellular matrix; F-actin, Filamentous actin; FAK, Focal adhesion kinase; GPCRs, G- Matlung HL and Kuijpers TW (2021) b2 protein coupled receptors; HPK1, Hematopoietic progenitor kinase 1; dHL-60, HL-60 cell line differentiated towards Integrin Signaling Cascade neutrophil-like cells; ITAMs, Immunoreceptor tyrosine-based activation motifs; LAD, Leukocyte adhesion deficiency in Neutrophils: More syndrome; fMLP, N-formylmethionine-leucyl-phenylalanine; NK cells, Natural killer cells; NETs, Neutrophil extracellular Than a Single Function. traps; PSI, Plexin–semaphorin–integrin; PI3K, Phosphoinositide 3-kinase; PMN, Polymorphonuclear; PKC, Protein kinase C; Front. Immunol. 11:619925. RACK-1, Receptor For Activated C Kinase 1; ROS, Reactive oxygen species; SYK, Spleen tyrosine kinase; TNFa, Tumor doi: 10.3389/fimmu.2020.619925 necrosis factor a; WASP, Wiskott–Aldrich syndrome protein. Frontiers in Immunology | www.frontiersin.org 1 February 2021 | Volume 11 | Article 619925 Bouti et al. b2 Integrin Signaling in Neutrophil Function INTRODUCTION family, the collagen, and some laminin-binding integrins, have an additional a-I domain inserted into a loop in the b-propeller Neutrophils domain, which employ ligand binding (20). A hallmark of Human neutrophils or polymorphonuclear cells (PMNs) form integrins is their ability to reversibly switch between different the frontline of the immune response against bacteria and fungi. ectodomain conformations with distinct affinities for the ligand. Despite their fundamental role in inflammation, recent evidence It is meanwhile well accepted that integrins can adopt at least gives prominence to the controversial role of neutrophils in three conformations: a low affinity bent conformation, an cancer (1, 2). Whereas recent data using deuterium labeling extended-closed conformation with intermediate affinity, and in vivo suggest that human neutrophils can survive up to 5.4 days an extended-open conformation with high ligand affinity (21– in the circulation (3), general belief based on traditional studies 23). Notably, the difference in affinity for the ligand between the still is that the high number of neutrophils emerging from the low and high affinity states varies strongly between different bone marrow on a daily basis (1011 per day) is balanced with integrins from several hundred to several thousand fold (24–26). their short half-life in the circulation (up to about 12h) (4). In The conformational changes are either induced and stabilized by contrast, the abundancy of neutrophil population in humans, the extracellular ligand (outside-in activation) or are triggered by representing up to 70% of circulating leukocytes, is not reflected tightly regulated intracellular signals (inside-out activation); the on mice in which the percentage of neutrophils varies from 10 to latter is particularly important for integrins expressed on blood 25% of white blood cells (5, 6). cells and will be described in detail in the following sections. Apart from the daily extravasation to offer that first natural Both integrin subunits have a single-pass transmembrane helical line of defense at the tissue level, neutrophils are able to domain, which are non-covalently associated within the plasma directionally migrate at larger numbers out of the bloodstream membrane. The two transmembrane domains interact at two at the site of infection and either clear the pathogen or induce the interfaces termed the “outer membrane clasp” and the “inner recruitment of adaptive immune cells by chemokine/cytokine membrane clasp”, which are mediated by helical packing of the production or antigen presentation (7–9). Neutrophils exit the N-terminal part and hydrophobic interactions and a salt bridge at circulation via trans-endothelial migration, which takes place in the C-terminal part of the transmembrane domains, respectively sequential steps (tethering, rolling, crawling, cell arrest/firm (27–29). The transmembrane domains are essential for integrin adhesion/transmigration); all have been thoroughly described function as they propagate conformational changes across the (10, 11). At the site of infection, neutrophils exert their killing plasma membrane in both directions. These changes occur upon properties to eliminate the pathogen extracellularly or upon binding of the ligand to the ectodomain or intracellular adapter phagocytosis. Neutrophils have the capacity to kill the invaded proteins to the cytoplasmic domain, which cause the disruption of microorganism by secreting their toxic granular content the transmembrane domain interactions and a separation of the (degranulation), producing reactive oxygen species (ROS), or transmembrane and intracellular domains. releasing their DNA and subsequently forming neutrophil The intracellular domains represent the control modules of extracellular traps (NETs) in a process of cell death known as integrins. These rather short peptides serve as docking stations NETosis (6, 12). These processes of neutrophil extravasation, for numerous intracellular proteins, which regulate the integrin’s migration, and neutrophil effector functions against infectious activity state and execute their functions. The majority of studies agents rely to a very large extent on integrins. were done on the b integrin tail, which has a length of only 30 to 70 amino acids with the exception of the much longer b4 integrin Integrins: Expression, Structure and cytoplasmic domain. Most b-tails contain a membrane proximal Activation NPxY/F and a membrane distal NxxY/F motif flanking a serine/ Integrins are a family of ubiquitously expressed, transmembrane threonine-rich motif (30). Many intracellular proteins interact receptors. They anchor cells within their ambient extracellular with these motifs in a phosphorylation dependent or matrix (ECM) and bind to counter-receptors expressed on other independent manner. The most prominent proteins that bind cells (13). Integrins are expressed on the cell surface as to these motifs are Talins and Kindlins, which are essential heterodimers of non-covalently associated a and b subunits integrin activators and will be described in detail below (31, 32). (14, 15). So far, 24 different ab heterodimers have been The cytoplasmic tails of the different a chains show low described in mammals, which exhibit specific ligand binding homology but share a GFFXR sequence in the membrane- properties. The extracellular part of the a subunits defines ligand proximal region (33). Much less is known about protein specificity.