Neutrophil arrest by LFA-1 activation

Abstract

Lymphocyte function-associated antigen-1 (LFA-1) is a heterodimeric integrin consisting of α(L) (gene name, Itgal) and β(2) (gene name, Itgb2) subunits expressed in all leukocytes. LFA-1 is essential for neutrophil recruitment to inflamed tissue. Activation of LFA-1 by chemokines allows neutrophils and other leukocytes to undergo arrest, resulting in firm adhesion on endothelia expressing intercellular adhesion molecules (ICAMs). In mice, CXCR2 is the primary chemokine receptor involved in triggering neutrophil arrest, and it does so through "inside-out" activation of LFA-1. CXCR2 signaling induces changes in LFA-1 conformation that are coupled to affinity upregulation of the ligand-binding headpiece (extended with open I domain). Unlike naïve lymphocytes, engagement of P-selectin glycoprotein ligand-1 (PSGL-1) on neutrophils stimulates a slow rolling behavior that is mediated by LFA-1 in a distinct activation state (extended with closed I domain). How inside-out signaling cascades regulate the structure and function of LFA-1 is being studied using flow chambers, intravital microscopy, and flow cytometry for ligand and reporter antibody binding. Here, we review how LFA-1 activation is regulated by cellular signaling and ligand binding. Two FERM domain-containing proteins, talin-1 and Kindlin-3, are critical integrin co-activators and have distinct roles in the induction of LFA-1 conformational rearrangements. This review integrates these new results into existing models of LFA-1 activation.

Keywords: LFA-1; chemokine; inflammation; integrin; neutrophil.

FIGURE 1

LFA-1 conformations. Integrin function is structurally regulated and three distinct conformations have been demonstrated. In its low affinity state (left), LFA-1 is compact with a sharp bend at the “genu” and headpiece closely approaching the plasma membrane. Extended conformations (middle) differ in the orientation of their hybrid domain (purple), with swingout of this domain representing a conversion from intermediate to high affinity state (right). Data suggest that the extended/closed conformation (middle) can have either a low or intermediate affinity αI domain.

FIGURE 2

Neutrophil inside-out activation of LFA-1. Two distinct signal transduction pathways modulate LFA-1 activation in neutrophils: (1) selectin binding to PSGL-1, and (2) chemokine receptor engagement. PSGL-1 signals talin-1-dependent LFA-1 extension, whereas chemokine GPCRs signal LFA-1 activation to a high affinity state that requires both talin-1 and Kindlin-3. Some of these signaling molecules may exist in preformed modules to facilitate rapid integrin activation. Abbreviations: Fgr, feline Gardner–Rasheed sarcoma kinase; FcRγ, immunoglobulin gamma Fc receptor; DAP12, DNAX activating protein of 12 kDa; Syk, spleen tyrosine kinase; ADAP, adhesion and degranulation promoting adaptor protein; SLP-76, SH2 domain-containing leukocyte phosphoprotein of 76 kDa; Btk, Bruton tyrosine kinase; PI3K, phosphoinositide 3-kinase; PLC, phospholipase C; p38, p38 mitogen activated protein kinase; IP₃, inositol trisphosphate; DAG, diacylglycerol; CalDAG-GEFI, calcium- and diacylglycerol-regulated guanine nucleotide exchange factor I; PSGL-1, P-selectin glycoprotein ligand-1; ICAM-1, intercellular adhesion molecule-1; GAG, glycosaminoglycan.