ICAMs in Immunity, Intercellular Adhesion and Communication

Interactions among leukocytes and leukocytes with immune-associated auxiliary cells represent an essential feature of the immune response that requires the involvement of cell adhesion molecules (CAMs). In the immune system, CAMs include a wide range of members pertaining to different structural and functional families involved in cell development, activation, differentiation and migration. Among them, β2 integrins (LFA-1, Mac-1, p150,95 and αDβ2) are predominantly involved in homotypic and heterotypic leukocyte adhesion. β2 integrins bind to intercellular (I)CAMs, actin cytoskeleton-linked receptors belonging to immunoglobulin superfamily (IgSF)-CAMs expressed by leukocytes and vascular endothelial cells, enabling leukocyte activation and transendothelial migration. β2 integrins have long been viewed as the most important ICAMs partners, propagating intracellular signalling from β2 integrin-ICAM adhesion receptor interaction. In this review, we present previous evidence from pioneering studies and more recent findings supporting an important role for ICAMs in signal transduction. We also discuss the contribution of immune ICAMs (ICAM-1, -2, and -3) to reciprocal cell signalling and function in processes in which β2 integrins supposedly take the lead, paying particular attention to T cell activation, differentiation and migration.

LFA-1 in T cell priming, differentiation, and effector functions

The integrin LFA-1 is crucial for T cell entry into mammalian lymph nodes and tissues, and for promoting interactions with antigen-presenting cells (APCs). However, it is increasingly evident that LFA-1 has additional key roles beyond the mere support of adhesion between T cells, the endothelium, and/or APCs. These include roles in homotypic T cell-T cell (T-T) communication, the induction of intracellular complement activity underlying Th1 effector cell polarization, and the support of long-lasting T cell memory. Here, we briefly summarize current knowledge of LFA-1 biology, discuss novel cytoskeletal regulators of LFA-1 functions, and review new aspects of LFA-1 mechanobiology that are relevant to its function in immunological synapses and in specific pathologies arising from LFA-1 dysregulation.

Integrins meet complement: The evolutionary tip of an iceberg orchestrating metabolism and immunity

Immunologists have recently realized that there is more to the classic innate immune sensor systems than just mere protection against invading pathogens. It is becoming increasingly clear that such sensors, including the inflammasomes, toll-like receptors, and the complement system, are heavily involved in the regulation of basic cell physiological processes and particularly those of metabolic nature. In fact, their "non-canonical" activities make sense as no system directing immune cell activity can perform such task without the need for energy. Further, many of these ancient immune sensors appeared early and concurrently during evolution, particularly during the developmental leap from the single-cell organisms to multicellularity, and therefore crosstalk heavily with each other. Here, we will review the current knowledge about the emerging cooperation between the major inter-cell communicators, integrins, and the cell-autonomous intracellularly and autocrine-active complement, the complosome, during the regulation of single-cell metabolism. LINKED ARTICLES: This article is part of a themed issue on Canonical and non-canonical functions of the complement system in health and disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.14/issuetoc.

Keratinocytes costimulate naive human T cells via CD2: a potential target to prevent the development of proinflammatory Th1 cells in the skin

The interplay between keratinocytes and immune cells, especially T cells, plays an important role in the pathogenesis of chronic inflammatory skin diseases. During psoriasis, keratinocytes attract T cells by releasing chemokines, while skin-infiltrating self-reactive T cells secrete proinflammatory cytokines, e.g., IFNγ and IL-17A, that cause epidermal hyperplasia. Similarly, in chronic graft-versus-host disease, allogenic IFNγ-producing Th1/Tc1 and IL-17-producing Th17/Tc17 cells are recruited by keratinocyte-derived chemokines and accumulate in the skin. However, whether keratinocytes act as nonprofessional antigen-presenting cells to directly activate naive human T cells in the epidermis remains unknown. Here, we demonstrate that under proinflammatory conditions, primary human keratinocytes indeed activate naive human T cells. This activation required cell contact and costimulatory signaling via CD58/CD2 and CD54/LFA-1. Naive T cells costimulated by keratinocytes selectively differentiated into Th1 and Th17 cells. In particular, keratinocyte-initiated Th1 differentiation was dependent on costimulation through CD58/CD2. The latter molecule initiated STAT1 signaling and IFNγ production in T cells. Costimulation of T cells by keratinocytes resulting in Th1 and Th17 differentiation represents a new explanation for the local enrichment of Th1 and Th17 cells in the skin of patients with a chronic inflammatory skin disease. Consequently, local interference with T cell–keratinocyte interactions may represent a novel strategy for the treatment of Th1 and Th17 cell-driven skin diseases.

MicroRNA-31 Reduces the Motility of Proinflammatory T Helper 1 Lymphocytes

Proinflammatory type 1 T helper (Th1) cells are enriched in inflamed tissues and contribute to the maintenance of chronic inflammation in rheumatic diseases. Here we show that the microRNA- (miR-) 31 is upregulated in murine Th1 cells with a history of repeated reactivation and in memory Th cells isolated from the synovial fluid of patients with rheumatic joint disease. Knock-down of miR-31 resulted in the upregulation of genes associated with cytoskeletal rearrangement and motility and induced the expression of target genes involved in T cell activation, chemokine receptor- and integrin-signaling. Accordingly, inhibition of miR-31 resulted in increased migratory activity of repeatedly activated Th1 cells. The transcription factors T-bet and FOXO1 act as positive and negative regulators of T cell receptor (TCR)-mediated miR-31 expression, respectively. Taken together, our data show that a gene regulatory network involving miR-31, T-bet, and FOXO1 controls the migratory behavior of proinflammatory Th1 cells.

LFA-1 in T Cell Migration and Differentiation

Maintenance of homeostatic immune surveillance and development of effective adaptive immune responses require precise regulation of spatial and temporal lymphocyte trafficking throughout the body to ensure pathogen clearance and memory generation. Dysregulation of lymphocyte activation and migration can lead to impaired adaptive immunity, recurrent infections, and an array of autoimmune diseases and chronic inflammation. Central to the recruitment of T cells, integrins are cell surface receptors that regulate adhesion, signal transduction, and migration. With 24 integrin pairs having been discovered to date, integrins are defined not only by the composition of the heterodimeric pair but by cell-type specific expression and their ligands. Furthermore, integrins not only facilitate adhesion but also induce intracellular signaling and have recently been uncovered as mechanosensors providing additional complexity to the signaling pathways. Among several leukocyte-specific integrins, lymphocyte function-associated antigen-1 (LFA-1 or α_Lβ₂; CD11a/CD18) is a key T cell integrin, which plays a major role in regulating T cell activation and migration. Adhesion to LFA-1's ligand, intracellular adhesion receptor 1 (ICAM-1) facilitates firm endothelium adhesion, prolonged contact with antigen-presenting cells, and binding to target cells for killing. While the downstream signaling pathways utilized by LFA-1 are vastly conserved they allow for highly disparate responses. Here, we summarize the roles of LFA-1 and ongoing studies to better understand its functions and regulation.