The integrin family of proteins is comprised of 24 heterodimeric transmembrane adhesion receptors. Each integrin is formed through the non-covalent association of 1 α-subunit and 1 β-subunit; currently, 16 α-subunits and 8 β-subunits have been identified. Their expression on virtually all human cells and their complex signaling mechanisms explain their wide variety of biological roles, including blood clotting, cell adhesion, and migration.

Due to their extensive importance in biological systems, elucidating integrin signaling and receptor function has been of great interest since their characterization as adhesion molecules over 30 years ago. Integrins are important signaling proteins that mediate interactions of the cell with extracellular matrix proteins and with other cells via cell-surface ligands. Integrins exist in a continuum between a folded inactive form with low affinity for their ligand and an extended high affinity conformation (1), although even bent integrins are able to bind ligand in rare instances (2). As immune cell adhesion and extravasation into lymph nodes and tissues forms part of initiating an effective immune response, β₂ integrin conformation on the surface of leukocytes needs to be tightly regulated. β₂ integrins on the surface of circulating leukocytes tend, therefore, to be largely inactive (2) until inside-out and outside-in signaling trigger integrin-mediated adhesion and extravasation into tissue (Figure 1).

Inside-out signaling modifies how cells interact with their environment by facilitating receptor affinity and avidity (4) to allow binding to extracellular ligands. Outside-in signaling, on the other hand, mediates intracellular events in response to their environment by eliciting downstream signaling cascades in response to receptor occupation. The complex details of integrin signaling are reviewed elsewhere (5, 6) and are beyond the scope of this review. Briefly, inside-out signaling is mediated by talin (7) and kindlin (8, 9) binding to the intracellular domain of the β₂ subunit, a process initiated by chemokine receptor or Toll-like receptor (TLR) engagement (10, 11), which results in a conformational change in the integrin from a low-affinity to a high-affinity state. Outside-in signaling is then initiated by ligand binding to high-affinity integrin receptors (Figure 1). Downstream signaling events mediate the formation of focal complexes and adhesions through rearrangement of the actin cytoskeleton. The relative importance of affinity and avidity on integrin signaling and function is heavily debated (12, 13), but dynamic interaction between these processes and both inside-out and outside-in signaling seems likely (14).

β₂ integrins are the focus of this review, as they are exclusively found on leukocytes and therefore of particular importance for the immune system. They mediate cell recruitment into lymphoid organs and inflamed tissues by facilitating firm leukocyte arrest on endothelial cells and extravasation after cell rolling (15); cellular interactions between leukocytes including immunological synapse formation (16); and intracellular signaling cascades that influence cytoskeletal rearrangement, activation, proliferation and impact on cellular responses to TLRs. Importantly, through these three processes, β₂ integrins can have either pro-inflammatory or anti-inflammatory outcomes. The β₂ integrin subunit (CD18) can pair with one of four α-subunits (α_L—CD11a, α_M—CD11b, α_X—CD11c, and α_D—CD11d), forming leukocyte function-associated antigen-1, Mac1/CR3 (macrophage-1 antigen, complement receptor 3), P150,95/CR4 (complement receptor 4), and CD18/CD11d, respectively (Figure 2). For consistency, this review will utilize only the CD nomenclature. Both function and cell-specific expression of β₂ integrins vary according to the α-subunit involved.

The main ligands for the β₂ integrin family members are outlined in Figure 2. Briefly, CD11a binds to intracellular adhesion molecule-1 (ICAM-1), -2, -3, and -5, which are expressed by a variety of cells including leukocytes and endothelial cells, thereby mediating leukocyte recruitment to lymph nodes and sites of inflammation as well as cell–cell adhesion. CD11b binds the complement proteins iC3b and C4b with high affinity, mediating phagocytosis of complement-coated particles but can also bind ICAM-1, fibrinogen, and more than 40 other ligands (17). The sequence of CD11c is very close to that of CD11b, and indeed CD11c binds several of the same ligands including iC3b, ICAM-1, and fibrinogen. Multi-ligand binding capacity of CD11d is proposed to largely overlap with CD11b and includes ECM-associated proteins fibronectin, fibrinogen, vitronectin, Cyr61, and plasminogen (18).

This review will provide an overview of β₂ integrin expression on monocytes, macrophages and DCs, before exploring the paradoxical pro-inflammatory and regulatory roles of β₂ integrins in immune regulation in three key aspects of immune function: recruitment and migration, cellular interactions, and downstream cell signaling (Figure 3). We will furthermore review how dysregulated integrin signaling could contribute to inflammatory and autoimmune conditions and introduce the therapeutic potential of targeting β₂ integrins.

The expression of β₂ integrin subunits varies in different leukocyte subsets and between mice and humans. In general terms, CD11a is expressed on all leukocytes at varying levels, while CD11b, CD11c, and CD11d are predominantly expressed by monocytes, macrophages and DCs. Specifically, in humans, monocytes express all four β₂ integrin-associated alpha subunits (CD11a, CD11b, CD11c, and CD11d) with CD11a and CD11b expression greater than CD11c (19, 20); macrophages express CD11a and CD11b at lower levels than monocytes together with CD11c at similar levels to monocytes (21); while DCs mainly express CD11c together with CD11a, though some DC subsets also express CD11b (22). While CD11d has received less attention than the other β₂ integrins due to the absence of commercially available human antibodies, Miyazaki and colleagues showed CD11d expression on monocyte-derived DCs and macrophages as well as most circulating monocytes (23). To complement the scarce available data, mRNA expression data for the CD11d subunit ITGAD were consulted. While Villani and colleagues (24) find monocytes to express highest levels of ITGAD mRNA, the Expression Atlas (25) reports highest expression in DCs, with ITGAD expression in monocytes remaining below detectable threshold. However, overall both RNAseq data sets show that CD11d mRNA expression is very low in monocytes, macrophages, and DCs. Table 1 provides the details of expression of all β₂ integrin subunits in human and murine monocytes, macrophages, and DCs. Where available, expression analysis on DC subsets is given using the Guilliams nomenclature (26), which was recently confirmed and expanded by Villani and colleagues (24).

Animal studies have been instrumental in elucidating integrin function in monocytes, macrophages, and DCs. β₂ integrins are highly conserved across species, with mice, rats, and rabbits most commonly used as models. Importantly β₂ integrin-deficient mice are considered an appropriate model of the human condition leukocyte adhesion deficiency (LAD) where β₂ integrin expression or function is lost (57). However, while β₂ integrin structure is largely similar between species, cellular expression levels can vary significantly. A common example is CD11c, which in mice is predominantly expressed by conventional (cDCs) and plasmacytoid DCs (pDCs), although can also be expressed on lymphocyte subsets. In humans, on the other hand, CD11c is expressed not only on DCs but also monocytes, macrophages, granulocytes, and natural killer cells (19, 38). Animal and human studies therefore have to be compared with great care, and validation of concepts conceived in animal models in human cells remains a priority in elucidating the functions of β₂ integrins.

Evidence for the role of β₂ integrins in contributing to the development and progression of inflammatory and autoimmune conditions is accumulating. Considering that β₂ integrin signaling can have opposing functions depending on subunit pairing and the immune cell type it is expressed on, it is not surprising that these receptors play important roles in both contributing to as well as negatively regulating inflammatory processes.

Human genetic studies point to a role of β₂ integrins in inflammation and autoimmunity. A polymorphism of ITGAM, the CD11b subunit, increases the risk for the autoimmune disease systemic lupus erythematosus (87) (SLE), which shares genetic risk factors with rheumatoid arthritis (RA) (88). Disease risk for inflammatory bowel disease, similarly characterized by dysregulation of immune function specifically in the intestine, increases with amplified expression of alleles for both ITGAL, encoding CD11a, and the β₂ integrin ligand ICAM1 (89). Gene expression of CD11d in humans and mice was found to be increased in white adipose tissue in obesity, a condition characterized by an increase in systemic inflammation (90). Furthermore, CD11d activation led to increased IL-1β expression (23), which when overproduced can contribute to a variety of autoinflammatory conditions (91). While dysregulation of β₂ integrin signaling seems likely to be involved in a variety of autoimmune diseases and inflammatory conditions, exact mechanisms are still unclear, and further investigation of both signaling pathways and genetic basis will be needed to fully elucidate their complex roles.

Recent studies have focused on β₂ integrin involvement in RA, which serves as an excellent example of the opposing roles β₂ integrins can take in disease. Expression of CD11a is increased in inflamed synovial tissue, where it is hypothesized to contribute to cell activation and on-going joint destruction (92, 93), but not in peripheral blood of RA patients. However, as CD11a is also involved in facilitating immune cell migration to sites of inflammation, clear-cut cause and effect of the presence of activated β₂ integrins in the synovium is difficult to establish. Blocking all β₂ integrins reduced inflammation in a rabbit RA model (94), while absence of CD11a led to complete resistance to disease induction in a KB × N serum transfer mouse model of arthritis (95). Furthermore, both a small molecule antagonist against CD11a and a CD11a-monoclonal antibody (mAb) proved to be similarly successful in reducing both inflammatory-mediated bone destruction and cytokine mRNA levels within the murine joint (96, 97). Mice with mutations in the β₂ integrin ligand ICAM-1 also show reduced susceptibility to the collagen-induced arthritis (CIA) model (98). Clearly, CD11a–ICAM-1 interactions are essential for leukocyte recruitment to the inflamed joint.

However, evidence is emerging that other β₂ integrins may function to control inflammation in arthritis. CD11b KO mice, for example, show exacerbated joint pathology in the KB x N serum transfer model of arthritis, underlining the starkly opposite roles different β₂ integrins can play (95). A recent study replicated these results in a CIA model and, furthermore, showed that exacerbated joint pathology resulted from elevated IL-6 levels and an increase in Th17 cell priming, which could be rescued by introducing a CD11b-expressing DC cell line (99). On the other hand, blocking CD11b immediately before onset of disease significantly reduced disease burden in two different models of arthritis (CIA and a DBA/1 to severe combined immunodeficiency transfer model of arthritis) (100), suggesting that the role of CD11b in inflammatory arthritis may differ depending on the cell type involved and the disease stage.

When considering the importance, as well as the obvious complexity, of β₂ integrin function in autoimmune diseases such as RA, therapeutically targeting β₂ integrins will have to be carefully balanced but also holds great promise to offer novel treatment options.

Modulating integrin function to improve mal-adaptation or excessive activation of the immune system is of great interest in a variety of autoimmune and inflammatory conditions. However, achieving efficacy without immunocompromising side effects might prove challenging. Here, we discuss the progress and failures in developing integrin-targeted therapies and speculate on the routes forward for success.

To date, targeting integrins therapeutically has had mixed success in the clinic. The only mAb targeting β₂ integrins, Efalizumab, which targets CD11a, was originally developed as a treatment for psoriasis (101). However, several patients presented with the potentially fatal disease progressive multifocal leukoencephalopathy (PML), caused by reactivation of the JC virus, which results in a white matter disorder of the brain (102). Although the mechanism of PML development in Efalizumab-treated patients was not investigated, we speculate that viral reactivation was likely either due to the loss of immune cell recruitment to the brain to control the virus (103) or due to the mAb itself crossing the blood–brain barrier (104). Due to the occurrence of PML, Efalizumab was withdrawn from European and American markets due to its associated safety issues in 2009.

Although targeting β₂ integrins has so far failed in the clinic, targeting other integrins for the treatment of colitis and Crohn’s disease has proved successful. The mAb against the α₄ integrin, Natalizumab, was developed for the treatment of multiple sclerosis and Crohn’s disease (105, 106). This mAb binds to α₄β₁ and α₄β₇. However, PML also occurs in some Natalizumab-treated patients (integrin α₄β₁ is also involved in leukocyte recruitment to the brain) and so is no longer used widely (107). More recently, a specific α₄β₇ targeting mAb Vedolizumab has shown success in safety efficacy in Crohn’s disease and ulcerative colitis. This success story underlines the potential of targeting integrins for therapeutic purposes.

In order to realize the potential of targeting β₂ integrins therapeutically, it will be necessary to improve the strategy. As indicated by the success of Vedolizumab over Natalizumab, one way to do this is to target the right integrin subunit(s) in order to reduce the likelihood of side effects. Targeting CD11a, in the form of Efalizumab, proved unsuccessful in the clinic. As CD11a is expressed by almost all leukocytes, has vital roles in leukocyte recruitment and has immunoregulatory effects in mononuclear phagocytes, the resulting serious side effects from targeting this molecule therapeutically are, perhaps, not surprising. Targeting other CD11 subunits might be a more effective strategy. For example, CD11b, CD11c, and CD11d have a more restricted pattern of expression in leukocytes (predominantly on monocytes, macrophages, and DCs), which may make these molecules more suitable targets. Importantly, it is vital that we consider the pro- and anti-inflammatory functions of β₂ integrin subunits and design drugs to target them appropriately. CD11b, for example, has clear regulatory roles in macrophages and DCs, meaning that we could potentially exploit this immuno-suppressive pathway by activating, rather than blocking, this integrin subunit. Such a strategy may have less risk of serious side effects. It is, therefore, essential that we fully understand the specific functions of individual integrin subunits in different leukocyte populations in order to target β₂ integrin subunits effectively in the clinic.

Another option to explore is blocking not the β₂ integrin itself, but the ligand of interest. Targeting the CD11a and CD11b ligand, ICAM-1, has shown beneficial results especially in early RA (108), although immunogenicity of the mAb in question restricts clinical use (109) and problems caused by impaired leukocyte recruitment prevail.

Further potential difficulties in developing integrin-targeting therapy include the close signaling relationships that exist in some integrins, potentially leading to complex downstream effects mediated even by an activating mAb highly specific for a β₂ integrin (110). Carefully elucidating downstream signaling pathways and further increasing drug specificity is therefore essential to bring more integrin therapeutics into the clinic.

Innovative avenues to explore include computationally designed integrin proteins with constitutively activated or inactivated subunits, which could find applications in both pharmacological testing and therapy (111). Furthermore, developing small molecular drugs targeting β₂ integrins viable for oral use remains a priority, as it could offer an alternative way to yield the same beneficial results without the dangerous side effects of mAbs. An example is the small molecule CD11b agonist, Leukadherin-1, which previous studies found to reduce monocyte-mediated TNF-release by mimicking natural ligand binding. When NK cells and monocytes were pre-treated with Leukadherin-1, innate inflammatory signaling in human ex vivo studies was suppressed (112). While the study noted some caveats, for example, the differences of CD11b function on different cell types (78), the drug is still being explored for the treatment of SLE. Another small molecule currently in development is the CD11a antagonist BMS-587101, which acts by reducing CD11a-mediated adhesion and to a lesser effect T cell proliferation. It significantly improved both murine models of lung inflammation and transplant viability (113).

Continuous effort to increase drug specificity and further understand their complex delicate signaling networks will be needed to bring β₂ integrin-targeting drugs into the clinic. But while the use of integrin-targeting drugs has been contentious in the past, their potential in treating a wide variety of immune diseases is enormous and should not be neglected.

This review explored the opposing nature of β₂ integrin pro- and anti-inflammatory functions in three main immune functions, making them prime candidates to be both important mediators and regulators of the immune system. The first is migration, which allows for targeted immune cell recruitment to sites of infection and tissue damage. The second is adhesion, not only preceding immune cell extravasation at sites of inflammation, but also an important factor in initiating the adaptive immune response by facilitating cellular interactions. Finally, immune cell signaling, which allows for fine-tuned cooperation between a wide variety of immune cells. Considering the fact that β₂ integrins play a complex role in three important areas of the immune system and their differential expression on monocytes, macrophages and DCs, it becomes clear that the variety of studies presented in this review is by no means exhaustive. The common message is evident: β₂ integrins are involved in complex immunoregulatory signaling pathways. However, in addition to their well-established pro-inflammatory roles in recruitment and activation, β₂ integrins also have essential immunoregulatory functions. Dysregulated integrin signaling, expression and surface activation is therefore likely to contribute to a variety of inflammatory and autoimmune conditions. Elucidating the function of β₂ integrins further therefore promises to provide novel therapeutic targets for various disorders, RA being just one example.

CH and VM designed the structure of the review. LS wrote the first draft. CH and VM revised the manuscript. LS composed the figures. All authors have seen and agreed on the finally submitted version of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The handling Editor declared a shared affiliation, though no other collaboration, with the authors LS and CH.