The role of CD18 and regulatory T cells in skin inflammation

Abstract

β2-integrins are essential for cell-cell communication and the extravasation of leukocytes from blood into inflamed tissues. These surface receptors are composed of a variable α chain and a constant β chain known as CD18 and are specifically expressed by leukocytes. Four different α chains pair intracellularly with the common β chain: αL (CD11a), αM (CD11b), αX (CD11c) and αD (CD11d). The leukocyte function-associated antigen 1 (LFA-1, also known as CD11a/CD18) is the only β2-integrin family member expressed on T cells. As part of the immunological synapse (IS), LFA-1 binds to intercellular adhesion molecules (ICAM) expressed by other leukocytes, especially by dendritic cells (DC) and other antigen-presenting cells (APC) and thereby enhances T cell receptor (TCR) mediated T cell activation. LFA-1 engaging ICAM-1 on endothelial cells also facilitates T cell entry into peripheral tissues, enabling migration to sites of inflammation and infection. Moreover, LFA 1/ICAM-1 signaling, in combination with TCR stimulation, influences human T cell polarization by modulating T helper cell 1 (TH1), TH2, and TH17 responses. Humans with reduced or defective β2-integrin expression develop Leukocyte Adhesion Deficiency Syndrome Type-1 (LAD-1). This rare disease results from mutations in the gene encoding for the common β-chain of the β2-integrin family (Itgb2, also known as CD18). The most prominent clinical symptoms are profound leukocytosis and high susceptibility to infections. At the same time, LAD-1 patients are prone to develop autoimmune diseases like inflammatory bowel diseases (IBD). However, the molecular and cellular mechanisms that result in coexisting immunodeficiency and autoimmunity are still unresolved. Regulatory T cells (Treg) are crucial for the maintenance of peripheral tolerance and the regulation of immune responses. The immunosuppressive function of Treg depends on different mechanisms, cytokine production, and suppressive receptor interactions with T cells, APC, and other types of leukocytes. Mutational inactivation of the transcription factor forkhead box p3 (Foxp3), a crucial key regulator of Treg, causes the loss of functional Treg, and leads to severe systemic multi-organ autoimmunity. So far, different mouse strains with deleted CD11a or CD18 expression, respectively, have been established to study the role of LFA-1 for T cell functions under homeostatic conditions and in disease models. Those strains, in which the β subunit of β2-integrins was constitutively deleted, recapitulate the leukocytosis, immune deficiency, and seemingly contradictory autoimmune aspects of LAD-1. In common with the CD18-deficient mouse strains, CD11a-deficient mice show a defect in Treg function but have no obvious skin phenotype. However, as all leukocytes are affected, it is difficult to delineate the direct contribution of specific leukocyte populations to the various phenotypic alterations. This work aimed to understand the role of LFA-1 for Treg functions and the manifestation of autoimmunity and inflammation. We hypothesized that LFA-1-deficient Treg are functionally distorted and thereby may substantially contribute to the manifestation of (auto)inflammatory skin diseases. To overcome the limitations of the existing mouse models, we generated mice specifically lacking CD18 on Treg (CD18Foxp3), resulting in defective LFA 1. These mice spontaneously developed systemic immune activation, severe skin inflammation, including ears and tails, and lung tissue damage resembling an atopy-like phenotype. Furthermore, a profound activation of the TH2 pathway in CD18Foxp3 mice was observed, which also exhibited massively elevated IgE, eosinophilia, and enhanced allergic contact hypersensitivity responses. Furthermore, gene expression profiles of LFA-1-deficient Treg revealed increased expression of several genes associated with atopic dermatitis and asthma. Besides the atopy-like phenotype, CD18Foxp3 mice develop a coexisting autoimmune phenotype characterized by the presence of autoantibodies in the serum against basement membrane components and/or basal keratinocytes. In this respect, the spontaneous phenotype of CD18Foxp3 mice shares many aspects with that of scurfy mice. In addition, LFA 1 deficient Treg developed less contacts with DC, which in turn increased DC activation, which may result in T cell hyperactivation and exacerbated type II immune responses due to TH2-promoting DC. Deficiency of LFA-1 on Treg thereby has a similar negative effect on self tolerance as the loss of Foxp3. These observations suggest an indispensable role of LFA 1 on Treg to maintain immune homeostasis and implicate LFA-1-deficient Treg in the pathogenesis of the autoimmunity seen in LAD-1 patients