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Introduction Systemic lupus erythematosus (SLE) is a group of systemic autoimmune disorders characterized by anti-nuclear Melanocyte stimulating hormone release inhibiting factor (ANA), rashes and photosensitivity, joint inflammation, nephritis, and other clinical criteria. SLE develops through the breakdown of three major checkpoints: adaptive immune tolerance, peripheral innate immune responsiveness, and end-organ inflammation [1]. Adaptive immune dysfunction produces autoantibodies leading to immune complex formation and deposition in the skin, joints, and kidneys. Innate immunity plays an important role in determining disease severity and progression [2]. Factors and pathways that modulate innate immunity impact the course of disease, particularly in end organ systems where most major, life-threatening disease manifestations occur. We focus here on the Axl receptor tyrosine kinase pathway. Axl is a member of the TAM family of receptor tyrosine kinases. Axl engagement by its ligand Gas6 causes receptor homodimerization, autophosphorylation, and downstream signaling. Axl and other TAM family members are well-studied in malignancy but also play significant roles in immunity [3], [4], [5], [6], [7], [8]. Among immune cells, Axl mRNA expression is highest in macrophages. Axl expression is also reported in some dendritic cells, γδT cells, fibroblasts, CD25+ T cells, and B1a B cells [9]. Disturbance of TAM family function contributes to autoimmunity. TAM family triple knockout mice develop severe systemic autoimmunity, and two members have been implicated directly in autoimmunity [10]. The TAM family member Mer aids in the clearance of apoptotic cells that may otherwise exacerbate inflammation and autoimmunity [11], [12]. Further, Axl knockout mice exhibit worse inflammation and demyelination than normal controls in induced experimental autoimmune encephalomyelitis (EAE) [13]. Two major functions of Axl may influence SLE. First, Gas6-stimulated Axl in the kidney is necessary for renal mesangial cell proliferation, which contributes to nephritis [14], [15], [16], [17]. Second, Axl signaling blocks the expression of inflammatory cytokines through the induction of Twist, a transcriptional inhibitor, in macrophages [18]. We and others quantified soluble Axl (sAxl) in the serum of SLE patients and healthy controls [19], [20], [21]. Patients with active SLE exhibit significantly elevated sAxl versus healthy controls and patients with inactive SLE. Others have proposed modulation of the Axl tyrosine kinase pathway as a new target for SLE, but no feasible mechanism has been determined [22]. We hypothesized that leukocyte Axl is cleaved in lupus, abrogating anti-inflammatory signaling and contributing to the disease.
Materials and methods
Results
Discussion and conclusion In the present study we observe that SLE patients and lupus-prone mice exhibit increased levels of sheared soluble Axl ectodomain (sAxl) in the blood and reduced surface Axl and active Y779-phosphorylated Axl on immune cells. This occurs despite increased Axl ligand Gas6 in SLE serum [19], [20], [21]. We demonstrate that matrix metalloproteases ADAM10 and TACE (ADAM17) cleave surface Axl on CD11b+ or CD14+ and CD19+ lupus leukocytes. Under normal conditions, Axl acts to transduce anti-inflammatory signals in macrophages through Twist-mediated suppression of inflammatory cytokines [18]. This represents a part of the innate immune checkpoint, and dysregulation of this system may contribute to the pathology of autoimmune disease. We demonstrate that the loss of surface Axl abrogates Axl-mediated anti-inflammatory activity in vitro and in vivo. Both Axl-deficient and lupus-prone mouse macrophages fail to induce Twist or block expression of IL-6 and TNF-alpha in response to Gas6. Axl-deficient BMDM worsen end organ damage in vivo. To our knowledge these are the first data to show the functional significance of Axl ectodomain shearing in lupus.