Myoungsun Son

C1q limits dendritic cell differentiation and activation by engaging LAIR-1.

C1q, the first component of complement, and leukocyte-associated Ig-like receptor 1 (LAIR-1; CD305), an inhibitory receptor expressed on hematopoietic cells, have both been associated with arrest of monocyte-derived dendritic cell (DC) differentiation and inhibition of Toll-like receptor activity in plasmacytoid DCs. Defects in both molecules have been implicated in susceptibility to, and progression of, systemic lupus erythematosus. Inhibitory signaling partners for C1q on monocytes and DCs remain undefined. Because C1q contains collagen-like motifs and LAIR-1 is a universal collagen receptor, we hypothesized that C1q is a functional ligand for LAIR-1. Binding analyses in cell-free systems and on the cell membrane demonstrate that C1q and its collagen tail associate with LAIR-1 and LAIR-2 (CD306), a soluble inhibitor of LAIR-1. Both C1q and its collagen tail trigger phosphorylation of LAIR-1 immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in monocytes. Functional analyses show that C1q-mediated inhibition of monocyte-DC differentiation and C1q-mediated inhibition of IFN-α production by plasmacytoid DCs were both reversed by LAIR-2. Moreover, C1q-mediated inhibition of DC differentiation was reversed by LAIR-1 siRNA. Thus, C1q is a functional ligand for LAIR-1 restricting immune cell differentiation and activation. The discovery of C1q interactions with LAIR-1 and LAIR-2 lends much needed insight into molecular mechanisms operating to prevent the loss of tolerance, particularly in systemic lupus erythematosus.

C1q-mediated repression of human monocytes is regulated by leukocyte-associated Ig-like receptor 1 (LAIR-1).

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by abnormal function of both the innate and the adaptive immune system, leading to a loss of tolerance to self-antigens. Monocytes are a key component of the innate immune system and are efficient producers of multiple cytokines. In SLE, inappropriate activation of monocytes is thought to contribute to the loss of self-tolerance. In this study, we demonstrate that type 1 interferon (IFN) production by CpG-challenged monocytes can be suppressed by C1q through activating leukocyte-associated Ig-like receptor-1 (LAIR-1), which contains immunoreceptor tyrosine-based inhibition motifs (ITIMs). The phosphorylation of LAIR-1 and the interaction of LAIR-1 with SH2 domain-containing protein tyrosine phosphatase-1 (SHP-1) were enhanced after LAIR-1 engagement by C1q. Moreover, engagement of LAIR-1 by C1q inhibited nuclear translocation of interferon regulatory factor (IRF)-3 and IRF5 in CpG-stimulated monocytes. These data suggest a model in which LAIR-1 engagement by C1q helps maintain monocyte tolerance, specifically with respect to Toll-like receptor-9-mediated monocyte activation.

Anti-DNA antibodies cross-react with C1q.

Systemic lupus erythematosus (SLE) is an autoimmune disorder that involves multiple organ systems and typically presents as a chronic inflammatory disease. Antibodies to double-stranded (ds) DNA are present in approximately 70% of patients and form nucleic acid containing immune complexes which activate dendritic cells through engagement of toll-like receptors, leading to a pro-inflammatory, pro-immunogenic milieu. In addition, anti-dsDNA antibodies deposit in kidneys to initiate glomerulonephritis. Antibodies to C1q have also been implicated in lupus nephritis and are found in 30-50% of patients. C1q is a known suppressor of immune activation and C1q deficiency is the strongest risk factor for SLE. We previously identified a subset of anti-DNA antibodies that binds the N-methyl-D-aspartate receptor. We now show that both mouse and human anti-DNA antibodies with this specificity bind C1q. These antibodies bind to Clq in glomeruli and exhibit decreased glomerular deposition in the absence of C1q. We propose that this subset of anti-DNA antibodies participates in lupus pathogenesis through direct targeting of C1q on glomeruli and also through removal of soluble C1q thereby limiting the ability of C1q to mediate immune homeostasis.

SLE-associated risk factors affect DC function

Numerous risk alleles for systemic lupus erythematosus (SLE) have now been identified. Analysis of the expression of genes with risk alleles in cells of hematopoietic origin demonstrates them to be most abundantly expressed in B cells and dendritic cells (DCs), suggesting that these cell types may be the drivers of the inflammatory changes seen in SLE. DCs are of particular interest as they act to connect the innate and the adaptive immune response. Thus, DCs can transform inflammation into autoimmunity, and autoantibodies are the hallmark of SLE. In this review, we focus on mechanisms of tolerance that maintain DCs in a non‐activated, non‐immunogenic state. We demonstrate, using examples from our own studies, how alterations in DC function stemming from either DC‐intrinsic abnormalities or DC‐extrinsic regulators of function can predispose to autoimmunity.

Fundamental role of C1q in autoimmunity and inflammation

C1q, historically viewed as the initiating component of the classical complement pathway, also exhibits a variety of complement-independent activities in both innate and acquired immunity. Recent studies focusing on C1q’s suppressive role in the immune system have provided new insight into how abnormal C1q expression and bioactivity may contribute to autoimmunity. In particular, molecular networks involving C1q interactions with cell surface receptors and other ligands are emerging as mechanisms involved in C1q’s modulation of immunity. Here, we discuss the role of C1q in controlling immune cell function, including recently elucidated mechanisms of action, and suggest how these processes are critical for maintaining tissue homeostasis under steady-state conditions and in preventing autoimmunity.

Evidence for C1q-mediated crosslinking of CD33/LAIR-1 inhibitory immunoreceptors and biological control of CD33/LAIR-1 expression

C1q collagen-like region (CLR) engaging and activating the LAIR-1 inhibitory immunoreceptor represents a non-complement mechanism for maintaining immune quiescence. Given the binding promiscuity of C1q’s globular region (gC1q), we hypothesized that C1q concurrently associates with distinct inhibitory immunoreceptors to produce C1q-mediated modulatory networking. Like LAIR-1, CD33 inhibitory immunoreceptors are highly expressed on monocytes. Binding CD33 restricts cell activation/differentiation; however, natural ligands for CD33 remain elusive. CD33 has IgC2-like domains potentially recognized by gC1q. Thus, we asked whether C1q binds to CD33 and if C1q mediates CD33/LAIR-1 crosslinking. Our findings demonstrate that C1q and gC1q interact with CD33 to activate its inhibitory motifs, while CLR does not. Whole C1q is required to crosslink CD33 and LAIR-1 and concurrently activate CD33/LAIR-1 inhibitory motifs. While C1q binds CD33C2 domains, decreased C1q-CD33 interactions resulting from sialic acid masking of CD33C2 domains suggests a process for regulating C1q-CD33 activity. Consistent with defective self-tolerance, CD33/LAIR-1 expression is reduced in systemic lupus erythematosus (SLE) myelomonocytes. The anti-inflammatory cytokine M-CSF, but not DC growth factors, sustains CD33/LAIR-1 expression on both healthy and SLE cells suggesting further biological control of C1q-CD33/LAIR-1 processes.

Constitutive Vagus Nerve Activation Modulates Immune Suppression in Sepsis Survivors

Patients surviving a septic episode exhibit persistent immune impairment and increased mortality due to enhanced vulnerability to infections. In the present study, using the cecal ligation and puncture (CLP) model of polymicrobial sepsis, we addressed the hypothesis that altered vagus nerve activity contributes to immune impairment in sepsis survivors. CLP-surviving mice exhibited less TNFα in serum following administration of LPS, a surrogate for an infectious challenge, than control-operated (control) mice. To evaluate the role of the vagus nerve in the diminished response to LPS, mice were subjected to bilateral subdiaphragmatic vagotomy at 2 weeks post-CLP. CLP-surviving vagotomized mice exhibited increased serum and tissue TNFα levels in response to LPS-challenge compared to CLP-surviving, non-vagotomized mice. Moreover, vagus nerve stimulation in control mice diminished the LPS-induced TNFα responses while having no effect in CLP mice, suggesting constitutive activation of vagus nerve signaling in CLP-survivors. The percentage of splenic CD4+ ChAT-EGFP+ T cells that relay vagus signals to macrophages was increased in CLP-survivors compared to control mice, and vagotomy in CLP-survivors resulted in a reduced percentage of ChAT-EGFP+ cells. Moreover, CD4 knockout CLP-surviving mice exhibited an enhanced LPS-induced TNFα response compared to wild-type mice, supporting a functional role for CD4+ ChAT+ T cells in mediating inhibition of LPS-induced TNFα responses in CLP-survivors. Blockade of the cholinergic anti-inflammatory pathway with methyllcaconitine, an α7 nicotinic acetylcholine receptor antagonist, restored LPS-induced TNFα responses in CLP-survivors. Our study demonstrates that the vagus nerve is constitutively active in CLP-survivors and contributes to the immune impairment.

High-Mobility Group Box 1-Induced Complement Activation Causes Sterile Inflammation

High-mobility group box 1 (HMGB1), a well-known danger-associated molecular pattern molecule, acts as a pro-inflammatory molecule when secreted by activated immune cells or released after necrotic cell damage. HMGB1 binds to immunogenic bacterial components and augments septic inflammation. In this study, we show how HMGB1 mediates complement activation, promoting sterile inflammation. We show that HMGB1 activates the classical pathway of complement system in an antibody-independent manner after binding to C1q. The C3a complement activation product in human plasma and C5b-9 membrane attack complexes on cell membrane surface are detected after the addition of HMGB1. In an acetaminophen (APAP)-induced hepatotoxicity model, APAP injection reduced HMGB1 levels and elevated C3 levels in C1q-deficient mouse serum samples, compared to that in wild-type (WT) mice. APAP-induced C3 consumption was inhibited by sRAGE treatment in WT mice. Moreover, in a mouse model of brain ischemia–reperfusion injury based on middle cerebral arterial occlusion, C5b-9 complexes were deposited on vessels where HMGB1 was accumulated, an effect that was suppressed upon HMGB1 neutralization. We propose that the HMGB1 released after cell necrosis and in ischemic condition can trigger the classical pathway of complement activation to exacerbate sterile inflammation.