Key Points: C1q can form a multimolecular signaling complex with HMGB1, RAGE, and LAIR-1 in lipid rafts. C1q and HMGB1 together promote monocytes to differentiate to an anti-inflammatory phenotype.

Significance: Monocyte to macrophage differentiation is critical to inflammation and resolution of inflammation and can be regulated by high-mobility group box 1 (HMGB1) and HMGB1 plus C1q, respectively. These are 2 evolutionarily old and highly conserved molecules. While HMGB1 causes a positive-feedback loop between the proinflammatory lipid mediator leukotriene B4 (LTB₄) and IRF5, an important regulator of inflammatory macrophage polarization, HMGB1 plus C1q increases production of specialized proresolving lipid mediators (SPMs) lipoxin A4, resolvin D1, and resolvin D2, which prevent IRF5 transcription. As nonresolving inflammation is a common condition, we designed an HMGB1–C1q mimetic peptide that can polarize monocytes to an antiinflammatory phenotype in vitro and in vivo.

Significance: Clinical studies have shown that defects in type I interferon (IFN) production or antibodies to IFN appear to correlate with severe COVID-19 infection. Here, we demonstrate that proinflammatory cytokines but not type I IFN are produced by hypoxic human monocytes. We show that hypoxia suppresses production of type I IFN but not nuclear factor-κB–dependent proinflammatory cytokines. We demonstrate that hypoxia-inducible factor-1α is a direct transcriptional suppressor of interferon regulatory factors, the transcriptional activators of type I IFN. These findings may aid in understanding and control of impaired IFN production by severe acute respiratory syndrome coronavirus 2 infection.

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.

The importance of the complement component C1q has been highlighted by its involvement in autoimmunity, infection, inflammatory diseases, and tumors. The unique tulip-like structure of C1q has both a collagen-like stalk (C1q tail) and heterotrimeric globular head (gC1q), each with different binding specificities, and the binding of these components to their respective receptors leads to functional complexities in the body and bridges innate and adaptive immunity. This review describes the fundamental roles of C1q in various microenvironments and focuses on the importance of the interactions of C1q and its receptors with the inhibitory receptor LAIR-1 in maintaining homeostasis. Current therapeutic opportunities modulating LAIR-1 are also discussed.

The disruption of the immune tolerance induces autoimmunity such as systemic lupus erythematosus and vasculitis. A chromatin-binding non-histone protein, high mobility group box 1 (HMGB1), is released from the nucleus to the extracellular milieu in particular environments such as autoimmunity, sepsis and hypoxia. Extracellular HMGB1 engages pattern recognition receptors, including Toll-like receptors (TLRs) and the receptor for advanced glycation endproducts (RAGE). While the HMGB1-RAGE axis drives inflammation in various diseases, recent studies also focus on the anti-inflammatory effects of HMGB1 and RAGE. This review discusses current perspectives on HMGB1 and RAGE’s roles in controlling inflammation and immune tolerance. We also suggest how RAGE heterodimers responding microenvironments functions in immune responses.