THE CIS ASSOCIATION OF CD47 WITH INTEGRIN Mac-1 REGULATES MACROPHAGE RESPONSES BY STABILIZING THE EXTENDED INTEGRIN CONFORMATION

Refolding of the recombinant IgV domain of CD47 from E. coli for NMR studies

CD47 is a widely expressed integrin-associated transmembrane protein that regulates many macrophage functions, including phagocytosis. Since many cancer cells overexpress CD47 to evade the immune system, targeting CD47 has been proposed as a strategy to enhance macrophage-mediated destruction of cancer cells. Consequently, developing antagonists that block the CD47-SIRPα interaction has drawn attention. However, the exclusive use of eukaryotic cell culture to produce CD47-IgV precludes isotope enrichment of the domain. This prevents the use of solution NMR as a tool for identifying CD47 inhibitors. Here, we describe a two-step refolding protocol for the CD47-IgV domain from inclusion bodies produced in E. coli. The yield of CD47-IgV is ∼30 mg/L of culture. The refolded domain interacts with the anti-CD47 antibody B6H12 with an affinity similar to glycosylated CD47-IgV produced in mammalian cells and binds the IgV domain of SIRPα. The method allowed us to produce 15N and 13C enriched CD47-IgV for NMR. The chemical shift assignments of the CD47-IgV domain backbone atoms confirmed that the refolded protein has the same secondary structure as the crystal structure of CD47-IgV produced in mammalian cells. Furthermore, NMR data showed refolded CD47-IgV interacts with SIRPα-IgV similarly to glycosylated CD47-IgV. The application of this method can advance the progress of biochemical investigations of the interaction between CD47-IgV and SIRPα and will be useful for the discovery of antibodies, small molecules, and peptides targeting the CD47/SIRPα axis in vivo.

Fibrin induces infiltration of macrophages and neutrophils via integrin αMβ2 and triggers aortic dissection

BACKGROUND AND PURPOSE

Infiltration of macrophages and neutrophils plays a crucial role in the occurrence of aortic dissection (AD), while the mechanism elucidating their infiltration remains unknown. The present study aimed to delineate the underlying mechanism and provide a potential therapeutic strategy to attenuate AD progression.

EXPERIMENTAL APPROACH

A model of AD was established in male mice using β-aminopropionitrile and angiotensin II. Proteomic analysis, histological evaluation, flow cytometry, western blot, multiple fluorescence staining and adhesion assays were used to evaluate fibrin and inflammatory cells during AD progression. Fibrinogen-lowering drugs and fibrinogen γ-chain knockout (Fgg+/-) mice were also used to evaluate the fibrin-integrin αMβ2 interaction.

KEY RESULTS

Fibrin deposition was confirmed by proteomic analysis and histological staining, accompanied by infiltration of macrophages and neutrophils detected by flow cytometry during the progression of AD. After confirming that macrophages and neutrophils infiltrated at the sites where fibrin was deposited by immunofluorescence, an association between fibrin and the integrin αMβ2 was disclosed using protein-protein interaction analysis and immunofluorescence. The pivotal role of interactions between fibrin and integrin αMβ2 in AD progression was confirmed by cell adhesion in vitro, down-regulation of fibrin using batroxobin and Fgg+/- mice in vivo. The relevance of fibrin and integrin αMβ2 was also found in patients with AD.

CONCLUSION AND IMPLICATIONS

Fibrin plays a crucial role in triggering AD through recruiting macrophages and neutrophils via integrin αMβ2. Regulation of fibrin deposition or inhibition of the interaction between fibrin and integrin αMβ2 provide a potential therapy against AD.

Biochemical and biophysical mechanisms macrophages use to tune phagocytic appetite

Macrophages phagocytose, or eat, pathogens, dead cells and cancer cells. To activate phagocytosis, macrophages recognize 'eat me' signals like IgG and phosphatidylserine on the target cell surface. Macrophages must carefully adjust their phagocytic appetite to ignore non-specific or transient eat me signal exposure on healthy cells while still rapidly recognizing pathogens and debris. Depending on the context, macrophages can increase their appetite for phagocytosis, to prioritize an effective immune response, or decrease their appetite, to avoid damage to healthy tissue during homeostasis. In this Review, we discuss the biochemical and biophysical mechanisms that macrophages employ to increase or decrease their sensitivity or capacity for phagocytosis. We discuss evidence that macrophages tune their sensitivity via several mechanisms, including altering the balance of activating and inhibitory receptor expression, altering the availability of activating receptors, as well as influencing their clustering and mobility, and modulating inhibitory receptor location. We also highlight how membrane availability limits the capacity of macrophages for phagocytosis and discuss potential mechanisms to promote membrane recycling and increase phagocytic capacity. Overall, this Review highlights recent work detailing the molecular toolkit that macrophages use to alter their appetite.

CD47 and thrombospondin-1 contribute to immune evasion by Porphyromonas gingivalis

Porphyromonas gingivalis is a gram-negative anaerobic bacterium linked to periodontal disease. Remarkably, P. gingivalis thrives in an inflamed environment rich in activated neutrophils. Toll-like receptor 2 (TLR2) recognition is required for P. gingivalis to evade innate immune killing; however, the mechanisms through which P. gingivalis uncouples host inflammation from bactericidal activity are only partially known. Since integrin activation and alternative signaling are implicated in P. gingivalis TLR2-mediated immune escape, we explored the role of CD47, a widely expressed integrin-associated protein known to suppress phagocytosis and implicated as an interacting partner with other innate immune receptors. We found that CD47 associates with TLR2, and blocking CD47 leads to decreased intracellular P. gingivalis survival in macrophages in a manner dependent on the bacterial major fimbria. In vivo, CD47 knock-out mice cleared P. gingivalis more efficiently than wild-type mice. Next, we found increased expression and secretion of the CD47 ligand thrombospondin-1 (TSP-1) following P. gingivalis infection. Secreted TSP-1 broadly protected P. gingivalis and other periodontitis-associated bacterial species from neutrophil bactericidal activity. Therefore, CD47-TLR2 cosignaling in response to P. gingivalis induces TSP-1 that in turn suppresses neutrophil activity, an effect that can explain how species such as P. gingivalis survive in an inflamed environment and cause dysbiosis.

The human CD47 checkpoint is targeted by an immunosuppressive Aedes aegypti salivary factor to enhance arboviral skin infectivity

The Aedes aegypti mosquito is a vector of many infectious agents, including flaviviruses such as Zika virus. Components of mosquito saliva have pleomorphic effects on the vertebrate host to enhance blood feeding, and these changes also create a favorable niche for pathogen replication and dissemination. Here, we demonstrate that human CD47, which is known to be involved in various immune processes, interacts with a 34-kilodalton mosquito salivary protein named Nest1. Nest1 is up-regulated in blood-fed female A. aegypti and facilitates Zika virus dissemination in human skin explants. Nest1 has a stronger affinity for CD47 than its natural ligand, signal regulatory protein α, competing for binding at the same interface. The interaction between Nest1 with CD47 suppresses phagocytosis by human macrophages and inhibits proinflammatory responses by white blood cells, thereby suppressing antiviral responses in the skin. This interaction elucidates how an arthropod protein alters the human response to promote arbovirus infectivity.

The role of CD47 in non-neoplastic diseases

CD47 is a 50 kDa five-spanning membrane receptor that plays a crucial role in multiple cellular processes, including myeloid cell activation, neutrophils transmigration, vascular remodeling, leukocyte adhesion and trans-endothelial migration. Recent studies have revealed that CD47 is a highly expressed anti-phagocytic signal in several types of cancer, and therefore, blocking of CD47 has shown an effective therapeutic potential in cancer immunotherapy. In addition, CD47 has been found to be involved in a complex interplay with microglia and other types of cells, and increasing evidence indicates that CD47 can be targeted as part of immune modulatory strategies for non-neoplastic diseases as well. In this review, we focus on CD47 and its role in non-neoplastic diseases, including neurological disorders, atherosclerosis and autoimmune diseases. In addition, we discuss the major challenges and potential remedies associated with CD47-SIRPα-based immunotherapies.

The TSP1-CD47-SIRPα interactome: an immune triangle for the checkpoint era

The use of treatments, such as programmed death protein 1 (PD1) or cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) antibodies, that loosen the natural checks upon immune cell activity to enhance cancer killing have shifted clinical practice and outcomes for the better. Accordingly, the number of antibodies and engineered proteins that interact with the ligand-receptor components of immune checkpoints continue to increase along with their use. It is tempting to view these molecular pathways simply from an immune inhibitory perspective. But this should be resisted. Checkpoint molecules can have other cardinal functions relevant to the development and use of blocking moieties. Cell receptor CD47 is an example of this. CD47 is found on the surface of all human cells. Within the checkpoint paradigm, non-immune cell CD47 signals through immune cell surface signal regulatory protein alpha (SIRPα) to limit the activity of the latter, the so-called trans signal. Even so, CD47 interacts with other cell surface and soluble molecules to regulate biogas and redox signaling, mitochondria and metabolism, self-renewal factors and multipotency, and blood flow. Further, the pedigree of checkpoint CD47 is more intricate than supposed. High-affinity interaction with soluble thrombospondin-1 (TSP1) and low-affinity interaction with same-cell SIRPα, the so-called cis signal, and non-SIRPα ectodomains on the cell membrane suggests that multiple immune checkpoints converge at and through CD47. Appreciation of this may provide latitude for pathway-specific targeting and intelligent therapeutic effect.