Unlocking CD47: Immune Regulation and Cancer's Achilles Heel

What is CD47?

CD47, also recognized as Cluster of Differentiation 47, is a transmembrane glycoprotein prominently displayed on diverse cell surfaces throughout the body. It stands as a pivotal player in an array of biological processes, encompassing cell adhesion, immune modulation, apoptosis, and immune evasion.

A core function of CD47 lies in its role as a "don't eat me" signal. It establishes an interaction with signal regulatory protein alpha (SIRPα), which is present on immune cells. This interaction transmits an inhibitory signal, communicating that the cell is part of the body's own "self" and should be spared from immune attack or engulfment. This mechanism safeguards normal tissues and cells from immune system attack, thus maintaining immune tolerance. However, some cancer cells exploit this defense mechanism by upregulating CD47 expression, effectively evading immune recognition and annihilation. This manipulation promotes unchecked tumor growth and immune evasion.

Research has substantiated that disrupting the CD47-SIRPα interaction can bolster the immune system's prowess to target cancer cells. In response, novel anti-cancer therapies are emerging, focusing on impeding CD47 function to heighten the immune system's efficiency in eradicating tumor cells. These therapies, often referred to as CD47 antibodies or CD47 antibody-drug conjugates, attach to CD47 molecules and intervene in their interaction with SIRPα. This intervention empowers immune cells to better identify and assail tumor cells. By interfering with CD47's protective role, these therapies hold the potential to reshape the landscape of cancer treatment by harnessing the immune system's capabilities to combat tumors more robustly.

Structure and expression of CD47

CD47, previously known as integrin-associated protein (IAP), is a prominent transmembrane glycoprotein with a molecular weight of approximately 50 kDa. Belonging to the immunoglobulin family, it features a heavily glycosylated extracellular N-terminal IgV domain, along with five transmembrane domains and a concise C-terminal cytoplasmic tail. Notably, the composition of these three domains varies among humans and animals, resulting in the existence of four alternative isoforms.

Unlike its earlier designation as IAP, CD47 is now recognized as its more suitable name due to its extensive presence across virtually all cell types within the body, even in cells that do not express integrins, such as erythrocytes. Additionally, CD47 can manifest in a larger and more complex form, linked with heparin and chondroitin sulfate glycosaminoglycan. This particular form is evident in both human and murine T-cells, as well as endothelial cells. It takes on the role of signal inhibition after T-cells bind to thrombospondins (TSPs), contributing to a more intricate understanding of CD47's functional diversity.

Interaction with extracellular ligands

The Relationship Between SIRPα and CD47

SIRPα, also referred to as CD172a, Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 (SHPS-1), or brain immunoglobulin-like molecule with tyrosine-based activation motifs (BIT), is a transmembrane glycoprotein expressed on myeloid cells like granulocytes, monocytes, macrophages, dendritic cells, and their precursors, including hematopoietic stem cells, as well as neuronal cells [6]. Structurally, it comprises an extracellular N-terminal domain with three immunoglobulin-like domains and a cytoplasmic domain housing two tyrosine phosphorylation sites and four immunoreceptor tyrosine inhibitory motifs (ITIMs) [7].

The interaction between CD47 and the N-terminal domain of SIRPα on phagocytic cells triggers a phosphorylation response of the ITIM. This activation subsequently engages protein tyrosine phosphatases (PTPases), particularly Src homology region 2 (SH2) domain-containing phosphatase 1 (SHP-1) and 2 (SHP-2) [8]. As a result, the dephosphorylation of immunoreceptor tyrosine activation motifs (ITAM) occurs, preventing the macrophages from engulfing the targeted cell and transmitting a "don't eat me" signal to the innate immune system. This newly identified signal appears to be exploited by cancer cells, allowing them to elude detection and destruction by the immune system [9].

The Relationship Between TSPs and CD47

Thrombospondins (TSPs) are extracellular matrix calcium-binding glycoproteins that play a crucial role in regulating cell motility, proliferation, and differentiation [10]. Currently, five isoforms of TSPs are recognized, with TSP-1 being the first endogenous ligand identified for CD47. TSP-1 is secreted by vascular and inflammatory cells, acting as a regulator of various cellular functions. Notably, it constitutes a significant component of platelet α granules and is released upon activation [11].

The interaction between TSP-1 and CD47 occurs through the binding of TSP-1's C-terminal binding domain (CBD) peptide 4N1K to the extracellular IgV region of CD47. This interaction holds pivotal roles in numerous biological processes, including inflammation, immune responses, cellular proliferation, apoptosis, adhesion, and migration [12]. Furthermore, this binding contributes to thrombus formation by influencing platelet activation and aggregation, as well as maintaining homeostasis. TSP-1's antiangiogenic properties stem from its ability to hinder nitric oxide (NO) pro-survival responses in endothelial and vascular smooth muscle cells (VSMC) [13]. The TSP1-CD47 interaction has been reported to suppress NO signaling in various cell types, subsequently promoting osteoclastogenesis [14].

The disruption of the CD47-TSP-1 interaction through TSP-1-blocking antibodies or the down-regulation of CD47 on tumor cells using RNA interference has been found to prevent tumor-induced osteoclast formation in conditions like multiple myeloma [15]. Additionally, research indicates that disrupting the TSP-1/CD47 interaction yields positive outcomes in cancer therapy [16].

Regulation of CD47 expression

The regulation of CD47 expression in cancer cells involves a complex interplay of various factors, including cytokines, oncogenes, microRNAs, and enzymes. Notably, IL-6 triggers the activation of the STAT3 signaling pathway, leading to an increase in CD47 expression. Both TNF-α and IL-1β contribute to the upregulation of CD47 at the transcriptional level, a process mediated by the activation of the NF-κB pathway and enhanced binding of NF-κB to the CD47 promoter. Similarly, IFN-γ has been shown to elevate CD47 expression, although the specific mechanisms remain to be further elucidated.

Under hypoxic conditions, the hypoxia-inducible factor 1 (HIF-1) directly binds to the CD47 promoter, promoting the transcriptional expression of CD47. Another regulatory mechanism involves MYC, which directly binds to the CD47 promoter to facilitate the transcriptional upregulation of CD47. On the microRNA front, miR-708, miR-192, miR-222, miR-133a, miR-155, miR-200a, and miR-340 exert their influence by suppressing CD47 expression through direct targeting of the 3′-untranslated region (UTR) of CD47 mRNA.

At the post-translational modification level, an enzyme called QPCTL modifies the N-terminal pyroglutamate formation of the CD47 protein, impacting the interaction between CD47 and SIRPα. This interaction is crucial for the "don't eat me" signal mechanism, wherein CD47 prevents immune cells from attacking healthy cells. Understanding these multifaceted regulatory pathways offers insights into potential therapeutic strategies targeting CD47 for various medical conditions.

Significance of CD47 Detection in Clinical Settings

The clinical detection of CD47 holds immense significance due to its pivotal role in the progression and management of various diseases. Here are the key reasons why CD47 clinical detection is of paramount importance:

1. Tumor Therapy: CD47 detection is crucial in antitumor therapy as it aids in assessing the interaction between tumor cells and the immune system. Tumor cells often overexpress CD47, which interacts with immune cell SIRPα, preventing immune cell phagocytosis. This immune evasion mechanism allows tumor cells to escape destruction. Detecting CD47 levels helps unravel the immune evasion strategies employed by tumors, leading to more effective treatment planning.
2. Immunotherapy Strategy: The expression of CD47 significantly impacts the outcome of immunotherapy. Numerous immunotherapy approaches aim to reinstate immune cell attack on tumors. Detecting CD47 expression assists in predicting patient responses to immunotherapy, guiding treatment selection, and monitoring treatment efficacy.
3. Autoimmune Diseases: Abnormal CD47 expression correlates with autoimmune diseases like rheumatoid arthritis and systemic lupus erythematosus. CD47 detection aids in identifying signs of immune system imbalance, facilitating better treatment and management of immune-related conditions.
4. Neurodegenerative Diseases: CD47 expression is linked to neurodegenerative diseases such as Parkinson's and Alzheimer's. Detecting CD47 expression provides insights into the pathogenesis of these diseases, paving the way for exploring novel therapeutic approaches.
5. Cardiovascular Disease: Altered CD47 expression is associated with inflammation and cardiovascular system injury. Detecting CD47 levels helps assess cardiovascular disease risk and progression, guiding patient management strategies.

In conclusion, clinical CD47 detection significantly contributes to the diagnosis and treatment of tumor therapy, immunotherapy, autoimmune diseases, neurodegenerative diseases, and cardiovascular diseases. It not only unveils disease mechanisms but also guides tailored treatment decisions, ultimately enhancing patient outcomes and survival rates.

CD47 Protein

Recombinant Human Leukocyte Surface Antigen Cd47 (CD47) Protein (His), Active

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