Stealth functionalization of biomaterials and nanoparticles by CD47 mimicry

Purpose: Biomaterials and nanoparticles (NPs) used for medical purposes often trigger immune recognition, inflammation, and rapid clearance by the reticuloendothelial system (RES)/mononuclear phagocyte system (MPS). PEGylation and cell-membrane camouflaging are common stealth strategies, but they have limitations, including anti-PEG immune responses. CD47, a “don’t-eat-me” signal, is emerging as an alternative stealth functionalisation to improve biocompatibility and prolong circulation.

Hypothesis: No formal experimental hypothesis. Central thesis: CD47 functionalisation—using CD47 protein or CD47-mimicking peptides—can confer “marker-of-self” properties to biomaterials and NPs, inhibit macrophage phagocytosis via the CD47–SIRPα axis, reduce inflammation, prolong blood circulation, and improve targeted drug delivery. CD47 mimicry peptides are advantageous over full-length CD47 protein due to lower cost, easier chemical conjugation, and better biocompatibility.

Aims: Describe the role of CD47 in the immune system, especially the CD47–SIRPα “don’t-eat-me” signalling pathway. - Review anti-phagocytic properties of CD47-modified biomaterials. - Discuss common stealth-nanoparticle strategies: PEGylation and cell membrane-camouflaged NPs. - Highlight CD47 functionalisation and CD47-mimicking peptides for stealth modification of biomaterials and NPs. - Summarise experimental evidence and potential clinical applications for tumours and autoimmune diseases.

Delivery system: Stealth strategies reviewed: PEGylation; cell membrane-camouflaged NPs (RBC membranes); CD47 functionalisation using recombinant CD47 or CD47-mimicking peptides. - Biomaterials/NPs: polyvinyl chloride (PVC), polyurethane (PU), poly(2-methoxyalkylacrylate) (PMEA)-modified tubing, polystyrene beads, PLGA NPs, graphene oxide (GO) nanosheets, biodegradable photoluminescent waterborne polyurethane (BPLP-WPU) micelles, gold NPs, liposomes, magnetic NPs, silica NPs. - Payloads/agents: paclitaxel, myelin peptide-loaded MHC complexes, TGF-β1, imaging probes. - Targeting/functionalization: CD47 extracellular domains; CD47-mimicking peptides such as “self-peptide” (SP), CD47 MEVG-WYRSPSFRSVVHLYRNGK, and other SIRPα-binding sequences. - Disease contexts: tumours (A549, SCC7, murine breast cancer), experimental autoimmune encephalomyelitis (EAE), inflammatory responses to cardiovascular biomaterials, autoimmune disorders.

Approach: Review of preclinical literature; no primary experiments. In vitro systems include CHO-K1 cells, macrophages, neutrophil migration assays, and cocultures. In vivo models include rat subdermal polymeric conduit perfusion, mouse tumour models (A549, SCC7, breast cancer), and EAE mice. Biomaterials include PVC/PU tubing, polystyrene beads, PLGA NPs, GO nanosheets, and BPLP-WPU micelles. No clinical trials are reported; the review focuses on experimental proof-of-concept studies.

Key methods: Phagocytosis assays: macrophage uptake, internalised beads per cell, flow cytometry. - Protein/biomaterial characterisation: CD47 density on surfaces, flow cytometry. - In vivo blood perfusion: Chandler Loop Apparatus, neutrophil activation (CD18 expression), cell attachment to blood-contacting surfaces. - Biodistribution and retention: tumour accumulation, brain retention in EAE, blood circulation half-life. - Therapeutic efficacy: antitumor activity, tumour necrosis, survival rate, immunomodulation. - Imaging: fluorescence and light microscopy; in vivo imaging of GO nanosheets and micelles.

Key results: CD47-coated polystyrene beads reduced macrophage phagocytosis by 50% compared with CD47-free beads; anti-CD47 antibody abolished the effect. - CD47 immobilisation on PVC tubing almost completely inhibited cell attachment to blood-contacting surfaces; unmodified PVC and PMEA-modified tubing showed ~30-fold greater cell attachment. - CD47-functionalised surfaces significantly reduced CD18 expression, a marker of neutrophil activation, compared with freshly drawn blood not exposed to PVC tubing. - CD47-expressing artificial antigen-presenting cells (aAPCs): concentration-dependent inhibition of macrophage ingestion, with 40–60% uptake of aAPC^CD47+; theoretical CD47 density on aAPCs ~350 molecules/μm² vs ~250 molecules/μm² on human RBCs. - Paclitaxel-loaded CD47-linked nanobeads: up to 4-fold increase in blood half-life, increased tumour drug retention, and better antitumor efficacy; only one CD47 molecule per 45,000 nm² required for a 60-nm NP to show stealth effect. - Tolerogenic PLGA NPs with CD47 peptide in EAE mice: CD47+ NPs retained in brain 175–275% more than CD47− NPs after 12 h; fewer co-localisation with dendritic cells and macrophages; longer in vivo retention. - CD47-mimic SP-coated graphene oxide nanosheets: tumour accumulation 2.0-fold and 2.3-fold greater than physical mixture and PEG-GO, respectively, at 8 h; maintained over 48 h. - SP-modified BPLP-WPU micelles: prolonged blood lifetime, complete tumour necrosis, and 100% survival over 100 days in a murine breast cancer model.

Interpretation: CD47 functionalisation is a promising alternative stealth strategy to PEGylation and cell-membrane camouflaging. CD47-mimicking peptides can reduce macrophage uptake, prolong circulation, enhance tumour accumulation, and improve therapeutic outcomes in preclinical models. The authors conclude that CD47-functionalised biomaterials and NPs may have future clinical applications in targeted therapies against tumours and autoimmune diseases, though clinical translation remains to be established.

Limitations: Review article; no primary data. - PEGylation limitations: anti-PEG antibodies, accelerated blood clearance (ABC), complement activation. - Cell membrane-camouflaged NPs: manufacturing, characterisation, and clinical validation remain limited. - CD47 functionalisation is still at the experimental/proof-of-concept stage; no clinical trials are reported. - Use of full-length CD47 protein is difficult due to size and folding requirements; CD47-mimicking peptides are an alternative but require further optimisation. - Long-term safety, immunogenicity, and efficacy of CD47-functionalised biomaterials/NPs in humans remain unknown. - Potential off-target effects of CD47–SIRPα modulation and species differences need further investigation.