CD47 / SIRPα Blocking Assay Design: Avoiding the Pitfalls That Invalidate Your DataPUBLISHED
CD47 / SIRPα Blocking Assay Design: Avoiding the Pitfalls That Invalidate Your Data
TL;DR
CD47–SIRPα is a low-affinity, high-on-rate interaction — which makes it easy to mis-measure and easy to over-block. Design around three traps: (1) the red blood cell problem (RBCs are loaded with CD47), (2) avidity/aggregation artifacts in SPR, and (3) the V1/V2 SIRPα allotype. Get those right and your blocking assay will actually predict phagocytosis.
1. Biology in one paragraph
CD47 ("don't eat me" signal) is expressed broadly — including on red blood cells — and engages SIRPα (CD172a) on macrophages to suppress phagocytosis. Tumor cells upregulate CD47 to escape innate immunity; anti-CD47 or anti-SIRPα blockade restores phagocytosis and is an active clinical strategy. The CD47–SIRPα interaction is low affinity (reported µM-range KD) with fast kinetics — a very different beast from high-affinity checkpoint pairs like TIGIT–CD155.
2. Pitfall #1 — The RBC problem
Because RBCs express high levels of CD47, any assay containing blood products (or poorly washed effectors) has a massive CD47 sink. Consequences:
- Anti-CD47 antibodies get titrated away → apparent low potency.
- Phagocytosis readouts are confounded by RBC binding and hemolysis background.
Fix: use purified CD47 (IgV domain) and SIRPα for binding assays; in cell-based work, rigorously wash macrophages and use CD47-blocking controls (e.g., pre-adsorb on CD47-negative cells) to prove specificity.
3. Pitfall #2 — Low-affinity kinetics in SPR/BLI
- µM-range KD with fast on/off rates → high-density surfaces cause rebinding and make the interaction look tighter than it is.
- Fix: low-density capture (Rmax ≤20 RU), high flow rate, and report both KD and kon/koff.
- Aggregated CD47 or SIRPα-Fc fusions create avidity → apparent nM binding that won't reproduce in cells. Always check SEC on the COA; prefer monomeric ECD reagents.
4. Pitfall #3 — SIRPα allotypes
SIRPα has common variants (notably V1 and V2) that differ in CD47 affinity. V1 is the most studied; V2 binds CD47 with different affinity and is common in some populations. If your program is clinical-facing, know which allotype your reagent is — and whether your antibody blocks both.
5. Assay design summary
| Component | Recommendation |
|---|---|
| Binding assay | Low-density SPR/BLI; CD47 IgV (or ECD) vs SIRPα V1 ECD; report KD, kon, koff |
| Blocking assay | Competition SPR (CD47 + anti-CD47 → SIRPα) and/or cell-based phagocytosis |
| Controls | CD47-negative cells; isotype antibody; SIRPα-blocking positive control |
| QC to request | SEC monomer %, endotoxin, SPR KD vs the partner protein, allotype statement (V1/V2) |
6. Related resources
- [CD47 / SIRPα Matched Pair](/pairs/cd47-sirpa)
- [SPR/BLI Characterization Service](/services/spr-bli)
- [Special Constructs (SIRPα V1/V2, CD47 variants)](/services/custom-protein)
What next?
- [Buy the CD47/SIRPα Pair](/request?utm_campaign=cd47-sirpa-blocking&utm_source=site&utm_medium=organic&utm_content=cta-buy) — monomeric ECD reagents with SPR data
- [Request a Sample](/request?utm_campaign=cd47-sirpa-blocking&utm_source=site&utm_medium=organic&utm_content=cta-sample) — validate the pair in your own setup
- [Discuss Your Experiment](/request?utm_campaign=cd47-sirpa-blocking&utm_source=site&utm_medium=organic&utm_content=cta-discuss) — get help with RBC interference and blocking assay design
Next steps