Signal-regulatory protein α from the NOD mouse binds human CD47 with an exceptionally high affinity-- implications for engraftment of human cells.

Lai Shan Kwong,A Neil Barclay,Deborah Hatherley,Marion H Brown

doi:10.1111/imm.12290

Abstract

One common way to study human leucocytes and cancer cells in an experimental in vivo situation is to use mice that have been genetically engineered to lack an immune system and prevent human cell rejection. These mice lack CD132 and either RAG2 or the catalytic subunit of the DNA-dependent protein kinase, to make the mice deficient in lymphocytes and natural killer cells. The NOD mouse strain provides a better background for engraftment than other strains due to stronger engagement of the signal-regulatory protein-α (SIRPα) inhibitory receptor with human CD47 (hCD47) resulting in a ‘don't-eat-me’ signal. To determine the molecular parameters that determine this major functional effect in the NOD mouse we measured the affinity of hCD47 for SIRPα from various mouse strains. Human CD47 bound SIRPα from the NOD mouse with an affinity 65 times greater than SIRPα from other mouse strains. This is due mainly to the NOD SIRPα lacking two amino acids in domain 1 compared with other mouse strains. Remarkably the SIRPα(NOD) binds hCD47 with 10 times the affinity of the syngeneic hCD47/hSIRPα interaction. This affinity is outside the normal range for affinities for leucocyte surface protein interactions and raises questions as to what is the optimal affinity of this interaction for engraftment and what other xenogeneic interactions involved in homeostasis may also not be optimal.

Highlights

The recent development of methods to manipulate the genetic makeup of mice to allow engraftment of human cells such as tumours or bone marrow cells into mice has become a major tool for research into the human immune system and cancer
The NOD mouse strain provides a better background for engraftment than other strains due to stronger engagement of the signal-regulatory protein-a (SIRPa) inhibitory receptor with human CD47 resulting in a ‘don’t-eat-me’ signal
To determine the molecular parameters that determine this major functional effect in the NOD mouse we measured the affinity of human CD47 (hCD47) for SIRPa from various mouse strains

Summary

Introduction

The recent development of methods to manipulate the genetic makeup of mice to allow engraftment of human cells such as tumours or bone marrow cells into mice has become a major tool for research into the human immune system and cancer. To prevent rejection of human cells, the host mouse is made immunodeficient by disabling lymphocyte development through halting the recombination of T-cell and B-cell receptors – either using SCID or RAG mutants combined with a CD132 (c chain of several cytokine receptors) knockout that prevents natural killer cell development.[1] The NOD mouse provides a good host for engraftment and a major contribution in working out the molecular basis for this advantage was made by Takenaka et al.,[2] who mapped the locus responsible to the SIRPa gene. High levels of CD47 correlate with poor prognosis, indicating the importance of the molecular parameters of the CD47/SIRPa interaction and providing the rationale for the current development of reagents to block the interaction, overcome inhibition of myeloid cells and lead to greater cancer cell death.[7,8,9]

Methods

Results

Discussion

Conclusion