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

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-alpha (SIRP alpha) 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 SIRPa from various mouse strains. Human CD47 bound SIRP alpha from the NOD mouse with an affinity 65 times greater than SIRP alpha from other mouse strains. This is due mainly to the NOD SIRPa lacking two amino acids in domain 1 compared with other mouse strains. Remarkably the SIRP alpha(NOD) binds hCD47 with 10 times the affinity of the syngeneic hCD47/hSIRP alpha 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.