The BALB/c-specific polymorphic SIRPA enhances its affinity for human CD47, inhibiting phagocytosis against human cells to promote xenogeneic engraftment

Abstract

It has been shown that in xenotransplantation of human cells into immunodeficient mice, the mouse strain background is critical. For example, the nonobese diabetic (NOD) strain is most efficient, the BALB/c is moderate, and the C57BL/6 is inefficient for human cell engraftment. We have shown that the NOD-specific polymorphism of the signal regulatory protein-alpha (Sirpa) allows NOD SIRPA to bind human CD47, and the resultant "don't eat me" signaling by this binding prevents host macrophages to engulf human grafts, thereby inhibiting rejection. Here we tested whether the efficient xenotransplantation capability of the BALB/c strain is also mediated by the SIRPA-CD47 self-recognition system. BALB/c SIRPA was capable of binding to human CD47 at an intermediate level between those of C57BL/6 SIRPA and NOD SIRPA. Consistent with its binding activity, BALB/c-derived macrophages exhibited a moderate inhibitory effect on human long-term culture-initiating cells in in vitro cultures, and showed moderate phagocytic activity against human hematopoietic stem cells. The increased affinity of BALB/c SIRPA for human CD47 was mounted at least through the BALB/c-specific L29V SNP within the IgV domain. Thus, the mouse strain effect on xenogeneic engraftment might be ascribed mainly to the binding affinity of strain-specific polymorphic SIRPA with human CD47. This information should be useful for developing a novel immunodeficient strain with superior efficiency for xenogeneic transplantation of human cells.

Anti‐human SIRPα antibody is a new tool for cancer immunotherapy

“…It was demonstrated that human CD47 minimally binds to SIRPα from either BALB/c, 129 or C57BL/6 mice . In contrast, human CD47 can strongly bind to SIRPα from NOD mice . Indeed, the specific binding of a recombinant human CD47‐Fc fusion protein to BMDM from DKO mice (129/BALB/c mixed background) was minimal and only observed at the high concentration of the recombinant protein (Figure , right panel).…”

“…The CD47‐SIRPα interaction has also been demonstrated to function as an inhibitory signal for phagocytosis of cancer cells by macrophages . It was also demonstrated that human CD47 can nicely bind to SIRPα from nonobese diabetic (NOD) mice, whereas it minimally interacts with SIRPα from either BALB/c, 129 or C57BL/6 mice . Such a difference between mouse SIRPα subtypes for their binding abilities to human CD47 is likely attributable to amino acid variations in the N‐terminal Ig‐V domain of SIRPα .…”

F4/80+Host Macrophages Are a Barrier to Murine Embryonic Stem Cell-Derived Hematopoietic Progenitor EngraftmentIn Vivo

“…To our knowledge, a comparison of the expression levels and functional abilities of macrophage inhibitory ligands and in the NSG versus 129 strains has not been performed. However, there is evidence of mouse strain differences in the binding of the ability of the NOD, BALB/s, and B6 forms of SIRP α to bind to the human macrophage inhibitor receptor CD47 and prevent phagocytosis of xenogeneic cells in mice [53]. Our observations that ESHPs are phagocytosed by host macrophages in vitro and that clodronate treatment promotes higher donor chimerism from ESHPs in mice in vivo strongly suggest that ESHPs stimulate innate immune responses and that control of macrophage-induced immune rejection should be considered in the field as new hematopoietic derivatives are produced from ESCs or ESC-like induced pluripotent stem cells for in vivo transplantation.…”

Targeting CD47: the achievements and concerns of current studies on cancer immunotherapy

“…BALB/c may possibly be such a candidate (22). It may be necessary to investigate and compare the affinities between human CD47-SIRPα and human CD47-Sirpα BALB/c , which would help determine whether BRG (BALB/c-Rag2 null Il2r γ null ) animal would be a better substitute for NSG as the xenograft host for the study of human CD47.…”