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

Abstract

Understanding how embryonic stem cells and their derivatives interact with the adult host immune system is critical to developing their therapeutic potential. Murine embryonic stem cell-derived hematopoietic progenitors (ESHPs) were generated via coculture with the bone marrow stromal cell line, OP9, and then transplanted into NOD.SCID.Common Gamma Chain (NSG) knockout mice, which lack B, T, and natural killer cells. Compared to control mice transplanted with adult lineage-negative bone marrow (Lin− BM) progenitors, ESHP-transplanted mice attained a low but significant level of donor hematopoietic chimerism. Based on our previous studies, we hypothesized that macrophages might contribute to the low engraftment of ESHPs in vivo. Enlarged spleens were observed in ESHP-transplanted mice and found to contain higher numbers of host F4/80+ macrophages compared to BM-transplanted controls. In vivo depletion of host macrophages using clodronate-loaded liposomes improved the ESHP-derived hematopoietic chimerism in the spleen but not in the BM. F4/80+ macrophages demonstrated a striking propensity to phagocytose ESHP targets in vitro. Taken together, these results suggest that macrophages are a barrier to both syngeneic and allogeneic ESHP engraftment in vivo.

Clodronate Improves Survival of Transplanted Hoxb8 Myeloid Progenitors with Constitutively Active GMCSFR in Immunocompetent Mice

“…We sought to improve the survival of either HDPs or HDP-on cells and hypothesized that the removal of endogenous MΦs may improve survival, since improved survival of other cell types has been reported when animals are pretreated with MΦ-killing liposomal clodronate. 38 Endogenous MΦs are found in tissue niches that transplanted HDPs might be able to occupy. Successful transplantation of MΦs has been reported in Csf2rb -KO mice, which have impaired MΦ activity and reduced numbers of MΦs.…”

“…Successful transplantation of MΦs has been reported in Csf2rb -KO mice, which have impaired MΦ activity and reduced numbers of MΦs. 34 , 39 Additionally, F4/80 + cells phagocytose stem-cell-derived hematopoietic progenitor cells 38 and could be involved in removing HDPs. Hence, the removal of endogenous MΦs may be beneficial for HDPs by creating a niche and interfering with active removal by endogenous MΦs.…”

“…Phagocytic endogenous MΦs are responsible for the low post-transplantation survival of embryonic-stem-cell-derived hematopoietic progenitors in the highly immunodeficient NOD/SCID (non-obese diabetic/severe combined immunodeficiency) mouse model. 38 Long-term post-transplantation survival of wild-type bone-marrow-derived MΦs have been observed in a Csf2rb KO mouse, which possesses small numbers of dysfunctional MΦs. 34 , 39 In this model, the lack of functional MΦs may have allowed the functional MΦs to engraft and survive.…”

“…In support of this observation, MΦs are primarily responsible for removing embryonic-stem-cell-derived hematopoietic progenitors. 38 The enhanced HDP survival after liposomal clodronate treatment is probably due to the removal of endogenous MΦs. The endogenous MΦs may phagocytose transplanted HDPs or occupy tissue niches required by HDPs for longer term survival.…”

Innate Immune Determinants of Graft-Versus-Host Disease and Bidirectional Immune Tolerance in Allogeneic Transplantation

“…The latter in turn activates M1 macrophages (reviewed in [65]). M1 macrophages are associated with T cell-independent acute allograft rejection in a murine model of MHC-mismatched allo-HSCT [70][71][72]. Recipient M1 macrophages can exhibit potent cytotoxicity against the donor BM via the TLR4/toll-interleukin-1 receptor domain-containing adapter inducing interferon-β (TRIF) [71].…”