Pre-Transplant CSF-1 Therapy Expands the Recipient Macrophage Pool and Modulates Graft Versus Host Disease After Allogeneic Hematopoietic Cell Transplantation
Authors: Hashimoto Daigo , Chow Andrew , Greter Melanie , Leboeuf Marylene , Ginhoux Florent , Teshima Takanori , Merad Miriam 
Published: October 2019

Abstract
Abstract 242
Host antigen presenting cells are critical to activate allo-reactive T cells and to initiate acute graft versus host disease (GVHD) after allogeneic bone marrow transplantation (BMT). However, while the role of host dendritic cells (DCs) in GVHD has been established, the contribution of host macrophages to GVHD has not been clearly addressed. In this study we revisited the role of host macrophages in GVHD. To this end we used an anti-CSF-1R monoclonal antibody (aCSF1R) to reduce macrophages but not DC numbers in lymphoid organs. We treated recipient C57BL/6 mice with αCSF1R on days -5 to -3 followed on day 0 by TBI and i.v. injection of BM cells and splenocytes isolated from MHC-mismatched BALB/c donor. We found that host macrophages that persist in the spleen and LN of recipient mice were severely reduced by αCSF1R, whereas host DCs remained unaffected by the treatment. We also found that αCSF1R-treatement significantly enhanced GVHD morbidity and mortality (Figure left panel) after allogeneic BMT, enhanced donor T cell expansion in recipient spleen, LN and liver, and increased IFN-gamma and TNF-alfa sera levels compared to mice treated with control IgG. Similar results were obtained when low dose Lip-Clod was administered 10 days prior to transplant in order to deplete macrophages in lymphoid tissues but not host DC whose half-life in lymphoid tissues does not exceed three days allowing them to recover to normal at the time of transplant. Our results revealed that in contrast to a previous report in which higher Lip-Clod doses administered 7 and 2 days prior to transplant led to the depletion of both DCs and macrophages and improved GVHD, low dose Lip-Clod administered 10 days prior to transplant, depleted host macrophages but not DCs and aggravated GVHD.
We also explored the mechanisms by which host macrophages could potentially control donor T cell expansion induced by host DC after allo-HCT. We found that host macrophages isolated from allogeneic recipients inhibited the proliferation of allo-reactive T cells co-cultured with host C57BL/6 DC partly through a cell-cell contact pathway. Host macrophages were also able to engulf and reduce alloreactive T cells after few hrs of culture. Consistently, 18 hrs after allo-HCT, the number of donor T cells accumulating in the recipient spleen and mesenteric LNs were higher in Ab-treated mice compared to the control groups and CFSE-labeled donor T cells were engulfed by splenic macrophages during the first day of transplant and prior to the initiation of donor T cell proliferation. These results suggest that host macrophages limit the expansion of donor T cells through their ability to engulf donor T cell and to inhibit donor T cell proliferation induced by host DC stimulation.
Further supporting the importance of host macrophages in regulating the development of GVHD, we found that pre-transplant administration of the cytokine CSF-1 increased the number of spleen macrophages, limited the expansion of donor allo-reactive T cells and improved GVHD outcome (Figure right panel). Altogether, we showed that in contrast to host DCs, host macrophages modulate GVHD through their ability to limit donor T cell activation and expansion. In addition, our data also identify pre-transplant CSF-1 injection as a novel clinical strategy for the prevention of GVHD in patient candidate for allo-HCT.

Disclosures:
No relevant conflicts of interest to declare.