IL-34 Actions on FOXP3+ Tregs and CD14+ Monocytes Control Human Graft Rejection

Abstract

Cytokines are major players regulating immune responses toward inflammatory and tolerogenic results. In organ and bone marrow transplantation, new reagents are needed to inhibit tissue destructive mechanisms and eventually induce immune tolerance without overall immunosuppression. IL-34 is a cytokine with no significant homology with any other cytokine but that acts preferentially through CSF-1R, as CSF-1 does, and through PTPζ and CD138. Although IL-34 and CSF-1 share actions, a detailed analysis of their effects on immune cells needs further research. We previously showed that both CD4 + and CD8 + FOXP3 + Tregs suppress effector T cells through the production of IL-34, but not CSF-1, and that this action was mediated through antigen-presenting cells. We showed here by single-cell RNAseq and cytofluorimetry that different subsets of human monocytes expressed different levels of CSF-1R, CD138, and PTPζ and that both CD4 + and CD8 + FOXP3 + Tregs expressed higher levels of CSF-1R than conventional T cells. The effects of IL-34 differed in the survival of these different subpopulations of monocytes and RNAseq analysis showed several genes differentially expressed between IL-34, CSF-1, M0, M1, and also M2 macrophages. Acute graftvs.-host disease (aGVHD) in immunodeficient NSG mice injected with human PBMCs was decreased when treated with IL-34 in combination with an anti-CD45RC mAb that depleted conventional T cells. When IL-34-differentiated monocytes were used to expand Tregs in vitro, both CD4 + and CD8 + FOXP3 + Tregs were highly enriched and this effect was superior to the one obtained with CSF-1. Human CD8 + Tregs expanded in vitro with IL-34-differentiated allogeneic monocytes suppressed human immune responses in an NSG mouse aGVHD model humanized with hPBMCs. Overall, we showed that IL-34 induced the differentiation of human monocytes with a particular transcriptional profile and these cells favored the development of potent suppressor FOXP3 + Tregs.

IL-34 and CSF-1, deciphering similarities and differences at steady state and in diseases

“…In rat liver and heart transplantation models, IL‐34 was protective. In a heart transplantation model, we demonstrated that IL‐34 administration differentiates monocytes into regulatory macrophages, which expand and potentiate the suppressive function of CD4 + and CD8 + Tregs, inducing allogeneic tolerance 25,159 . CSF‐1 is also expressed by rat CD8 + Tregs but is not involved in their suppressive function since its blockade did not restore Teff cells proliferation in a suppressive assay, in contrast to IL‐34 blockade 38 .…”

“…CSF‐1 in association with TNFα induces in vitro MDSCs that decrease GVHD severity upon injection 173 . We showed that IL‐34‐macrophages are more efficient to expand Tregs than CSF‐1‐macrophages and those Tregs are highly suppressive as they delay GVHD development in immune humanized NSG mice 25 . In human clinical trials, the protective role of CSF‐1 administration needs to be further investigated because some results showed improved recovery associated with less development of chronic GVHD 174,175 .…”

“…The affinity of IL‐34 for PTPζ is lower than that of other ligands, however, IL‐34 is able to induce signaling even at low concentrations through interaction with chondroitin sulfate chains of the receptor 4 . PTPζ expression is high in all CD14 + blood monocytes, but higher levels are observed in non‐classical and intermediate monocytes versus classical ones 25 .…”

“…Although the expression and role of CSF‐1R is mostly described in monocytes and tissue macrophages (including osteoclasts and microglia) with a clear importance in their physiology, development, and survival, CSF‐1R is also expressed by conventional dendritic cells, B lymphocytes, smooth muscle cells of the vessels, intestinal epithelium (Paneth cells and colonic epithelial cells), neurons (including neural progenitor cells), kidney (renal proximal tubule epithelial cells), hematopoietic stem cells, oocytes, preimplantation embryos, decidual and trophoblastic cells 2,8,27–29 . Additionally, we have recently shown an expression of CSF‐1R by activated FoxP3 + Tregs raising the question about the function of these cytokines on Tregs 25 .…”

“…Slightly different phenotypes were also observed in IL‐34‐ versus CSF‐1‐differentiated macrophages as shown by over‐expression of HLA‐DR and eotaxin 2, and down‐regulation of CD54 and MCP‐1 22 . CSF‐1‐derived macrophages showed a better capacity for bacteria phagocytosis than IL‐34‐derived macrophages as shown by the up‐regulation of phagocytosis‐associated genes ( MARCO, A2M, VSIG4 , and COLEC12 ) 25 . IL‐34‐macrophages and CSF‐1‐macrophages express similar level of IL‐10 and low level of IL‐12 26 .…”