IL-34 deficiency impairs FOXP3+ Treg function in a model of autoimmune colitis and decreases immune tolerance homeostasis

Abstract

Background: Immune homeostasis requires fully functional Tregs with a stable phenotype to control autoimmunity. Although IL-34 is a cytokine first described as mainly involved in monocyte cell survival and differentiation, we recently described its expression by CD8⁺ Tregs in a rat model of transplantation tolerance and by activated FOXP3⁺ CD4⁺ and CD8⁺ Tregs in human healthy individuals. However, its role in autoimmunity and potential in human diseases remains to be determined.

Methods: We generated Il34^-/- rats and using both Il34^-/- rats and mice, we investigated their phenotype under inflammatory conditions. Using Il34^-/- rats, we further analyzed the impact of the absence of expression of IL-34 for CD4⁺ Tregs suppressive function. We investigated the potential of IL-34 in human disease to prevent xenogeneic GVHD and human skin allograft rejection in immune humanized immunodeficient NSG mice. Finally, taking advantage of a biocollection, we investigated the correlation between presence of IL-34 in the serum and kidney transplant rejection.

Results: Here we report that the absence of expression of IL-34 in Il34^-/- rats and mice leads to an unstable immune phenotype, with production of multiple auto-antibodies, exacerbated under inflammatory conditions with increased susceptibility to DSS- and TNBS-colitis in Il34^-/- animals. Moreover, we revealed the striking inability of Il34^-/- CD4⁺ Tregs to protect Il2rg^-/- rats from a wasting disease induced by transfer of pathogenic cells, in contrast to Il34^+/+ CD4⁺ Tregs. We also showed that IL-34 treatment delayed EAE in mice as well as GVHD and human skin allograft rejection in immune humanized immunodeficient NSG mice. Finally, we show that presence of IL-34 in the serum is associated with a longer rejection-free period in kidney transplanted patients.

Conclusion: Altogether, our data emphasize on the crucial necessity of IL-34 for immune homeostasis and for CD4⁺ Tregs suppressive function. Our data also shows the therapeutic potential of IL-34 in human transplantation and auto-immunity.

Highlights: -Absence of expression of IL-34 in Il34^-/- rats and mice leads to an unstable immune phenotype, with a production of multiple auto-antibodies and exacerbated immune pathology under inflammatory conditions. -Il34^-/- CD4⁺ Tregs are unable to protect Il2rg^-/- rats from colitis induced by transfer of pathogenic cells. -IL-34 treatment delayed EAE in mice, as well as acute GVHD and human skin allograft rejection in immune-humanized immunodeficient NSG mice.

Keywords: CRISPR/Cas9; Foxp3; IL-34; Treg; autoimmunity; immunotherapy; knockout; rat; tolerance.