The Role of PPARd in Alloreactive CD8 T cell Metabolism

Abstract

Graft-versus-host disease (GVHD) is the most common cause of non-relapse mortality following allogeneic hematopoietic stem cell transplantation. Using a pre-clinical model of GVHD, previous work has found that CD8 T cells recovered on day 7 post-transplant upregulate the transcription factor, peroxisome proliferator activated receptor delta (PPARd). While PPARd has been shown to be a key regulator of fatty acid oxidation (FAO) in other tissues, it’s role in T cells is not well studied. We hypothesize that PPARd drives FAO in alloreactive CD8 T cells and that CD8 T cells lacking PPARd will be unable to oxidize fat and thus unable to cause GVHD. To investigate this hypothesis, ex vivo FAO was measured in day 7 WT versus PPARd KO CD8 T cells by quantitating conversion of 3H-palmitate to 3H2O. Production of 3H2O decreased by 50% in KO CD8 T cells, suggesting an inability to fully oxidize fat. To understand the mechanism underlying this decrease, we performed RNA sequencing and identified thirty genes that were differentially expressed in WT versus KO CD8s including 8 genes related to FAO. Strikingly, despite a decreased ability to oxidize fat, PPARd KO CD8 T cells were recovered in equal numbers on day 7 post transplantation, suggesting that CD8 T cells lacking PPARd adopt alternative metabolic pathways to generate sufficient energy for in vivo proliferation and activation. Ultimately, our analyses indicate that PPARd is required for FAO in alloreactive CD8 T cells, but this process is dispensable for short term proliferation and/or survival of CD8 T cells during GVHD. Future directions will identify compensatory metabolic pathways utilized by PPARd KO T cells through a combination of in vitro nutrient drop out experiments and metabolomic flux analyses.