Peroxisome Proliferator Activated Receptor-Delta Controls Fat Oxidation in Alloreactive T Cells
Authors: Waltz Gail , Rajeevan Arati , Dobbs Andrea , Denby Elisabeth , Byersdorfer Craig 
Published: October 2019

Abstract
Allogeneic hematopoietic stem cell transplantation (alloHSCT) is a curative treatment for high-risk leukemia and multiple non-malignant hematologic disorders. However, the routine use of alloHSCT remains limited by acute graft-versus-host disease (GVHD), where activated donor T cells attack and destroy host tissues in the skin, gastrointestinal tract, and liver. We have previously shown that GVHD-causing T cells increase fat oxidation compared to both syngeneic and naive T cells. To explore this adaptation mechanistically, we studied the role of the transcription factor Peroxisome Proliferator Activated Receptor delta (PPAR-δ) in alloreactive donor T cells during the initiation of GVHD. By day 7 post-transplant, alloreactive T cells up-regulated PPAR-δ >5-fold compared to pre-transplant naive T cells (p<0.0001, Figure 1A). Furthermore, PPAR-δ was necessary for maximally severe GVHD, as major-MHC mismatched B6xDBA2 F1 mice receiving donor T cells deficient in exon 4 of PPAR-δ (PPAR-δ KO) survived longer than mice receiving wildtype (WT) T cells (p<0.007, Figure 1B).
We next investigated the mechanism underlying this observed decrease in GVHD severity. As a transcription factor, PPAR-δ controls expression of multiple genes involved in fat transport and oxidation. To determine its role in alloreactive cells, RNA was collected from CD4 and CD8 T cells on day 7 post-transplant and levels of 8 known PPAR-δ targets quantitated by RT-PCR. These 8 targets were selected from a longer list of genes known to be up-regulated in alloreactive cells. Transcript levels of both carnitine palmitoyl transferase-1a (CPT-1a) and CD36 decreased in PPAR-δ KO CD8 T cells (Figure 2A), with decreases in CD36 protein levels confirmed by immunoblot (Figure 2B). Interestingly, changes in CPT-1a and CD36 did not occur in PPAR-δ KO CD4 T cells. To assess the functional consequence of these changes, day 7 WT versus PPAR-δ KO CD8 T cells were plated with 3H-palmitate and fat oxidation measured ex vivo. Consistent with a decrease in expression of genes involved in fat transport and mitochondrial fat import, fat oxidation decreased by >75% in PPAR-δ KO CD8 cells (Figure 2C). However, despite these decreases, the number of PPAR-δ KO CD8 T cells recovered on day 7 post-transplant was equivalent to WT T cells (Figure 3A, left panel). In contrast, PPAR-δ KO CD4 T cell numbers decreased by 30% on day 7, despite equivalent levels of CD36 and CPT1a (Figure 3A, right panel).
Finally, we addressed whether pharmacologic inhibition of PPAR-δ might also effectively mitigate GVHD. Administration of the PPAR-δ inhibitor GSK3787 on days 3-6 post-transplant substantially decreased the number of donor T cell recovered on day 7 (Figure 3B), with PPAR-δ impairment corroborated by a decrease in CPT1a gene transcription. However, instead of improving recipient health, GSK3787 treatment instead worsened weight loss and increased rates of post-transplant morbidity and mortality. From these data, we conclude that PPAR-δ is necessary in alloreactive T cells to cause maximally severe GVHD and that mechanistically, an absence of PPAR-δ impairs fat oxidation in CD8 T cells without impacting CD8 T cell numbers. In contrast, PPAR-δ deficiency decreases the number of CD4 T cells post-transplant, but does so without impacting CPT1a or CD36 levels, highlighting clear differences in metabolic reprogramming between CD4 and CD8 alloreactive cells. Finally, our data suggest that systemic inhibition of PPAR-δ post-transplant is not feasible given a sharp increase in toxicity. Future work will elucidate the mechanism of PPAR-δ in CD4 T cells, define the additional metabolic adaptations of CD8 cells which lack PPAR-δ, and determine if similar changes occur in human T cells. Together, these studies will test whether cellular inhibition of PPAR-δ represents a clinically-relevant, future therapy for GVHD.

Disclosures
No relevant conflicts of interest to declare.