UCP3 reciprocally controls CD4+ Th17 and Treg cell differentiation.

Emma B O’Connor,Gemma Leon,Natalia Muñoz-Wolf,Patrick T Walsh,Ed C Lavelle,Kingston H G Mills,Richard K Porter

doi:10.1371/journal.pone.0239713

Abstract

Uncoupling proteins (UCPs) are members of the mitochondrial anion carrier superfamily that can mediate the transfer of protons into the mitochondrial matrix from the intermembrane space. We have previously reported UCP3 expression in thymocytes, mitochondria of total splenocytes and splenic lymphocytes. Here, we demonstrate that Ucp3 is expressed in peripheral naive CD4+ T cells at the mRNA level before being markedly downregulated following activation. Non-polarized, activated T cells (Th0 cells) from Ucp3-/- mice produced significantly more IL-2, had increased expression of CD25 and CD69 and were more proliferative than Ucp3+/+ Th0 cells. The altered IL-2 expression observed between T cells from Ucp3+/+ and Ucp3-/- mice may be a factor in determining differentiation into Th17 or induced regulatory (iTreg) cells. When compared to Ucp3+/+, CD4+ T cells from Ucp3-/- mice had increased FoxP3 expression under iTreg conditions. Conversely, Ucp3-/- CD4+ T cells produced a significantly lower concentration of IL-17A under Th17 cell-inducing conditions in vitro. These effects were mirrored in antigen-specific T cells from mice immunized with KLH and CT. Interestingly, the altered responses of Ucp3-/- T cells were partially reversed upon neutralisation of IL-2. Together, these data indicate that UCP3 acts to restrict the activation of naive T cells, acting as a rheostat to dampen signals following TCR and CD28 co-receptor ligation, thereby limiting early activation responses. The observation that Ucp3 ablation alters the Th17:Treg cell balance in vivo as well as in vitro suggests that UCP3 is a potential target for the treatment of Th17 cell-mediated autoimmune diseases.

Highlights

Since the discovery of uncoupling protein 1 (UCP1) in brown adipose tissue (BAT) in the 1970s [1], other Uncoupling proteins (UCPs) of the mitochondrial inner membrane have been reported
Using the comparative cholera toxin (CT) method to calculate relative gene expression, Ucp3 cDNA converted from mRNA was first normalised to Hprt as an endogenous control to account for variability in the initial concentration and quality of the total RNA and in the conversion efficiency of the reverse transcription reaction
Ucp3 expression is significantly decreased in Th0 cells within 24 h of stimulation compared to naive T cells and remains at a significantly lower level up to 72 h post-stimulation (Fig 1A)

Summary

Introduction

Since the discovery of uncoupling protein 1 (UCP1) in brown adipose tissue (BAT) in the 1970s [1], other UCPs of the mitochondrial inner membrane have been reported. UCP3 has an amino acid sequence more similar to that of the archetypal UCP, UCP1, than any other member of the mitochondrial anion carrier superfamily (excluding UCP2), at 57% homology. It followed naturally that, upon discovery, this protein was assumed to have uncoupling activity similar to that of UCP1. Uncoupling activity of UCP3 has been demonstrated in mitochondria isolated from skeletal muscle of 3,4-methylenedioxymethamphetamine (MDMA)-treated rats and mice [16]. Since its discovery in skeletal muscle [4], UCP3 protein has been reported in BAT [12], spleen, thymus, reticulocytes, monocytes, lymphocytes [17], pancreatic β-cells [18] and heart [14]

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