Background/Purpose: Rheumatoid arthritis (RA) is a chronic inflammatory disease, genetically associated with polymorphisms in HLA class II molecules. CD4 cells produce proinflammatory cytokines and orchestrate multiple disease-relevant functions in the inflamed joint. How threshold settings in intracellular signaling cascades in such CD4 T cells affect arthritogenic functions is insufficiently understood. 

Reactive oxygen species (ROS) are classically considered harmful as they can cause oxidative stress. However, they also have critical cytoprotective functions by modulating cellular signal transduction. Intracellular ROS derive from mitochondria as a byproduct of metabolic activity. Cells possess a complex machinery for ROS removal, with the principal cellular reductant NADPH deriving from the pentose phosphate pathway (PPP), where glucose-6-phosphate dehydrogenase (G6PD) functions as the rate-limiting enzyme.

Methods: Naive CD4 T cells from patients with seropositive RA and age-matched controls were isolated and their T cell receptors were cross-linked. The following parameters were measured in resting and poststimulation T cells: ROS production, cell cycle progression and apoptotic susceptibility; commitment to the Th1, Th17, Th2 and Treg lineage. Expression of G6PD transcripts was quantified by RT-PCR. To attenuate intracellular ROS levels, cells were treated with the SOD mimetic Tempol. Synovial membrane biopsies were typed for HLA-DRB1*04 by RT-PCR. Mitochondrial mass was quantified by measuring copy numbers of mitochondrial DNA.

Results: Intracellular ROS in RA T cells were consistently decreased below 70% of those in controls (p=0.01). The ROS loss in RA T cells was associated with faster cell cycle progression (p<0.001), increased apoptotic susceptibility (p=0.01) and premature conversion of the naïve to memory phenotype (p=0.05). Treating T cells with the ROS scavenger Tempol could mimic this differentiation defect. RA T cells were prone to differentiate into IFN-γ and IL-17-producing cells, whereas the frequencies of IL-4-producing and FoxP3-expressing cells were indistinguishable in RA and control cells. RA T cells had a reduced mitochondrial mass and their intracellular NADPH concentrations were increased (p=0.04). Further evidence for a more active PPP in RA T cells came from increased expression levels of G6PD (p=0.03). The reduction of mitochondrial mass was reproduced in synovial tissue biopsies, where HLA-DRB1*04+ patients expressed significantly reduced mitochondrial DNA (p=0.05).

Conclusion: Intracellular ROS levels in RA T cells are reduced, imposing reductive stress. ROS loss may result from reduced mitochondrial mass, but also from enhanced production of the reductant NADPH. ROS depletion fundamentally shifts the functional behavior of human T cells, enhancing their cell cycle progression and swaying their differentiation towards proinflammatory Th1 and Th17 cells. The data delineate a mechanistic connection between intracellular redox imbalance and athritogenic T cell functions, with the prospective of therapeutically influencing such T cell defects via restoration of ROS production.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/depletion-of-reactive-oxygen-species-biases-t-cells-to-proinflammatory-cytokine-production-in-rheumatoid-arthritis/