Background/Purpose: T lymphocytes hold a pinnacle position in the pathogenesis of rheumatoid arthritis (RA) and through their longevity contribute to disease chronicity. The protein kinase Ataxia telangiectasia mutated (ATM) is traditionally considered critical in DNA damage repair and cell cycle checkpoint regulation. Recent studies have extended ATM function from the nucleus to the cytoplasm where it acts as an oxidative stress sensor and regulates cellular metabolism and apoptosis, affecting cellular fate decisions and survival. CD4 T cells from RA patients express low levels of ATM protein and are inefficient in repairing damaged DNA. Whether ATM deficiency affects reactive oxygen species (ROS) sensing, metabolic competence, apoptotic sensitivity as well as T cell functional differentiation is unknown.

Methods: Naïve T cells (CD4⁺CD45RO^–) were purified from the blood of RA patients (n=59) and age-matched controls (n=62). T cell proliferation, cell cycle progression and IL-2 production was compared 48 and 72 hours following TCR ligation. Naïve-to-Memory conversion was monitored by cytometric phenotyping. Resting and activated T cells were loaded with fluorogenic dyes (DCF and DHE) to quantify H₂O₂ and O^2-production. The G2/M cell cycle checkpoint was measured with anti-phospho-Histone H3. Expression of ATM, p-ATM (Ser1981), AMPKa1, and p-AMPKa1 (Thr172) were quantified by immunoblotting. ATM signaling was blocked through the inhibitor ku-55933 or siRNA-mediated knockdown.

Results: Compared to age-matched control T cells, RA T cells produced low levels of ATM transcripts, ATM monomers and ATM dimers. In search for upstream signals triggering ATM activity, control and RA T cells were compared for the intracellular levels of H₂O₂ and O^2-. Both ROS were diminished in RA T cells. Reduced ATM signaling in RA T cells resulted in delayed histone H2Ax phosphorylation. ATM-deficient RA T cells passed through the G2/M cell cycle checkpoint without proper arrest, accelerating their differentiation into IL-2-producing effector cells. The modified cell cycle behavior led to the rapid conversion of naïve into memory T cells, depleting the naïve T cell reserve in RA patients.  Abnormalities in cell cycle progression and T cell differentiation were reproduced in healthy T cells by blocking ATM signaling with ku-55933 or by siRNA-mediated ATM knockdown. Further downstream effects of insufficient ATM signaling in RA T cells included decreased activation of AMPK, a kinase critically involved in energy sensing and metabolic regulation.

Conclusion: Deficiency of ATM redirects cell cycle behavior, metabolic fitness and functional differentiation of RA T cells. ATM-low RA T cells fail to adhere to cell cycle checkpoints, proliferate faster and differentiate prematurely. Also, ATM deficiency impairs triggering of the energy sensor AMPK, leaving the cell without sufficient energy supply. Consequences include the depletion of the naïve T cell reserve, accelerated T cell aging and excessive production of T cell effector cytokines. Overall, these abnormalities in T cell homeostasis and function bias the immune system toward chronic non-resolving inflammation.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/deficiency-of-the-protein-kinase-ataxia-telangiectasia-mutated-accelerates-t-cell-aging-in-rheumatoid-arthritis/