Daan Pieren

81 Compromised DNA repair promotes the accumulation of regulatory T cells accumulation of DNA damage over life. Accumulation of Tregs in response to IL-6 induced by DNA-damage may be an attempt by the immune system to counteract pro-inflammatory conditions that occur during aging. Diminished T-cell proliferation is a hallmark of T-cell aging [3,37]. Our data show that compromised DNA repair hampers TCR- and IL-2-mediated T-cell proliferation. Reduced expression of the IL-2 receptor (IL-2R) chains CD25 and CD122 may explain reduced IL-2-mediated proliferation [44,45]. Indeed, memory Th and Tc cells of Ercc1 -/ Δ 7 mice showed reduced CD122 + cell frequencies and expression levels. Moreover, Ercc1 -/ Δ 7 Tregs and Tc cells showed reduced upregulation of CD25 expression after stimulation. Compromised DNA repair did not impact all T-cell stimulatory pathways since co-stimulation via CD28 in the presence of CD3 stimulation could trigger proliferation. This was in contrast to findings in WT aged mice as these mice show impaired proliferation in response to CD28 and CD3 stimulation. Together, our findings indicate that defects in IL-2-mediated T-cell proliferation observed with age can be attributed to compromised DNA repair. Conversely, compromised DNA repair did not hamper CD28-mediated T-cell proliferation. Therefore, impaired CD28-mediated proliferation likely develops via a mechanism other than Ercc1 -/ Δ 7 -mediated compromised DNA repair. As mTORC1 negatively interferes with the ATM checkpoint that promotes DNA damage repair [31], the mTOR pathway may be linked to DNA damage response signaling [30], and eRapa reduces aging-related phenotypical T-cell changes in WT mice [29], we investigated whether rapamycin could slow down the aging-related T-cell changes imposed by compromised DNA repair. In vivo treatment with eRapa did not slow down the accumulation of Tregs in Ercc1 -/ Δ 7 mice, suggesting that compromised DNA repair promotes the accumulation of Tregs independent of mTOR. Our findings concur with a previous study in eRapa-fed WT mice that shows no change to Treg numbers [29]. In contrast, Neff et al. report a decrease in Treg numbers after eRapa supplementation [46]. These conflicting results may be explained by the difference in Treg characterization. Whereas we and others [29] characterize Tregs by expression of the Treg master transcription factor FoxP3 [47], Neff et al. assessed Tregs as CD25 + cells without including FoxP3 [46]. WT aging lowers the frequency and expression of CD25 on FoxP3 + Tregs [3,7], which we also observed in Ercc1 -/ Δ 7 mice. Therefore, characterization of Tregs based on CD25 only may underestimate the total amount of Tregs present. eRapa could not prevent decreased proliferation of T cells we observed in Ercc1 -/ Δ 7 mice. These findings are in contrast to previously observed findings in eRapa-fed WT mice, where eRapa improved T-cell proliferation [29]. An 3