Response by Bansal et al., to Letter Regarding Article, “Dysfunctional and Proinflammatory Regulatory T-Lymphocytes Are Essential for Adverse Cardiac Remodeling in Ischemic Cardiomyopathy”

In Response:

We thank Wang et al for their interest in our work,¹ and their comments on the molecular mechanisms related to insulin resistance (IR) that may underlie the loss of regulatory T cell (Treg) immunomodulatory capacity in heart failure (HF). Wang et al raise interesting concepts worthy of exploration in future experiments, ones that we have considered as potential next steps. IR may indeed be an important contributor to Treg dysfunction as insulin signaling and Treg function appear to be closely interlinked. Whereas insulin may regulate Treg cellular activity, normal Treg function is critical for maintaining global insulin sensitivity and suppressing inflammation in visceral adipose tissue.² Although the specific mechanisms for this bidirectional regulation are likely multifaceted as indicated by Wang et al, tumor necrosis factor-α (TNF) signaling via TNF receptor 1 (TNFR1) may be of importance, as in vivo administration of a TNFR1 blocking peptide-Fc fusion protein markedly improved insulin sensitivity in Wistar rats with diet-induced obesity, and enhanced tyrosine phosphorylation of insulin receptor substrate-1 in muscle and fat.³ However, in our study, although TNFR1⁺ Tregs were significantly increased in HF, TNFR1 deficient Tregs did not exhibit restoration of immunosuppressive function (but did reverse direct anti-angiogenic effects). Activation of PI(3) kinase-Akt-mTOR signaling with attendant increase of glycolysis and Glut1 expression has been shown to suppress Treg function.⁴ In our studies, we demonstrated increased abundance of Tregs expressing high levels of Glut1 in HF,¹ suggesting a potential role for Akt-mTOR signaling. As HF is well known to be an IR state, the role of this signaling pathway in producing dysfunction of HF Tregs, as well as its dependence on insulin signaling and downstream mediators, clearly requires further study.

We also observed that Tregs in HF exhibited increased expression of interferon-γ (IFNγ). As indicated by Wang et al, loss-of-function of the E3 ubiquitin ligase VHL and subsequent augmentation of hypoxia-inducible factor 1α (HIF-1α) has been shown to induce IFNγ expression and glycolytic reprogramming in Tregs, with their conversion into Th1 effector type T-cells with Foxp3 downregulation and loss of suppressive potential. Whether this pathway is of importance in the production of the dysfunctional Treg phenotype is unclear and requires further investigation. However, it is worth noting that in our study, HF Tregs exhibited increased IFNγ expression despite maintenance of Foxp3 expression. Moreover, HIF-1α has also been shown to induce conversion of effector T-cells into potent Tregs with enhanced immunosuppressive capacity,⁵ suggesting dichotomous, context-dependent effects of HIF-1α in T-cells. Finally, we were intrigued by the findings from Epstein’s group that epicardial loss of YAP and TAZ reduced Treg infiltration and worsened acute inflammation, fibrosis, and mortality after myocardial infarction (MI). In contrast, our study examined the role of Tregs well beyond the acute phase after MI, after the development of chronic ischemic cardiomyopathy, with Treg modulation initiated at 4 w post-MI. Post-MI mortality in mice after coronary ligation mainly occurs by day 10; mortality rates beyond the 4 w post-MI period were low in both Treg-modulated and unmodulated groups in our study.