Schematic showing that physiological T-cell development correlated with the loss of cardiac regenerative potential in post-natal mice, and adoptive transfer of adult T cells into neonates prolonged inflammation and inhibited myocardial regeneration in an IFN-γ-dependent manner (Dolejsi et al.). Related to other immune cells in heart regeneration, the specific contribution of T-cell subpopulations are depicted. Dotted arrows represent possible future studies, including the converse adoptive cell transfer.
The discovery of a mammalian heart regenerative window after birth has boosted studies of endogenous heart regeneration, for the exciting possibility of therapeutics for myocardial infarction (MI) and heart failure (HF). Seminal studies have proposed that post-natal shifts in tissue environment oxygenation, endothermy, metabolic substrate utilization, and cardiomyocyte plasticity explain the loss of regenerative capacity in adults. The adaptive immune response to MI acts as a 'double-edged sword' to cardiac protection and regeneration. While tissue-resident macrophages are essential in this process, other immune cells, such as monocytes, monocyte-derived macrophages, and B cells, also participate in the ensuing inflammation and exacerbate myocardial injury. Nonetheless, the roles of the immune cell repertoire have remained elusive. In particular, it is controversial as to what extent lymphocytes comprising CD4+ and CD8+ T cells are beneficial or deleterious in the MI context.[4,5] Concordantly, clarifying how T cells impact heart regeneration is of great relevance. In this issue of European Heart Journal, Dolejsi et al. demonstrate that physiological T-cell development coincided with loss of cardiac regenerative potential in post-natal mice, and adoptive transfer of adult T cells into neonates prolonged inflammation, promoted fibrotic scarring, and inhibited myocardial regeneration in an interferon (IFN)-γ-dependent manner.
Dolejsi et al. first applied an established surgical MI model to post-natal day 1 (P1) and P7 mice. As anticipated, P1 mice showed early inflammation followed by complete heart regeneration and functional recovery within 7 days post-injury, while P7 mice exhibited delayed inflammatory resolution with irreversible, collagen-rich scarring and dysfunction, the hallmarks of adult disease. Flow cytometry analysis revealed an interesting influx of γδ-T cells (CD4– CD8–) in post-MI P1 hearts, more so than in P7 mice, but most evident was a dominant abundance in αβ-T-cell numbers (CD4+ and CD8+) in all P7 heart tissues. This was mirrored by increased systemic CD4+ and CD8+ T-cell prevalence in the young maturing mouse, and notably coincided with the closing of the P1 to P7 cardiac regenerative window.
Following this discovery, adoptive transfer of adult mouse CD3+ T cells into P1 neonates generated a novel 'heterochronic chimera' model, with a neonatal heart but an adult-like T-cell component, comprising mostly T-helper cells (CD4+ Foxp3–) and cytotoxic T cells (CD8+). Surprisingly, this P1 mouse model lost heart regenerative capacity after MI. Instead, the heart developed fibrotic scars resembling those of P7 and adult mice. Enhanced cardiac leucocyte influx was observed in adult-T-cell recipients, with increased neutrophils, CD4+ and CD8+ T cells, and proinflammatory monocyte-derived macrophage recruitment. Transcriptomic analysis pointed to a mechanistic role for IFN-γ, a known T-cell-secreted inflammatory factor. Dolejsi et al. confirmed the centrality of this pathway in conferring impaired regeneration, by transferring adult T cells from Ifng-null mice to wild-type P1 neonates, which restored heart regeneration and functional recovery. Moreover, treating non-regenerative P7 mice with IFN-γ-neutralizing antibody resulted in a reduced myocardial inflammatory response after MI. Taking all this together, the authors shed light on a new and interesting mechanistic trade-off, wherein the development of a mature, mammalian adaptive T-cell immune compartment contributes to limiting cardiac regenerative potential.
This study elucidating the inhibitory role of adult T cells on heart regeneration represents one of the pioneering pieces of work in the field. Notably, impaired tissue regeneration following development of T-cell competency has parallels in evolutionary history, and is consistent with work by Li et al. who demonstrated that ablation of CD4+ T cells using anti-CD4 lytic antibody in P8 juvenile mice reduced myocardial fibrosis and promoted heart regeneration after cryoinjury and apical resection. Li et al. also highlighted that ablation of CD8+ T cells did not restore heart regenerative capacity in juvenile mice, suggesting CD4+ T-helper cells as the key regulatory players here, which agrees with their numerical abundance, determined at 75% of adult T cells in the current study. Still, it would be interesting to separate CD4+ and CD8+ subsets during the adoptive transfer of adult T cells into P1 mice.
Future cell transfer studies could focus on CD4+ Foxp3+ regulatory T cells (Tregs). Although Dolejsi et al. and others have found Tregs to represent a small subset (1–4%) of adult T cells, Li et al.[11,12] have previously validated an indispensable role in promoting mammalian heart regeneration via secreted paracrine factors. Using single-cell RNA-sequencing (scRNA-seq), multiple Treg factors (e.g. CCL24, GAS6, and AREG) have been predicted, and suggested to promote cardiomyocyte cell cycle re-entry at least in vitro. One wonders whether application of scRNA-seq to the transferred adult T-cell compartment would reveal additional relevant T-cell subtypes and factors. Further characterization of the observed neonatal γδ-T cells in the cardiac regeneration setting would also be interesting. A currently unanswered question is whether the transferred adult T-cell compartment functions simply as a numerical boost to the neonatal T-cell numbers, or, are the transferred adult T cells also inherently different from their neonatal T-cell counterparts. scRNA-seq might again offer insights. If they are different, then could a converse transfer of the neonatal T-cell compartment to adult mouse myocardium impart protective or therapeutic benefits in the MI context? Obtaining sufficient cell numbers could be a challenge. Perhaps utilizing neonatal T-cell secretomes, or immunologically immature Rag2-deficient mice, could help.
The findings here are exciting, but there is reason to express caution when considering therapeutic implications for human disease. In contrast to P8 mice, ablating CD4+ T cells in adult mice was recently found to exacerbate post-MI cardiac dysfunction and fibrotic deposition, highlighting that key differences exist between juvenile P7–8 mice and adults. Results from one may not extrapolate well to the other, or to aged humans, where the majority of clinical need lies. IFN-γ is another example of a mediator, ablation of which is strongly beneficial to cardiac regeneration and post-MI function in the current study. Indeed, IFN-γ is a cytokine well known to elicit pathological inflammation, in addition to cardiomyocyte atrophy, apoptosis, and dilated cardiomyopathy. Yet, IFN-γ inhibition in adult mice can also drive cardiac hypertrophy, fibrosis, and deleterious post-MI outcomes. Of note, while IFN-γ-neutralizing antibody reduced inflammatory macrophage recruitment in the current study, whether this translated to improved cardiac function is not clear. Overzealous targeting of either CD4+ T cells or IFN-γ is likely to be undesirable as a therapy for MI, and more subtle, time-sensitive interventions would be required. These complexities and the importance of a specific biological and temporal context are carefully elaborated by Dolejsi et al. in their Discussion.
Finally, this work adds to a new and growing resource of literature implicating key roles for the immune system in aspects of cardiac biology, pathophysiology, regeneration, and therapeutics. Researchers could put additional thought into immune components in relevant models of human heart disease, including in vitro, where this often is absent. It is also worth reflecting on what commonalities might underlie the various identified regulators of mammalian cardiac regeneration. Could metabolism, or DNA damage, be critical underlying regulatory mechanisms? Or are T cells a new and independent piece of a large, balanced, co-regulatory puzzle, now brought to light. In summary, Dolejsi et al. reveal T cells as an attractive, hidden corner, to explore for new treatments of heart disease.
The authors are supported by grants from the National Medical Research Council (Singapore), and the Human Longevity Catalyst Award (Singapore).
Eur Heart J. 2022;43(28):2710-2712. © 2022 Oxford University Press
Copyright 2007 European Society of Cardiology. Published by Oxford University Press. All rights reserved.