Rejection-associated Mitochondrial Impairment After Heart Transplantation

Out of the ice age: Preservation of cardiac allografts with a reusable 10 °C cooler

Objective

Static cold storage with ice has been the mainstay of cardiac donor preservation. Early preclinical data suggest that allograft preservation at 10 °C may be beneficial. We tested this hypothesis by using a static 10 °C storage device to preserve and transport cardiac allografts.

Methods

In total, 52 allografts were recovered between July 2023 and March 2024 and transported using a 10 °C storage cooler. Results were compared to a 3:1 propensity match of allografts transported on ice. Patients were excluded for the following reasons: dual viscera transplant, previous heart transplant, complex congenital heart disease, or allograft injury during procurement.

Results

Among the 10 °C cooler cohort, median total ischemic time was 222 minutes at 10 °C versus 193 minutes on ice (P < .0001). Intraoperative change in lactate was statistically lower at 10 °C (3.6 vs 5.1 mmol/L, P = .0016). Cardiac index score was greater in 10 °C cooler hearts at 24 (3.2 vs 3.0, P = .016) and 72 hours (3.3 vs 2.9, P = .037), despite similar vasoactive inotrope scores. There was no difference in severe primary graft dysfunction (1.9 vs 2.6%, P > .99). 10 °C hearts demonstrated less change in lactate but no difference in vasoactive inotrope scores or cardiac index. In hearts with extended ischemic time, delta lactate was lower in 10 °C cooler hearts. There was no statistical difference in outcomes for donor hearts >40 years old.

Conclusions

This is an early experience of static preservation in a 10 °C cooler. Postoperative allograft function was excellent, and lactate profiles lower in those allografts with extended ischemic times. Static cold storage targeting 10 °C may offer an inexpensive method for extended heart preservation. Further investigation is needed to assess long-term outcomes of 10 °C storage.

Unsupervised mRNA-seq classification of heart transplant endomyocardial biopsies

BACKGROUND

Endomyocardial biopsy (EMB) is currently considered the gold standard for diagnosing cardiac allograft rejection. However, significant limitations related to histological interpretation variability are well-recognized. We sought to develop a methodology to evaluate EMB solely based on gene expression, without relying on histology interpretation.

METHODS

Sixty-four EMBs were obtained from 47 post-heart transplant recipients, who were evaluated for allograft rejection. EMBs were subjected to mRNA sequencing, in which an unsupervised classification algorithm was used to identify the molecular signatures that best classified the EMBs. Cytokine and natriuretic peptide peripheral blood profiling was also performed. Subsequently, we performed gene network analysis to identify the gene modules and gene ontology to understand their biological relevance. We correlated our findings with the unsupervised and histological classifications.

RESULTS

Our algorithm classifies EMBs into three categories based solely on clusters of gene expression: unsupervised classes 1, 2, and 3. Unsupervised and histological classifications were closely related, with stronger gene module-phenotype correlations for the unsupervised classes. Gene ontology enrichment analysis revealed processes impacting on the regulation of cardiac and mitochondrial function, immune response, and tissue injury response. Significant levels of cytokines and natriuretic peptides were detected following the unsupervised classification.

CONCLUSION

We have developed an unsupervised algorithm that classifies EMBs into three distinct categories, without relying on histology interpretation. These categories were highly correlated with mitochondrial, immune, and tissue injury response. Significant cytokine and natriuretic peptide levels were detected within the unsupervised classification. If further validated, the unsupervised classification could offer a more objective EMB evaluation.

Human myocardial mitochondrial oxidative capacity is impaired in mild acute heart transplant rejection

Aims

Acute cellular rejection (ACR) following heart transplantation (HTX) is associated with long‐term graft loss and increased mortality. Disturbed mitochondrial bioenergetics have been identified as pathophysiological drivers in heart failure, but their role in ACR remains unclear. We aimed to prove functional disturbances of myocardial bioenergetics in human heart transplant recipients with mild ACR by assessing myocardial mitochondrial respiration using high‐resolution respirometry, digital image analysis of myocardial inflammatory cell infiltration, and clinical assessment of HTX patients. We hypothesized that (i) mild ACR is associated with impaired myocardial mitochondrial respiration and (ii) myocardial inflammation, systemic oxidative stress, and myocardial oedema relate to impaired mitochondrial respiration and myocardial dysfunction.

Methods and results

We classified 35 HTX recipients undergoing endomyocardial biopsy according International Society for Heart and Lung Transplantation criteria to have no (0R) or mild (1R) ACR. Additionally, we quantified immune cell infiltration by immunohistochemistry and digital image analysis. We analysed mitochondrial substrate utilization in myocardial fibres by high‐resolution respirometry and performed cardiovascular magnetic resonance (CMR). ACR (1R) was diagnosed in 12 patients (34%), while the remaining 23 patients revealed no signs of ACR (0R). Underlying cardiomyopathies (dilated cardiomyopathy 50% vs. 65%; P = 0.77), comorbidities (type 2 diabetes mellitus: 50% vs. 35%, P = 0.57; chronic kidney disease stage 5: 8% vs. 9%, P > 0.99; arterial hypertension: 59% vs. 30%, P = 0.35), medications (tacrolimus: 100% vs. 91%, P = 0.54; mycophenolate mofetil: 92% vs. 91%, P > 0.99; prednisolone: 92% vs. 96%, P > 0.99) and time post‐transplantation (21.5 ± 26.0 months vs. 29.4 ± 26.4 months, P = 0.40) were similar between groups. Mitochondrial respiration was reduced by 40% in ACR (1R) compared with ACR (0R) (77.8 ± 23.0 vs. 128.0 ± 33.0; P < 0.0001). Quantitative assessment of myocardial CD3⁺‐lymphocyte infiltration identified ACR (1R) with a cut‐off of >14 CD3⁺‐lymphocytes/mm² (100% sensitivity, 82% specificity; P < 0.0001). Myocardial CD3⁺ infiltration (r = −0.41, P < 0.05), systemic oxidative stress (thiobarbituric acid reactive substances; r = −0.42, P < 0.01) and myocardial oedema depicted by global CMR derived T2 time (r = −0.62, P < 0.01) correlated with lower oxidative capacity and overt cardiac dysfunction (global longitudinal strain; r = −0.63, P < 0.01).

Conclusions

Mild ACR with inflammatory cell infiltration associates with impaired mitochondrial bioenergetics in cardiomyocytes. Our findings may help to identify novel checkpoints in cardiac immune metabolism as potential therapeutic targets in post‐transplant care.

Mitochondrial Consequences of Organ Preservation Techniques during Liver Transplantation

Allograft ischemia during liver transplantation (LT) adversely affects the function of mitochondria, resulting in impairment of oxidative phosphorylation and compromised post-transplant recovery of the affected organ. Several preservation methods have been developed to improve donor organ quality; however, their effects on mitochondrial functions have not yet been compared. This study aimed to summarize the available data on mitochondrial effects of graft preservation methods in preclinical models of LT. Furthermore, a network meta-analysis was conducted to determine if any of these treatments provide a superior benefit, suggesting that they might be used on humans. A systematic search was conducted using electronic databases (EMBASE, MEDLINE (via PubMed), the Cochrane Central Register of Controlled Trials (CENTRAL) and Web of Science) for controlled animal studies using preservation methods for LT. The ATP content of the graft was the primary outcome, as this is an indicator overall mitochondrial function. Secondary outcomes were the respiratory activity of mitochondrial complexes, cytochrome c and aspartate aminotransferase (ALT) release. Both a random-effects model and the SYRCLE risk of bias analysis for animal studies were used. After a comprehensive search of the databases, 25 studies were enrolled in the analysis. Treatments that had the most significant protective effect on ATP content included hypothermic and subnormothermic machine perfusion (HMP and SNMP) (MD = -1.0, 95% CI: (-2.3, 0.3) and MD = -1.1, 95% CI: (-3.2, 1.02)), while the effects of warm ischemia (WI) without cold storage (WI) and normothermic machine perfusion (NMP) were less pronounced (MD = -1.8, 95% CI: (-2.9, -0.7) and MD = -2.1 MD; CI: (-4.6; 0.4)). The subgroup of static cold storage (SCS) with shorter preservation time (< 12 h) yielded better results than SCS ≥ 12 h, NMP and WI, in terms of ATP preservation and the respiratory capacity of complexes. HMP and SNMP stand out in terms of mitochondrial protection when compared to other treatments for LT in animals. The shorter storage time at lower temperatures, together with the dynamic preservation, provided superior protection for the grafts in terms of mitochondrial function. Additional clinical studies on human patients including marginal donors and longer ischemia times are needed to confirm any superiority of preservation methods with respect to mitochondrial function.