3. Discussion
heart failure, ischemic heart disease
ISCHEMIC HEART DISEASE, INFLAMMATION, DISEASE, MINOR, HEART FAILURE
This is the first study investigating circulating small EVs and their miRNA cargo in patients with ischemic heart disease treated with percutaneously intramyocardially administered allogeneic ADSCs.The patient collective described within our analyses were classified as untreatable with conventional medical interventions, as both their clinical presentation and cardiac parameters indicated the presence of significant heart failure despite the highest degree of feasible optimal pharmacological management, whilst being deemed unlikely to benefit from further interventions or surgery. Fortunately, only a small number of patients meet this criterium. Nonetheless, it is precisely this patient cohort that is in desperate need of novel regenerative therapies. In the SCIENCE study, the concept of allogeneic ADSCs derived from healthy donors was studied as an off-the-shelf product alternative with the potential to offer an easy, comparably quick, and standardized regenerative cell-based therapy. Despite the high expectations, and the fact that the SCIENCE study once again managed to confirm the safety aspect of cell-based regenerative therapy, it did not report an effect on the primary endpoint of improved cardiac function 6 months after treatment (assessed by left ventricular ESV in echocardiography) [The reasons for the mostly unimpressive results of cell-based regenerative therapies have long been a topic of discussion. Whilst not yet clearly elucidated, the lack of substantial benefits of these therapies might be at least partially attributable to the low long-term cell engraftment, which has previously been reported as only 5% after 2 h, and 1% after 20 h. Thus, the limited survival of transplanted cells has often been discussed [The aim of this subanalysis was to investigate the effect of ADSC treatment on small EVs and their miRNA cargo. In the light of the lack of measurable effects of ADSC treatment on functional indicators or quality of life in this study, the only minor effects on small EV levels do not come as a surprise. While miR-126 was significantly reduced in ADSC patients after 12 months, no other group differences were observed. Furthermore miR-24 and miR-126 showed a significant negative correlation with NT-proBNP, while miR-21 and miR-24 further negatively correlated with troponin. Additionally, small EV miRNAs also correlated with liver function, inflammation, and lipid metabolism parameters.EVs have emerged as a subject of interest within the past years, and a myriad of data have been published on this topic. However, despite the extensive effort made in clarifying definitions in EV research, upon studying the currently available literature, one is quickly confronted with varying and mostly un- or vaguely defined EV subclasses. This makes comparing or interpretating previously published findings, as well as establishing the novelty and reproducibility of newly obtained results, difficult. In its most recent position statement published in 2018, the International Society for Extracellular Vesicles (ISEV) has simplified the nomenclature [EV RNA profiles generally reflect the transcriptome of their cells of origin. However, some distinct differences can be observed, suggesting selective packing of miRNAs in small EVs [We chose six cardiac miRNAs as the focus of our investigation. Of those, only three were expressed in plasma small EVs in our patients. Despite being previously described as cardiac miRNAs with systemic expression patterns, miR-1, miR-133a and miR-499 were not (sufficiently) expressed in our samples, and were thus not included in further analyses [Several studies have attempted to confirm and harness the regenerative potential of the above-mentioned miRNAs delivered as miR-loaded small EVs in in vitro and small animal studies. miR-126 is crucially involved in regulating vascular integrity and angiogenesis [With regard to the interpretation of the results of our study, several limitations must be mentioned. Firstly, fifteen patients is a prohibitively small size, especially considering that only five patients received the placebo, due to the randomization protocol, thus offering the additional challenge of unbalanced group sizes to an already small initial cohort (2:1 randomization of ADSCs and placebo). We are aware that such a small sample size can only generate hypotheses. The very strong inclusion and exclusion criteria allowed us to include a limited number of patients in a limited time frame, even though many more patients were reviewed. In addition, due to well-balanced patient characteristics between the centers in the multicenter study, a pre-defined number of patients per center was defined. Additionally, in the light of the non-existent clinical effect of the ADSC treatment itself, and the primary hypothesis postulating that the observed small EV (cargo) changes would be linked to the treatment success, the lack of any observable significant differences between ADSC and placebo patients is in line with our predictions. However, small EV miRNAs are known to point towards subclinical effects, and those might have simply been missed as a result of the small sample size. Furthermore, only men were included in this study. Even though female patients were contacted, none were willing to participate in this study. Due to the well-established relevance of gender-specific differences in disease presentation and therapy response, a possible treatment effect might have been missed due to low inclusion rates of women, a consideration not only relevant for this study, but the area of regenerative cardiac therapy in general.