Background Unloading of failing hearts by left ventricular assist devices induces an extensive cardiac remodeling which may lead to a reversal of the initial phenotype–or to its deterioration. The mechanisms underlying these processes are unclear. Hypothesis Heterotopic heart transplantion (hHTX) is an accepted model for the study of mechanical unloading in rodents. The wide variety of genetically modified strains in mice provides an unique opportunity to examine remodeling pathways. However, the procedure is technically demanding and has not been extensively used in this area. To support investigators adopting this method, we present our experience establishing the abdominal hHTX in mice and describe refinements to the technique. Methods In this model, the transplanted heart is vascularised but implanted in series, and therefore does not contribute to systemic circulation and results in a complete mechanical unloading of the donor heart. Training followed a systematic program using a combination of literature, video tutorials, cadaveric training, direct observation and training in live animals. Results Successful transplantation was defined as a recipient surviving > 24 hours with a palpable, beating apex in the transplanted heart and was achieved after 20 transplants in live animals. A success rate of 90% was reached after 60 transplants. Operative time was shown to decrease in correlation with increasing number of procedures from 200 minutes to 45 minutes after 60 operations. Cold/warm ischemia time improved from 45/100 to 10/20 minutes. Key factors for success and trouble shootings were identified. Conclusion Abdominal hHTX in the mouse may enable future examination of specific pathways in unloading induced myocardial remodeling. Establishment of the technique, however, is challenging. Structured training programs utilising a variety of training methods can help to expedite the process. Postoperative management, including daily scoring increases animal wellbeing and helps to predict survival.

Complement activation may drive the pathology of hypertension through its effects on innate and adaptive immune responses, aside from direct effects on tissue integrity. Recently it was suggested that hypertension is a disease characterized by a decreased number of forkhead box protein 3 (Foxp3)+ regulatory T cells (Tregs) and that combined deficiency of the anaphylatoxin receptors C3aR (complement component 3a receptor) and C5aR1 (complement component 5a receptor) upregulates Tregs and heals hypertension and hypertensive end-organ damage. Using data from the European Renal cDNA Bank, renal single cell sequencing data and immunohistochemistry we found increased expression of C3aR, C5aR1 and Foxp3 in kidney biopsies of patients with hypertensive nephropathy. Expression of C3aR and C5aR1 was mainly found in myeloid cells and almost absent in lymphocytes. Next we aimed to determine whether C3aR or C3aR/C5aR1 double deficiency decreases blood pressure and hypertensive injury in Ang II infused mice. However, no difference was found for blood pressure, renal injury (albuminuria, glomerular and tubulointerstitial injury, inflammation) and cardiac injury (cardiac fibrosis, heart weight, gene expression) between C3aR KO and WT mice as well as C3aR/C5aR1 double KO respectively. Ang II as well as DOCA salt induced hypertension resulted in an increased number of Tregs in the kidney. This was valid in mice of the Balb/c and C57black strain. In summary, hypertensive nephropathy in patients is characterized by an increased expression of anaphylatoxin receptors and Tregs. However, deficiency of C3aR alone or C3aR and C5aR1 combined does not influence blood pressure or hypertensive end-organ damage in Ang II induced hypertension. Targeting the anaphylatoxin receptors C3aR alone or in combination with C5aR1 is not useful to treat cardiovascular disease in hypertension.

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