Modulating the immune response and the pericardial environment with LPS or prednisolone in the axolotl does not change the regenerative capacity of cryoinjured hearts

Abstract

IntroHumans are incapable of regenerating after suffering from a myocardial infarction. Instead a fibrotic scar without contractile or electrically conducting properties is formed. This results in cardiac remodeling, which steers the heart towards heart failure. It is hypothesized that the time frame and composition of infiltrating immune cells play a role in the fibrotic outcome.A popular model organism capable of cardiac regeneration is the axolotl (Ambystoma mexicanum). It is believed that the immune response to cardiac injury in the axolotl progresses with a timing and distribution of infiltrated immune cells that supports regeneration rather than extensive inflammation and scar formation. Patients suffering from cardiovascular disease display a systemic pro‐inflammatory state, which includes an upregulation of inflammatory factors, and will likely influence the immune response mounted in the incidence of a myocardial infarction. Though fully healthy mammals are not able to functionally regenerate from cardiac injury such as a myocardial infarction, it is interesting to evaluate how a strong modulation of the immune response will affect an animal model capable of regeneration. Therefore, we have chosen to modulate the immune response in a pro‐ and anti‐inflammatory direction by injecting LPS or prednisolone respectively.MethodsIn this study 24 axolotls were given a cryoinfarction injury on the ventricle, thereby mimicking a myocardial infarction. The regenerative potential was observed in three different environments; a pro‐inflammatory (10 μg LPS), an anti‐inflammatory (20 μg prednisolone) and a saline control (amphibian Ringer’s solution) by injecting 20 μL of one of the three chosen substances into the pericardium of the axolotl every five days. LPS and prednisolone were hypothesized to alter the timing and distribution of infiltrating immune cells and inhibit regeneration. Echocardiography and quantitative histology were applied to measure function (ventricular stroke volume) and anatomical regeneration (healed volumetric fraction) of injured hearts.ResultsAll treatment groups showed extensive heart regeneration and no statistically significant differences in stroke volume or healed fraction between groups were observed.ConclusionSince neither echocardiographic nor histological data, showed any statistically significant differences in the functional or anatomical regeneration, this brings into question the importance of the immune system during regeneration in the axolotl.