The Role of Macrophages During Zebrafish Injury and Tissue Regeneration Under Infectious and Non-Infectious Conditions

Abstract

The future of regenerative medicine relies on our understanding of the mechanistic processes that underlie tissue regeneration, highlighting the need for suitable animal models. For many years, zebrafish has been exploited as an adequate model in the field due to their very high regenerative capabilities. In this organism, regeneration of several tissues, including the caudal fin, is dependent on a robust epimorphic regenerative process, typified by the formation of a blastema, consisting of highly proliferative cells that can regenerate and completely grow the lost limb within a few days. Recent studies have also emphasized the crucial role of distinct macrophage subpopulations in tissue regeneration, contributing to the early phases of inflammation and promoting tissue repair and regeneration in late stages once inflammation is resolved. However, while most studies were conducted under non-infectious conditions, this situation does not necessarily reflect all the complexities of the interactions associated with injury often involving entry of pathogenic microorganisms. There is emerging evidence that the presence of infectious pathogens can largely influence and modulate the host immune response and the regenerative processes, which is sometimes more representative of the true complexities underlying regenerative mechanics. Herein, we present the current knowledge regarding the paths involved in the repair of non-infected and infected wounds using the zebrafish model.

Keywords: infectious condition; macrophage; non-infectious condition; regeneration; tissue injury; zebrafish.

Figure 1

Kinetic of caudal fin and heart regeneration under non-infectious conditions. Transection of the zebrafish embryo caudal fin in non-infected condition leads to early recruitment of MФ, which complements the pool of resident MФ already present in the tail. At this very early stage, resident macrophages (MФ) phagocytose debris and dead cells. During the inflammatory phase, some MФ undergo polarization into tnfa-positive pro-inflammatory MФ, reaching a peak at 6 hpa. The tnfa-positive MФ disappear and a majority of non-inflammatory tnfa-negative MФ accumulate. During resolution of the inflammation, the undifferentiated blastema cells proliferate, peaking at 24 hpa, resulting in new limb formation. Cryoinjury of the heart of the adult zebrafish leads to an early recruitment of MФ, completing the pool of resident MФ already present in the heart, between 1 and 3 dpc. Tnfa-positive pro-inflammatory MФ predominantly arrive at the site and the proliferation of undifferentiated cells around the wound constituting the blastema begins. Already differentiated cardiomyocyte cells participate in tissue restoration. Non-inflammatory MФ represent the major cells present until the end of the regeneration process, resulting in a healed organ.

Figure 2

Caudal fin regeneration under infectious conditions. Amputation of the caudal fin in an infectious condition, with a scalpel pre-soaked in a solution containing pathogenic microorganisms (for instance Listeria monocytogenes) impairs tissue regeneration. Exacerbated inflammation is caused by an excessive number of tnfa-positive MФ at the wound. Resolution of inflammation is impeded at 24 hpa, with many tnfa-positive MФ that remain present. This results in incomplete tissue restoration and fibrosis at 7 dpa onward, characterized by the presence of disorganized collagen fibers.