Transgenic zebrafish that express fluorophores under the control of mpeg1.1 (mpeg1) and csf1ra (c-fms) promoters have been widely used to study the dynamics and functions of mononuclear phagocytes (MNPs) in larval zebrafish, unveiling crucial roles for these innate immune cells in many processes, including tissue repair. Adult zebrafish are also being increasingly used as a model organism for such studies because of their regenerative capacity and presence of innate and adaptive immune cells. For example, recent investigations highlight roles of MNPs in the regulation of diverse cellular processes during heart regeneration, including scarring, cardiomyocyte proliferation, and neovascularization. However, transgenic lines that stratify MNP subpopulations (monocytes, macrophages, and dendritic cells) are not yet available, preventing functional analysis of these populations. In an attempt to better segregate cardiac MNPs, we assessed the coexpression of mpeg1.1 and csf1ra reporter transgenes in adult zebrafish hearts. Unexpectedly, this also identified a discrete population of mpeg1.1 + csf1ra 2 lymphoid-like cells, which respond to cardiac cryoinjury in a different temporal pattern to mpeg1.1 + MNPs. mpeg1.1 + lymphoid cells were also abundant in the skin, spleen, and blood, and their frequency was unaffected in the hearts of csf1ra j4e1/j4e1 mutant zebrafish, which display deficiencies in MNP populations. Flow cytometry, imaging, and cytological and gene expression analyses collectively indicate that these cells comprise a mixed population of B cells and NK-like cells. Our study therefore highlights the need to identify novel MNP lineage markers but also suggests undetermined roles of B cells and NK-like cells in cardiac homeostasis and repair in adult zebrafish. ImmunoHorizons, 2020, 4: 464-474.

The study of heart repair post-myocardial infarction has historically focused on the importance of cardiomyocyte proliferation as the major factor limiting adult mammalian heart regeneration. However, there is mounting evidence that a narrow focus on this one cell type discounts the importance of a complex cascade of cell–cell communication involving a whole host of different cell types. A major difficulty in the study of heart regeneration is the rarity of this process in adult animals, meaning a mammalian template for how this can be achieved is lacking. Here, we review the adult zebrafish as an ideal and unique model in which to study the underlying mechanisms and cell types required to attain complete heart regeneration following cardiac injury. We provide an introduction to the role of the cardiac microenvironment in the complex regenerative process and discuss some of the key advances using this in vivo vertebrate model that have recently increased our understanding of the vital roles of multiple different cell types. Due to the sheer number of exciting studies describing new and unexpected roles for inflammatory cell populations in cardiac regeneration, this review will pay particular attention to these important microenvironment participants.