A population of injury-responsive lymphoid cells expresses mpeg1.1 in the adult zebrafish heart

Transgenic zebrafish which 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 utilized as a model organism for such studies due to 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 co-expression of mpeg1.1 and csf1ra reporter transgenes in adult zebrafish hearts. Unexpectedly, this also identified a discrete population of mpeg1.1+csf1ra- 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 csf1raj4e1/j4e1 mutant zebrafish, which display deficiencies in MNP populations. Flow cytometry, imaging, cytological and gene expression analyses collectively indicates these cells comprise a mixed population of B cells and natural killer-like (NKL) cells. Our study therefore highlights the need to identify novel MNP lineage markers but also suggests undetermined roles of B cells and NKL cells in cardiac homeostasis and repair in adult zebrafish.