The macrophage-expressed gene (mpeg) 1 identifies a subpopulation of B cells in the adult zebrafish

Abstract

The mononuclear phagocytic system (MPS) consists of many cells, in particular macrophages, scattered throughout the body. However, there is increasing evidence for the heterogeneity of tissue-resident macrophages, leading to a pressing need for new tools to discriminate MPS subsets from other hematopoietic lineages. Mpeg1.1 is an evolutionary conserved gene encoding perforin-2, a pore-forming protein associated with host defense against pathogens. Zebrafish mpeg1.1:GFP and mpeg1.1:mCherry reporters were originally established to specifically label macrophages. Since, more than 100 peer-reviewed publications have made use of mpeg1.1-driven transgenics for in vivo studies, providing new insights into key aspects of macrophage ontogeny, activation and function. However, while the macrophage-specific expression pattern of the mpeg1.1 promoter has been firmly established in the zebrafish embryo, it is currently not known whether this specificity is maintained through adulthood. Here we report direct evidence that beside macrophages, a subpopulation of B-lymphocytes is marked by mpeg1.1 reporters in most adult zebrafish organs. These mpeg1.1+ lymphoid cells endogenously express mpeg1.1 and can be separated from mpeg1.1+ macrophages by virtue of their light-scatter characteristics using FACS. Remarkably, our analyses also revealed that B-lymphocytes, rather than mononuclear phagocytes, constitute the main mpeg1.1-positive population in irf8null myeloid-defective mutants, which were previously reported to recover tissue-resident macrophages in adulthood. One notable exception are skin macrophages, whose development and maintenance appear to be independent from irf8, similar to mammals. Collectively, our findings demonstrate that irf8 functions in myelopoiesis are evolutionary conserved and highlight the need for alternative macrophage-specific markers to study the MPS in adult zebrafish. SUMMARY SENTENCEMpeg1 is not a restricted macrophage marker, but also labels B cells in the adult zebrafish. Therefore, previously identified irf8-independent macrophages likely consist of B lymphocytes. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=183 SRC="FIGDIR/small/836098v1_ufig1.gif" ALT="Figure 1"> View larger version (30K): org.highwire.dtl.DTLVardef@128a738org.highwire.dtl.DTLVardef@1fa6097org.highwire.dtl.DTLVardef@db61b2org.highwire.dtl.DTLVardef@eb2e2e_HPS_FORMAT_FIGEXP M_FIG C_FIG