The evolutionarily conserved immune system of the zebrafish (Danio rerio), in 
combination with its genetic tractability, position it as an excellent model 
system in which to elucidate the origin and function of vertebrate immune cells. 
We recently reported the existence of antigen-presenting mononuclear phagocytes 
in zebrafish, namely macrophages and dendritic cells (DCs), but have been 
impaired in further characterizing the biology of these cells by the lack of a 
specific transgenic reporter line. Using regulatory elements of a class II major 
histocompatibility gene, we generated a zebrafish reporter line expressing green 
fluorescent protein (GFP) in all APCs, macrophages, DCs, and B lymphocytes. 
Examination of mhc2dab:GFP; cd45:DsRed double-transgenic animals demonstrated 
that kidney mhc2dab:GFP(hi); cd45:DsRed(hi) cells were exclusively mature 
monocytes/macrophages and DCs, as revealed by morphologic and molecular 
analyses. Mononuclear phagocytes were found in all hematolymphoid organs, but 
were most abundant in the intestine and spleen, where they up-regulate the 
expression of inflammatory cytokines upon bacterial challenge. Finally, 
mhc2dab:GFP and cd45:DsRed transgenes mark mutually exclusive cell subsets in 
the lymphoid fraction, enabling the delineation of the major hematopoietic 
lineages in the adult zebrafish. These findings suggest that mhc2dab:GFP and 
cd45:DsRed transgenic lines will be instrumental in elucidating the immune 
response in the zebrafish.