A comparison of amphibian (Xenopus laevis) tadpole and adult frog macrophages.

The amphibian declines are compounded by emerging pathogens that often preferentially target distinct amphibian developmental stages. While amphibian immune responses remain relatively unexplored, macrophage (Mφ)-lineage cells are believed to be important to both amphibian host defenses and to their pathogen infection strategies. As such, a greater understanding of tadpole and adult amphibian Mφ functionality is warranted. Mφ biology is interdependent of interleukin-34 (IL-34) and colony-stimulating factor-1 (CSF-1) cytokines and we previously showed that CSF-1- and IL-34-derived Mφs of the Xenopus laevis frog are morphologically, transcriptionally, and functionally distinct. Presently, we directly compared the cytology and transcriptomes of X. laevis tadpole and frog CSF-1- and IL-34-Mφs. Our results indicate that tadpole and frog CSF-1-Mφs possess greater non-specific esterase activity, typically associated with Mφ-lineage cells. By contrast, both tadpole and frog IL-34-Mφs have greater specific esterase activity, which is typically attributed to granulocyte-lineage cells. Our comparisons of tadpole CSF-1-Mφ transcriptomes with those of tadpole IL-34-Mφs indicate that the two tadpole populations possess significantly different transcriptional profiles of immune and non-immune genes. The frog CSF-1-Mφ gene expression profiles are likewise significantly disparate from those of frog IL-34-Mφs. Compared to their respective tadpole Mφ subtypes, frog CSF-1- and IL-34-Mφs exhibited greater expression of genes associated with antigen presentation. Conversely, compared to their frog Mφ counterparts, tadpole CSF-1- and IL-34-Mφs possessed greater levels of select Fc-like receptor genes. Presumably, these cytological and transcriptional differences manifest in distinct biological roles for these respective tadpole and frog Mφ subtypes.