Exploring the relationships between amphibian (Xenopus laevis) myeloid cell subsets

The differentiation of distinct leukocyte subsets is governed by lineage-specific growth factors that elicit disparate expression of transcription factors and markers by the developing cell populations. For example, macrophages (Mφs) and granulocytes (Grns) arise from common granulocyte-macrophage progenitors in response to distinct myeloid growth factors. In turn, myelopoiesis of the Xenopus laevis anuran amphibian appears to be unique to other studied vertebrates in several respects while the functional differentiation of amphibian Mφs and Grns from their progenitor cells remains poorly understood. Notably, the expression of colony stimulating factor-1 receptor (CSF-1R) or CSF-3R on granulocyte-macrophage progenitors marks their commitment to Mφ- or Grn-lineages, respectively. CSF-1R is activated by the colony stimulating factor-1 (CSF-1) and interleukin (IL-34) cytokines, resulting in morphologically and functionally distinct Mφ cell types. Conversely, CSF-3R is ligated by CSF-3 in a process indispensable for granulopoiesis. Presently, we explore the relationships between X. laevis CSF-1-Mφs, IL-34-Mφs and CSF-3-Grns by examining their expression of key lineage-specific transcription factor and myeloid marker genes as well as their enzymology. Our findings suggest that while the CSF-1- and IL-34-Mφs share some commonalities, the IL-34-Mφs possess transcriptional patterns more akin to the CSF-3-Grns. IL-34-Mφs also possess robust expression of dendritic cell-associated transcription factors and surface marker genes, further underlining the difference between this cell type and the CSF-1-derived frog Mφ subset. Moreover, the three myeloid populations differ in their respective tartrate-resistant acid phosphatase, specific- and non-specific esterase activity. Together, this work grants new insights into the developmental relatedness of these three frog myeloid subsets.