ABSTRACT
Pathogens such as the Frog Virus 3 (FV3) ranavirus are contributing to the worldwide amphibian declines. While amphibian macrophages (MÏs) are central to the immune defenses against these viruses, the pathogen recognition capacities of disparate amphibian MÏ subsets remain unexplored. In turn, MÏ differentiation and functionality are interdependent on the colony-stimulating factor-1 receptor (CSF-1R), which is ligated by colony-stimulating factor-1 (CSF-1) and the unrelated interleukin-34 (IL-34) cytokines. Notably, the Xenopus laevis frog CSF-1- and IL-34-derived MÏs are functionally distinct, and while the CSF-1-MÏs are more susceptible to FV3, the IL-34-MÏs are highly resistant to this pathogen. Here, we elucidate the pathogen recognition capacities of CSF-1- and IL-34-differentiated MÏs by evaluating their baseline transcript levels of key pathogen pattern recognition receptors (PRRs). Compared to the frog CSF-1-MÏs, their IL-34-MÏs exhibited greater expression of PRR genes associated with viral recognition as well as PRR genes known for recognizing bacterial pathogen-associated molecular patterns (PAMPs). By contrast, the CSF-1-MÏs displayed greater expression of toll-like receptors (TLRs) that are absent in humans. Moreover, although the two MÏ types possessed similar expression of most downstream PRR signaling components, they exhibited distinct outcomes upon stimulation with hallmark PAMPs, as measured by their tumor necrosis factor-alpha and interferon-7 gene expression. Remarkably, stimulation with a TLR2/6 agonist conferred FV3 resistance to the otherwise susceptible CSF-1-MÏs while treatment with a TLR9 agonist significantly ablated the IL-34-MÏ resistance to FV3. These changes in MÏ-FV3 susceptibility and resistance appeared to be linked to changes in their expression of key immune genes. Greater understanding of the amphibian macrophage pathogen-recognition capacities will lend to further insights into the pathogen-associated causes of the amphibian declines.
