Metabolites Involved in Immune Evasion by Batrachochytrium dendrobatidis Include the Polyamine Spermidine

Amphibians have been declining around the world for more than four decades. One recognized driver of these declines is the chytrid fungus, Batrachochytrium dendrobatidis , which causes the disease chytridiomycosis. Amphibians have complex and varied immune defenses against B. dendrobatidis , but the fungus also has a number of counter-defenses. Previously, we identified two small molecules produced by this fungus that inhibit frog lymphocyte proliferation, methylthioadenosine (MTA) and kynurenine (KYN). Here we report on the isolation and identification of the polyamine spermidine (SPD) as another significant immunomodulatory molecule produced by B. dendrobatidis . SPD and its precursor, putrescine (PUT), are the major polyamines detected, and SPD is required for growth. The major pathway of biosynthesis is from ornithine through putrescine to spermidine. An alternative pathway from arginine to agmatine to putrescine appears to be absent. SPD is inhibitory at concentrations ≥ 10 μM and is found at concentrations between 1-10 μM in active fungal supernatants. Although PUT is detected in the fungal supernatants, it is not inhibitory to lymphocytes even at concentrations as high as 100 μM. Two other related polyamines, norspermidine (NSP) and spermine (SPM), also inhibit amphibian lymphocyte proliferation, but a third polyamine, cadaverine (CAD) does not. A suboptimal (non-inhibitory) concentration of MTA (10 μM), a byproduct of spermidine synthesis, enhances the inhibition of SPD at 1 and 10 μM. We interpret these results to suggest that B. dendrobatidis produces an “armamentarium” of small molecules that alone, or in concert, may help it to evade clearance by the amphibian immune system.