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

Abstract

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 counterdefenses. Previously, we identified two small molecules produced by the 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 of ≥10 μM and is found at concentrations between 1 and 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 (noninhibitory) concentration of MTA (10 μM), a by-product 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.

Asymmetric cross-strain protection for amphibians exposed to a fungal-metabolite prophylactic treatment

“…Some of the metabolites Bd produces (e.g. methylthioadenosine, tryptophan and spermidine) are immunosuppressive [21,22]. These factors can suppress immunity by decreasing lymphocyte functioning and proliferation and inducing apoptosis [21,23].…”

“…These factors can suppress immunity by decreasing lymphocyte functioning and proliferation and inducing apoptosis [21,23]. Given that our Bd metabolite treatments are composed of all of the soluble chemicals Bd produces, the Bd metabolites we used to induce acquired resistance also presumably contain these immunosuppressive factors [21,22].…”

A Call for a Better Understanding of Aquatic Chytrid Biology

“…A major knowledge gap in understanding the ecological function of aquatic chytrids from a biological view is their wider role as parasites and pathogens, particularly of algae, because most of this knowledge comes from amphibian and plant hosts. Culture-based studies have characterized the impact of B. dendrobatidis secretions on amphibian skin (Moss et al, 2008;Rollins-Smith et al, 2019) and investigations of growth physiology have allowed for general phenotypic profiling (Berger et al, 2005;Voyles, 2011), but quantitative biological investigations into chytrid parasites of other aquatic hosts are largely lacking.…”

Immunological Aspects of Chytridiomycosis

“…When tested in vivo, Bd supernatants significantly inhibited phytohemagglutinin-induced inflammatory swelling (testing the extent of lymphocyte infiltration into the injection site) [ 152 ]. Several immunomodulatory metabolites released by Bd (but not by related non-pathogenic chytrids) have since been identified and characterised (including methylthioadenosine, kynurenine and spermidine) and confirmed to inhibit lymphocyte proliferation and cause apoptosis in vitro [ 171 , 172 ]. However, if adaptive immune recoveries can occur in some species, it is not yet clear what factors may mediate the extent of Bd-associated immunosuppression.…”