Rainbow trout gut epithelial cells upregulate MHCII transcript expression in response to experimental infection with the anglerfish parasite Spraguea americanus

The microsporidia are a group of obligate intracellular parasites implicated in the collapse of numerous North American fisheries and aquaculture industries over the past century. Although the majority of fish-infecting microsporidia are transmitted via ingestion, very little is known regarding the role of gut epithelial cells in anti-microsporidian immunity. In the absence of cell lines from Lophiid fishes, the present study investigates this phenomenon via experimental infection of the rainbow trout gut epithelial cell line RTgutGC with spores of the anglerfish parasite Spraguea americanus. As early as 1-day post-infection, mature spores attached to the plasma membranes of RTgut cells and induced spore uptake via an endocytic process. After uptake, spores persisted within individual, tight-fitting endosomes for 2 weeks, during which no lysosome/phagolysosome fusion or spore degradation was observed. Moreover, infected RTgut cells upregulated transcripts encoding the MHCII (Major histocompatibility class 2) alpha and beta chains while downregulating transcripts encoding the invariant chain (14-1), the 35 kDa subunit of IL-12, IL-1β and the class I component β2 m (beta-2 microglobulin). While S. americanus spores were ultimately incapable of germinating and developing within RTgut cells, these observations indicate that the cell line RTgutGC may possess M-cell-like characteristics and that gut epithelial cells may play a crucial role in sampling and presenting exogenous antigens during the initial stages of microsporidia infection in teleosts. To this end, future use of this novel in vitro infection can inform the development of novel strategies to protect susceptible finfish stocks from microsporidia outbreaks, thus helping these industries keep pace with growing global demands for fish protein.