(483) Detection of secretory vesicles linked to Schistocephalus solidus-mediated immunomodulation

Introduction/Rationale: Parasite-induced immunosuppression increases the host’s susceptibility to secondary infection, thereby decreasing its fitness. Parasites primarily mediate host immunomodulation via the production of excretory secretory products (ESPs). The cestode, Schistocephalus solidus, modulates the immune response, behavior, and physiology of its host, the threespine stickleback fish, via ESPs. However, as previous work has almost exclusively focused on the host responses during late-stage S. solidus infection, the anatomical origins and timing of ESP production are unknown.

Methods: To address this gap, we used differential staining techniques to detect S. solidus secretory organs at multiple developmental stages: day 21 (immature, unsegmented) and 42 (mature, segmented) plerocercoids. These techniques involved Hematoxylin and Eosin (H&E) and immunohistochemical analysis through lectin staining.

Results: H&E staining revealed that d21 samples had a nucleated tegument, and did not contain secretory vesicles. Conversely, d42 samples showed an acellular tegument, and secretory vesicles. Lectin staining indicated the presence of abundant glycoprotein-containing secretory vesicles in the parenchyma of d42 samples, which were absent at d21. These results suggest that ESPs may be produced and stored in the secretory vesicles of the parenchyma, and that significant ESP production, and therefore immunomodulation, may not occur until later stages of plerocercoid development.

Conclusion: Supporting this finding, we observe significant, and wide-reaching immune suppression in d42-infected fish, with reductions in splenic melanomacrophages, IgM+ B cells, and CD4+ T cells. These preliminary findings and tools lay the foundation for future work, including confirmation of secretory vesicle contents and the characterization of ESP composition across cestode development.