The parasitic ciliate Ichthyophthirius multifiliis infecting skin, fins and gills of fish induces a protective
immune response in rainbow trout (Oncorhynchus mykiss) surviving the infection and a similar
protection can be conferred by i.p. injection of live theronts. A combined molecular and immunohistochemical
approach has been used in this work for pinpointing cellular and humoral immune factors in
gill tissue involved in the response and indicating interactions between the systemic and local responses.
Fish were immunized by intra-peritoneal injection of live I. multifiliis theronts, control fish were injected
with PBS and subgroups were treated with the immuno-suppressant hydrocortisone before fish were
challenged with live theronts. Significant up-regulations of genes encoding IgM, IgT, C3, SAA, IL-8, IL-22
and IFN-g were induced by immunization and challenge. Hydrocortisone treatment had a significant
down-regulating effect on genes incoding IgT, IgM, CD4, CD8, IFN-g, IL-8 and IL-22 in all groups.
Immunohistochemistry, using monoclonal antibodies to detect cellular markers, demonstrated active
involvement of CD8, MHC II, IgT and IgM positive cells in gill tissue. Putative T-cells (CD8 positive cells)
were detected in the intraepithelial lymphoid tissue located at the base of gill filaments and in hyperplastic
gill tissue but following infection a clear efflux of these cells was detected. MHC II positive cells
were distributed across the gill filaments and accumulated in hyperplastic tissue but hydrocortisone
treatment affected their density negatively in both immunized and non-immunized fish. IgT positive
cells were present in the epithelial lining of the gill lamellae (suggesting a primary role of this protein in
the mucosal defence against the ciliate) whereas IgM positive cells were found only in gill arterioles and
the lamellar capillaries. The present work indicates an intensive activity and specialized function of
immune cells (B-cells, T-cells and macrophages) and humoral elements such as immunoglobulins IgT and
IgM which are orchestrated by cytokines in gill tissue reacting against I. multifiliis.