Detalle de: Transcriptomic profiling and IgM and IgT repertoire dynamics in rainbow trout gills following primary and secondary challenge with Lactococcus petauri

Fish mucosal tissues lack the organized lymphoid structures present in mammals. Although this is also true for the gills, with the gill-associated lymphoid tissue (GIALT) being mainly composed of B and T cells scattered throughout the epithelium and the lamellae, in some species such as rainbow trout ( Oncorhynchus mykiss ) or Atlantic salmon ( Salmo salar ), a more organized structure designated as the interbranchial lymphoid tissue (ILT) is also identified. Yet, although many studies have investigated the immune response of the gill to stimulation or pathogen encounter, which have revealed the activation of both innate and adaptive immune elements, how the gills respond to secondary stimulation or whether memory responses are locally orchestrated is still unknown. To provide insights into this matter, in the current study, we have infected rainbow trout by bath with Lactococcus petauri and then re-infected or mock-infected the survivor fish as well as mock-infected controls. At day 4 after the last infection, the fish were sacrificed and gills used to perform an RNAseq transcriptomic analysis and an IgM and IgT repertoire analysis. The number of cells secreting total and specific IgM in gills was also established by ELISpot. Our results demonstrate that memory responses are locally organized in the rainbow trout gills, given that re-infected fish did not up-regulate genes related to inflammation (as happens in the primary infection), but preferentially modified genes related to adaptive immunity, mainly to B cell function, experiencing a significant IgM and IgT clonal expansion. Accordingly, re-infected fish produced increased levels of specific IgM both locally and in serum, although no clonal selection was apparent. These findings advance our understanding of mucosal immunological memory in teleost fish and reveal the gills as a site of localized adaptive immune regulation during secondary responses.