University of Southern Denmark Acquired Protective Immunity in Atlantic Salmon Salmo salar against the Myxozoan Kudoa thyrsites Involves Induction of MHIIβ+ CD83+ Antigen-Presenting Cells

25 The histozoic myxozoan parasite Kudoa thyrsites causes post-mortem myoliquefaction and 26 is responsible for economic losses to salmon aquaculture in the Pacific Northwest. Despite 27 its importance, little is known about the host-parasite relationship, including the host 28 response to infection. The present work sought to characterize the immune response in 29 Atlantic salmon during infection, recovery and re-exposure to K. thyrsites. After exposure to 30 infective seawater, infected and uninfected smolts were sampled three times over 4275 31 degree-days. Histological analysis revealed infection severity decreased over time in 32 exposed fish while in controls there was no evidence of infection. Following a secondary 33 exposure of all fish, severity of infection in the controls was similar to that measured in 34 exposed fish at the first sampling time, but was significantly reduced in re-exposed fish, 35 suggesting the acquisition of protective immunity. Using immunohistochemistry, we 36 detected a population of MHII+ cells in infected muscle that followed a pattern of 37 abundance concordant with parasite prevalence. Infiltration of these cells into infected 38 myocytes preceded destruction of the plasmodium and dissemination of myxospores. Dual39 labelling indicated a majority of these cells were CD83+/MHII+. Using RT-qPCR, we 40 detected significant induction of cellular effectors including macrophage/dendritic cells 41 (mhii/cd83/mcsf), B cells (igm/igt), and cytotoxic T cells (cd8/nkl) in the musculature of 42 infected fish. These data support a role for cellular effectors such as antigen presenting 43 cells (monocyte/macrophage and dendritic cells) along with B and T cells in the acquired 44 protective immune response of Atlantic salmon against K. thyrsites. 45 46 INTRODUCTION 47 on A uust 2, 2018 by gest http/iai.asm .rg/ D ow nladed fom