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To date, intraperitoneal (i.p.) injection seems to be the most effective vaccination route in aquaculture, as many i.p. administered fish vaccines are capable of conferring strong and long-lasting immune responses. Despite this, how peritoneal leukocytes are regulated upon antigen encounter has only been scarcely studied in fish. Although, in the past, myeloid cells were thought to be the main responders to peritoneal inflammation, a recent study revealed that IgM + B cells are one of the main cell types in the teleost peritoneal cavity in response to pathogenic bacteria. Thus, in the current work, we have focused on establishing how IgM + B cells are recruited into the peritoneum in rainbow trout (Oncorhynchus mykiss) comparing different antigens: Escherichia coli as a bacterial model, E. coli-derived lipopolysaccharide (LPS) or viral hemorrhagic septicemia virus (VHSV). In addition to studying their capacity to dominate the peritoneal cavity, we have established how these IgM + B cells are regulated in response to the different antigens, determining their levels of IgM secretion, surface MHC II expression, cell size and phagocytic abilities. Our results reveal that IgM + B cells are one of the main cell types amplified in the peritoneum in response to either bacterial or viral antigens and that these immunogenic stimulations provoke a differentiation of some of these cells towards plasmablasts/plasma cells whereas others seem to be implicated in antigen presentation. These findings contribute to a better understanding of the immune processes that regulate peritoneal inflammation in teleost fish., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Microsporidia are intracellular obligate parasites which have recently been found to be related to fungi. They have a unique extrusion apparatus that is able to inject the sporoplasm directly into the target cell without using receptors. Encephalitozoon microsporidia are a source of morbidity and mortality in humans. It has been suggested that microsporidia may modulate the host cell cycle and apoptosis. We report here that caspase-3 cleavage is inhibited at different times of Vero cell infection by Encephalitozoon microsporidia and that the phosphorylation and translocation of p53 to the nucleus, previous steps for the activation of this protein, do not occur after infection of Vero cells. Consequently, the transcriptional function of p53 is impaired during the infection cycle as demonstrated by luciferase reporter assays. Thus, to our knowledge, for the first time it is shown that an intracellular parasite may be able to multiply in the host cell without activating the p53 apoptotic pathway of that cell. However, changes in the expression of Bcl-2 or Bax levels were not observed.