Serological memory is one of the most important mechanisms activated by the immune system when threatened by viral infections. The knowledge of how to potentiate this response may lead to development of more efficient fish viral vaccines. This is a long p rocess requiring studies on cultivated leukocytes as well as in vivo vaccination trials. In vitro studies, as planned here, will be a mean to dissect whether selected Toll-like receptor (TLR) ligands take part in shaping Atlantic salmon B cell responses. This will be followed up by immunization experiments to test whether these ligands in combination with selected viral antigens affect B cell responses in live fish, including both IgM and IgT positive B cell subsets. Here, we are applying for three months funding to start collaboration with Penn State Institute of Immunology, Philadelphia, USA, where B cell proliferation and studies on immunoglobulin subsets (IgM/IgT) after TLR-ligand stimulation will be performed. Penn State Institute of Immunology is wo rld leading in its field. Our previous studies, both in vitro and in vivo has handed us a main hypothesis - that specific TLR ligands can directly activate salmon B cells to proliferate and mature into plasma cells and in the presented project further exp eriments are planned to test this prediction. In turn, we aim at measuring IgM and IgT responses, both in vitro and in vivo, and look at what roles TLRs have in shaping antibody responses to viral infections. A more complete understanding of how TLRs regu lates B cell activation may lead to improved vaccine design.