In fish, intramuscular (i.m) injection of plasmid
DNA encoding
viral proteins has proved a highly effective vaccination strategy against some viral pathogens. The efficacy of
DNA vaccination in teleost fish is based on the high level of
viral antigen expression in muscle cells inducing a strong and long-lasting protection. However, the mechanisms through which this protection is established and effectuated in fish are still not fully understood. Moreover, similarities to mammalian models cannot be established since
DNA vaccination in mammals usually induces much weaker responses. In this work, we have focused on the characterization of the immune cells that infiltrate the muscle at the site of
DNA injection in vaccinated fish and the
chemokines and
chemokine receptors that may be involved in their infiltration. We have demonstrated through diverse techniques that B lymphocytes, both IgM⁺ and IgT⁺ cells, represented a major infiltrating cell type in fish vaccinated with a
viral haemorrhagic septicaemia virus (VHSV)
glycoprotein-encoding
DNA vaccine, whereas in control fish injected with an oil adjuvant mainly granulocyte/monocyte-type cells were attracted. Among twelve
chemokine genes studied, only CXCL11_L1, CK5B and CK6
mRNA levels were up-regulated in
DNA vaccinated fish compared to fish injected with the corresponding vector backbone. Furthermore, the transcription of CXCR3B, a possible receptor for CXCL11_L1 was also significantly up-regulated in vaccinated fish. Finally, experiments performed with recombinant trout CK5B and CK6 and
chemokine expression plasmids revealed that these
chemokines have chemotactic capacities which might explain the recruitment of B cells to the site of
DNA injection. Altogether, our results reveal that there is an early
chemokine-related B cell recruitment triggered by i.m.
DNA vaccination against VHSV which might play an important role in the initial phase of the immune response.