Immune-Endocrine Interactions in the Fish Gonad during Infection: An Open Door to Vertical Transmission

The interaction between the immune and endocrine systems has long been recognized in vertebrates. In fish, it is known that the prevalence and intensity of such infections are higher in males than in females and probably related to sex steroid hormone levels. In addition, the immune response in the fish gonad tissues is specifically regulated to prevent infertility. This condition is used by some pathogens to colonize the fish gonad, evade the systemic immune response, and so spread to the progeny. This review brings up to date our knowledge concerning fish gonad immunity and its regulation, immune-endocrine interactions, and how some pathogens use this tissue to spread to the progeny through vertical transmission. More specifically, we will look at the case of the European sea bass and nodavirus (NNV). Sea bass is a very susceptible fish species to NNV infections, and this virus has been associated to vertical transmission since it is detected in gonad fluids as well as in testicular gametes. In fact, sea bass immunity in the gonad is regulated in a very different way to other target tissues (brain and retina) or immune-relevant tissues (head-kidney or spleen).