SUPPRESSION OF NATURAL-KILLER-CELL ACTIVITY IN HARBOR SEALS (PHOCA-VITULINA) FED BALTIC SEA HERRING

Mass mortalities among marine mammal populations in recent years have raised questions about a possible contributory role of contaminants ac cumulated through the marine food chain. While viruses were shown to b e the primary cause of the outbreaks, an immunotoxic action by organoc hlorine chemicals in affected animals could not be ruled out. We carri ed out a 2 1/2-year immunotoxicological experiment in which two groups of 11 harbour seals each were fed herring from either the relatively contaminated Baltic Sea or the relatively uncontaminated Atlantic Ocea n. Seals in the Baltic Sea group accumulated 3-4 times higher levels o f Ah-receptor-mediated 2,3,7,8-TCDD Toxic Equivalents in blubber than did their Atlantic counterparts following 2 years on the respective di ets. Blood was sampled a total of 17 times during the course of the ex periment for immunological evaluation, during which time the natural c ytotoxic activity of peripheral blood mononuclear cells isolated from seals fed Baltic Sea herring declined to a level approximately 25% low er than that observed in seals fed Atlantic herring (P < 0.01). Natura l killer (NK) cell activity has not been previously described for a ma rine mammal species. We characterized the natural cytotoxic activity o f harbour seal peripheral blood mononuclear cells (PBMC), and found th is to be interleukin-2 (IL-2) responsive, sensitive to antibody anti-a sialo GM1, and it was higher against a virus-infected target cell, lik e NK cells described for other mammals. As NK cells are leukocytes whi ch play an important role in the first line of defence against viruses , the observed impairment of NK cell activity in the seals feeding on the Baltic Sea herring suggests that exposure to contaminants may have an adverse effect on the defence against virus infections in seals in habiting polluted waters in Europe. This may therefore have affected t he severity of the infections, the survival rates and the spread of in fections during recent epizootics.