Discovery of cold-tolerant phagocytic activity in polymorphonuclear leucocytes (PMNs) of marine mammals

Marine mammals have evolved a variety of thermoregulatory mechanisms to adapt to the marine environment, in which significant heat loss is common. However, the mechanisms by which normal biological defense mechanisms are maintained in areas directly exposed to external environmental temperatures, such as injury sites and peripheral tissues, remain unclear. This study focused on polymorphonuclear leukocytes (PMNs), which play a key role in the first line of defense, and examined the temperature dependence of phagocytosis against microspheres, zymosan, and Escherichia coli in bottlenose dolphin and spotted seal PMNs. To elucidate the characteristics of marine mammals, the same analyses were performed on PMNs from terrestrial mammals (human, dog, pig, and cattle) and comparative studies were conducted across animal species. While terrestrial mammal PMNs exhibited a temperature-dependent decline in phagocytosis across all particles, bottlenose dolphin PMNs maintained stable activity even at low temperatures. Additionally, the study revealed that spotted seal PMNs exhibited a temperature-dependent decline in phagocytic activity against microspheres. However, these cells retained robust phagocytic activity against zymosan and E. coli, even at low temperatures. These results suggest that the PMNs of marine mammals have undergone evolutionary adaptation to enable effective biological defense mechanisms in cold marine environments.