STEWART, J.J.; NYHOLM, S.V.; MCFALL-NGAI, M.J.: Squid hemocytes are able to discriminate the specific symbiont from other types of bacteria.

The establishment of the light organ symbiosis between the Hawaiian bobtail squid Euprymna scolopes and the marine luminous bacterium Vibrio fischeri depends on specific selection for the bacterial symbiotic partner and exclusion of other potentially detrimental strains of bacteria. Unlike vertebrates, E. scolopes and other cephalopods do not have an antibody based non-self recognition system and must rely totally on innate immunity to discriminate between pathogenic and non-pathogenic bacteria. Because the light organ is highly vascularized, likely components of the innate immune system for recognizing and interacting with foreign bacteria are the squid’s phagocytic cells, or hemocytes. In this study, the ability of squid hemocytes to recognize and bind different strains of marine bacteria was measured. V. fischeri and V. parahaemolyticus adhered to hemocytes at low levels, one to two cells per hemocyte, whereas V. harveyi attached to the hemocytes at 15 to 20 cells per hemocyte. However, when the squid were cured of the symbiotic bacteria with antibiotics and then exposed to the same strains of marine bacteria, V. fischeri showed a five-fold increase in adherence to the hemocytes. This difference in attachment of bacteria to the hemocytes from cured animals was specific for V. fischeri, since no change in the adherence of other strains of bacteria upon curing was observed. These results suggest that the host hemocytes can modify their response to V. fischeri, a capacity that may be important in the establishment and maintenance of the specific association. Supported by NIH RR-12294 and NSF IBN-9904601 to MM-N.