KOROPATNICK, T.; ESTRADA, A.; APICELLA, M.; MCFALL-NGAI, M.: Messengers or mercenaries? Characterizing the role of macrophage-like hemocytes in the bacteria-induced morphogenesis of the juvenile light organ of Euprymna scolopes

The symbiotic association between the squid Euprymna scolopes and the luminous marine bacterium Vibrio fischeri begins anew each generation. The hatchling host gathers symbionts from the environment using currents created by four ciliated epithelial appendages protruding from the light organ surface. Upon infection with V. fischeri, the light organ undergoes a 4 d, symbiont-induced morphogenesis involving widespread apoptosis followed by the regression of the ciliated appendages. This study investigated the role of macrophage-like hemocytes in this dramatic morphogenesis. Confocal microscopy was used to characterize hemocyte trafficking within the central blood sinuses of the ciliated appendages over time in both symbiotic and aposymbiotic (uninfected) animals, and in animals exposed to various microbial products (e.g., lipopolysaccharide, peptidoglycan). In the presence of the symbiont a statistically significant increase in hemocyte number could be seen in the blood sinuses as early as 2 h post-infection, and a peak in number was reached by 24 h. Hemocyte trafficking was irreversible in fully symbiotic animals that were subsequently cured of symbionts with antibiotics. Experiments with agents that delay hemocyte migration indicated that an increase in hemocyte number was not required for the onset of apoptosis. However, high numbers of hemocytes continued to be present in the blood sinuses at later time points during the regression phase of morphogenesis, suggesting a role for these cells in the eventual loss of the epithelial appendages. Supported by NSF IBN 9904601 and NIH RR12294 to M. McFall-Ngai and E.G. Ruby.