Symbiotic bacteria, immune-like sentinel cells, and the response to pathogens in a social amoeba

Abstract

Some endosymbionts living within a host must modulate their hosts' immune systems in order to infect and persist. We studied the effect of a bacterial endosymbiont on a facultatively multicellular social amoeba host. Aggregates of the amoebaDictyostelium discoideumcontain a subpopulation of sentinel cells that function akin to the immune systems of more conventional multicellular organisms. Sentinel cells sequester and discard toxins fromD. discoideumaggregates and may play a central role in defense against pathogens. We measured the number and functionality of sentinel cells in aggregates ofD. discoideuminfected by bacterial endosymbionts in the genusParaburkholderia. InfectedD. discoideumproduced fewer and less functional sentinel cells, suggesting thatParaburkholderiamay interfere with its host's immune system. Despite impaired sentinel cells, however, infectedD. discoideumwere less sensitive to ethidium bromide toxicity, suggesting thatParaburkholderiamay also have a protective effect on its host. By contrast,D. discoideuminfected byParaburkholderiadid not show differences in their sensitivity to two non-symbiotic pathogens. Our results expand previous work on yet another aspect of the complicated relationship betweenD. discoideumandParaburkholderia, which has considerable potential as a model for the study of symbiosis.