Aggregatibacter actinomycetemcomitans is an oral pathogen and etiologic agent of localized
aggressive periodontitis. The bacterium is also a cardiovascular pathogen causing
infective endocarditis. A. actinomycetemcomitans produces
leukotoxin (LtxA), an important
virulence factor that targets white blood cells (WBCs) and plays a role in immune evasion during disease. The functional receptor for LtxA on WBCs is leukocyte function antigen-1 (LFA-1), a β-2
integrin that is modified with N-linked
carbohydrates. Interaction between toxin and receptor leads to cell death. We recently discovered that LtxA can also lyse red blood cells (RBCs) and
hemolysis may be important for pathogenesis of A. actinomycetemcomitans. In this study, we further investigated how LtxA might recognize and lyse RBCs. We found that, in contrast to a related toxin, E. coli α-
hemolysin, LtxA does not recognize
glycophorin on RBCs. However,
gangliosides were able to completely block LtxA-mediated
hemolysis. Furthermore, LtxA did not show a preference for any individual
ganglioside. LtxA also bound to
ganglioside-rich C6 rat
glioma cells, but did not kill them. Interaction between LtxA and C6 cells could be blocked by
gangliosides with no apparent specificity.
Gangliosides were only partially effective at preventing LtxA-mediated cytotoxicity of WBCs, and the effect was only observed when a high ratio of
ganglioside:LtxA was used over a short incubation period. Based on the results presented here, we suggest that because of the similarity between N-linked
sugars on
LFA-1 and the structures of
gangliosides, LtxA may have acquired the ability to lyse RBCs.