Activation of human natural killer cells by lipopolysaccharide from Actinobacillus actinomycetemcomitans

Lipopolysaccharide (LPS) from the Y4 strain of this bacterium, which is implicated in the pathogenesis of juvenile periodontitis, was incubated with human peripheral blood lymphocytes (PBL) and its action compared to that of LPS from Escherichia coli. Both LPS augmented cytotoxicity measured against natural killer (NK) cell-resistant tumour targets within 24 h of incubation. Cytotoxicity was exclusively found in NK-enriched low-density large granular lymphocyte fractions, as separated by Percoll gradient. LPS activated NK cells without stimulating high levels of proliferation. The minimum concentration of A. actinomycetemcomitans LPS required to activate NK cells was 1 microgram/ml; higher concentrations did not significantly increase this activation. LPS had no synergistic effect on the induction of PBL cytotoxicity by interleukin-2. In contrast, LPS pre-activated monocytes inhibited the induction of lymphocyte cytotoxicity by either interleukin-2 or LPS.