Induction of Fas ligand in murine bone marrow NK cells by bacterial polysaccharides

Bacterial polysaccharides have a wide range of activities in mammals. We have studied the effect of LPS and poly-beta-(1-->4)-D-mannuronate (mannuronan, poly-M), an exopolysaccharide from Pseudomonas aeruginosa, on the cytotoxicity mediated by murine bone marrow cells (BMC). Addition of LPS or mannuronan to BMC induced a time- and dose-dependent cytotoxicity against Jurkat cells. The LPS- or mannuronan-induced cytotoxicity was due to increased Fas ligand (FasL) expression by BMC, since 1) Fas-transfected L1210-Fas target cells were more susceptible to lysis than the Fas(low)-expressing parent L1210 cells, 2) stimulated BMC from FasL-defective gld/gld mice were not cytolytic and, 3) the cytolytic activity of normal BMC was inhibited by a Fas-Fc fusion protein. Flow cytometry showed an increase in surface FasL in LPS-stimulated BMC. RT-PCR analysis of BMC revealed constitutive expression of FasL mRNA, which was increased after LPS stimulation. Immunomagnetic depletion of NK1.1-, CD2-, or CD32/16-expressing cells from BMC abrogated the LPS-induced BMC cytotoxicity against L1210-Fas cells, suggesting that NK cells were the cytotoxic effector cells. Depletion of CD45R/B220-, Gr-1-, or CD11b/Mac-1-expressing cells only partially decreased BMC-mediated cytotoxicity, and depletion of CD4- or CD8a-expressing cells had no effect. The results support the conclusion that LPS and mannuronan induce expression of cytotoxic FasL on bone marrow NK cells.