Activation of murine lymphocytes by lipopolysaccharide incorporated in fusogenic, reconstituted influenza virus envelopes (virosomes).

We have studied the in vitro activation of murine lymphocytes with LPS incorporated in the membranes of both phospholipid vesicles (liposomes) and vesicles composed of fusogenic, reconstituted influenza virus envelopes (virosomes). The incorporation of Salmonella minnesota rough-LPS in liposomes reduced the potency of LPS to stimulate splenocyte proliferation and cell surface kappa-light chain expression on 70 Z/3 pre-B cells by over 100-fold. Salmonella minnesota rough-LPS inserted into virosomes was at least 10-fold more potent than free LPS, both when prebound virosomes were allowed to be taken up by the cells at neutral pH and when the virosomes were fused into the plasma membrane by low pH treatment. Inactivation of the virosomes by low pH pretreatment reduced the potency of the virosomal LPS to the level of liposome-incorporated LPS. The association of the various LPS forms with the cells was quantitated using radio-iodinated LPS. Correcting for uptake, virosomal LPS remained 2- to 10-fold more potent than free LPS in stimulating B lymphocytes and at least 100-fold more active than liposomal LPS or fusion-inactivated virosomes. After low pH-induced fusion with the plasma membrane, the majority (80%) of the prebound virosomes had fused with the cells, compared with about 8% after neutral uptake. From these results we conclude that LPS inserted into the plasma or endosomal membranes efficiently activates murine lymphocytes. The fusion data suggest that the incorporation into endosomal membranes might be a more effective stimulus.