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 (liposome s) and vesicles composed of fusogenic, reconstituted influenza virus e nvelopes (virosomes), The incorporation of Salmonella minnesota rough- LPS in liposomes reduced the potency of LPS to stimulate splenocyte pr oliferation and cell surface k-light chain expression on 70 Z/3 pre-B cells by over 100-fold. Salmonella minnesota rough-LPS inserted into v irosomes was at least 10-fold more potent than free LPS, both when pre bound 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 reduce d the potency of the virosomal LPS to the level of liposome-incorporat ed LPS. The association of the various LPS forms with the cells was qu antitated 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 fu sion-inactivated virosomes. After low pH-induced fusion with the plasm a membrane, the majority (80%) of the prebound virosomes had fused wit h the cells, compared with about 8% after neutral uptake, From these r esults we conclude that LPS inserted into the plasma or endosomal memb ranes efficiently activates murine lymphocytes. The fusion data sugges t that the incorporation into endosomal membranes might be a more effe ctive stimulus.