MEMBRANE-CHANGES IN LIPOPOLYSACCHARIDE-STIMULATED MURINE-B LYMPHOCYTES ASSOCIATED WITH CELL ACTIVATION

The lateral diffusion of the fluorescent lipid analog 3,3'-dioctadecylindocarbocyanine iodide (Dil) was measured in the membranes of murine B lymphocytes treated with the B cell mitogen lipopolysaccharide (LPS). The mobility of Dil, as measured by fluorescence photobleaching recovery (FPR) techniques, was temperature-dependent with a value of 6.1 . 10(-9) cm2 s-1 at 37-degrees-C. Untreated cells exhibited this diffusion coefficient over 72 h in culture. In contrast, Dil mobility decreased to 2.0 . 10(-9) cm2 s-1 at 37-degrees-C in membranes of LPS-stimulated lymphocytes 24 h following LPS exposure. Interestingly, this decreased lipid lateral diffusion was not accompanied by any change in surface immunoglobulin lateral diffusion which remained essentially unchanged at 3.6-4.3 . 10(-11) cm2 s-1 over 72 h. To determine whether LPS effects on lipid lateral diffusion were due to insertion of LPS into the cell plasma membrane, we examined TRITC-LPS diffusion in B lymphocytes from LPS-responsive Balb/c and C3Heb/FeJ mice and from hypo-responsive C3H/HeJ mice. DiI and TRITC-LPS mobility decreased more than 50% in LPS-stimulated Balb/c and C3Heb/FeJ cells by 72 h. On C3H/HeJ lymphocytes, there was, no change in Dil or TRITC-LPS lateral diffusion throughout the incubation period. These data indicate that B lymphocyte membrane composition is altered in LPS-activated lymphoblasts and that the decreased lateral diffusion of lipid probes does not result from membrane perturbation by LPS insertion into the lipid bilayer. Further, similarities between TRITC-LPS and Dil lateral diffusion suggest that most LPS molecules interact non-specifically with B cell membranes, presumably by acyl chain insertion of the lipid A moiety.