Immunogenic liposomes were prepared from mixtures containing different relative amounts of 2,4-dinitrophenyl-6-N-aminocaproyl-phosphatidylethanolamine (DNP-Cap-PE) and L-α-dipalmitoylphosphatidylcholine (DPPC) to study the effect of hapten epitope density on the in vitro immunogenicity of these model membranes. Anti-TNP plaque-forming cell (PFC) responses to the liposomes in a Mishell-Dutton culture system were strictly dependent on the ratio of DNP-Cap-PE to DPPC, confirming previous and current in vivo results. Liposomes containing 0.625 or 1.25% DNP-Cap-PE did not induce responses; liposomes with 2.5% DNP-Cap-PE stimulated significant responses; optimal responses were, however, obtained with liposomes containing 5 and 10% DNP-Cap-PE. This density effect profile was observed in all four mice strains tested (C57BL/6, BALB/ c, DBA/1, DBA/2). Furthermore, liposomes containing another hapten, mono-(azophenyltrimethylammonium)-tyrosyl-phosphatidylethanolamine (APT-Tyr-PE), gave a similar density profile for the in vitro induction of anti-APT PFC responses. Assuming homogeneous distribution of the hapten-PE derivatives in the lipid bilayers, using 5% as the critical hapten density required for a maximal response, and assuming 48 A2 as the molecular area occupied by DPPC, the average distance between two adjacent determinants would be approximately 32 A. This calculation suggests that both sites on one or more membrane immunoglobulin receptors must be occupied for optimal B cell activation.
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Tadakuma et al. (1980) studied this question.