Modulation of Murine Macrophage Nitric Oxide Synthesis by Liposomal Phospholipids: Correlation with Liposome Immune Adjuvant Activity

Abstract

The influence of alum and liposomal phospholipids on interferon-γ-(IFN-γ), IFN-γ/N-acetylmuramyl-L-alanyl-D-isoglutamine- (MDP) or IFN-γ/tumor necrosis factor-α- (IFN-γ/TNF-α) induced macrophage nitric oxide (NO) synthesis has been investigated. IFN-γ induced NO synthesis in a dose-dependent manner. TNF-α and MDP did not induce NO synthesis, but interacted synergistically with sub-optimal doses of IFN-γ. Alum strongly inhibited IFN-γ-induced NO synthesis (ID₅₀25 μ/ml). Liposomes composed of dipalmitoylphosphatidylcholine (DPPC) had no effect on IFN-γ-induced NO synthesis. IFN-γ-induced NO synthesis was stimulated by DPPC/dimyristoylphosphatidylglycerol (DMPG) liposomes (9:1 mol ratio, ED₅₀ 45 nmol phospholipid/ml), and inhibited by DPPC/dipalmitoylphosphatidylethanolamine (DPPE) liposomes (9:1 mol ratio, ID₅₀ > 500 nmol phospholipid/ml), and DPPC/phosphatidylserine (PS) liposomes (7:3 mol ratio, ID₅₀ 150 nmol phospholipid/ml). Alum, DPPC/PE and DPPC/PS liposomes also inhibited IFN-γ/MDP- and IFN-γ/TNF-α-induced NO synthesis. Neither alum or the liposome preparations had significant toxicity towards macrophages in vitro at concentrations that induced maximal inhibition or stimulation of IFN-α-induced NO synthesis. Immunization of mice with alum-adsorbed and liposome-incorporated bovine serum albumin (BSA) demonstrated that enhancement or reduction of both IgG antibody and the proportion of IgG2a/IgG2b was correlated with stimulation or inhibition of IFN-γ-induced NO synthesis.