The in vivo efficacy of three cell wall-active antibiotics, imipenem, meropenem, and ceftazidime, was compared in mice rendered hypersusceptible to the pathophysiologic effects of lipopolysaccharide by treatment with D-galactosamine. When CF-1 mice were administered Escherichia coli, D-galactosamine, and saline intraperitoneally, an LD50 was achieved at an inoculum of approximately 2 x 10(4) cfu. Administration of antibiotic at 20 mg/kg resulted in significant but widely variable protective efficacy from E. coli lethality among the three antibiotics. At this dose, an approximately 3-fold increase in LD50 was observed with either meropenem or ceftazidime, whereas administration of imipenem resulted in an approximately 8-fold increase in LD50 (P = .0053). When the dose of antibiotic was decreased to 2 mg/kg, neither meropenem nor ceftazidime could provide measurable protection, whereas imipenem was almost fully protective (P < .002). These differences in protective efficacy were also noted with experimental Pseudomonas aeruginosa but not Staphylococcus aureus infection.

Treatment of log phase cultures ofEscherichia coli with cell active antibiotics results in increased exposure of immunologically reactive lipid A epitopes of lipopolysaccharide (LPS) and release of soluble LPS into culture supernatants. Comparison of the efficacy of two cell wall active antibiotics, ceftazidime, a penicillin-binding protein 3 selective antibiotic, and imipenem, a penicillin-binding protein 2 selective antibiotic, for their relative efficacy in mediating LPS release indicated quantitative but not qualitative differences, with the former antibiotic manifesting a significantly broader range of concentrations at which LPS release could be demonstrated. Comparison of the relative efficacy of these two antibiotics in a mouse bacteraemia model in which animals were made hypersensitive to the lethal effects of endotoxin by treatment with D-galactosamine indicated that the latter antibiotic may provide a greater level of protection. These studies suggest that the release of endotoxin mediated by antibiotic treatment may contribute to the pathogenesis of disease in infections due to gram-negative organisms.

We have employed neutralizing monoclonal antibodies to mouse interferon-γ (IFNγ) and to the receptor for mouse IFNy in studies designed to assess the protective efficacy of each of these monoclonals, administered either separately or in combination, in endotoxin-induced lethality. While pretreatment with either antibody alone, at doses of 200 μg per mouse, provided limited protection (70-50% lethality) in comparison to non-neutralizing antibody controls (100% lethality), the two monoclonal antibodies administered together provided a substantially greater level of protection (17% lethality). Although administration of 100 μg per mouse of either monoclonal alone was not protective (more than 65% lethality), a combination of both antibodies at this dose provided significant protection (19% lethality). In addition, administration of both antibodies 30 min post-endotoxin challenge also demonstrated significant protection in comparison to single antibody immunotherapy. In vitro studies using mouse peritoneal macrophages stimulated with LPS and IFNy have established confirmatory data for a synergistic effect of neutralizing antibody to IFNy and the IFNγ receptor in inhibiting macrophage activation as assessed by production of nitric oxide. These results provide a strong rationale for dual targeting of ligand and receptor in single cytokine immunotherapy.