Data indicate that monophosphoryl lipid A, in a dose 10,000 times that of endotoxin, used in experimental pyrogenicity trials, is well tolerated in human volunteers. Pretreatment of normal human volunteers with monophosphoryl lipid A attenuated the systemic response to bacterial endotoxin. These data support further clinical testing of monophosphoryl lipid A for the prevention or amelioration of the severe sequelae of sepsis.

Background: Monophosphoryl lipid A (MPL^®) is a detoxified derivative of the lipopolysaccharide (LPS) moiety of Salmonella minnesota R595, which has retained immunostimulatory activities. MPL^® has been administered to many subjects in clinical trials as an adjuvant component of infectious disease vaccines and is currently a component of a licensed cancer vaccine, Melacine^® (Corixa Inc., Schering Plough). MPL^® has, in particular, been shown to promote Th1-type antigen specific responses. L-tyrosine is a depot adjuvant which is fully metabolisable and has been successfully employed in allergy vaccines for a number of years. Methods: Mice were immunised with MPL^® adjuvant in conjunction with separate preparations of either ovalbumin or glutaraldehyde-modified ragweed pollen extract both coprecipitated with L-tyrosine. The specific antibody isotypes IgG1, IgG2a, IgG2b and also IgE were measured. Rats received booster injections of keyhole limpet haemocyanin (KLH) in conjunction with MPL^® adjuvant following priming with KLH in alum alone. KLH-specific antibody responses were measured. Results: It was shown that a combination of L-tyrosine and MPL^® were synergistic in enhancing murine antigen specific IgG antibody responses without enhancing antigen specific IgE responses. Furthermore, this adjuvant combination promoted strong IgG2 antigen specific responses indicative of a Th1 directed response. In KLH sensitised rats, treatment with MPL^® was shown to prevent a secondary IgE antibody response when injected with booster injections of antigen. Conclusions: Immunisation of mice with two different antigens adsorbed to L-tyrosine induced a Th1-skewed primary response when in conjunction with MPL^® adjuvant which also generally enhances a specific IgG response. Incorporation of MPL^® adjuvant in the immunisation of rats prevented a secondary specific IgE response. These results suggest that the employment of this new adjuvant in clinical allergy vaccination formulations may result in an improved efficacy which could be utilised in various ways to improve compliance.

The synthesis of a series of novel analogues of lipid A, the active principle of lipopolysaccharide, is reported. In these compounds, the 1-O-phosphono and (R)-3-hydroxytetradecanoyl moieties of native Salmonella minnesota R595 lipid A have been replaced with hydrogen and the length of the normal fatty acyl residues has been systematically varied. Normal fatty acid chain length in the 3-O-desacyl monophosphoryl lipid A (MLA) series is shown to be a critical determinant of iNOS gene expression in activated mouse macrophages and the induction of proinflammatory cytokines in human peripheral monocytes. Examination of pyrogenicity in rabbits and lethal toxicity in D-galactosamine-treated mice shows that toxic effects in the MLA series can be ameliorated by modifying fatty acid chain length. When used as an adjuvant for tetanus toxoid vaccines, certain MLA derivatives enhance the production of tetanus toxoid-specific antibodies in mice.