セルフアジュバントワクチンの創製を目指した高次機能化リピドAの開発

概要

Lipopolysaccharide (LPS) is a component of the outer membrane of Gram-negative bacteria, and known as endotoxin for its potent inflammatory activities. Terminal glycolipid moiety, “lipid A” is an active principal of LPS. Monophosphoryl lipid A (MPL), which lacks one phosphate group has lower immunostimulatory activity than lipid A. MPL derivative 3D-MPL1) (GlaxoSmithKline) has been practically used as a vaccine adjuvant. On the other hand, it has been reported that a self-adjuvant vaccine composed of an antigen and an adjuvant can promote efficient antibody production2). As 3D-MPL has already in practical use, it is almost certain that the MPL based self-adjuvant vaccine will lead to the development of innovative vaccines. Meanwhile, the lipid A conjugation strategy maintaining its innate immunological function is still under developing. In the case of MPL as a substrate, since it is easy to modify the 1-position by glycosylation, many lipid A derivatives have been reported3-8) using the 1-position of MPL as a foothold to introduce antigens, but there have been few reports on the innate immune function of MPL, although its antibody production enhancing effect has been investigated. On the other hand, we have applied a lipid A modification strategy to mimic natural LPS structure by introducing a modification group from the 6'-position of lipid A via a hydrophilic linker that mimic a sugar chain9). However, there are no examples of natural type lipid A modification retaining its immune functions in vitro, and in most cases the activity was reduced even without inactivation9).
We examined the lipid A modification method to create a self-adjuvant vaccine based on MPL504, attenuated lipid A developed by us, as an adjuvant. To determine the position of the modification, we synthesized MPL504 analog libraries 2-6 (Figure 1), in which the 1-position was converted to a hydroxy or propy group and the 6'-position was converted to a hydroxy, amino, or acetamide group, and evaluated their functions. As a result, although, the activity of derivative 3-6 was reduced, MPL504 (1-OH, 6'-NHAc) (2) with an aminoacetyl group at the 6'-position showed the same level of activity as unmodified MPL504 (1). Therefore, the position of the modification group was determined to be 6'-position.We developed a new hydrophilic linker mimicking sugar chain structure for the development of highly functionalized lipid A. We previously found that the linker influenced the activity of lipid A, i.e., polyethylene glycol linker significantly decreased the lipid A activity, whereas the more hydrophilic linkers maintained the activity9). In this study, we designed and synthesized a novel hydrophilic sugar mimic linker (SML) 7 from D-mannitol (Scheme 1). Compound 9 was synthesized from the disaccharide intermediate 810). Coupling of 9 with the SML 7 afforded 10. We performed a functional evaluation of 10 and found that the SML 7 introduction maintained the lipid A activity. Furthermore, we synthesized self-adjuvant vaccine MPL504-SML-Tn (11) containing Tn antigen, one of the tumor-associated carbohydrate antigens as a highly functionalized lipid A. The immunological functions of MPL504-SML-Tn (11) was also evaluated.