A Simple Mycobacterial Monomycolated Glycerol Lipid Has Potent Immunostimulatory Activity

Abstract

It is a long held belief that the strong immunostimulatory activity of the Mycobacterium bovis bacillus Calmette-Guérin vaccine and Freund’s complete adjuvant is due to specific mycobacterial cell envelope components, such as lipids and polysaccharides. Implicated mycobacterial lipids include, among others, the so-called cord factor or trehalose dimycolate, but limited information is available regarding the precise molecular nature of the stimulatory components responsible for the interaction with human APCs. In this regard, the majority of research aimed at identifying and characterizing individual immunostimulatory mycobacterial lipids has been performed in the murine system using bone marrow-derived dendritic cells. In this study, it is documented that potent immunostimulatory activity lies within the bacillus Calmette-Guérin nonpolar lipid class. This activity can be narrowed down to a remarkably simple monomycolyl glycerol (MMG) with the ability to stimulate human dendritic cells as assessed by enhanced expression of activation markers and the release of proinflammatory cytokines. A synthetic analog of MMG based on 32 carbons (C32) was found to exhibit comparable levels of immunostimulatory activities. Immunization of mice with the tuberculosis vaccine candidate, Ag85B-ESAT-6, in MMG or the synthetic analog using cationic liposomes as the delivery vehicle was found to give rise to a prominent Th1 response characterized by significant levels of IFN-γ. Together, this development opens up the possibility of producing a novel class of chemically defined lipid adjuvants to enhance the activity of new vaccine formulations, directed against infectious agents including tuberculosis.

Towards understanding the functional diversity of cell wall mycolic acids of Mycobacterium tuberculosis

“…4) has been isolated from the Mycobacterium avium-Mycobacterium intracellulare complex [59]. Even a very simple monomycolylglycerol (MMG) has the ability to stimulate human dendritic cells, while a model based on just a C 32 chain has similar effects [60].…”

Novel Generation Mycobacterial Adjuvant Based on Liposome-Encapsulated Monomycoloyl Glycerol from Mycobacterium bovis Bacillus Calmette-Guérin

“…Hence, TDM induced an increase in the number of mononuclear cells and the appearance of classical granulomatous lesions, whereas MMG gave rise to a more rapid and dense infiltrate of polymorphonuclear cells, which resolved after 4 wk. Further evidence supporting the differences between MMG and TDM comes from recent studies using human monocyte-derived DC (7). In these studies, stimulation with MMG resulted in enhanced expression of activation markers and the release of proinflammatory cytokines, whereas stimulation with TDM resulted in negligible activation comparable to that obtained with medium alone.…”

“…Individual MMG, based on ␣-mycolic acids and keto-mycolic acids, and a C 32 synthetic MMG analog (3-hydroxy-2-tetradecyl-octadecanoic acid-2,3-dihydroxypropyl ester) were prepared as described previously (7,9). The same synthetic strategy was used to prepare a C 18 mono-acyl glycerol analog (C 18 MAG; octadecanoic acid-2,3-dihydroxypropyl ester).…”

“…In addition, the biological activity could be reproduced with a synthetic analog of MMG based on 32 carbons atoms. The recent finding that natural MMG and a synthetic analog also activate human dendritic cells (DC) (7) further supports the use of MMG compounds as vaccine adjuvants.…”

Pathophysiological Implications of Cell Envelope Structure in Mycobacterium tuberculosis and Related Taxa

“…Glucose monomycolates (GMMs) are common in mycobacteria, but in highly variable proportions [25]. Consistent proportions, however, are recorded for monomycoloyl glycerols (MMGs) (Figure 3) in the Mycobacterium bovis members of the M. tuberculosis complex [26,27], thereby suggesting some cell envelope structural involvement. Mycobacteria also produce very complex mixtures of di-and triacylglycerols, some of which contain non-hydroxylated fatty acids that correspond to the meromycolate portion of mycolic acids [28,29], the so-called "mycobacteric" acids [2].…”