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Enhancement of endotoxin activity by muramyldipeptide
Haruhiko Takada1, Shigekuni Yokoyama, Shuhua Yang
1Department of Microbiology and Immunology, Tohoku University School of Dentistry, Sendai, Japan. dent-ht@mail.cc.tohoku.ac.jp
Abstract:
Synthetic muramyldipeptide (MDP), the minimum structural moiety of bacterial peptidoglycan for adjuvant and related activities, sensitized mice for two types of lethal shock induced by lipopolysaccharide (LPS): an early anaphylactoid shock and late endotoxin shock. In relation to the late reaction in MDP-primed mice, enhanced production of inflammatory cytokines was induced in response to various bacterial components. MDP showed a priming effect in mice not only when administered parentally but also via the oral route. MDP activated human monocytic THP-1 cells in a CD14-, Toll-like receptor 2 (TLR2)- and TLR4-independent manner to increase expression of MyD88, a common adaptor and signaling molecule for TLRs, and exhibited synergistic cytokine inducing effects with TLR4 agonists (LPS, synthetic lipid A), TLR2 agonist (synthetic lipopeptide), and TLR9 agonist (bacterial CpGDNA) in THP-1 cells in culture. Consistent with these findings, MDP primed TLR2 knockout mice as well as wild-type controls, but not TLR4-mutated C3H/HeJ mice, to enhance production of tumor necrosis factor-alpha upon stimulation with synthetic lipid A. In contrast to the BCG- and Propionibacterium acnes-priming system, MDP primed mice in an interferon-gamma-independent manner. Further studies are required to elucidate the mechanisms of the synthetic and priming activities of MDP for various bacterial components.
Insights
Synthetic muramyldipeptide (MDP) primes mice to enhance inflammatory responses to bacterial components, acting independently of Toll-like receptors (TLRs) and interferon-gamma. This priming occurs through both parenteral and oral administration routes.
Area of Science:
- Immunology
- Microbiology
- Pharmacology
Background:
- Synthetic muramyldipeptide (MDP) is the minimal structural unit of bacterial peptidoglycan responsible for adjuvant activities.
- Bacterial components like lipopolysaccharide (LPS) can induce lethal shock in susceptible individuals.
- Understanding immune priming mechanisms is crucial for developing novel therapeutic strategies.
Purpose of the Study:
- To investigate the immune-priming effects of synthetic muramyldipeptide (MDP) in mice.
- To elucidate the mechanisms underlying MDP-induced sensitization to bacterial components.
- To explore the role of Toll-like receptors (TLRs) and interferon-gamma in MDP-mediated immune responses.
Main Methods:
- Mice were sensitized with MDP and subsequently challenged with LPS to assess shock responses.
- Cytokine production was measured in response to various bacterial components in MDP-primed mice.
- Human monocytic THP-1 cells were used to study MDP activation and synergistic effects with TLR agonists.
- TLR2 knockout and TLR4-mutated mice were employed to investigate the role of specific TLRs in MDP priming.
Main Results:
- MDP sensitized mice to both early anaphylactoid and late endotoxin shock induced by LPS.
- MDP-primed mice exhibited enhanced inflammatory cytokine production upon stimulation with bacterial components.
- MDP activated THP-1 cells independently of CD14, TLR2, and TLR4, increasing MyD88 expression.
- MDP showed synergistic cytokine induction with TLR4, TLR2, and TLR9 agonists in THP-1 cells.
- MDP primed TLR2 knockout mice but not TLR4-mutated mice to enhance tumor necrosis factor-alpha production upon lipid A stimulation.
- MDP-induced priming was independent of interferon-gamma.
Conclusions:
- Synthetic muramyldipeptide (MDP) acts as an immune priming agent, enhancing responses to bacterial components through both parenteral and oral routes.
- MDP's priming activity involves MyD88 upregulation and synergistic cytokine induction with various TLR agonists, independent of TLR2 and TLR4 in initial activation.
- The priming effect is observed in TLR2 knockout mice but not in TLR4-mutated mice, suggesting a complex interplay with TLR signaling pathways.
- MDP-mediated immune priming occurs independently of interferon-gamma, differentiating it from other known priming systems.