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Definition
Fragments of the bacterial cell wall peptidoglycan, murein, have long been known to have potent adjuvant properties. The synthetic versions of these molecules and murein-component derivatives are known as adjuvant peptides.

Discovery
Johannsen L et al., in 1989 described the
properties of small cell wall monomers (muramyl peptides) that cause a number of short-lived effects in vivo and in vitro. The most studied of these molecules has been muramyl dipeptide (adjuvant peptide; N-acetyl-muramyl-L-alanine-D-isoglutamine [MDP]), the minimal active subunit of bacterial peptidoglycan. However, other muramyl peptides that contain 1,6-anhydromuramyl residues have been reported to have greater potency as an adjuvant peptides. Acute effects of muramyl peptides include breakdown of the blood-aqueous humor barrier, arthritis, cytoxicity, and slow-wave sleep. That cell wall monomers can cause sleep has excited much interest, since it explains one major side effect of infection-listlessness. More controversial has been a suggestion that muramyl peptides play a role in the sleep process of healthy people  1,2,3

Structural Characteristics
The murein component N-acetylmuramylalanyl-d-isoglutamine (MDP) has been shown to functionally substitute for killed mycobacteria in Freund’s Complete Adjuvant, and the repeating unit of murein, N-acetylglucosaminyl-1,4-N-acetylmuramylalanyl-d-isoglutamine (glucosaminylmuramyl dipeptide, GMDP) has an even stronger immunomodulatory activity than MDP. Synthetic versions of these molecules and other murein-component derivatives have been extensively studied as potential adjuvants for human use, but significant side effects such as pyrogenicity and uveitis have prevented their acceptance for clinical use. The peptides were isolated by using a GMDP-specific antibody to screen a phage-display library, selecting for peptides that mimic the biochemical properties and/or the biophysical shape of the GMDP molecule. 4. Sequence of muramyl adjuvant peptide is Ac-muramyl-D-Ala-D-Glu-NH2.

Mode of Action
The capacity of five different adjuvants, AlPO4, a muramyldipeptide formulation (MDP.TSL), Freund's adjuvant, immunostimulating complex and its matrix components to elicit humoral and cellular responses in rabbits immunized with the human immunodeficiency virus type 1 (HIV-1) envelope protein rgp160IIIB was compared. The highest antibody titers against gp160 and gp41/gp120 epitopes were seen with rgp160 in MDP.TSL or Freund's adjuvant, whereas the broadest responses were seen in rabbits immunized with rgp160 in matrix or MDP.TSL. The broadest spectrum of high-avidity antibodies was also induced by rgp160 in MDP.TSL. Neutralizing titers against HIV-1IIIB, low titers to HIV-1MN, and the most efficient inhibition of viral cell-to-cell spread was seen with rgp160 in MDP.TSL. The strongest and most persisting cellular responses were induced by rgp160 in AlPO4 or MDP.TSL. Using MDP.TSL as the adjuvant, also improved the immune response against gp120 epitopes by boosting rgp160-primed rabbits with rgp160, multiple antigenic peptides (MAPs), or unconjugated peptides. The MAPs induced high neutralizing titers and were superior to rgp160 alone in inducing both humoral and cellular reactivity
5.The investigation of the adjuvant activity of muramyl peptide on humoral and cellular immune responses showed that it augments the mitogenic responses of splenic lymphocytes to phytohemagglutinin and lipopolysaccharide and the formation of antibodies to sheep erythrocytes in mice. In addition, it potentiated polyclonal B cell activation in vivo and in vitro. Muramyl peptides are involved in sleep associated with bacterial infection, their role in normal physiological sleep is less certain. It has been speculated that "natural" muramyl peptides, derived from degraded gut flora, may pass into the bloodstream, where they play a role in normal sleep. Muramic acid serves as a chemical marker for muramyl peptides, since it is not synthesized by mammals. After injection of synthetic muramyl dipeptide in rabbits, muramic acid was readily detected (after release by acid hydrolysis) in the circulation 6.

Functions

Normal physiological sleep, muramyl peptides are involved in sleep associated with bacterial infection, their role in normal physiological sleep is less certain. It has been speculated that "natural" muramyl peptides, derived from degraded gut flora, may pass into the bloodstream, where they play a role in normal sleep. Rapid elimination of a synthetic adjuvant peptide from the circulation after systemic administration and absence of detectable natural muramyl peptides in normal serum was observed 6.

Peptidoglycan-polysaccharide complexes remain in host tissues, since they are resistant to degradation by mammalian enzymes and avoid rapid excretion through the kidney. The extremely large size of these molecules is a major factor in their ability to cause chronic inflammation 7.

Chemical marker, Muramic acid served as a chemical marker for the presence of muramyl peptides, since it is not found in nature other than in peptidoglycan. These are used as peptide adjuvants 8.

References

1.     Johannsen L, Rosenthal RS,Martin SA, Cady AB,Obal Jr. F,Guinand M,Krueger JM (1989). Somnogenic activity of O-acetylated and dimeric muramyl dipeptides. Infect. Immun., 57(9):2726-2732.

2.     Karnovsky M (1986). Muramyl peptides in mammalian tissues and their effects at the cellular level. Fed. Proc., 45:2556-2560.

3.     Kufoy EA, Fox K, Fox A, Parks C, Pakalnis VA (1990). Modulation of the blood-aqueous barrier by Gram positive and Gram negative bacterial cell wall components in the rat and rabbit. Exp. Eye Res., 50(2):189-195.

4.     Laman AG, Shepelyakovskaya AO, Berezin IA, Boziev KM, Rodionov IL, Chulina IA, Malakhova GV, Brovko FA, Murashev AN, Korpela TK, Nesmeyanov VA (2007). Identification of pentadecapeptide mimicking muramyl peptide. Vaccine, 25(15):2900-2906.

5.     Levi M, Rudén U, Birx D, Loomis L, Redfield R, Lövgren K, Akerblom L, Sandström E, Wahren B (1993). Effects of adjuvants and multiple antigen peptides on humoral and cellular immune responses to gp160 of HIV-1. J Acquir Immune Defic Syndr., 6(8):855-864.

6.     Fox A, Fox K (1991). Rapid elimination of a synthetic adjuvant peptide from the circulation after systemic administration and absence of detectable natural muramyl peptides in normal serum at current analytical limits. Infect Immun., 59(3):1202-1205.

7.     Gilbart J, Fox A (1987). Elimination of group A streptococcal cell walls from mammalian tissues. Infect. Immun., 55:1526-1528.

8.    Gilbart J, Fox A, Whiton RS, Morgan SL (1986). Rhamnose and muramic acid: chemical markers for bacterial cell walls in mammalian tissues. J. Microbiol. Methods., 5(5-6):271-282.

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