Failure of synthetic muramyl dipeptide to increase antibacterial resistance

Augmentation of resistance of mice to bacterial infection by a polysaccharide-peptidoglycan complex (PSPG) extracted from Lactobacillus casei

A water-soluble polysaccharide-peptidoglycan complex (PSPG) was prepared from heat-killed Lactobacillus casei by digesting the bacteria with N-acetylmuramidase. The molecular weight of PSPG was over 30,000, and the polysaccharide portion of PSPG, its main component was composed of rhamnose, glucose, galactose, glucosamine and galactosamine. Mice pretreated intraperitoneally with PSPG survived after a lethal infection with Listeria monocytogenes or Pseudomonas aeruginosa. The growth of infecting bacteria (L. monocytogenes, P. aeruginosa, Salmonella typhimurium, Escherichia coli) in both the peritoneal cavity and the liver was inhibited markedly in the mice that had been treated with PSPG. It was suggested that macrophages may be the main effector for the anti-infectious effect of PSPG since treatment of mice with carrageenan, a selective macrophage blocker, markedly reduced the anti-infectious effect of PSPG.

Effect of free or liposome-encapsulated muramyl dipeptide on uptake and intracellular survival of Listeria monocytogenes in mouse peritoneal macrophages in vitro

The effect of free versus liposome-encapsulated muramyl dipeptide (MDP) on the uptake and intracellular survival of Listeria monocytogenes in mouse peritoneal macrophages in vitro was investigated. Macrophages in monolayer culture were exposed to free MDP at various concentrations during different time periods before incubation with Listeria monocytogenes. An increase in bacterial uptake dependent on the concentration of MDP and the length of exposure was observed. Exposure of macrophages to 200 micrograms of free MDP per milliliter for 15 h led to a threefold increase in bacterial uptake, resulting in an average of 15 bacteria per macrophage after 30 min of incubation. In addition, intracellular bacteria were killed in the MDP-exposed macrophages, in contrast to the intracellular multiplication of Listeria monocytogenes in macrophages not exposed to MDP. Encapsulation of MDP within liposomes resulted in a significant enhancement of its activity: liposomal encapsulation led to a 1,000-fold reduction in the amount of MDP required to obtain these effects on bacterial uptake and intracellular killing, whereas empty liposomes had no effect at all. Liposomal encapsulation may be an appropriate means of increasing delivery of the muramyl peptides to the macrophages.

Alteration of non-specific resistance to infection with Listeria monocytogenes

The experimental infection of murine hosts with Listeria monocytogenes is often used as a model for cell-mediated immunity. However, the natural immunity or non-specific resistance to listeriosis can be influenced by the parasite itself and also by a wide array of endogenous and exogenous host factors. The most important host factor in inbred mouse strains is their genetically determined susceptibility or resistance to Listeria monocytogenes. Secondly, the age of the mice is crucial for the outcome of infection. Resistance is only slowly developed by newborn mice, while aged mice possess an increased non-specific resistance as compared to young adult animals. Resistance is further influenced by the nutritional status, by pregnancy or by a simultaneous second antigenic stimulation. Regarding exogenous factors, macrophage blocking agents can totally abolish the resistance to listeriosis, while a lot of immunomodulating agents, such as BCG, killed Bordetella pertussis or Propionibacterium acnes organisms, lipopolysaccharides, suramin etc., can either increase or decrease the resistance. The mononuclear phagocyte system seems to be the main target of all these immunomodifiers. The timing between listeria infection and application of the immunomodulator determines the effect on non-specific resistance. A simultaneous injection of parasite and immunomodulator results in a decrease of resistance, while the application of immunoadjuvants several days before infection can dramatically increase the resistance to listeriosis. The delicate equilibrium of the mononuclear phagocyte system must therefore be taken into account, when infection with Listeria monocytogenes is used to test for immune-modifying agents, which are intended for use in humans or animals.

Compensation of cyclophosphamide immunosuppression by a bacterial immunostimulant (Broncho-Vaxom) in mice

The compensatory effect of a bacterial lysate, Broncho-Vaxom (BV) on the immunosuppressive action of cyclophosphamide (CY) was investigated. In CY immunosuppressed mice, BV treated animals recovered to normal levels of IgM and IgG in serum as well of IgA and IgG in gut secretions significantly earlier than controls. Furthermore, normal cell proliferation in thymus, as estimated by measuring the relative size of this organ was achieved earlier in BV treated mice than in control mice. Oral treatment with BV restores the number of IgM anti SRBC producing cells in spleen, in CY immunosuppressed mice. Since immunosuppression induced by CY increases the susceptibility to various infections, we tested in immunosuppressed animals the protective effect of BV towards IP challenge infections with Streptococcus pneumoniae, Staphylococcus aureus, Klebsiella pneumoniae var ozaenae, Pseudomonas aeruginosa and Candida albicans. BV led to an enhanced resistance towards both pneumococci and staphylococci challenge infections but not to the other challenge microorganisms.

Synthetic immunomodulators for prevention of fatal infections in a burned guinea pig model

Individuals who have suffered severe trauma, such as burns, have a high incidence of infection associated with impaired host resistance. Nonspecific stimulators of host defense mechanisms, i.e., immunomodulators, may be of benefit in such situations. A small animal model (guinea pigs) was developed to study the efficacy of immunomodulators in burns. Anesthetized animals received a 20% total body surface area, full-thickness, scald burn. There was no mortality associated with this injury, but these animals were highly susceptible to challenge with Pseudomonas aeruginosa strain 1244 by direct injection into the burn wound within 24 hours of injury. This susceptibility persisted about 7 days. The standard model adopted was to injure animals, then challenge with 1 median lethal dose (LD50) of P. aeruginosa 96 hours after injury. Using this model, six synthetic immunomodulators were tested: CP-20,961, CP-46,665, muramyl dipeptide, thymopoietin pentapeptide (TP-5), levamisole, and lithium. Drug administration began 24 hours after injury and ended prior to challenge with P. aeruginosa at 96 hours. CP-20,961, muramyl dipeptide, levamisole, and lithium all had no beneficial effect on survival. A single dosage (0.3 mg/kg, I.V.) of CP-46,665, administered 24 hours postinjury, increased the survival rate from 50% to 85% and mean survival time (MST) from 8.2 days to 12.4 days. TP-5, given in four doses (0.1 mg/kg, I.V. each) every 24 hours, increased the survival rate from 40% to 80% and MST from 6.9 days to 11.6 days. These data show that immunomodulators could be of benefit in burns, but also that not all agents are effective in this particular situation.

Effects of treatment with muramyl dipeptide on resistance to Mycobacterium leprae and Mycobacterium marinum infection in mice

Studies were carried out to determine whether treatment of mice with the synthetic adjuvant muramyl dipeptide (MDP) afforded resistance to infection with Mycobacterium leprae or M. marinum in mice. Regardless of the timing, dose, or route of administration, there was no evidence that treatment with MDP or 3 of its analogs (desMDP, abuMDP, valMDP) enhanced resistance to food pad infection with M. leprae or M. marinum. In parallel studies, systemic treatment with Propionibacterium acnes failed to protect against either M. leprae or M. marinum. Administration of P. acnes locally into the foot pad afforded marginal resistance to M. marinum when injected prior to infection. Local treatment with P. acnes afforded marked resistance to growth of M. leprae, even when administered months after infection.

Muramyl dipeptides: prospect for cancer treatments and immunostimulation

The immunopharmacological activities of bacterial cell walls and muramyl peptides were collected in table form with a comprehensive literature. The past and present studies emphasizing the host-defense enhancing activities of muramyl peptides for antitumor immunotherapy were surveyed along three possible approaches: 1) the nonspecific enhancement of natural defense ability of host against tumor cells themselves; 2) the enhancement of nonspecific resistance of host to microbial infections which are frequently encountered and difficult to treat in the advanced stage of tumor patients; and 3) the stimulation of immunity against tumor-specific or tumor-associated immunogens. Finally, the prospects of successful antitumor immunotherapy with muramyl peptides and their derivatives was discussed.

Effect of L18-MDP(Ala), a synthetic derivative of muramyl dipeptide, on nonspecific resistance of mice to microbial infections

By subcutaneous treatment with an aqueous solution of 6-O-stearoyl-N-acetylmuramyl-L-alanyl-D-isoglutamine [6-O-CH3-(CH2)16-CO-MurNAc-L-Ala-D-isoGln] [referred to here as L18-MDP(Ala)], an augmentation of the resistance of mice to Escherichia coli, Pseudomonas aeruginosa. Staphylococcus aureus, and Candida albicans infections was observed, but not to infections with Klebsiella pneumoniae and Listeria monocytogenes. Against E. coli infections, L18-MDP(Ala) was highly protective, irrespective of the administration route. Bacteremia occurring at an early phase of such infections was almost completely prevented by subcutaneous treatment 1 day before infection. Single or multiple doses were also effective against C. albicans infection. The phagocytosis of E. coli by mouse peritoneal polymorphonuclear cells was enhanced by treatment with the adjuvant, and the phagocytosis of K. pneumoniae was also enhanced, but only when the mice were treated either with rabbit normal serum or with a specific immune serum. The growth of the fungus in the kidneys was significantly inhibited, and growth was eliminated from the kidneys by treatment with the adjuvant once a day for 4 consecutive days, starting 1 day before infection. However, no growth suppression of L. monocytogenes in the livers or spleens of infected mice was observed when they were treated with a single dose of the adjuvant. This difference may be ascribed to the differences in the effector mechanisms of defense and to the different degree of augmentation of each defense mechanism by L18-MDP(Ala).

Protective effect of muramyl dipeptide analogs against infections of Pseudomonas aeruginosa or Candida albicans in mice

Two analogs of N-acetylmuramyl-L-alanyl-D-isoglutamine (muramyl dipeptide) were found to give better protection than muramyl dipeptide against intraperitoneal Pseudomonas aeruginosa infection or intravenous Candida albicans infection in mice. The analogs tested were N-acetyl-nor-muramyl-L-alanyl-D-isoglutamine and N-acetylmuramyl-L-alpha-amino-butyryl-D-isoglutamine. The optimum treatment was 80 mg/kg per day given once daily for 4 consecutive days before infection by the intraperitoneal, intravenous, or subcutaneous route. Dose response was limited. The compounds were not orally active. Synergism was seen between N-acetyl-nor-muramyl-L-alanyl-D-isoglutamine and gentamicin. No postinfection protection was observed. A nonspecific stimulation of macrophage cells by muramyl dipeptide analogs may contribute to the protection because antiinfective activity against Listeria monocytogenes given intraperitoneally was achieved with CBA mice.

Stimulation of nonspecific host resistance to infection induced by muramyldipeptides

The effect of muramyldipeptide (MDP), N-acetylmuramyl-L-alanyl-D-isoglutamine [MDP(Ala)], and its analogs on bacterial infection was studied using the experimental model of sepsis infection in mice. Injection of MDP(Ala) gave mice definitive protection against E. coli infection, but only partial protection against P. aeruginosa or K. pneumoniae infection. Several factors influencing the protective activity of MDP(Ala) on E. coli infection were studied, and it was demonstrated that the activity was induced by various routes of administration of MDP(Ala), including the oral route, and was markedly influenced by the bacterial inoculum size. It was also shown that the effective dose of MDP(Ala) was 100 micrograms per mouse for intraperitoneal, intravenous or subcutaneous injections and 1,000 microgram per mouse when administered orally. Furthermore, the optimal interval between MDP-treatment and infection was 24 hr when the treatment was carried out before infection. Clearance of bacterial cells in blood was observed after E. coli infection in mice treated with MDP(Ala). The efficacy of MDP(Ala) and two analogs, N-acetylmuramyl-L-valyl-D-isoglutamine [MDP(Val)] and N-acetylmuramyl-L-seryl-D-isoglutamine [MDP (Ser)], was evaluated for the E. coli infection; MDP(Val) was proven to be slightly less active than MDP(Ala), and MDP(Ser) to be the least effective, although MDP(Val) or MDP(Ser) was reported to have higher adjuvanticity than MDP (Ala) for the development of delayed-type hypersensitivity.

Effects of muramyl dipeptide treatment on resistance to infection with Toxoplasma gondii in mice

Studies were carried out to determine whether treatment of mice with the synthetic adjuvant muramyl dipeptide afforded any resistance to infection with the obligate intracellular protozoan Toxoplasma gondii. Marked resistance to lethal challenge infection was observed in CBA but not C57BL/6 mice pretreated with muramyl dipeptide. In CBA mice, a single muramyl dipeptide treatment administered 14, 7, or 4 days before Toxoplasma challenge did not afford protection, whereas mice treated at -1 day were highly resistant. Additional studies carried out to investigate the mechanisms underlying the enhanced resistance to Toxoplasma in muramyl dipeptide-treated mice failed to reveal either enhanced cytolytic antibodies to the parasite or evidence that peritoneal macrophages from treated mice were activated as determined in vitro by their microbicidal capacity for Toxoplasma or cytotoxic capacity for tumor target cells.

Activation of macrophages for enhanced release of superoxide anion and greater killing of Candida albicans by injection of muramyl dipeptide

The adjuvant muramyl dipeptide (MDP) has been shown to affect a number of macrophage functions in vitro. We studied the effect of subcutaneous injection of MDP into mice. Cultured peritoneal macrophages from treated mice displayed increased spreading, total cell protein, and specific activity of beta-glucosaminidase a constituent of macrophage lysosomes, and of lactate dehydrogenase. Generation of superoxide anion (O2-) by MDP-treated macrophages stimulated by contact with phorbol myristate acetate was enhanced by over fivefold to levels achieved by macrophages from bacillus Calmette-Guérin-infected mice. The enhancement in stimulated O2- release was noted by 1 h after injection of MDP, peaked by 3 h, and remained high for at least 48 h. Priming for enhancement of O2- release by MDP was similar in athymic nude mice and in normal littermates, suggesting that mature T lymphocytes are not involved in this MDP effect. Priming for enhanced stimulated O2- release, and morphologic and enzymic changes, were not achieved by injection of the D-D stereoisomer of MDP. Phagocytosis of Candida albicans was only slightly greater by macrophages from mice give MDP, but MDP-stimulated cells killed two times more C. albicans in vitro than did cells from untreated animals. When MDP was given 18 h before, simultaneously with, or 24 h after lethal infectious challenge with C. albicans, treated mice were protected compared with controls. These results suggest that injection of MDP effectively and rapidly activates macrophages in the recipient animal. This agent should serve as an important probe of macrophage physiology and, perhaps ultimately, as a means of enhancing host defense in humans.

Effects of treatment with muramyl dipeptide and certain of its analogs on resistance to Listeria monocytogenes in mice

Studies were carried out to determine whether treatment of mice with the synthetic adjuvant muramyl dipeptide (MDP) afforded any resistance to infection with Listeria monocytogenes. Regardless of the timing, dose, or route of administration, there was no evidence that treatment with either MDP or two of its analogs (des-MDP or MDP-D-D) induced any resistance to listeria infection in BALB/c, CBA/J, or C57BL/6J mice. In contrast, pretreatment with MDP induced marked protection to infection with Streptococcus pneumoniae (type III).

Lipophilic derivative of muramyl dipeptide is more active than muramyl dipeptide in priming macrophages to release superoxide anion

Mouse peritoneal macrophages, when treated with a lipophilic derivative of muramyl dipeptide either in vitro or in vivo by intraperitoneal injection, showed a more than fivefold increase in their ability to generate superoxide anion after stimulation of the macrophages with phorbol myristate acetate. This response was more than twice that observed with the parent molecule, muramyl dipeptide (MDP). Unlike MDP, which has a systemic effect, the lipophilic derivative, [B30]-MDP, did not alter the response of peritoneal macrophages when given subcutaneously in the flank, suggesting that [B30]-MDP remains localized at the site of injection. The enhanced effect of [B30]-MDP over MDP appeared to be due to the inherent lipophilicity of the molecule, and was probably not due to either stimulation of T lymphocytes or activation of the alternative pathway of complement.