Macrophage activation by muramyl dipeptide (MDP) without lymphocyte participation

Antibody repertoire development in fetal and neonatal piglets. VIII. Colonization is required for newborn piglets to make serum antibodies to T-dependent and type 2 T-independent antigens

Cesarean-derived piglets were reared for 5 wk under germfree conditions or monoassociated with a benign Escherichia coli (G58-1) or a enterohemorrhagic strain (933D) derived from O157:H7, and immunized i.p. with the T-dependent (TD) Ags fluorescein-labeled (FL) keyhole limpet hemocyanin or trinitrophenylated (TNP) keyhole limpet hemocyanin and the type 2 T-independent Ags TNP-Ficoll or FL-Ficoll. Only colonized piglets showed an increase in serum IgG, IgA, and IgM and had serum Abs to FL, TNP, and colonizing bacteria. While serum Abs to FL or TNP appeared following colonization alone, secondary responses were restricted to piglets immunized using TD carriers. While animals colonized with 933D had significantly higher total serum IgG and IgM levels and specific IgG Abs than those colonized with G58-1, no differences were seen in serum IgA levels, B cell diversification in the ileal Peyer's patches, and specific activity (ELISA activity per micrograms of Ig) of pre-boost serum IgG and IgM anti-TNP and anti-FL Abs. Serum IgA Abs to TNP, FL, or bacteria were not detected. Ag-driven responses, as measured by an increase in specific Ab activity, were only observed in secondary responses to TD Ags and to colonizing, pathogenic E. coli. We propose that germline-encoded, isotype-switched B cells in newborn piglets differentiate to Ab-secreting cells 1) after stimulation by bacteria-activated APCs or 2) through direct stimulation by bacterial products. We further propose that Ag-driven systemic responses require both bacterial colonization and TD Ags translocated to the peritoneum.

Some cellular and pathophysiological correlates of the inflammatory effects of a synthetic immunomodulatory agent, muramyl dipeptide (MDP)

Acute, fully reversible paw edema was produced in mice after systemic administration of muramyl dipeptide (MDP, i.e. N-acetylmuramyl-L-alanyl-D-isoglutamine). Its stereoisomer N-acetylmuramyl-D-alanyl-D-isoglutamine (MDP-D,D) was much less effective. The swelling of paws occurred very soon (1 h) after MDP injection, reached the maximum severity at an interval of 6 h and declined afterwards. While no substantial quantitative differences were found in the sensitivity of the various inbred strains of mice to edemagenic activity of MDP, athymic nude mice were completely resistant to the induction of edema. Formation of edema was blocked by silica, indomethacin (partially also by nordihydroguaiaretic acid), monoclonal antibodies against T-cells and their TH-subpopulation. It is suggested that the MDP-induced edema is a macrophage- and T-cell-dependent, prostaglandin- (and partially leukotriene)-mediated acute reaction associated with increased vascular permeability. Possible engagement of immune/inflammatory cytokines like IL-1 has been discussed. The data support the view that this type of edema is a consequence of changes in the activity of important cellular components of the immune system.

Species dependency of in vitro macrophage activation by bacterial peptidoglycans

The effect of various bacterial cell wall components on in vitro biological function of murine peritoneal exudate macrophages was evaluated. We examined four different parameters of metabolic activity and monokine secretion. Peritoneal exudate macrophages from rats and guinea pigs, all of the strains tested, were stimulated by whole bacterial cell wall preparations, purified bacterial cell wall peptidoglucans, its water-soluble peptidolglycan fragments, muramyl dipeptides and amphipathic substances. Murine peritoneal exudate macrophages were activated by amphipathic substances of gram-positive bacteria. However, macrophages from mice, irrespective of strains, were not stimulated in the in vitro assay systems by purified bacterial cell wall peptidoglycan, water-soluble bacterial peptidoglycan fragments or muramyl dipeptides. These results suggest that macrophage activation by bacterial peptidoglycan in vitro is animal species specific.

Macrophages are stimulated by muramyl dipeptide to induce polymorphonuclear leukocyte accumulation in the peritoneal cavities of guinea pigs

N-Acetylmuramyl-L-alanyl-D-isoglutamine (muramyl dipeptide [MDP]) injected intraperitoneally significantly increased the number of cells entering the peritoneal cavity of guinea pigs primed with liquid paraffin or thioglycollate. There was a close relationship between peritoneal polymorphonuclear leukocyte (PMN) accumulation and the uptake of glucosamine by macrophages in guinea pigs treated with a variety of bacterial cell surface components such as cell wall peptidoglycan subunits and bacterial or synthetic lipid A. The PMN accumulation was also facilitated by the intraperitoneal transfer of the peritoneal macrophages that had been stimulated by MDP in vitro. Furthermore, cell-free lavage fluids taken from the peritoneum of MDP-treated guinea pigs also initiated the influx of PMNs when introduced into the peritoneal cavities of liquid paraffin-pretreated guinea pigs. These results suggest that a soluble factor which attracts neutrophils is produced by MDP-treated macrophages. Partial characterization of the factor is described.

The carbohydrate- and lipid-containing cell wall of mycobacteria, phenolic glycolipids: structure and immunological properties

Phenolic glycolipids were first discovered as cell-wall constituents of M. bovis, M. bovis BCG, M. marinum, and M. kansasii. Recently, such compounds were also isolated from M. leprae and have been shown to be specific-species serological markers. Moreover, they seem to be involved, in the case of lepromatous leprosy, in the stimulation of the suppressor T-cells. The functional activities of these phenolic glycolipids over the immune cells stimulation emphasized the role played by these molecules in the mycobacteria pathogenicity. Phenolic glycolipids have also been found in M. gastri and M. tuberculosis strain Canetti. From a structural point of view, these glycolipids contain the same aglycon moiety mainly assigned to phenolphthiocerol diester while the sugar part structure confers to some of these glycolipids their antigenic specificity. The search of immunoreactive glycolipids and their function analysis remain a challenge for chemists and immunologists for the understanding of the mycobacteria pathogenicity.

Granuloma formation by muramyl dipeptide associated with branched fatty acids, a structure probably essential for tubercle formation by Mycobacterium tuberculosis

Muramyl dipeptide, which does not induce epithelioid granuloma when injected alone dissolved in phosphate-buffered saline, could induce extensive granulomas in guinea pigs when chemically conjugated with branched, but not linear, fatty acids. Peptidoglycan fragments of Staphylococcus epidermidis could evoke epithelioid granulomas when incorporated in a water-in-oil emulsion. These findings suggest the importance of a lipid bound to muramyl dipeptide for granuloma formation. In view of the fact that mycobacteria uniquely contain large amounts of branched fatty acids, it was proposed that the complex of muramyl dipeptide and branched fatty acids, mostly mycolic acids, is a structure in tubercle bacilli responsible for tubercle formation.

Inhibition of macrophage DNA synthesis by immunomodulators. II. Characterization of the suppression by muramyl dipeptide or lipopolysaccharide [3H]thymidine incorporation into macrophages

Guinea pig peritoneal exudate macrophages actively incorporated [3H]thymidine into trichloroacetic acid-insoluble fraction in vitro. The incorporation of [3H]thymidine was almost completely inhibited by aphidicolin, an inhibitor of DNA polymerase alpha and an autoradiograph showed heavy labeling in nuclei of 15% of macrophage populations. These results indicate that the observed thymidine incorporation was due to a nuclear DNA synthesis. The [3H]thymidine incorporation was markedly suppressed when macrophages were activated by immunoadjuvants such as muramyl dipeptide (MDP) or bacterial lipopolysaccharide (LPS). The suppression of [3H]thymidine incorporation by MDP was neither due to the decrease in thymidine transport through the cell membrane, nor due to dilution by newly synthesized "cold" thymidine. An autoradiograph revealed that MDP markedly decreased the number of macrophages the nuclei of which were labeled by [3H]thymidine. These results suggest that the suppression of [3H]thymidine incorporation by the immunoadjuvants reflects a true inhibition of DNA synthesis. The inhibition of DNA synthesis by MDP was also observed in vivo. Further, it was strongly suggested that the inhibition was not caused by some mediators, such as prostaglandin E2, released from macrophages stimulated by the immunoadjuvants but caused by a direct triggering of the adjuvants at least at the early stage of activation. Cyclic AMP appears to be involved in the inhibitory reaction.

Enhanced superoxide anion release from phagocytes by muramyl dipeptide or lipopolysaccharide

Muramyl dipeptide (MDP) and lipopolysaccharide (LPS) from Escherichia coli were tested for the ability to influence superoxide anion (O2-) release from guinea pig phagocytes. Both MDP and LPS alone did not, by themselves, stimulate O2- release by macrophages and polymorphonuclear leukocytes. However, the preincubation of macrophages with MDP or LPS primed the macrophages to release an enhanced amount of O2- when stimulated by cytochalasin E and wheat germ agglutinin. When polymorphonuclear leukocytes were treated in the same way, only LPS showed an enhancing effect. MDP enhanced NADPH oxidase activity of macrophages, which is probably the reason for enhanced O2- release by MDP.

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.

Epithelioid granuloma induced by muramyl dipeptide in immunologically deficient rats

When WKA rats were either neonatally thymectomized or injected with anti-rat thymocyte sera, their T-cell functions were effectively suppressed. When neonatally thymectomized plus anti-rat thymocyte serum-treated rats were injected with non-immunogenic muramyl dipeptide in water-in-oil emulsion, they produced massive epithelioid granulomas. Essentially, no morphological difference was noticed between granulomas induced in untreated rats and in thymectomized plus anti-rat thymocyte serum-treated rats. These findings strongly suggest that muramyl dipeptide-induced epithelioid granulomas required no T cells for their formation. In contrast, the induction of adjuvant arthritis appeared to depend on the presence of T cells.