Abstract

Muramyl peptides have a variety of biological effects in mammals, including enhancement of the immune response, sleep, and body temperature. Although mammals lack biosynthetic pathways for muramyl peptides, they are found in mammals and are well known as components of bacterial cell walls. This suggests that phagocytic mammalian cells digest bacterial cell walls and produce biologically active muramyl peptides. Staphylococcal cell walls were radioactively labeled during growth of the bacteria. During the digestion of these radiolabeled bacteria, murine bone marrow macrophages produced low-molecular-weight substances that coeluted chromatographically with the radioactive cell wall marker. Further separation of these substances using reversed-phase high-performance liquid chromatography resulted in the isolation of substances with high specific biological activity. Intracerebroventricular injection of rabbits with these substances induced an increase in slow-wave sleep and body temperature and a suppression of rapid-eye-movement sleep. The characteristics of the biological responses and the chromatographic behavior of the active components are consistent with those of muramyl peptides. The ability of macrophages to tailor muramyl peptides from peptidoglycan may provide an amplification step for the immune response. Muramyl peptides released by macrophages may also act as mediators for various facets of the acute phase response elicited by bacterial infections such as fever and sleep.