Isolation, Structure Elucidation, and Synthesis of a Macrophage Stimulatory Lipopeptide from Mycoplasma fermentans Acting at Picomolar Concentration

Macrophages are typically stimulated by components of microbial cell walls. Surprisingly, cell wall–less mycoplasmas can also very efficiently stimulate macrophages. We showed recently that mycoplasma-derived lipopeptides constitute the active principle. We have now isolated a clone of Mycoplasma fermentans expressing mainly one macrophage-stimulating lipopeptide. This lipopeptide was detergent-extracted and isolated by reversed-phase high-performance liquid chromotography, using nitric oxide release from C3H/HeJ mouse macrophages as bioassay for detection. In contrast to “conventional” bacterial lipoproteins, this lipopeptide had a free NH₂ terminus. Amino acid composition, sequence, and the molecular weight of 2,163.3 are consistent with the following structure: S-(2,3-bisacyloxypropyl)cysteine-GNNDESNISFKEK with one mole C16:0, and a further mole of a mixture of C18:0 and C18:1 fatty acid per lipopeptide molecule. The sequence could not be found in either the protein identification resource nor the Swiss Prot data bank. We named this 2-kD lipopeptide, macrophage-activating lipopeptide-2 (MALP-2). Synthetic dipalmitoyl MALP-2 and mycoplasma-derived MALP-2 were compared with the bioassay. Both lipopeptides showed an identical dose dependency with a half-maximal response at 10⁻¹¹ M concentration. MALP-2 may be one of the most potent natural macrophage stimulators besides endotoxin.