Mitogenic effects of bacterial cell walls and their components on murine splenocytes

Stimulation of splenocytes from ICR mice by cell walls of 17 species of gram-positive bacteria, and peptidoglycans and water-soluble enzymatic digests prepared from some of them, and by synthetic N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) was studied in terms of mitogenic activity. All the cell walls tested except those of Micrococcus lysodeikticus, regardless of their mycolic acid content and immunopotentiating activity, had definite stimulatory activity on splenocytes. In general, the cell walls of mycobacteria, nocardia, and stretomyces had stronger mitogenic activity than other cell walls. The mitogenic activity of cell walls was mainly attributable to a peptidoglycan moiety, and the activity was retained even when the peptidoglycan had been degraded into a monomer or a dimer of its subunit, though a polymerized form of the subunits exhibited stronger activity than the monomer or dimer. Definite mitogenicity of synthetic MDP on splenocytes from ICR mice was also confirmed. The origin of the unknown principle(s) that is found in the cell walls of nocardia, mycobacteria and streptomyces and is responsible for their potent mitogenic activity on murine splenocytes was discussed.