Muramyl dipeptide and polymorphonuclear leukocyte chemotaxis in vitro.

Muramyl dipeptide (MDP) is an immunostimulatory agent that has been shown repeatedly to provide protection against the effects of some forms of experimental surgical infection. The mechanisms of this protection are incompletely understood, and in order to further define them, the impact of MDP upon in vitro polymorphonuclear (PMN) leukocyte chemotaxis was examined in the presence of both normal and opsonic depleted serum. We have also evaluated the effect of MDP as a chemoattractant itself and the response of neutrophils along with MDP to a standard chemoattractant, n-formyl-L-methionyl-L-leucyl-L-phenyl-alanine (FMLP), using the agarose assay for chemotaxis. MDP has weak but significant chemotactic activity itself, with a peak effect at a concentration of 5 X 10(-8) micrograms/ml for neutrophils. These effects are enhanced by the addition of 10% serum. MDP more strongly enhances neutrophil chemotaxis to FMLP. The enhancement of chemotaxis may be a factor in the protection that MDP exerts against some experimental bacterial challenges.