N-acetylglucosamine-containing muramyl peptides directly affect macrophages

In this study flow cytometry was used to show that macrophages were the major population of murine peritoneal exudate cells (MPEC), increasing Ia expression upon treatment with N-acetylglucosaminyl-beta 1-4-N-acetylmuramyl-alanyl-D-isoglutamine (GMDP). Modulation of Ia expression resulted from direct action of GMDP on macrophages, rather than from effect of cytokines released by T-cells. The effect of GMDP on two populations of macrophages, namely, slow and rapid responding, was studied in detail. Rapid responding cells were represented by Ia-positive macrophages: GMDP augmented their Ia expression. In contrast, slow responding subpopulation was represented by initially Ia-negative macrophages, in which GMDP induced de novo synthesis of Ia-antigens. The ability to induce Ia expression was also characteristic for other adjuvant-active N-acetylglucosamine-containing muramyl peptides (GMPs). Macrophages were shown to engulf GMPs by endocytosis. Activation of macrophages by GMDP resulted in an increase in their phagocytic activity.