Nonspecific stimulation of host defenses against bacterial challenge in immunosuppressed mice.

Muramyl dipeptide (MDP) has been shown to be a safe and effective host defense stimulant in both normal and malnourished hosts exposed to a bacterial challenge. We investigated its effect in mice compromised by pretreatment with either cyclophosphamide or steroids. The bacterial challenge, designed to simulate a surgical wound infection, consisted of a Klebsiella pneumoniae-laden suture. The MDP treatment resulted in lower local and systemic bacterial spread and increased survival in mice immunosuppressed by cyclophosphamide than in untreated controls. No such effect was demonstrated in animals compromised by steroids. These results suggest that MDP exerts its effect primarily on the macrophage.