Role of macrophage phospholipase D in natural and CpG-induced antimycobacterial activity.

The present study addresses the differential ability of macrophages to control intracellular growth of non-pathogenic Mycobacterium smegmatis (Msm) and pathogenic M. tuberculosis (MTB). Results reported herein show that 3 h post infection, intracellular Msm, but not MTB, was significantly killed by macrophages. As the role of human macrophage phospholipase D (PLD) in the activation of antimicrobial mechanisms has been documented, we hypothesised the role of such enzyme in antimycobacterial mechanisms. To this aim, macrophage PLD activity was analysed at different times after exposure with either pathogenic MTB or non-pathogenic Msm. Results showed that, starting from 15 min after mycobacterial exposure, MTB did not induce macrophage PLD activity, whereas the environmental non-pathogenic Msm stably increased it. The direct contribution of PLD in intracellular mycobacterial killing was also analysed by inhibiting enzymatic activity with ethanol or calphostin C. Results show that PLD inhibition significantly increases intracellular Msm replication. In order to see whether the innate PLD-mediated antimicrobial mechanisms against MTB are also induced after CpG ODN stimulation, the role of PLD has been analysed in the course of CpG-mediated intracellular MTB killing. CpG DNA increased PLD activity in both uninfected and MTB-infected macrophages, and the inhibition of PLD activity resulted in a significant reduction of CpG-induced MTB killing. Taken together, our data suggest a relationship between host PLD activation and the macrophage ability to control intracellular mycobacterial growth.