EFFECT OF RECOMBINANT HUMAN GAMMA-INTERFERON ON INTRACELLULAR ACTIVITIES OF ANTIBIOTICS AGAINST LISTERIA-MONOCYTOGENES IN THE HUMAN MACROPHAGE CELL-LINE THP-1

Listeria monocytogenes is a facultative intracellular pathogen which e nters cells by endocytosis and reaches phagolysosomes from where it es capes and multiplies in the cytosol of untreated cells. Exposure of ma crophages to gamma interferon (IFN-gamma) restricts L. monocytogenes t o phagosomes and prevents its intracellular multiplication. We have te sted whether IFN-gamma also modulates the susceptibility of L. monocyt ogenes to antibiotics. We selected drugs from three different classes displaying marked properties concerning their cellular accumulation an d subcellular distribution, namely, ampicillin (not accumulated by cel ls but present in cytosol), azithromycin (largely accumulated by cells but mostly restricted to lysosomes), and sparfloxacin (accumulated to a fair extent but detected only in cytosol). We used a continuous lin e of myelomonocytic cells (THP-1 macrophages), which display specific surface receptors for IFN-gamma, and examined the activity of these an tibiotics against L. monocytogenes Hly(+) (virulent variant) and L. mo nocytogenes Hly(-) (a nonvirulent variant defective in hemolysin produ ction). Untreated THP-1 and phorbol myristate acetate-differentiated T HP-1 were permissive for infection and multiplication of intracellular L. monocytogenes Hly(+) (virulent variant). All three antibiotics tes ted were bactericidal against this Listeria strain when added to an ex tracellular concentration of 10x their MIC. After preexposure of THP-1 to IFN-gamma, L. monocytogenes Hly(+) was still phagocytosed but no l onger grew intracellularly. The activity of ampicillin became almost u ndetectable (antagonistic effect), and that of azithromycin was unchan ged (additive effect with that of IFN-gamma), whereas that of sparflox acin was markedly enhanced (synergy). A similar behavior (lack of bact erial growth, associated with a loss of activity of ampicillin, an enh anced activity of sparfloxacin, and unchanged activity of azithromycin ) was observed in cells infected L. monocytogenes Hly(-). This modulat ion of antibiotic activity, which we ascribe to the change of subcellu lar localization of L. monocytogenes caused by IFN-gamma or by the lac k of virulence factor, could result from a change in bacterial respons iveness to antibiotics, a modification of the drug activity, or differ ences in drug bioavailabilities between cytosol and phagosomes.