[en] Myeloperoxidase (MPO) is an enzyme located within polymorphonuclear neutrophils capable of producing cytotoxic oxidant species that are particularly active against bacteria with polysaccharide capsules. Pseudomonas aeruginosa (10(6) bacteria per 1ml) are killed within 1 h in vitro by a MPO/H2O2/C1- system (48mU=132ng of MPO). The question arose as to whether human macrophages would acquire cytotoxic activity when loaded with this enzyme. Monocytes were therefore isolated from human blood and cultured for up to ten days to induce maturation to macrophages. These cells lost endogenous MPO within five days while H2O2 production in response to stimulation by phorbol myristate acetate (10(-6)M) decreased to 23% within ten days. On the other hand, their capacity to take up exogenous MPO increased fourfold from day three to day ten. Human macrophages cultured from eight days (when both H2O2 production and MPO uptake were sufficient) were therefore used to study the effects of MPO uptake on cytocidal activity against Pseudomonas aeruginosa. After a 1 h MPO loading period, macrophages (5X10(5) cells per ml) were incubated in the presence of bacteria (0.5 to 2X10(6) bacteria per ml) for 2 h at 37 degrees C. At a bacteria/macrophage ratio of 1, only 34.8+/-7.0% of bacteria survived (compared to killing by non-loaded macrophages), while 74.4+/-9.3% survived at a ratio of 4. From these results, we conclude that loading macrophages with exogenous MPO could enhance their microbicidal activity, suggesting a potentially useful therapeutic application.