Non-chemotactic influence of CXCL7 on human phagocytes. Modulation of antimicrobial activity against L. pneumophila

We have investigated the role of CXCL7 in the immune response of human phagocytes against the intracellular bacteria Mycobacterium tuberculosis and Legionella pneumophila. We have observed that polymorphonuclear neutrophil (PMN) chemotaxis induced by the supernatants of infected monocyte derived macrophages (MDM) may be attributed to CXCL8 rather than CXCL7, although both chemokines are present in large quantities. We have also found that CXCL7 is present not only in the supernatants of MDM, but also in the supernatants of PMN of some, but not all, individuals. Western blot analysis revealed that, in both MDM and PMN supernatants appeared two bands with molecular weights consistent with the platelet basic protein (PBP) and the neutrophil activating protein-2 (NAP-2) sizes. Regarding the influence on infected cells, recombinant NAP-2 enhanced the antimicrobial activity of IFNγ activated MDM against L. pneumophila, but not against M. tuberculosis. In addition, U937 cells transfected with a NAP-2 construct inhibited the intracellular multiplication of L. pneumophila, supporting its role in the modulation of the antimicrobial activity. Finally, U937 cells transfected with the NAP-2 construct showed an adherence that was dramatically enhanced when the substrate was fibronectin. We conclude that human phagocytes produce CXCL7 variants that may have a significant influence on the immune response against bacterial pathogens.