The zebrafish-Mycobacterium marinum infection model has been used for the development of this project. My host laboratory has established the zebrafish as a model for tuberculosis by infecting it with M. marinum, its natural pathogen and a close genetic relative of M. tuberculosis. The zebrafish-M. marinum infection model presents both the biological advantage that they are a host-pathogen pair that has evolved together, and the logistical advantage that M. marinum can be handled more safely than M. tuberculosis. This recognised model has provided insights into human tuberculosis with important therapeutic implications.
The development of this project by using mainly phagocyte recruitment assays and high definition confocal microscopy, led us to discover that the first phagocytes recruited very early after infection are the resident macrophages, that rapidly engulf the mycobacteria. Then, mycobacteria actively induce resident macrophages to produce and release lipoxins, that are pro-resolving lipid mediators that inhibit neutrophil migration and function. Our research with lipoxin-deficient animals has demonstrated that after removing this inhibitory signal, neutrophils are able to migrate to the infection site and then mycobacterial infection is dramatically attenuated. Importantly, one of the drugs used to inhibit lipoxin production is zileuton, that has already been approved for the treatment of asthma in humans. We have confirmed that neither neutrophil extracellular traps nor different neutrophil subpopulations are playing any relevant roles at the mycobacterial initial infection site. Surprisingly, the infection attenuation observed in the absence of the inhibitory lipoxins is not due to the killing of mycobacteria by neutrophils themselves, but to the enhancement of macrophage microbicidal activity when neutrophils are close to them. Going further in the study of this striking mechanism, we have discovered that neutrophils recruited in the absence of lipoxins release the enzyme myeloperoxidase, that is able to bind the macrophage mannose receptors that are present in the macrophage membrane resulting in the enhancement of macrophage activity. The existence of this mechanism in response to mycobacteria has never been shown before, and its knowledge will be crucial for the development of new strategies to treat tuberculosis by avoiding the inhibition of neutrophil response in patients.
All these results have been showed and discussed with the scientific community in different international meetings and workshops, and are included in a manuscript that is now in preparation and that will be sent for publication as an open-source article in Immunity, an international and prestigious scientific journal.
