| Summary: | Bibliography leaves 60-70. === Tuberculosis remains to be a leading infectious cause of death worldwide. This is in spite of the BCG vaccine against tuberculosis that has been in use for over 80 years as well as several chemotherapies. Mycobacterium tuberculosis, the causative agent of tuberculosis, is a facultative intracellular pathogen targeting host cells, predominantly macrophages, to establish an infection. Endothelial cells form a barrier that has to be crossed by the bacilli in the establishment, and subsequent dissemination of mycobacterial infection. The present study was undertaken to investigate the phagocytosis of mycobacteria by endothelial cells and macrophages and the subsequent activation of these host cells after infection with the bacilli. Endothelial cells, obtained from the human umbilical vein (HUVEC) were infected with BCG-GFP under various conditions and uptake determined by means of flow cytometry. Endothelial cells phagocytised mycobacteria in a dose- and a time-dependant manner. Exposure of mycobacteria to serum opsonins enhanced the uptake of the bacilli by endothelial cells. However, heat killing of mycobacteria inhibited its uptake by endothelial cells. Data from the fluorescent microscope showed the association of BCG-GFP signal with endothelial cells detected on the FACS caliber. Analysis by confocal microscopy confirmed internalisation of endothelial cells by mycobacteria. Endothelial cells were further investigated for an acquired phenotype following infection with mycobacteria. CD31, a marker for endothelial cells, was neither down regulated nor up regulated. However, ICAM-1 expression, one of the adhesion molecules was down regulated upon infection of endothelial cells with mycobacteria. No TNF-α and IL-6 were detected in culture supernatants of infected endothelial cells. Macrophages obtained from murine bone marrow, phagocytosed mycobacteria in both a dose- and a time-dependant manner. Unlike endothelial cells, heat killing of mycobacteria did not obliterate their uptake by macrophages. However, macrophages preferentially phagocytosed viable mycobacteria in a 3h infection period, but not in an 18h period. Macrophages from the Mac-l mouse strain, lacking a phagocytic receptor 3 (CR3), were included in this study. The uptake of pathogenic mycobacteria, H37Rv by macrophages from Mac-1, was reduced in cell cultures infected for 4 hours but not those infected at 1 and 2 hours. Similarly, reduced uptake of avirulent mycobacteria strains H3 7Ra and BCG in the absence of CR3 was pronounced in cell cultures infected for longer periods. The activation state macrophages acquire after infection with mycobacteria was investigated with respect to the expression of MHC glycoproteins, and secretion of IL-10 and IL-12. The activation state of macrophages with respect to these parameters studied is critical in the interaction with T-lymphocytes, for subsequent containment of mycobacteria infection. Production of IL-12, a critical Th-1 cytokine, was proportional to MOI, and enhanced by viability of pathogenic mycobacteria. Furthermore, prior exposure of mycobacteria to serum opsonins inhibited the secretion of IL-12, while exposure of the bacilli to bronchoalveolar factors greatly enhanced it. IL-10 production by infected macrophages was on the other hand inhibited by prior exposure of mycobacteria to both serum and bronchoalveolar fluid factors. Macrophages consitutively expressed MHC I. After infection with pathogenic mycobacteria, cells positive for MHC I, were hardly detected in macrophage cultures infected with mycobacteria that had been exposed to fresh serum and bronchoalveolar fluid opsonins. MHC II on the other hand, was not consitutively expressed on macrophages. Following infection with pathogenic mycobacteria, the highest percentage of cells positive for the antibody against MHC II were observed in macrophage cultures infected both without any opsonin and in the presence of bronchoalveolar fluid factors. - In conclusion, the present study demonstrates that endothelial cells bind and internalise mycobacteria. That they get activated as evidenced in the down-regulation of ICAM-1 following infection with mycobacteria. Thus endothelial cells may not just be a passive, physical barrier but host cells that may have an active role in mycobacterial infection. Macrophages in comparison to endothelial cells were more effective in phagocytosis of mycobacteria in a time- and dose-dependant manner, differentiating themselves as professional phagocytes in the internalisation of heat-killed bacteria where the endothelial cells lacked the ability for the uptake of mycobacteria.
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