Multinucleated Giant Cell Formation Induced by IFN-γ/IL-3 Is Associated with Restriction of VirulentMycobacterium tuberculosisCell to Cell Invasion in Human Monocyte Monolayers

One of the hallmarks of an effective immune response againstMycobacterium tuberculosisis the formation of granulomas containing multinucleated giant cells. IFN-γ and interleukin-3 (IL-3) promote Langhans-type multinucleated giant cell formation and have been identified in T cell clones reacting toM. tuberculosisantigens. The ability of human monocytes treated with IFN-γ and IL-3 to limit the spread ofM. tuberculosisin anin vitroinfection assay was examined. Monocytes were incubated with control medium, IFN-γ, TNF-α, and calcitriol, a combination permissive toM. tuberculosisgrowth, or IFN-γ and IL-3 and infected with a low inoculum ofM. tuberculosis(Erdman). IFN-γ/IL-3 treatment reducedM. tuberculosisCFU relative to both untreated and IFN-γ/TNF-α/calcitriol-treated monocytes. Specifically, CFU were reduced by 79% at 14 days in the IFN-γ/IL-3 treatment group relative to the IFN-γ/TNF-α/calcitriol treatment group, an effect that was not due to toxic monocyte metabolites.M. tuberculosisgrowth restriction by IFN-γ/IL-3-treated monocyte monolayers was associated with the development of Langhans-type multinucleated giant cells. At the light microscope level, dense growth ofM. tuberculosissurrounded by a ring of nuclei localized to the center of individual cells. The intracellular location ofM. tuberculosiswas confirmed by electron microscopy. In contrast, monocyte monolayers treated with IFN-γ/TNF-α/calcitriol consisted of a syncitium of cells containing monocyte aggregates. Nonlocalized linear arrays ofM. tuberculosiswere observed to be growing throughout such aggregates. These results suggest that physical sequestration ofM. tuberculosisby Langhans-type multinucleated giant cells may limit cell to cell spread of this pathogen, thereby restricting growth.