Macrophage replacement in Mycobacterium tuberculosis-induced murine lung granulomas

Abstract

Abstract Monocytes/macrophages are the dominant cell type in mycobacterial granulomas. Macrophages both provide a survival niche for the bacteria and play a central role in bacterial control. Most of them are recruited from the blood with a Ly6C+ phenotype and they differentiate into F4/80+ macrophages in the lung lesions. It is well demonstrated that monocytes have access to pre-formed granulomatous lesions, but the rate of cell recruitment and the fate of the recruited cells are not well understood. Previously, testing BCG-induced liver granulomas using a transplantation model, we detected continuous cell replacement in the preformed lesions. Here, using intravital staining and cytofluorimetric detection of CD45+ blood cells in combination with IHC, we show that CD11b+ cells can access the lung parenchyma and Mtb-induced lesions over the course of 24 hours in Mtb H37Rv infected animals. We measured the level of monocyte/macrophage infiltration in both acute and chronic infection in C57BL/6 and C3HeB/FeJ mice. Although monocytes have a short lifespan in the blood, using intravenous cell transfer of tdTomato-expressing sentinel cells we show that they can live longer than a week in the lung lesions of Mtb-infected animals. Interestingly, a population of adoptively transferred CD11b+tdTomato+ cells can be detected for more than 10 days in the blood vessels in the infected lung. Gene expression profiling of recently or earlier recruited (CD11b+F4/80− or CD11b+F4/80+) cells sorted from the lung shows differential inflammatory gene expression between these populations. The continuous cell replacement that is required to maintain granulomatous lesions offers a novel possibility to manipulate granuloma pathology for a therapeutic advantage.