Diabetes mellitus (DBM) reduces immunological activity and increases susceptibility to various infections, including tuberculosis. Alveolar macrophages (AMs) functions, such as pathogen phagocytosis, removal of senescent cells, and tissue repair are altered in DBM. Animal studies have shown that during hyperglycemic states, the phagocytic function of AMs and the expression of adhesion molecules may be altered, interfering with the recruitment of immune cells to the inflammatory site.
To mimic conditions occurring in the lung alveolus, we co-cultured human AM (hAM) cell line (Daisy cells) with human umbilical vein endothelial cells (HUVEC) for 48 hours in the presence of culture media, normal glucose (5 mMol/L), and high glucose (22 mMol/L). Using flow cytometry we analyzed the expression of cell surface markers CD 11b, 11c, 14, 16, 23, 24, 32, 36, 64, 163, and 206. Phagocytic function was measured by fluorescence microscopy after inoculation of cells with GFP-expressing Mycobacterium smegmatis and BCG.
We confirmed that Daisy cells closely resemble primary hAMs. Exposure of HUVEC cells to high glucose decreased CD11c and increased CD163 expression on hAMs in co-culture, suggesting a shift towards M2 polarization. Exposure of HUVEC cells to high glucose also decreased the expression of CD169 in hAMs in co-culture, a marker which promotes bacterial killing. A decrease in phagocytosis of mycobacteria by hAMs was observed when hAMs were directly exposed even to normal glucose. When hAMs were in co-culture with HUVEC cells, phagocytosis only decreased when HUVEC cells were exposed to high glucose concentrations.
Exposing hAMs directly to glucose and indirectly via a co-culture system yields different phenotypic and functional changes. A hyperglycemic state may induce the endothelium to signal to hAMs promoting a shift towards M2 polarization and limiting pathogen phagocytosis and clearance. These findings may explain the defective sentinel function of hAMs, promoting tuberculosis susceptibility in a diabetic host.