The CD14 receptor does not mediate entry of Mycobacterium tuberculosis into human mononuclear phagocytes.

Prior reports have suggested that CD14 mediates uptake of Mycobacterium tuberculosis into porcine alveolar macrophages and human fetal microglia, but the contribution of CD14 to cell entry in human macrophages has not been studied. To address this question, we used flow cytometry to quantify uptake by human monocytes and alveolar macrophages of M. tuberculosis expressing green fluorescent protein. Neutralizing anti-CD14 antibodies did not affect bacillary uptake and the efficiency of bacillary entry was similar in THP-1 cells expressing low and high levels of CD14. However, most internalized bacteria were found in CD14+ but not in CD14- monocytes because M. tuberculosis infection upregulated CD14 expression. We conclude that: (1) CD14 does not mediate cellular entry by M. tuberculosis; (2) M. tuberculosis infection upregulates CD14 expression on mononuclear phagocytes, and this may facilitate the pathogen's capacity to modulate the immune response.