Mycobacterium tuberculosis induces CCL18 expression in human macrophages

The interaction of Mycobacterium tuberculosis (MTB) with the immune system is mediated by cytokine and chemokine responses of macrophages and/or dendritic cells. Chemokine (C-C motif) ligand 18 (CCL18) and interleukin (IL)-10 are major factors secreted by phagocytes, postulated to recruit naïve T lymphocytes and inhibit pro-inflammatory cells. Our study investigated the role of CCL18 and IL-10 in an in vitro model of infection by MTB in human macrophages. CD14(+) monocytes, obtained from the peripheral blood of eight healthy donors, differentiated in monocyte-derived macrophages (MDM) with monocyte-colony stimulating factor (100 ng/ml) for 6 days, were stimulated in vitro with lipopolysaccharide (LPS) (1 microg/ml) and with heat killed MTB Hv37Ra (multiplicity of infection 1:5) for 24 h. Alveolar macrophages from five healthy donors were infected with MTB Hv37RA. CCL18 protein and mRNA were detected by enzyme-linked immunosorbent assay (ELISA) and real-time PCR, IL-10 levels by ELISA. Stimulation of MDM with LPS or MTB led to a significant increase in CCL18 protein (control 2.67 +/- 0.46 ng/ml, LPS 4.05 +/- 0.56 ng/ml, with MTB 6.70 +/- 1.59 ng/ml, n = 5, P < 0.05) and specific mRNA levels (control 0.09 +/- 0.01, LPS 0.24 +/- 0.11, with MTB 0.34 +/- 0.08 CCL18/Glyceraldehyde 3-phosphate dehydrogenase (GAPDH), n = 3, P < 0.05). A significant increase of the production of CCL18 was observed in infected alveolar macrophages. IL-10 levels increased from 38.52 +/- 26.38 pg/ml in control cells to 1129.32 +/- 235.00 and 974.25 +/- 164.46 pg/ml in LPS and MTB treated cells, respectively (P < 0.05). Up-regulation of CCL18 and IL-10 in macrophages by MTB may be involved in the recruitment of naïve T cells in association with local suppressive immunity against intracellular pathogens. This could represent a mechanism of tolerance during the early phases of infection.