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 μg/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.