Mannan-Binding Lectin Inhibits Candida albicans-Induced Cellular Responses in PMA-Activated THP-1 Cells through Toll-Like Receptor 2 and Toll-Like Receptor 4

Background: Candida albicans (C. albicans), the most common human fungal pathogen, can cause fatal systemic infections under certain circumstances. Mannan-binding lectin (MBL), a member of the collectin family in the C-type lectin superfamily, is an important serum component associated with innate immunity. Toll-like receptors (TLRs) are expressed extensively, and have been shown to be involved in C. albicans-induced cellular responses. We first examined whether MBL modulated heat-killed (HK) C. albicans-induced cellular responses in phorbol 12-myristate 13-acetate (PMA)-activated human THP-1 macrophages. We then investigated the possible mechanisms of its inhibitory effect. Methodology/Principal Finding: Enzyme-linked immunosorbent assay (ELISA) and reverse transcriptasepolymerase chain reaction (RT-PCR) analysis showed that MBL at higher concentrations (10-20 mu g/ml) significantly attenuated C. albicans-induced chemokine (e.g., IL-8) and proinflammatory cytokine (e.g., TNF-alpha) production from PMA-activated THP-1 cells at both protein and mRNA levels. Electrophoretic mobility shift assay (EMSA) and Western blot (WB) analysis showed that MBL could inhibit C. albicans-induced nuclear factor-kB (NF-kB) DNA binding and its translocation in PMA-activated THP-1 cells. MBL could directly bind to PMA-activated THP-1 cells in the presence of Ca2+, and this binding decreased TLR2 and TLR4 expressions in C. albicans-induced THP-1 macrophages. Furthermore, the binding could be partially inhibited by both anti-TLR2 monoclonal antibody (clone TL2.1) and anti-TLR4 monoclonal antibody (clone HTA125). In addition, coimmunoprecipitation experiments and microtiter wells assay showed that MBL could directly bind to the recombinant soluble form of extracellular TLR2 domain (sTLR2) and sTLR4. Conclusions/Significance: Our study demonstrates that MBL can affect proinflammatory cytokine and chemokine expressions by modifying C. albicans-/TLR-signaling pathways. This study supports an important role for MBL on the regulation of C. albicans-induced cellular responses.