Insoluble beta-glucan from the cell wall of Candida albicans induces immune responses of human THP-1 monocytes through Dectin-1

Abstract

Background: beta-glucan is the major structure component of Candida albicans (C. albicans) cell wall. It has been demonstrated that Dectin-1 as the principal C-type lectin pattern-recognition receptor (PRR) can recognize fungal beta-glucan and induce immune responses. In this study, we sought to clarify whether insoluble beta-glucan from the cell wall of C. albicans (CaIG) could induce immune responses in human THP-1 monocytes (a human acute monocytic leukemia cell line) and to determine the underlying mechanisms.

Methods: Human THP-1 monocytes were challenged with CaIG in vitro. The mRNA expression of Dectin-1, Toll-like receptors (TLR2), proinflammatory cytokine (TNF-alpha) and chemokine (IL-8) was assayed by real-time reverse transcription polymerase chain reaction (RT-PCR). The secretion of TNF-a and IL-8 were measured by enzyme-linked immunosorbent assay (ELISA). H(2)O(2) release was determined by microplate fluorescent assay. Western blotting was used to analyze IkappaB-a phosphorylation and degradation.

Results: Exposure of THP-1 monocytes to CaIG led to increased gene expression and secretion of TNF-alpha and IL-8. CaIG induced H(2)O(2) release in a time-dependent manner. CaIG hydrolyzed with zymolyase failed to induce gene expression and secretion of TNF-alpha, IL-8 and H(2)O(2) release. CaIG up-regulated the mRNA of Dectin-1, whereas the mRNA level of TLR2 was not altered. THP-1 monocytes challenged with CaIG resulted in the activation of NF-kappaB in a time-dependent manner. Dectin-1 inhibitor laminarin blocked the CaIG-induced production of TNF-alpha and H(2)O(2) in THP-1 monocytes, but no such effect was observed in pretreatment with anti-TLR2 neutralizing antibody and the LPS inhibitor (polymyxin B).

Conclusion: CaIG may play a role in activation of immune responses in human THP-1 cells through Dectin-1, not TLR2.