Immune recognition of Candida albicans beta-glucan by dectin-1

Abstract

Beta (1,3)-glucans represent 40% of the cell wall of the yeast Candida albicans. The dectin-1 lectin-like receptor has shown to recognize fungal beta (1,3)-glucans and induce innate immune responses. The importance of beta-glucan-dectin-1 pathways for the recognition of C. albicans by human primary blood cells has not been firmly established. In this study we demonstrate that cytokine production by both human peripheral blood mononuclear cells and murine macrophages is dependent on the recognition of beta-glucans by dectin-1. Heat killing of C. albicans resulted in exposure of beta-glucans on the surface of the cell wall and subsequent recognition by dectin-1, whereas live yeasts stimulated monocytes mainly via recognition of cell-surface mannans. Dectin-1 induced cytokine production through the following 2 pathways: Syk-dependent production of the T-helper (Th) 2-type anti-inflammatory cytokine interleukin-10 and Toll-like receptor-Myd88-dependent stimulation of monocyte-derived proinflammatory cytokines, such as tumor necrosis factor-alpha . In contrast, stimulation of Th1-type cytokines, such as interferon-gamma , by C. albicans was independent of the recognition of beta-glucans by dectin-1. In conclusion, C. albicans induces production of monocyte-derived and T cell-derived cytokines through distinct pathways dependent on or independent of dectin-1.

Figure 1

Mean levels of tumor necrosis factor (TNF)-α (A), interleukin (IL)-6 (B), IL-10 (C), and interferon (IFN)-γ (D) produced in peripheral blood mononuclear cells after incubation with heat-killed (HK) or live Candida albicans. All results are pooled triplicate data from 2 separate experiments, with a total of 8 volunteers per group. Whisker bars, SDs. *P < .05; **P < .01.

Figure 2

A, Mean levels of tumor necrosis factor (TNF)-α and interleukin (IL)-6 in peripheral blood mononuclear cells (PBMCs), according to the presence or absence of dectin-1 blocking by glucan phosphate (glucanP), after incubation with live or heat-killed (HK) Candida albicans. B, Mean levels of TNF-α in murine peritoneal macrophages from dectin-1^+/+ mice and dectin-1^-/- mice after incubation with RPMI 1640 (control), live C. albicans, or HK C. albicans. C, Mean levels of TNF-α in PBMCs in which dectin-1 receptor, Toll-like receptor (TLR) 4, and mannose receptor (MR) were blocked by glucanP, anti-TLR4 antibody (aTLR4), and anti-MR antibody (aMR), respectively. TNF-α levels in PBMCs stimulated with lipopolysaccharide (LPS) in growth medium were included as a control. All results are pooled triplicate data from 2 separate experiments, with a total of 6 volunteers per group. Whisker bars, SDs. *P < .05; **P < .01.

Figure 3

Mean levels of tumor necrosis factor (TNF)-α, interleukin (IL)-6, interferon (IFN)-γ, and IL-10 produced in peripheral blood mononuclear cells after incubation with Candida albicans in the absence of inhibitors (control; open bars), in the presence of glucan phosphate (glucanP; solid bars), or in the presence of Syk inhibitor (Syk-inh; hatched bars). Results are pooled triplicate data from 2 separate experiments, with a total of 8 volunteers per group. Whisker bars, SDs. ^aLevel of IL-10 is ×10 ng/mL.