Background
β-1→3-D-glucans occur as a principal component of microbial cell walls or can be secreted from both, non-pathogenic and pathogenic fungi such as S. cerevisae and C. albicans [1]. These β-1→3-D-linked glucose polymers are characterized as a fungal pathogen-associated molecular pattern (PAMP) [2]. The primary cellular recognition of β-1→3-D-glucans is mediated by several β-1→3-D-glucan receptors on phagocytes [3,4] and other cells [5,6]. Human as well as murine Dectin-1 has been demonstrated to be the major pattern recognition receptor (PRR) for intact yeast and β-1→3-D-glucan-containing particles (i.e. zymosan) on monocytes/macrophages as well as neutrophils and on primary cells [7-11]. In the murine system, binding of zymosan to Dectin-1 resulted in production of TNFα through Toll-like receptor 2 and the adaptor protein MyD88 [12]. Another water-soluble β-1→3-D-glucan (PGG-glucan) has been described to activate NFκB and NFIL-6 in murine cell lines [13,14]. Similarly, it has been shown that β-1→3-D-glucans activate NFκB in a human monocyte-like cell line [15] and in human polymorphonuclear neutrophils (PMN), in the latter case without secretion of pro-inflammatory cytokines (IL-1, IL-6, TNFα) [16]. One study proposed that the production of the anti-inflammatory IL-1RA, but not IL-1 by human monocytes may be a potentially protective mechanism induced by β-1→3-D-glucan [17]. Three other investigations have reported that human leukocytes and human vascular endothelial cells produce IL-8 in response to zymosan [18] or a water-soluble β-1→3-D-glucan [6,19].
In addition, β-1→3-D-glucans seem to be able to modify the response to pro-inflammatory stimuli or even sepsis. In a murine polymicrobial sepsis model, β-1→3-D-glucan [20] treatment resulted in decreased septic morbidity and mortality mediated via inhibition of NFκB and stimulation of the phosphoinositide-3-kinase (PI3K) pathway [21,22]. These and other animal studies [23,24] as well as a clinical trial [25] support a protective role of β-1→3-D-glucan in certain pro-inflammatory conditions. The mechanisms underlying these beneficial effects of β-1→3-D-glucan are only partially resolved, especially in humans. Thus, the aim of this study was to elucidate molecular and cellular mechanisms of β-1→3-D-glucans on human leukocytes in pro-inflammatory conditions with special emphasis on the cytokine profile and its transcriptional regulation. For this purpose, peripheral blood mononuclear cells (PBMC) were exposed to a well-defined β-1→3-D-glucan, i.e. glucan phosphate (GP) [20,26], alone or simultaneously with LPS from gram-negative bacteria or the superantigen TSST-1 from gram-positive bacteria over 48 h. Because of the potential effect of β-1→3-D-glucan on cytokine production [12,16-19], TNFα, IL-1β, IL-6, IL-8 and IL-1RA were measured as well as IFNγ, IL-2, IL-4, IL-10, IL-12 and TGFβ1. Correspondingly, four NFκB sites from the TNFα promoter (κ consensus, κ1, κ2, κ3) [27], a κ consensus site from the IL-8 promoter [28], an NFAT site from the IFNγ promoter (ATP2) [29] and a consensus NFIL-6 site from the IL-6 promoter [13] were examined. Because of the anti-inflammatory role of IL-1RA, we focused on binding of transcription factors to the IL-1RA promoter. An inhibitory element and three positive-acting LPS-response elements (LRE-1, LRE-2 and LRE-3) in the IL-1RA promoter, including NFκB, PU.1 and NFIL-6 sites, have been characterized previously [30-33]. Using computational analysis for homology search [34], we looked for new binding motifs in the IL-1RA promoter.