Macrophage internalization of fungal beta-glucans is not necessary for initiation of related inflammatory responses.

Cell wall beta-glucans are highly conserved structural components of fungi that potently trigger inflammatory responses in an infected host. Identification of molecular mechanisms responsible for internalization and signaling of fungal beta-glucans should enhance our understanding of innate immune responses to fungi. In this study, we demonstrated that internalization of fungal beta-glucan particles requires actin polymerization but not participation of components of caveolar uptake mechanisms. Using fluorescence microscopy, we observed that uptake of 5-([4,6-dichlorotriazin-2-yl] amino)-fluorescein hydrochloride-Celite complex-labeled Saccharomyces cerevisiae beta-glucan by RAW macrophages was substantially reduced in the presence of cytochalasin D, which antagonizes actin-mediated internalization pathways, but not by treatment with nystatin, which blocks caveolar uptake. Interestingly, beta-glucan-induced NF-kappaB translocation, which is necessary for inflammatory activation, and tumor necrosis factor alpha production were both normal in the presence of cytochalasin D, despite defective internalization of beta-glucan particles following actin disruption. Dectin-1, a major beta-glucan receptor on macrophages, colocalized to phagocytic cups on macrophages and exhibited tyrosine phosphorylation after challenge with beta-glucan particles. Dectin-1 localization and other membrane markers were not affected by treatment with cytochalasin D. Furthermore, dectin-1 receptors rather than Toll-like receptor 2 receptors were shown to be necessary for both efficient internalization of beta-glucan particles and cytokine release in response to the fungal cell wall component.