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. 2011 Dec 15;10(6):603-15.
doi: 10.1016/j.chom.2011.10.009.

The β-glucan receptor Dectin-1 activates the integrin Mac-1 in neutrophils via Vav protein signaling to promote Candida albicans clearance

Xun Li  1 , Ahmad Utomo, Xavier Cullere, Myunghwan Mark Choi, Danny A Milner Jr, Deepak Venkatesh, Seok-Hyun Yun, Tanya N Mayadas
Affiliations

The β-glucan receptor Dectin-1 activates the integrin Mac-1 in neutrophils via Vav protein signaling to promote Candida albicans clearance

Xun Li et al. Cell Host Microbe. .

Abstract

Resistance to fungal infections is attributed to engagement of host pattern-recognition receptors, notably the β-glucan receptor Dectin-1 and the integrin Mac-1, which induce phagocytosis and antifungal immunity. However, the mechanisms by which these receptors coordinate fungal clearance are unknown. We show that upon ligand binding, Dectin-1 activates Mac-1 to also recognize fungal components, and this stepwise process is critical for neutrophil cytotoxic responses. Both Mac-1 activation and Dectin-1- and Mac-1-induced neutrophil effector functions require Vav1 and Vav3, exchange factors for RhoGTPases. Mac-1- or Vav1,3-deficient mice have increased susceptibility to systemic candidiasis that is not due to impaired neutrophil recruitment but defective intracellular killing of C. albicans yeast forms, and Mac-1 or Vav1,3 reconstitution in hematopoietic cells restores resistance. Our results demonstrate that antifungal immunity depends on Dectin-1-induced activation of Mac-1 functions that is coordinated by Vav proteins, a pathway that may localize cytotoxic responses of circulating neutrophils to infected tissues.

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Figures

Figure 1
Figure 1. Dectin-1 activates Mac-1 in neutrophils and both collaborate in responses to fungal components
(A) Wild-type (WT) and Mac-1-/- neutrophils were stimulated with PMA and presented with IgM (negative control) or IgM-iC3b coated RBCs (Bar= 25μm). (B) Analysis of Mac-1 activation. WT and Dectin-1-/- neutrophils were incubated with IgM-iC3b RBCs in the absence (control) or presence of zymosan. (C) Neutrophils (PMN) from Dectin-1-/-, Mac-1-/- or MyD88-/-/TRIF-/- mice and their respective wild-type counterparts were incubated with FITC-labeled zymosan ±10mM MgCl2 (Mg2+). Left panel: Cells at 37°C were evaluated for internalized fluorescent particles. Right panel: Binding of FITC-labeled zymosan was evaluated by FACS analysis in cells at 4°C. (D) Zymosan phagocytosis in Mac-1-/- and WT macrophages (MΦ) was assessed as described for neutrophils. Data represent mean (±s.e.m.) of 3-4 independent experiments in (A-D). (E) Zymosan induced ROS was analyzed in Dectin-1-/-, Mac-1-/-, MyD88-/-/TRIF-/- and WT neutrophils. A graph of the mean (±s.e.m) of the relative light units (RLU) peak value for samples normalized to WT cells is shown. (F) WT and Mac-1-/- neutrophils were treated with zymosan in the presence of dectin-1 antibody 2A11 or IgG control, and real-time ROS generation was monitored. Representative ROS profiles and a graph, plotted as described in E) is shown. The reduction in ROS in Mac-1-/- neutrophils following IgG isotype control treatment was less significant than without (E) and may result from the engagement of FcγRs by the antibody. (G) ROS generation was monitored in WT and Mac-1-/- neutrophils treated with zymosan in the presence or absence of laminarin. Data are shown as in F. *, p<0.05, **, p<0.01; ***, p<0.001 (one-way or two-way ANOVA with Bonferroni post-test (B,C,E,F,G); unpaired t-test (A,D)). See also Figure S1.
Figure 2
Figure 2. Vav activated by Dectin-1 is required for Mac-1 activation and function
(A) Lysates of WT, Dectin-1-/- and Mac-1-/- neutrophils stimulated with zymosan for the times in minutes were analyzed for Vav-Tyr174 phosphorylation. (B) iC3b-RBC rosetting in wild-type and Vav1,3-/- neutrophils following treatment with vehicle control, zymosan or PMA. Representative pictures and a graph of the mean (±s.e.m) of 3 experiments are shown. FACS analysis of Dectin-1 and Mac-1 expression in wild-type (solid line) and Vav1,3-/- (dotted line) neutrophils compared to IgG isotype control is shown (C) C3-RBC rosetting was analyzed in unstimulated (-) and zymosan-stimulated wild-type cells treated with vehicle control (DMSO) or inhibitors of Syk (Piceatannol), Src (PP2), PLCγ (U73122), PI3K (PI-103, LY294002) or ERK (U0126) or a chelator of intracellular Ca2+ (BAPTA). Mean (±s.e.m) of results is given. n.d.= non-detectable (D) Upper left and bottom panel: Phagocytosis of FITC-labeled unopsonized or iC3b opsonized zymosan by WT and Vav1,3-/- neutrophils (PMN). Representative pictures are shown for unopsonized zymosan (Bar= 25μm). Upper right panel: Binding of FITC-labeled zymosan was evaluated by FACS analysis. Mean (±s.e.m) of results is given. (E) Zymosan phagocytosis in WT and Vav1,3-/- macrophages (MΦ). Mean (±s.e.m) of results is given. (F) Respiratory burst of WT and Vav1,3-/- neutrophils in response to zymosan. Representative ROS profiles and graph of the mean (±s.e.m) of the relative light units (RLU) peak value for samples (inset) is shown. n=5 independent experiments. ***, p<0.001 (one-way or two-way ANOVA with Bonferroni post-test (B,C,D upper left panel); one sample t-test (F); unpaired t-test (D upper right panel)). See also Figure S2.
Figure 3
Figure 3. Vav is dispensable for zymosan induced Rap GTPase activation but is required for ITAM-based signals
Wild-type (WT) and Vav1,3-deficient (V1,3-/-) neutrophils were stimulated with zymosan for times given in minutes (min) and cell lysates were prepared. (A) Western blots of GTP-loaded Rap1 (Rap1GTP; activated Rap1) as compared with total Rap1 and β-actin. (B) Western blot analysis of phosphorylation (p-) of Syk and Src, PLCγ2, PAK1/2, p40(phox), MAPK and Pyk2. β-actin served as a loading control. (C) Dectin-1-/- and Mac-1-/- neutrophils were analyzed similarly to that described in B). For all western blots representative data from 1 of 3 experiments is shown. (D) Indo-1 preloaded wild-type and Vav1,3-/- neutrophils were stimulated with zymosan (arrow). Ca2+ flux was evaluated by measuring a ratio of fluorescence signal at 405nm to 485nm over time. Data shown is a representative profile of the percentage of responding cells as a function of time calculated. Mean (±s.e.m) of the maximum percentage of responding cells from 4 independent experiments is given. *, p<0.05 (unpaired t-test). (E) Western blot analysis as described in B) of wild-type cells stimulated with zymosan and pretreated without (-) or with an inhibitor to PLCγ (U73122) or a chelator of intracellular Ca2+ (BAPTA). See also Figure S2.
Figure 4
Figure 4. Mac-1 is required for resistance to systemic C.albicans infection
(A) Mac-1-/- mice and their wild-type cohorts (n=8 in each) were infected intravenously with 5×104 C.albicans and monitored daily for survival. P=0.0011 (log-rank test) (B) Kidneys were harvested at day 4 after infection for gross morphology and visual inspection of abscesses (arrow) (Left panel), and for quantitation of fungal burden in kidney homogenates (Right panel). Each symbol represents data from an individual animal. (C) A representative photomicrograph of periodic acid Schiff (PAS)-stained kidney sections from wild-type and Mac-1-/- mice is shown. Within kidney tissues from Mac-1-/- mice, large collections of neutrophils (abscess formation) surrounded by mixed inflammatory cells are present with abundant fungal forms, which are virtually absent in kidneys from wild-type mice. (D) Kidneys harvested from mice at 16 and 48hrs after infection were homogenized and myeloperoxidase enzyme (MPO) activity was determined and normalized to total protein content. Mean (±s.e.m) of results is given. (E) Confocal intravital microscopy of wild-type and Mac-1-/- mice at the indicated times after injection with live GFP-labeled C.albicans. Mice were also given rhodamine-dextran to delineate blood vessels and anti-Gr-1 to identify neutrophils. Left panels: Representative pictures show neutrophils (blue), blood vessels (red) and C.albicans fungal elements (green). Right panel: Quantitation of percent of neutrophils outside of blood vessels. Data represent mean (±s.e.m.) of 3 independent experiments (F) Analysis of intracellular killing of live unopsonized C.albicans by neutrophils and macrophages. Data represent mean (±s.e.m.) of four experiments. (G) Kidney fungal burden at day 4 in wild-type, CD18-deficient (CD18-/-) and CD18-/- mice with human CD18 restored in neutrophils (hCD18+/mCD18-/-). (H) Renal neutrophil accumulation was evaluated in wild-type and CD18-/- mice at indicated time points after C.albicans infection. Data represent mean (±s.e.m.) of 3 independent experiments **, p<0.01; ***, p<0.001 (unpaired t-test (F); Mann-Whitney test (B,G)). See also Figure S3 and Movie S1.
Figure 5
Figure 5. Vav1,3 proteins are required for resistance to C.albicans infection
(A) Survival curves of wild-type and Vav1,3-deficient mice infected intravenously with 5×104 CFU of C.albicans (n=8 in each). P=0.0009 (log-rank test) (B) Images of kidneys harvested from wild-type and Vav1,3-/- mice 4 days after infection (Left panel) and quantification of fungal burden in kidney homogenates (Right panel). (C) A representative photomicrograph of PAS-stained renal sections from wild-type and Vav1,3-/- mice is shown. (D) Evaluation of tissue neutrophil accumulation in kidney at the indicated days after infection. Mean (±s.e.m) of results is given. (E) Laser-scanning confocal intravital microscopy of wild-type and Vav-1,3-/- mice as described in Fig. 4E. Data represent mean (±s.e.m.) of 3 independent experiments (F) Analysis of intracellular killing of live unopsonized C.albicans by neutrophils and macrophages. Data represent mean (±s.e.m.) of 3 independent experiments. (G) Wild-type (WT) and Vav 1,3 deficient (V1,3-/-) recipient mice were reconstituted with bone marrow from wild-type or Vav1,3-/- donors. At day 4 after C.albicans innoculation, the CFU in kidney homogenates was determined. **, p<0.01; ***, p<0.001 (unpaired t-test (F); Mann-Whitney test (B,G)). See also Figure S4.
Figure 6
Figure 6. Model of neutrophil mediated anti-fungal cytotoxicity
C.albicans blastoconidia (yeast) engage macrophages, which generate cytokines that activate the endothelium to recruit circulating neutrophils. Engagement of Dectin-1 on transmigrated neutrophils leads to phosphorylation (-P) of its ITAM by Src kinases, and recruitment and phosphorylation of Syk kinase leading to Vav/PLCγ activation and Ca2+ release. These steps result in Mac-1 activation and it's binding to C.albicans. Ligand-bound Mac-1 recruits p-Vav and contributes to the Dectin-1 initiated signaling complexes leading to activation of downstream targets including Pyk-2, PAK and ERK. These events together promote cytoskeletal rearrangement required for phagocytosis and activation of the NADPH oxidase.
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