Aspergillus fumigatus forms airborne spores (conidia) that humans typically clear in a silent 46 and asymptomatic manner. Due to the growing number of humans that live in immune 47 compromised states, A. fumigatus is the most common and lethal agent of mold pneumonia 48 worldwide (Brown et 2009). Phagocyte NADPH oxidase is central to fungal clearance, as evidenced by a 40-55% 54 lifetime prevalence of IA in patients with chronic granulomatous disease (Marciano et al., 2015) . 55
Reactive oxygen species induce a regulated cell death process in neutrophil-engulfed conidia, 56 and thereby protect the respiratory tract from tissue-invasive hyphae and fungal dissemination 57 (Shlezinger et al., 2017) . 58
Using targeted cell depletion strategies based on murine CCR2 promoter-dependent 59 diphtheria toxin receptor (DTR) transgene expression (Espinosa et al., 2014; Hohl et al., 2009) , 60 recent studies implicated CCR2 + inflammatory monocytes and Mo-DCs in sterilizing antifungal 61 immunity, both by direct fungal killing and by regulating the lung inflammatory milieu 62
To define the cellular target of CXCR3 signaling, we measured CXCR3 expression on a 176 wide range of leukocytes. Bone marrow (BM)-resident pDCs from naïve mice and lung pDCs 177 from A. fumigatus-infected mice expressed CXCR3, unlike other myeloid cell subsets analyzed 178 ( Figure 4A ). As expected, CD4 + T cells, CD8 + T cells and NK cells expressed CXCR3 ( Figure  179 S4A), but mice that lack the lymphoid lineage (Rag2 -/-Il2rg -/-) did not exhibit heightened 180 susceptibility to A. fumigatus challenge ( Figure S4B ), as reported previously (Espinosa et al., 181 2014) . 182
On the basis of these results and published work (Ramirez-Ortiz et al., 2011), we 183 hypothesized that CXCR3-dependent pDC trafficking is critical for innate antifungal immunity. 184
In prior studies, we did not identify pDCs in lung cell suspensions due to the inclusion of 185 collagenase type IV. This preparation method degraded a pDC epitope utilized for their 186 identification ( Figure S4C ). Using a collagenase-free method to prepare single cell lung 187 suspensions, we observed that pDCs started to infiltrate A. fumigatus-infected lungs at 48 h pi, 188 with a peak at 72 h pi ( Figure 4B ). pDC lung infiltration followed the observed peak in lung 189 CXCL9 and CXCL10 levels and the peak influx of CXCL9-and CXCL10-producing innate 190 immune cells ( Figures S4D-S4F ). Cxcr3 -/mice exhibited a ~50% reduction in lung pDCs 191 ( Figure 4C ), while the number of lung monocytes, Mo-DCs, and neutrophils were similar to 192 control mice at 72 h pi ( Figures S4G-S4I ). CXCR3 surface expression was reduced on lung 193 pDCs compared to circulating pDCs, consistent with the idea that CXCR3 surface expression is 194 downregulated following engagement by CXCR3 ligands ( Figure 4A ). To examine whether 195 CXCR3 acts in series or in parallel with other chemokine receptors to mediate pDC trafficking 196 from the BM to the lung, we measured pDC surface expression of additional chemokine 197 receptors in the resting and fungus-infected state. Consistent with prior studies (Fujimura et al., 198 2015; Sawai et al., 2013; Serbina and Pamer, 2006; Swiecki et al., 2017) , we found that CCR2 is 199 expressed by lung-infiltrating pDCs ( Figure 4D ). pDC lung recruitment was attenuated by 60-200 70% in Ccr2 -/mice at 72 h pi compared to controls ( Figure 4E ). As expected (Hohl et al., 201 2009), lung monocyte and Mo-DC numbers were significantly reduced in Ccr2 -/mice (Figures 202 S4J and S4K), while lung neutrophil accumulation was not affected ( Figure S4L ). In sum, these 203 data indicate that CXCR3 and CCR2 both mediate pDC trafficking. 204 Sequential CCR2-and CXCR3-dependent signals mediate pDC lung recruitment 205
To determine at which steps CXCR3 and CCR2 regulate pDC trafficking, we generated 206 mixed BM chimeric mice to compare the trafficking of congenically marked gene-deficient 207 (Cxcr3 −/− or Ccr2 -/-) and gene-sufficient (Cxcr3 +/+ or Ccr2 +/+ ) pDCs ( Figure 5A ). In one model, 208
CXCR3 and CCR2 may act in parallel at the same trafficking step. In this scenario, the ratio of 209 gene-knockout to gene-sufficient pDCs would be increased in the same compartment for both 210 chemokine receptors, prior to the dependent trafficking step. In an alternate model, CXCR3 and 211 CCR2 may act in series at different trafficking steps. In this scenario, the ratio of gene-deficient 212 to gene-sufficient pDCs would be elevated in distinct compartments. 213
The ratio of Cxcr3 −/− to Cxcr3 +/+ pDCs was elevated in the blood compared to the BM and 214 lung compartments ( Figures 5B and S4A) , both during infection and at baseline ( Figure 5B and 215 Figure S5A ). These data support a model in which CXCR3 primarily, but not exclusively, 216 mediates pDCs trafficking from the circulation into the lung in A. fumigatus-infected mice. In 217 contrast to pDCs, the ratio Cxcr3 −/− to Cxcr3 +/+ neutrophils and monocytes was similar in all 218 compartments analyzed, consistent with CXCR3-independent trafficking ( Figure S5B -S5C).
The ratio of Ccr2 −/− to Ccr2 +/+ pDCs was elevated in BM compared to the blood and lung 220 under all conditions examined. These data are consistent with a CCR2-dependent defect in pDC 221 exit from the BM during infection ( Figure 5C ) and in the steady state ( Figure S5D ). As 222 anticipated, monocyte bone marrow egress was highly dependent on CCR2 ( Figure 5SE ), while 223 neutrophil bone marrow was not ( Figure S5F ). In sum, these data show that CCR2-and CXCR3-224 dependent signals act in series to mediate pDC entry into the circulation and pDC egress from 225 the circulation into the lung, respectively. 226
To link CXCR3 signaling to pDC-dependent antifungal immunity, our model would predict 228 that the chemokine receptor, CXCR3, and its cellular target, the pDC, are both essential for 229 innate antifungal immunity. To test this conjecture, we infected BDCA2-DTR +/-(pDC Depleter) 230 mice in which pDCs are ablated at rest and under inflammatory conditions . 231
Following DT treatment, pDCs were depleted fully in the steady state and at 72 h pi, while lung-232 infiltrating monocytes, Mo-DCs, neutrophils, and all other leukocyte subsets examined were not 233 ( Figures 6A-6E ). In contrast to BDCA2-DTR +/mice, CCR2-DTR +/mice facilitated DT-induced 234 ablation of pDCs in addition to other known CCR2 + cells, i.e. monocytes, Mo-DCs, and NK cells 235 (Espinosa et al., 2014; Hohl et al., 2009) . 236
To define the functional role of pDCs during A. fumigatus infection, we compared the 237 mortality, lung LDH levels, lung fungal burden and histopathology in pDC-depleted mice and in 238 littermate controls. pDC-depleted mice were more susceptible to A. fumigatus challenge than 239 control mice ( Figure 6F ) and mortality correlated with greater lung damage, as measured by 240 BAL fluid lactate dehydrogenase (LDH) levels ( Figure 6G ), an increased fungal burden 241 following infection with two commonly used A. fumigatus isolates ( Figures 6H and 6I) , and a failure to control fungal germination in the lung at 72 h pi ( Figure 6J ). Importantly, pDCs did not 243 contribute to the production of CXCL9 and CXCL10 in the lung during A. fumigatus challenge, 244 as predicted by prior data (Figures 6K and 6L ). In sum, CXCR3 + pDCs were essential for innate 245 antifungal immunity in the lung after A. fumigatus challenge. 246
To determine how pDCs shape innate antifungal immunity in the lung, we hypothesized that 248 pDCs are critical for antifungal effector functions, since lung neutrophil and Mo-DC numbers 249 were unaffected in pDC depleted mice. To test this hypothesis, we utilized the fluorescent 250
Aspergillus reporter (FLARE) conidia that encode a dsRed viability fluorophore and are labeled 251 with an Alexa Fluor 633 tracer (AF633). FLARE conidia enable us to distinguish live 252 (DsRed + AF633 + ) and dead (DsRed -AF633 + ) conidia during cellular interactions with leukocytes 253 with single-encounter resolution ( Figure 7A ). Using the FLARE strain, we quantified leukocyte 254 conidial uptake and killing in pDC-depleted and littermate control mice. pDCs did not bind to or 255 engulf A. fumigatus conidia, as measured by acquisition of AF633 fluorescence ( Figures 7B and  256 S7A). pDC ablation did not affect lung neutrophil fungal uptake at 72 h pi compared to lung 257 neutrophils in pDC-sufficient controls ( Figures 7C and 7D ). However, the frequency of 258 neutrophils that contained live conidia was substantially increased in pDC-depleted mice 259 compared to control mice ( Figures 7C and 7E ). In other words, neutrophil-engulfed conidia were 260 more likely to be killed in control mice than in pDC-depleted mice ( Figures 7C and 7E ). pDC 261 ablation did not alter conidial uptake or significantly reduce conidial killing by monocytes and 262
Mo-DCs at 72 h pi ( Figures 7D and 7E ). These findings indicate that pDCs enhanced neutrophil 263 fungicidal activity. 264
Neutrophils generate reactive oxygen species (ROS) via NADPH oxidase as a primary 265 effector mechanism against A. fumigatus (Marciano et al., 2015) . Neutrophil NADPH oxidase 266 induces a regulated cell death process in A. fumigatus conidia (Shlezinger et al., 2017) . To 267 determine whether pDCs regulate the neutrophil oxidative burst in A. fumigatus-infected mice, 268
we measured neutrophil ROS production in pDC-depleted mice. The ROS median fluorescence 269 intensity in ROS + lung neutrophils isolated from pDC-depleted mice was significantly reduced 270 compared to ROS + neutrophils isolated from littermate control mice at 72 h pi ( Figure 7E and 271 7F). These data show that lung pDCs regulate neutrophil ROS generation during A. fumigatus 272 challenge.
Humans maintain lifelong sterilizing immunity to inhaled mold conidia. Breaches in the 275 respiratory innate immune system lead to mold pneumonia, a life-threatening disease in patients 276 with functional or numeric deficits in myeloid cell function, particularly with the loss of the 277 antifungal properties of neutrophils. In this study, we demonstrate a three-cell circuit that 278 involves pDCs, monocytes and derivative Mo-DCs, and neutrophils to regulate sterilizing 279 immunity against Aspergillus conidia. This circuit acts as a feedforward amplification 280 mechanism, by coupling fungal recognition and fungus-induced inflammation to CXCR3 281 signaling-dependent pDC recruitment in the lung. pDCs regulate neutrophil ROS induction and 282 fungal killing to mediate sterilizing immunity at the respiratory barrier. 283
Our data demonstrate that conidial uptake by lung-infiltrating monocytes and Mo-DCs 284 triggered Dectin-1-and CARD9-dependent production of CXCL9 and type I and type III 285 interferon signaling-dependent production of CXCL10; the latter process was readily detectable 286 in neutrophils as well. The relative contribution of CXCL9 and CXCL10 and potential molecular 287 redundancy with regard to downstream CXCR3-dependent processes during fungal infection Cre-lox candidate gene deletion strategies is essential to test these candidates formally. 414
In this study, we uncover essential functions for pDCs in innate antifungal defense that 
The authors declare no competing interests. 
This study did not generate new unique reagents. 557
This study did not generate or analyze new datasets or codes. 559
Mice 561 Tween-20 at a concentration of 0.6-1.2Í10 9 cells and 50 µl of cell suspension was administered 603 via the intranasal route to mice anesthetized by isoflurane inhalation. 604
To analyze of conidia uptake and killing, FLARE conidia were used to infect mice (in vivo) or 607 co-culture with BM leukocyte (in vitro). In data analyses for a given leukocyte subset, conidial 608 uptake refers to the frequency of fungus-engaged leukocytes, i.e. the sum of dsRed + AF633 + and 609 dsRed -AF633 + leukocytes. Conidial viability within a specific leukocyte subset refers to the 610 frequency of leukocytes that contain live conidia (dsRed + AF633 + ) divided by the frequency of 611 all fungus-engaged leukocytes (dsRed + AF633 + and dsRed -AF633 + ).
To ablate specific cells, CCR2-DTR, CCR2-DTR, Rag2 -/-Il2rg -/-, ROSA26-iDTR Mrp8-Cre , 614 BDCA2-DTR, and non-transgenic littermate controls were injected i.p. with 10 ng/g body weight 615
DT on Day -1, Day 0, and Day +2 pi (Espinosa et Neutrophils were identified as CD45 + CD11b + Ly6C lo Ly6G + cells, inflammatory monocytes 627 as CD45 + CD11b + CD11c -Ly6G -Ly6C hi cells, Mo-DCs as CD45 + CD11b + CD11c + Ly6G -628
Ly6C hi MHC class II + cells, and pDCs as CD45 + CD11c int SiglecF -CD19 -NK1.1 -CD11b -629 B220 + SiglecH + cells. 630
To analyze the lung fungal burden, perfused murine lungs were homogenized using a PowerGen 631 125 homogenizer (Fisher) in 2 mL PBS, 0.025% Tween-20, and plated on Sabouraud dextrose 632 agar. To analyze cytokine levels by ELISA, whole lungs were weighed and mechanically 633 homogenized in 2 mL PBS containing protease inhibitor. To analyze cytokine levels by qRT-634 PCR, total RNA from cells was extracted with TRIzol (Invitrogen). One to two micrograms of total RNA were reverse-transcribed using High-Capacity cDNA Reverse Transcription Kit 636 (Applied Biosystems). TaqMan Fast Universal Master Mix (2×), TaqMan probes (Applied 637 Biosystems) for each gene were used and normalized to glyceraldehyde-3-phosphate 638 dehydrogenase. Gene expression was calculated using the DDCt method relative to the naïve 639 sample. For histology, perfused lungs were fixed in 4% paraformaldehyde, embedded in paraffin, 640 sectioned in 4 µm slices, stained with hematoxylin and eosin (H&E) or modified Gomori 641 methenamine silver (GMS), and digitally scanned using a Zeiss Mirax Desk Scanner. Images 642 were captured from whole slide images, acquired with an Aperio ScanScope (Aperio 643 Technologies) using 5× and 20× objectives at the Molecular Cytology Core Facility (MSKCC). 644 BAL LDH and albumin levels were measured with a CytoTox96 non-radioactive cytotoxicity 645 assay kit and a BCG albumin assay kit, respectively. 646 