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but the precise biochemical functions ofthe ATPase in the evolutionarily divergent
apicomplexa remain to be established.
As transmembrane proteins, the hybrid
GCs could be receptors or sensors thatlink extracellular signals to cellular responses in the parasite. It is tempting
to speculate that the ATPase domains
play a role in transducing such signals.
Whether ookinete gliding can be triggeredor directed toward the mosquito midgut
epithelium by extracellular signals isunknown. However, tachyzoites egressfrom host cells in response to a low potassium environment that occurs upon hostcell rupture (Moudy et al., 2001), andPlasmodium gametocytes get activatedby the mosquito environment through acGMP/PKG dependent pathway (Brochetet al., 2014). If and how GCs transduce
these signals is now the next question.

hybrid P-type ATPase-guanylate cyclase. Cell
Host Microbe 24, this issue, 804–816.

REFERENCES
Baker, D.A., Drought, L.G., Flueck, C., Nofal, S.D.,
Patel, A., Penzo, M., and Walker, E.M. (2017).
Cyclic nucleotide signalling in malaria parasites.
Open Biol. 7, 7.
Beck, J.R., Rodriguez-Fernandez, I.A., de Leon,J.C., Huynh, M.H., Carruthers, V.B., Morrissette,N.S., and Bradley, P.J. (2010). A novel family ofToxoplasma IMC proteins displays a hierarchicalorganization and functions in coordinating parasite
division. PLoS Pathog. 6, e1001094.
Billker, O., Lourido, S., and Sibley, L.D. (2009).
Calcium-dependent signaling and kinases in apicomplexan parasites. Cell Host Microbe 5, 612–622.
Brochet, M., Collins, M.O., Smith, T.K., Thompson,E., Sebastian, S., Volkmann, K., Schwach, F.,
Chappell, L., Gomes, A.R., Berriman, M., et al.
(2014). Phosphoinositide metabolism links
cGMP-dependent protein kinase G to essential
Ca2+ signals at key decision points in the life cycleof malaria parasites. PLoS Biol. 12, e1001806.
Brown, K.M., and Sibley, L.D. (2018). Essential
cGMP signaling in Toxoplasma I initiated by a

Carucci, D.J., Witney, A.A., Muhia, D.K., Warhurst,D.C., Schaap, P., Meima, M., Li, J.L., Taylor, M.C.,
Kelly, J.M., and Baker, D.A. (2000). Guanylyl
cyclase activity associated with putative bifunctional integral membrane proteins in Plasmodium
falciparum. J. Biol. Chem. 275, 22147–22156.
Fang, H., Gomes, A.R., Klages, N., Pino, P., Maco,B., Walker, E.M., Zenonos, Z.A., Angrisano, F.,Baum, J., Doerig, C., et al. (2018). Epistasis studies
reveal redundancy among calcium-dependent
protein kinases in motility and invasion of malaria
parasites. Nat. Commun. 9, 4248.
Gao, H., Yang, Z., Wang, X., Qian, P., Hong, R.,
Chen, X., Su, X.Z., Cui, H., and Yuan, J. (2018).
ISP1-anchored polarization of GCbeta/CDC50A
complex initiates malaria ookinete gliding motility.
Curr. Biol. 28, 2763–2776 e2766.
Moudy, R., Manning, T.J., and Beckers, C.J.
(2001). The loss of cytoplasmic potassium
upon host cell breakdown triggers egressof Toxoplasma gondii. J. Biol. Chem. 276,41492–41501.

Exploring the Gut Fungi-Lung Allergy Axis
Akira Shibuya1,2,* and Kazuko Shibuya2
1Department of Immunology, Life Science Center for Survival Dynamics, Tsukuba Advanced Research Alliance (TARA), University of Tsukuba,
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8575, Japan
2Faculty of Medicine, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8575, Japan*Correspondence: ashibuya@md.tsukuba.ac.jp
https://doi.org/10.1016/j.chom.2018.11.012

Gut fungal dysbiosis exacerbates colitis and lung allergy. In recent studies published in Science (Leonardiet al., 2018) and Cell Host & Microbe (Li et al., 2018), gut-resident CX3CR1+ mononuclear phagocytes were
found to mediate crosstalk between intestinal fungi and systemic immunity along the lung-gut axis duringhomeostasis and inflammation.
Most animals, including humans androdents, live symbiotically with vastarrays of microorganisms. The gut is amajor organ in which microorganisms
reside; it contains trillions of microbes,
including bacteria, viruses, and fungi. An
imbalance in gut microbial composition—dysbiosis—can be caused bymany factors, including host genetics,
lifestyle, exposure to microorganisms,
and medical treatment. Gut dysbiosis
has been associated not only with intestinal inflammation but also with many diseases distant from the gut, such as atopic
dermatitis, allergy, cancer, obesity, anddiabetes (Honda and Littman, 2016).

Fungal dysbiosis can be caused by atleast two factors, namely antibiotic orantimycotic treatment. Antibiotic treatment affects not only gut bacterial
composition but also sometimes fungalcomposition (Noverr et al., 2004; Kimet al., 2014), perhaps due to the elimination of bacterial species that promoteresistance to fungal growth in the intestine. In contrast, ironically, antimycotic
therapy can induce fungal dysbiosis bypromoting the overgrowth of drug-resistant filamentous fungi. For example, prolonged oral treatment of mice with theantifungal drug fluconazole suppresses
the growth of Candida spp. but increases

the abundance of Aspergillus, Wallemia,and Epicoccum in the intestine (Wheeleret al., 2016). Interestingly, fluconazole
treatment can result in the overgrowth ofdrug-resistant gut fungi for several weeks
after the cessation of treatment (Li et al.,2018). Although gut fungal populations
are minor components of the gut microbiota, fungal dysbiosis can lead to thedevelopment of intestinal inflammatory
diseases such as Crohn’s disease or ulcerative colitis, alcoholic liver disease, orallergic inflammation in the lung (Wheeleret al., 2016). In this regard, an elegant
study by Li et al. (2018) in this issue provides formal evidence that gut fungal

Cell Host & Microbe 24, December 12, 2018 ª 2018 Elsevier Inc. 755

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Figure 1. The Gut CX3CR1+ Mononuclear Phagocytes as Key Links between Gut Fungi and Inflammation in the Lung and Intestine
Fluconazole-induced dysbiotic fungal community directly stimulates CX3CR1+ MNPs via the fungal C-type lectin receptors, resulting in the Th17 and Th2 responses in the intestine. These responses in turn lead to suppression of intestinal inflammation and exacerbation of allergic airway inflammation, respectively.

dysbiosis activates the gut fungi-lung allergy axis. They used gnotobiotic micecolonized with altered Schaedler flora: aminimal bacterial community consistingof a defined set of functionally diverse
anaerobic and aerobic bacteria, butlacking fungi. Fluconazole treatmentof these mice had no effect on housedust-mite (HDM)-induced allergic lunginflammation. However, HDM-induced
lung inflammation was exacerbated byoral supplementation of the fluconazole
treatment with a dysbiotic fungal community, including Aspergillus amstelodami,Epicoccum nigrum, and Wallemia sebi
(Li et al., 2018), which grow in response
to fluconazole treatment in specific-pathogen-free mice, as described above(Wheeler et al., 2016). Fluconazoleinduced growth of dysbiotic fungi in thegut was thus sufficient to exacerbatelung allergy. Because fluconazole is often
used to treat immunocompromised patients or patients with mucosal or vulvovaginal candidiasis, careful attention shouldbe paid to fungal dysbiosis induced bythis drug. However, how fungal dysbiosis
induces inflammation of both the intestineand organs distant from it remains poorly
understood.
In an effort to determine how gut fungi
modulate host immunity and inflammation,
Leonardi et al. (2018) postulated that acertain subset of immune cells in the intestinal lamina propria might interact with gut
fungi. They found that, among the myeloid
cells in the intestine, only mononuclear
phagocytes (MNPs) expressing the fractalkine receptor CX3CR1 upregulated theexpression of CD40 and CD86 costimulatory molecules when the mouse gut wascolonized with the opportunistic humancommensal Candida albicans. CX3CR1+MNPs expressed the fungal C-type lectin
receptors (CLRs) Dectin-1 (Clec7a), Dectin-2 (Clec6a), and Mincle (Clec4e) andwere able to phagocytose Candida albicans in the intestine. Dectin-1 contains animmunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic tail.
Dectin-2 and Mincle associate with theITAM-bearing adaptor protein Fc recep-756 Cell Host & Microbe 24, December 12, 2018

tor-g chain (FcRg) through a positively
charged amino acid residue in their transmembrane regions. Because the ITAM isphosphorylated and mediates an activating signal via spleen tyrosine kinase(Syk) in myeloid cells, the response of
CX3CR1+ MNPs to fungi is Syk dependent(Leonardi et al., 2018). Genetic ablation of
CX3CR1+ MNPs in mice leads to gut fungal
dysbiosis, and, unlike in wild-type mice, adramatic decrease in the numbers ofTh17 cells in the colon and mesentericlymph nodes and severe dextran sulfate
sodium-induced colitis; there is also a
defect in antifungal antibody production(Leonardi et al., 2018). A recent study
demonstrated that IL-17F, but not IL-17A,
is involved in the development of inflammatory bowel diseases (Tang et al.,2018). IL-17A instead targets epithelial
cells in the intestine and promotes theactivation of regulatory pathways that protect the gastrointestinal tract (Lee et al.,2015), suggesting that CX3CR1+ MNPs
are involved in the activation or differentiation of Th17 cells producing IL-17A, but

Cell Host & MicrobePreviews
not IL-17F. Moreover, CX3CR1+ MNPs
produce IL-23, which supports group 3innate lymphoid cells (ILC3s) to produce
IL-22, thus protecting epithelial cells
from microbiota-induced colitis (Longmanet al., 2014). Interestingly, Leonardi et al.
(2018) found that CX3CR1+ polymorphisms are associated with impaired antifungal responses in patients with Crohn’sdisease, who are unable to produce antibodies against multiple fungal species.
Leonardi et al. (2018) concluded thatCX3CR1+ MNPs play an important role ininnate and adaptive immune responses
to fungi in a Syk-dependent mannerand mediate the interaction between gut
fungi and host intestinal homeostasis andinflammation (Figure 1).
CX3CR1+ MNPs are abundant not
only in the intestine but also in the lung.
To address the question of whether intestinal CX3CR1+ MNPs, which interact directlywith gut fungi, are also involved in inflammatory diseases in organs distant fromthe intestine, such as the lung, Li et al.
(2018) generated mice in which CX3CR1+
MNPs were specifically deleted from theintestine, but not from the lung. Comparedwith control mice, after fluconazole treatment, these mice showed a dramatic
amelioration of HDM-induced allergic
airway inflammation and a reduction inTh2 responses (Figure 1). Interestingly,
Syk inhibition in intestinal CX3CR1+ MNPs
suppressed HDM-induced lung inflammation and decreased the numbers of Th2
cells in both the intestine and lung of these
fluconazole-treated mice. Because a positive correlation between IL-17A production and asthma severity has been established (Chesné et al., 2014), it is possiblethat Th2 cytokines and IL-17A—the production of both of which is increased inassociation with CX3CR1+ MNP activity inresponse to gut fungal dysbiosis—cooperate in the exacerbation of allergic airway
inflammation. Fungi produce prostaglandins that promote M2 macrophage polarization in the lung, thus exacerbatingallergic airway inflammation (Kim et al.,2014). Therefore, there are two gut-fungus-associated pathways that lead toallergic airway inflammation: the production of metabolites such as prostaglandin
derived from the fungi themselves, andthe production of immune cells such as
CX3CR1+ MNPs that directly recognize
fungi in the intestine.
Although Iliev and colleagues havedemonstrated that gut CX3CR1+ MNPs
play both a protective role in gut-fungusassociated colitis and a pathogenic rolein allergic airway inflammation (Leonardiet al., 2018; Li et al., 2018) (Figure 1),
several important issues remain unclear.
First, although fluconazole treatment
enhances the growth of A. amstelodami,E. nigrum, and W. sebi (Leonardi et al.,2018), it would be interesting to determine
which fungus (or fungi) specifically stimulates CX3CR1+ MNPs to activate either
Th17 cells or Th2 cells, or both, andto elucidate how CX3CR1+ MNPs set
the pathway leading to adaptive immunity through the Th17 response or thatthrough the Th2 response. Second,although gut CX3CR1+ MNPs are involvedin an increase in Th2 cell numbers in boththe lung and intestine in response toHDM in mice treated with fluconazole (Liet al., 2018), it is unclear how the intestinalTh2 cells are involved in the increase in
numbers of lung Th2 cells. In other words,
are the origins of both intestinal and lungTh2 cells the same (i.e., do the intestinalTh2 cells migrate into the lung) and, if so,
what is the antigen specifically recognizedby the Th2 cells? Alternatively, it is possiblethat the increase in the number of intestinalTh2 cells via the action of CX3CR1+ MNPs
in response to gut fungal dysbiosis promotes the activation of HDM-induced
lung Th2 cells via cytokines such as IL-4.
Elucidation of these issues would definitively help further our understanding of
the pathophysiology of inflammatory diseases associated with gut fungal dysbiosis
and to develop novel therapeutic approaches to these conditions.
ACKNOWLEDGMENTS
We thank Mariana Silva Almeida for illustration ofthe graphic. This work was supported in part bygrants provided by the Japan Society for thePromotion of Science (KAKENHI) (grant numbers
16H06387 and 18H05022 to A.S. and grant number 16H05169 to K.S.).

REFERENCES
Chesné, J., Braza, F., Mahay, G., Brouard, S.,
Aronica, M., and Magnan, A. (2014). IL-17 in severe
asthma. Where do we stand? Am. J. Respir. Crit.
Care Med. 190, 1094–1101.
Honda, K., and Littman, D.R. (2016). The microbiota in adaptive immune homeostasis and disease. Nature 535, 75–84.
Kim, Y.G., Udayanga, K.G., Totsuka, N., Weinberg,J.B., Núñez, G., and Shibuya, A. (2014). Gut dysbiosis promotes M2 macrophage polarization andallergic airway inflammation via fungi-induced
PGE2. Cell Host Microbe 15, 95–102.
Lee, J.S., Tato, C.M., Joyce-Shaikh, B., Gulen,M.F., Cayatte, C., Chen, Y., Blumenschein, W.M.,
Judo, M., Ayanoglu, G., McClanahan, T.K., et al.
(2015). Interleukin-23-independent IL-17 production regulates intestinal epithelial permeability.
Immunity 43, 727–738.
Leonardi, I., Li, X., Semon, A., Li, D., Doron, I.,
Putzel, G., Bar, A., Prieto, D., Rescigno, M.,
McGovern, D.P.B., et al. (2018). CX3CR1+ mononuclear phagocytes control immunity to intestinalfungi. Science 359, 232–236.
Li, X., Leonardi, I., Semon, A., Doron, I., Gao, I.H.,
Putzel, G., Kim, Y., Kabata, H., Artis, D., Fiers,W., et al. (2018). CX3CR1+ mononuclear phagocytes mediate the effects of gut fungal dysbiosis
on allergic airway disease. Cell Host Microbe 24,
this issue, 847–856.
Longman, R.S., Diehl, G.E., Victorio, D.A., Huh,J.R., Galan, C., Miraldi, E.R., Swaminath, A.,
Bonneau, R., Scherl, E.J., and Littman, D.R.
(2014). CX3CR1+ mononuclear phagocytes support colitis-associated innate lymphoid cell production of IL-22. J. Exp. Med. 211, 1571–1583.
Noverr, M.C., Noggle, R.M., Toews, G.B., andHuffnagle, G.B. (2004). Role of antibiotics andfungal microbiota in driving pulmonary allergic responses. Infect. Immun. 72, 4996–5003.
Tang, C., Kakuta, S., Shimizu, K., Kadoki, M.,
Kamiya, T., Shimazu, T., Kubo, S., Saijo, S.,Ishigame, H., Nakae, S., and Iwakura, Y. (2018).
Suppression of IL-17F, but not of IL-17A, provides
protection against colitis by inducing Treg cells
through modification of the intestinal microbiota.
Nat. Immunol. 19, 755–765.
Wheeler, M.L., Limon, J.J., Bar, A.S., Leal, C.A.,
Gargus, M., Tang, J., Brown, J., Funari, V.A.,
Wang, H.L., Crother, T.R., et al. (2016).
Immunological consequences of intestinal fungaldysbiosis. Cell Host Microbe 19, 865–873.

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