Th17 Cells Require You to Chew before You Swallow

Immunity

Previews the limited availability of appointments. Should all vaccinations be administered at 9 AM? Using electronic medical records, hospitals are now beginning to track clinical outcomes of various procedures with time of admittance. However, most studies are interested in how the attentiveness of healthcare workers or shift changes impact clinical outcomes, rather than how the patient’s own circadian rhythms might affect a biological response to treatment. In summary, we now know that cellintrinsic circadian rhythms alter both the entry and egress of lymphocytes via Bmal1-dependent changes in the expression of homing receptors and ligands on their surface. Despite the relatively modest magnitude of these changes and their short periodicity, they seem to have an outsized effect on subsequent immune responses. Future studies will have to

take into account the timing of immunization and infection on the immunological outcome of any particular immune response and determine the mechanisms that temporally control immunity and host defense. Meanwhile, one is left to ponder whether the compositions of Johann Sebastian Bach were influenced by the circadian periodicity of his own immune system.

REFERENCES Druzd, D., Matveeva, O., Ince, L., Harrison, U., He, W., Schmal, C., Herzel, H., Tsang, A., Kawakami, N., Leliavski, A., et al. (2017). Immunity 46, this issue, 120–132. Gibbs, J., Ince, L., Matthews, L., Mei, J., Bell, T., Yang, N., Saer, B., Begley, N., Poolman, T., Pariollaud, M., et al. (2014). Nat. Med. 20, 919–926. Hailemichael, Y., Dai, Z., Jaffarzad, N., Ye, Y., Medina, M.A., Huang, X.F., Dorta-Estremera,

S.M., Greeley, N.R., Nitti, G., Peng, W., et al. (2013). Nat. Med. 19, 465–472. Halberg, F. (1963). Proc. R. Soc. Med. 56, 253–257. Haspel, J.A., Chettimada, S., Shaik, R.S., Chu, J.H., Raby, B.A., Cernadas, M., Carey, V., Process, V., Hunninghake, G.M., Ifedigbo, E., et al. (2014). Nat. Commun. 5, 4753. Hemmers, S., and Rudensky, A.Y. (2015). Cell Rep. 11, 1339–1349. Mohawk, J.A., Green, C.B., and Takahashi, J.S. (2012). Annu. Rev. Neurosci. 35, 445–462. Nguyen, K.D., Fentress, S.J., Qiu, Y., Yun, K., Cox, J.S., and Chawla, A. (2013). Science 341, 1483–1488. Scheiermann, C., Kunisaki, Y., and Frenette, P.S. (2013). Nat. Rev. Immunol. 13, 190–198. Yu, X., Rollins, D., Ruhn, K.A., Stubblefield, J.J., Green, C.B., Kashiwada, M., Rothman, P.B., Takahashi, J.S., and Hooper, L.V. (2013). Science 342, 727–730.

Th17 Cells Require You to Chew before You Swallow Marc Veldhoen1,* 1Instituto de Medicina Molecular, Faculdade de Medicina da Universidade de Lisboa, Av. Professor Egas Moniz, Lisbon 1649-028, Portugal *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2016.12.016

How immunity is regulated at distinct epithelial tissues that vary in microbial occupancy and environmental and tissue specific cues isn’t clear. Dutzan et al. (2017) report that mechanical-derived signals, not those from micro-organisms, are key to maintaining interleukin-17-expressing T helper (Th17) cells at the oral epithelia. Detailed knowledge of CD4+ T helper (Th) cell biology has been gained over the last decades. Initially split in two subsets, Th1 cells and Th2 cells, the discovery of Th17 cells did not contribute just a third effector arm of the CD4+ lineage. Th17 cells are rare in secondary lymphoid organs but enriched at epithelial barriers. There, they can be generated under the influence of microorganisms. In this issue of Immunity, Dutzan et al. (2017) report an important role for mechanical-damageinduced factors in specifically maintaining a Th17 cell population at the oral mucosa. Epithelia are prime locations at risk to microbial invasion, occupied by a variety

of microorganisms that make contributions to the health of the tissue as well as the host. Immune networks at epithelia balance tolerance, keep potential pathogens at bay, enable moderate immunity when minor breaches occur, and initiate comprehensive responses when encountering danger. Due to the large area it constitutes and the varied composition of its microorganisms, the intestine has attracted the most scrutiny and has been used to study and define these immune networks. Subsequently, attention was focused on the skin and lungs, with the realization that, similar to the intestine, these tissues are under the influence of micro-organisms as well. It became

8 Immunity 46, January 17, 2017 ª 2017 Published by Elsevier Inc.

apparent that not just the microbial players differ between barrier organs, but that immune networks and cellular compositions vary as well. For example, the mouse skin and small intestine contain a specialized population of lymphocytes, the intraepithelial lymphocytes, that is not present in the lung or the colon and are not able to secrete IL-17 (Li et al., 2011). Immune networks are tuned to tissuespecific needs. IL-17-producing lymphocytes present at all barrier sites are critical for protection against extracellular bacteria and fungi. They consist of those expressing a T cell receptor (TCR) formed with a and b chains (Th17 cells), such as

Immunity

Previews Limited chewing (young)

Chewing and/or damage (older)

IL-6

IFNγ

IL-17

IL-6Rα

gp130

Th1 Th17

Figure 1. Chewing and Tissue Damage Results in the Accumulation of Th17 Cells in the Gingiva The gingiva, forming the soft tissue surrounding the teeth, contain populations of lymphocytes. Young mice and humans have limited numbers of T cells producing IL-17 (Th17) and IFN-g (Th1) in the gingiva (left). Dutzan et al. (2017) show that over time, mechanical stress from mastication and tissue damage results in the secretion of IL-6 from epithelial cells, which, via binding to IL-6Ra-gp130, facilitates the accumulation of Th17 cells via increased proliferation (right).

in the lamia propria of the intestine, or gd chains such as are prevalent in lungs and dermis. The role of IL-17-producing lymphocytes encompasses regulating microbial occupancy via co-production of IL-17 and IL-22, synergistically increasing the production of antimicrobial factors (Kolls et al., 2008). The activity of Th17 cells is, however, not restricted to making IL-17 and IL-22, and they show a large degree of plasticity (Hirota et al., 2011, 2013). Th17 cells are hence an important component of the immune network at epithelia, maintaining epithelial integrity, managing microbial occupation, and responding to microbial threats robustly and with flexibility. Th17 cell differentiation and maintenance has been closely associated with components of the microbiota such as, but not exclusively, segmented filamentous bacteria (SFB) (Berer et al., 2011; Ivanov et al., 2008). Under the influence of micro-organisms, the cytokines IL-6, IL1b, tumor necrosis factor (TNF), transforming growth factor (TGF)-b, and IL-23 are produced with direct effects on the generation, function, and maintenance of Th17 cells. It is clear that SFB are not pre-

sent outside the small intestine, raising the question of how IL-17-mediated immunity is regulated at other tissues. The oral cavity is known to contain a substantial population of IL-17 cells, especially offering protecting against fungi (Conti et al., 2014). Loss of the ability to make or respond to IL-17 results in mucocutaneous candidiasis on the skin, vagina, and mouth. The cellular source of IL-17 in the oral cavity was shown to consist of at least tongue-resident populations of gd T cells as well as Th17 cells, with redundancy between these populations. The response of Th17 cells upon candida infection is swift and can be mobilized within a few hours to days (Conti et al., 2014; Hirota et al., 2011), suggesting that Th17 cell responses may be derived from a preexisting ‘‘natural’’ Th17 cell population derived from the thymus (Marks et al., 2009). The Th17 and gd T cells found in the tongue did, however, adhere to similar cues as those in the small intestine and completely depended on the presence of a microbiota for their presence. Dutzan et al. (2017) investigated Th17 cells present in the gingiva, the soft tissue

surrounding the teeth. The gingiva supports dentition and harbors a complex microbiome. Periodontitis is a common disease in humans that can result in loss of teeth and is linked with cardiovascular disease and rheumatoid arthritis (RA), yet there is limited understanding of the immune network present in the gingiva. Besides a protective role, uncontrolled Th17 cell responses at the gingiva can be detrimental and, like in RA, promotes bone loss. Hence a better understanding of Th17 cell accumulation and maintenance at this site is important. In this study by Dutzan et al. (2017), Th17 cells, in contrast to Th1 and regulatory T cells, were found to accumulate in the gingiva with age, from 8 to 24 weeks in mice and 18–25 to 40–50 years in humans. This accumulation was specific to the gingiva and was not observed at the skin, intestine, spleen, or even oral-draining lymph node. This enrichment of Th17 cells was linked with increased proliferation (Dutzan et al., 2017). Dutzan et al. (2017) next investigated the reason for the accumulation of Th17 cells in the gingiva. They did not observe significant differences in load, diversity, or composition of the microbiota in mice at 8 versus 24 weeks of age. SFB were not found in the oral cavity. Furthermore, presence or absence of intestinal SFB had no effect on Th17 cell accumulation in the gingiva. Gingiva Th17 cells accumulated in mice in the absence of dectin-1 and mannose receptor signaling, thereby excluding a role for fungi. TCR transgenic mice did not show an increase in Th17 cells in the gingiva, suggesting that Th17 cell accumulation was antigen specific. Interestingly, in marked contrast to the intestine, skin, or tongue, the maintenance of gingival Th17 cells was independent of the microbiota, as germ-free mice showed a similar accumulation of Th17 cells with age. The authors went on to look for the signals driving Th17 cell accumulation in the gingiva. They ruled out a role for IL-1a, IL1b, IL-12, and IL-23 by using genetically modified animals. However, they found that IL-6 was essential for the accumulation of Th17 cells in the gingiva and it acted intrinsically on Th17 cells. This result was surprising since IL-6 production was linked to microbial stimulation. Since the authors had found that the microbiota were dispensable for the Immunity 46, January 17, 2017 9

Immunity

Previews accumulation, they looked for the source of IL-6 in the gingiva. Surprisingly, they found that IL-6, but not IL-1b, was derived from the epithelium due to tissue damage or mechanical pressure alone, occurring physiologically in the mouth through abrasion and by chewing, thereby enabling the accumulation of Th17 cells. In line with this hypothesis, mice harbored increasing numbers of Th17 cells in the gingiva based on the level of abrasion experienced. Mice placed on soft diet had fewer Th17 cells than those on standard chow, which had fewer Th17 cells than those receiving additional rubbing of the gingiva (Figure 1). Although mechanical pressure may result in expression of chemoattractants, Th17 cells were not selectively recruited to the gingiva. Instead, the presence of IL-6 correlated with increased local proliferation of Th17 cells. This highlights that the cues required to accumulate Th17 cells at barrier sites involve IL-6 but that the induction of this cytokine can be achieved in more ways than via microbial stimulation. It remains to be investigated whether damage or mechanical sensing in the gingiva constitute a tissue-specific cue. The involvement of IL-6 would suggest this could be a general mechanism of relevance at other sites, albeit that the mastication provides continues signal in

the gingiva. In line with this, Dutzan et al. (2017) showed that damage induced in the skin also resulted in the accumulation of Th17 cells, suggesting that microbial induction as well as the tissuedamage response can enhance Th17 cell numbers. IL-17 stimulation of many cell types, such as fibroblasts, macrophages, chondrocytes, and osteocytes, often in synergy with other cytokines, results in the production of IL-6, IL-1b, and TNF, as well as chemokines such as CCL20. This positive-feedback loop would strengthen the accumulation of Th17 cells and recruitment of granulocytes. The results by Dutzan et al. (2017) emphasize the importance of IL-6 in maintaining Th17 cells. The mechanism may find additional relevance in autoimmune disorders such as RA, where continuous bone destruction by Th17 cells is a major symptom of disease, but which may also be directly involved, via the damageinduced secretion of IL-6, in sustaining disease. The work by Dutzan et al. (2017) shows that tissues have the ability to maintain numbers of Th17 cells via different mechanisms. In the case of the gingiva, the maintenance of a resident population of Th17 cells depends on the how well you chew your food.

REFERENCES Berer, K., Mues, M., Koutrolos, M., Rasbi, Z.A., Boziki, M., Johner, C., Wekerle, H., and Krishnamoorthy, G. (2011). Nature 479, 538–541. Conti, H.R., Peterson, A.C., Brane, L., Huppler, A.R., Herna´ndez-Santos, N., Whibley, N., Garg, A.V., Simpson-Abelson, M.R., Gibson, G.A., Mamo, A.J., et al. (2014). J. Exp. Med. 211, 2075–2084. Dutzan, N., Abusleme, L., Bridgeman, H., Greenwell-Wild, T., Zangerle-Murray, T., Fife, M.E., Bouladoux, N., Linley, H., Brenchley, L., Wemyss, K., et al. (2017). Immunity 46, this issue, 133–147. Hirota, K., Duarte, J.H., Veldhoen, M., Hornsby, E., Li, Y., Cua, D.J., Ahlfors, H., Wilhelm, C., Tolaini, M., Menzel, U., et al. (2011). Nat. Immunol. 12, 255–263. Hirota, K., Turner, J.E., Villa, M., Duarte, J.H., Demengeot, J., Steinmetz, O.M., and Stockinger, B. (2013). Nat. Immunol. 14, 372–379. Ivanov, I.I., Frutos, Rde.L., Manel, N., Yoshinaga, K., Rifkin, D.B., Sartor, R.B., Finlay, B.B., and Littman, D.R. (2008). Cell Host Microbe 4, 337–349. Kolls, J.K., McCray, P.B., Jr., and Chan, Y.R. (2008). Nat. Rev. Immunol. 8, 829–835. Li, Y., Innocentin, S., Withers, D.R., Roberts, N.A., Gallagher, A.R., Grigorieva, E.F., Wilhelm, C., and Veldhoen, M. (2011). Cell 147, 629–640. Marks, B.R., Nowyhed, H.N., Choi, J.Y., Poholek, A.C., Odegard, J.M., Flavell, R.A., and Craft, J. (2009). Nat. Immunol. 10, 1125–1132.

How Many Subsets of Innate Lymphoid Cells Do We Need? Kafi N. Ealey1 and Shigeo Koyasu2,* 1Laboratory

for Innate Immune Systems for Immune Cell Systems RIKEN Center for Integrative Medical Sciences, 1-7-22 Suehiro-cho, Tsurumi-ku, Yokohama 230-0045, Japan *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2016.12.018 2Laboratory

Innate lymphoid cells (ILCs) are composed of three main subsets. In this issue of Immunity, Simoni et al. (2017) show using mass-cytometry that human ILCs are highly heterogeneous between individuals and tissues and lack a previously proposed helper-type ILC1 population. Innate lymphoid cells (ILCs) are composed of cytotoxic natural killer (NK) cells and the more recently discovered

helper-like ILCs. All members of the ILC family lack expression of rearranged antigen receptors and do not display anti-

10 Immunity 46, January 17, 2017 ª 2017 Elsevier Inc.

gen specificity. ILCs play a critical role in the maintenance of tissue homeostasis, early defense against infection,