Commensal Microbiome Expands Tγδ17 Cells in the Lung and Promotes Particulate Matter-Induced Acute Neutrophilia

Particulate matter (PM) induces neutrophilic inflammation and deteriorates the prognosis of diseases such as cardiovascular diseases, cancers, and infections, including COVID-19. Here, we addressed the role of γδ T cells and intestinal microbiome in PM-induced acute neutrophilia. γδ T cells are a heterogeneous population composed of Tγδ1, Tγδ2, Tγδ17, and naïve γδ T cells (TγδN) and commensal bacteria promote local expansion of Tγδ17 cells, particularly in the lung and gut without affecting their Vγ repertoire. Tγδ17 cells are more tissue resident than Tγδ1 cells, while TγδN cells are circulating cells. IL-1R expression in Tγδ17 cells is highest in the lung and they outnumber all the other type 17 cells such as Th17, ILC3, NKT17, and MAIT17 cells. Upon PM exposure, IL-1β-secreting neutrophils and IL-17-producing Tγδ17 cells attract each other around the airways. Accordingly, PM-induced neutrophilia was significantly relieved in γδ T- or IL-17-deficient and germ-free mice. Collectively, these findings show that the commensal microbiome promotes PM-induced neutrophilia in the lung via Tγδ17 cells.


INTRODUCTION
gd T cells are abundant in mucosal tissues, such as the conjunctiva, skin, and lung (1)(2)(3). Amongst them, Tgd17 cells play an important role in the rapid response to foreign antigens by immediately secreting IL-17 and recruiting neutrophils into inflamed mucosal tissues (4,5). In particular, the lungs are constantly exposed to various environmental insults such as airborne pathogens and inorganic pollutants. In this process, the commensal microbiome acts as an important immune modulator (6,7). Recent studies have shown that dysregulated microbiota causes immune dysfunction, leading to poor control of respiratory infections, allergic asthma, or tumor immune surveillance (8)(9)(10). In the liver, the critical role of intestinal bacteria in the homeostasis of hepatic Tgd17 cells has been described (11). However, the immune crosstalk between the commensal microbiota and lung-resident Tgd17 cells has not yet been elucidated.
Air pollution is a serious problem worldwide, and accumulating evidence indicates that particulate matter (PM) has a significant effect on immune systems (12). Long-term exposure to PM induces IL-1 and TNFa secretion from alveolar macrophages (AM) and airway epithelial cells (AECs) (13)(14)(15) and is closely associated with increased mortality, morbidity, and hospitalization of asthma patients (16,17). In particular, traffic-driven particles (diesel exhaust particles, DEPs) exacerbate house dust mite (HDM)induced allergic asthma by enhancing Th17 cells in lungs (18,19). In addition, recent studies have shown that the severity of air pollution is highly correlated with the mortality rate of COVID-19 patients; for every 1 mg/m3 increase in airborne fine dust concentration, the mortality of patients increases by 11% (20). To date, however, the role of gd T cells in PM-induced pulmonary inflammation has not been well addressed.
gd T cells are innate T cells that develop in the thymus. In a previous study, we showed that there are three distinct effector subsets in the thymus according to their transcriptional profiles designated as Tgd1, Tgd2, and Tgd17 cells (21). These effector cells develop from common progenitors, and we showed that lineage differentiation models rather than TCR-mediated instructions and explained their ontogeny. In this study, we extended our previous research by analyzing gd T cells in the peripheral lymphoid and non-lymphoid organs, including the lungs and defined their critical role in PM-mediated acute pulmonary neutrophilia. We defined naïve gd T (TgdN) cells corresponding to conventional gd T cells (CD44 lo CD45RB hi ) described previously (21,22) and categorized all gd T cells as TgdN, Tgd1, Tgd2, and Tgd17 cells. Using anti-Vg1, Vg1/2, Vg4, Vg5, Vg6, and Vg7 antibodies in a single staining panel, we comprehensively analyzed Vg TCR usage in all four sub-types of gd T cells in peripheral tissues and compared them in specific pathogen-free (SPF) and germ-free (GF) mice. As a result, we found that commensal microbiota is critical for the maintenance of the peripheral pool of lung-resident IL-1R + Tgd17 cells, which contributed to the development of PM-induced acute airway neutrophilic inflammation, but not a chronic model of IL-17dominant HDM/PM-induced allergic asthma. Consistent with this, PM-induced neutrophila was significantly relieved in GF mice compared to SPF mice. Collectively, these findings provide mechanistic insight into the immune crosstalk between commensal microbiome, lung-resident gd T cells, and PMinduced neutrophilia.

Mice
B6 WT (C57BL/6) and Tcrd -/-(B6.129P2-Tcrd tm1Mom/J ) mice were purchased from the Jackson Laboratory and bred in our facility under specific pathogen-free (SPF) conditions. Il17a/f -/-(B6) mice were received from Dr. Charles D. Surh (POSTECH, Korea). CD45.1/2 B6 mice were received from Dr. Sin-Hyeog Im (POSTECH, Korea). All mice were used at the age of 6-12 weeks unless indicated, and age-and sex-matched animals were used as controls. Germ-free (GF) mice were bred and used as previously described (24). All mouse experiments were performed using protocols approved by the Institutional Animal Care and Use Committees (IACUC) of the POSTECH.

Mouse Models of Chronic HDM/ PM-Induced Allergic Asthma
We used a previously described house dust mite (HDM)-induced mouse asthma model with minor modification (25). HDM (Dermatophagoides pteronyssinus) extracts were purchased from Greer laboratories. Mice were intranasally administered with 20 mg of HDM daily for 4 days and challenged again with 20 mg of HDM for 4 days later. Fine dust was administered via the intranasal route with 250 mg of PM.

Parabiosis
Five-week-old B6 CD45.1/2 and CD45.2/2 female mice were joined together by parabiosis for 2 or 7 weeks, as previously described (11). Weight-matched mice were anesthetized and shaved. An incision was made along the side of each mouse and the skin was connected using surgical clips.

Cell Preparation
Mice were sacrificed at the indicated time points and BAL fluids were collected in 1 mL PBS. To remove circulating cells, 15 ml of PBS was injected into the heart after incision of the abdominal aorta. Harvested lung tissues were minced by McIlwain Tissue Chopper and digested in 5 mL of RPMI-1640 containing collagenase D (400 Mandl Units; ROCHE) and DNase I (1 mg/ml; 9003-98-9) on a shaker at 37°C for 45min, followed by filtration through a 70 mm strainer and 40%, 70% Percoll (Merck) gradient centrifugation (20 min at 2,000 rpm at room temperature). To isolate LP cells from the intestine, we followed previous report (26). Single-cell suspensions were prepared and separated by Percoll gradient centrifugation. Adipose tissues were minced and digested with collagenase type IV (100 units, Gibco) and collagenase D (400 Mandl Units, ROCHE) on a shaker at 37°C for 45min. The ears were excised and cut into small pieces. The ear epithelial cell layer was removed by vigorous stirring in PBS containing 3% FBS, 20 mM HEPES, 100 U/ml penicillin, 100 mg/ml streptomycin, 1 mM sodium pyruvate, and 10 mM EDTA at 37°C for 20 min. The tissue samples were then digested in PRMI containing collagenase type V (1 mg/ml, Sigma) at 37°C for 45 min. Total cells were counted using a VI-CELL Cell Viability Analyzer (BECKMAN COULTER) and stained for FACS analysis.

Immunofluorescence
Immunofluorescence staining was performed as described previously (29), with modifications. Briefly, tissues were fixed with 4% paraformaldehyde (PFA) for 1 hour and snap frozen. Five micrometer tissue sections were blocked with 5% bovine serum albumin and goat sera (Jackson Laboratory) for 1 hour at 25°C and stained with antibodies. Images were obtained using Leica DM6B with THUNDER system.

Statistical Analysis
Prism software (GraphPad, Version 8.4.2) was used for statistical analysis, and all data were represented as mean ± SD. Unpaired two-tailed t-tests and one-way ANOVAs were used for data analysis and the generation of P values. P < 0.05 was considered significant.

Peripheral Homeostasis of Tgd17 Cells Is Dependent on Commensal Microbiome
To analyze gd T cells systematically, we used a combination of transcription factors and surface markers as previously described (21), and newly defined naïve gdT (TgdN) cells as PLZF lo/-RORgt -Tbet -CD44 lo cells in the thymus ( Figure 1A, upper panels). We further analyzed TCR Vg usage using a panel of antibodies specific for TCR Vg1, Vg1/2, Vg4, Vg5, Vg6, and Vg7 in a single staining panel. In this way, we analyzed gd T cells in the thymus and periphery, and phenotyped different subsets of gd T cells with different TCR Vg chain usages, except TCR Vg3, which is a pseudogene.
As previously reported (30), Tgd17 cells mainly consist of PLZF lo Vg4 + and PLZF int Vg6 + cells both in the thymus and periphery, including the lung and skin ( Figure 1A and Supplementary Figure 1). In the thymus, all subtypes of gd T cells were present and most intraepithelial lymphocytes (IEL) gd T cells were TBET + Tgd1 cells. The majority of thymic TgdN and Tgd1 cells consisted of Vg1 + and Vg4 + cells, whereas more than half of IEL Tgd1 cells expressed TCR Vg7, indicating that the Vg TCR usage of gd T cells varies depending on the tissue type, despite the same effector lineage. As shown in previous studies (31), in the skin, GL3 hi dendritic epidermal T cells (DETC) were TCR Vg5 + and GL3 int dermal gd T cells were RORgt + Tgd17 cells expressing TCR Vg4 or Vg6 ( Figure 1A, lower right panels).
In the thymus and periphery of SPF mice, naïve and effector subsets of gd T cells were variably distributed, except Tgd2 cells that were exclusively present in the thymus ( Figure 1B  ( Figure 1C). TgdN and Tgd1 cells were also slightly reduced in the spleen, mesenteric lymph node (mLN), and IEL of GF mice compared SPF control. Interestingly, there were decreased numbers of Tgd2 cells, but not other subsets in thymi of GF mice ( Figure 1D), indicating that commensal microbiota affects thymic development of gd T cells. We further investigated the effect of microbiome on Vg TCR chain usage; however, there were no noticeable differences in the thymus and periphery between SPF and GF mice ( Figure 1E, Supplementary Figure  3 and Supplementary Table 1). Overall, these findings indicate that TCR Vg repertoire determined in the thymus is not affected by commensal microbiome in the periphery, suggesting that innate signaling rather than TCR engagement by specific antigens regulates the peripheral pool of gd T cells.

Microbial Colonization of GF Mice Restores Peripheral Pool of Tgd17 Cells
Because maternal commensal microbiome affects the fetal immune system (33)(34)(35), we analyzed its effect on the development of Tgd17 cells using new-born (day 1) mice, which have a fetal repertoire of gd T cells. The numbers of thymic Tgd17 cells were not different between SPF and GF mice at all ages (Figure 2A), and there were no substantial differences in their numbers and TCR Vg usage of thymic immature Tgd17 (CD24 hi RORgt + ) and mature Tgd17 (CD24 lo RORgt + ) cells in the neonatal mice (Supplementary Figures 4A, B). In 3-weekold pre-weaned GF mice, there was increased usage of TCR Vg1 in Tgd17 cells compared SPF mice ( Figure 2B and Supplementary Figures 4C, D). In the periphery, Vg4 + or Vg6 + peripheral Tgd17 cells were variably decreased in GF mice compared to SPF mice ( Figure 2C). The development of mucosal associated invariant T (MAIT) cells is dependent on the microbiome, and later colonization of GF mice failed to reconstitute their development (36). We tested this in gd T cells by cohousing 6-week-old GF mice with SPF mice for 6 weeks ( Figure 2D). However, unlike MAIT cells, in these mice, we observed that not only peripheral Tgd17 cells, including the lung and siLP ( Figure 2E), but also thymic Tgd2 ( Figure 2F) and IEL Tgd1 ( Figure 2G) cells were all restored to equivalent levels of SPF mice. This features indicate that later colonization of the commensal microbiome is sufficient for the restoration of gd T cells in adulthood.
Tgd17 Cells Are Tissue Resident gd T cells are generally known to be tissue resident. To better understand the circulating dynamics of each subset of gd T cells in the periphery, we generated a parabiosis model using C57BL/6 congenic mice and analyzed them 2 or 7 weeks later ( Figure 3A). We first confirmed that 50% of B cells in the LN were from paired parabionts ( Figure 3B and Supplementary Figure 5A) and analyzed gd T cells. Interestingly, 50% of TgdN cells in most lymphoid and non-lymphoid organs, except thymus and siLP, were derived from paired parabionts, indicating that they are a circulating population similar to B cells. Tgd17 cells were mostly tissue resident, especially in fat, ear skin, siLP, and lung. Tgd1 cells were also tissue resident, especially in IEL and siLP.
Generally, Tgd1 cells showed a less tendency of tissue residency compared to Tgd17 cells ( Figures 3C, D). As known that Vg5 + DETCs are only generated during the fetal period and reside in the skin (37)(38)(39), 97% of them were tissue resident at 2 and 7 weeks after parabiosis (Supplementary Figure 5). Therefore, each subset of gd T cells has different residential or circulatory characteristics with some variability depending on the tissues they localize. Consistent with a previous report (40), we also observed that invariant natural killer T (iNKT) cells, including both NKT1 and NKT17 cells, are tissue resident (Supplementary Figure 6). However, there was not much difference in their tissue residency between NKT1 and NKT2 cells at the second week of parabiosis, and there were no naïve NKT cells. Taken together, unlike previous thoughts, these findings indicate that each subset of gd T cells has unique pattern of tissue residency, that is Tgd17 cells are mostly resident in the peripheral tissues compared to Tgd1 cells, and TgdN cells are circulating cells.
We additionally compared tissue residency of gd T cells using intravascular staining of anti-CD45 antibodies ( Figures 3E, F and Supplementary Figure 7) and found no significant differences between SPF and GF mice. Although intravascular staining does not necessarily differentiate tissue resident population as some cells are intravascular resident, this result suggests that the absence of microbiome does not affect circulating tendency of gd T cells.

Type 17 Innate T Cells Express IL-1R in the Lung
Tgd17 cells are activated and rapidly produce IL-17 in response to IL-1 without TCR engagement (32,41,42). In the lung, gd T cells express copious amounts of IL-1R and their over activation due to excessive IL-1 leads to poor control of lung adenocarcinoma (8,43). Based on this, we further analyzed the expression pattern of IL-1R in gd T cells and compared it with those in other types of innate T cells such as NKT, MAIT, conventional CD4 T cells, and innate lymphoid cells (ILCs) ( Figures 4A, B and Supplementary Figure 8). Frequencies of IL-1R expression in type 17 innate T cells including Tgd17, NKT17, and MAIT17 cells and in ILC3s were comparable with one another at approximately 60-70%, whereas only about 20% of conventional Th17 cells expressed IL-1R. However, the number of IL-1R-expressing cells was highest in Tgd17 cells occupying 68% ( Figure 4B). In addition, pulmonary Tgd17 cells expressed the highest level of IL-1R compared to those in thymus, spleen, and mediastinal lymph node (medLN) ( Figure 4C). Together, these findings suggest that Tgd17 cells would be the main population that responds to exogenous IL-1 and produces IL-17.

PM Induces IL-1b Secretion and Acute Neutrophilia via Tgd17 Cells
To investigate the pathogenic role of IL-1R + lung-resident Tgd17 cells in vivo, we used a mouse model of PM-induced acute airway inflammation. We intranasally administered mice with 250 mg of PM and analyzed at each time points after exposure. Lung  epithelial cells are known to produce IL-1b in response to PM (14) and we further analyzed CD45 + leukocytes by flow cytometry. The mean fluorescence of intensity of IL-1b was sharply increased, as well as, the number of IL-1b-producing cells was increased after PM exposure (Supplementary Figure 9A). We analyzed the intracellular IL-1b in the CD45 + leukocytes using a gating strategy as depicted in Supplementary Figure 9B to include T cells, B cells, AM, interstitial macrophages (IM), neutrophils, and other undefined CD11b + cells. After 4 hours of PM exposure, AM and neutrophils remarkably produced IL-1b ( Figures 5A, B). Interestingly, the major cellular sources of IL-1b were neutrophils (33%) and CD11b + cells (37%) in normal lungs, and neutrophils produced most of IL-1b (approximately 80%) in PM-exposed lungs ( Figure 5C). Consistent with previous reports (18,19), we found that PM causes an acute expansion of alveolar macrophages and strong neutrophilia in the airways ( Figure 5D). The kinetics of neutrophil influx was similar to that of alveolar macrophages and peaked at 12 hours after PM administration ( Figure 5E). Since IL-17-producing Tgd17 cells are associated with neutrophilia in the lung after bacterial or viral infection (44,45), we next examined whether PM induces the production of IL-17 from gd T cells. Using a gating strategy as shown in Supplementary Figure 10, we found that the frequencies of IL-17-producing Tgd17, MAIT17, and NKT17 cells significantly increased ( Figures 5F-H), whereas there were no changes in IL-17 production from Th17 cells and ILC3s (Figures 5I, J) 24 hours after PM exposure. We also confirmed that the total number of IL-17-producing cells was approximately 2.68 times higher in PM-treated lungs compared to that in PBS-treated group ( Figure 5K, pie charts). Notably, we discovered that Tgd17 cells produce 75% of IL-17 under both normal and inflammatory conditions ( Figure 5K). PM exposure not only enhanced IL-17 production from Tgd17 cells ( Figure 5F), but also expanded their numbers upon its consecutive exposure for 4 days (Supplementary Figures 11A, B). However, numbers of Tgd1 or TgdN cells were not increased and there were rather decreased IFNg secretion from Tgd1 cells (Supplementary Figures  11C, D). Taken together, these findings indicate that innate T cells, but not Th17 CD4 T cells or ILC3s, are the source of early IL-17 upon PM exposure.

Commensal Microbiota Promotes PM-Induced Acute Neutrophilic Airway Inflammation
We showed that homeostasis of Tgd17 cells is dependent on commensal microbiomes (Figures 1, 2), and PM induces acute neutrophilia with Tgd17 expansion ( Figure 5). Therefore, we tested whether GF mice have reduced neutrophilic inflammation upon PM exposure (Figures 6A, B). Indeed, GF mice had reduced infiltration of neutrophils and AMs upon PM exposure. Immunofluorescence staining of the lungs revealed that neutrophils clustered together with Tgd17 cells around airways in SPF mice and GF mice had less infiltration of these cells ( Figure 6C). Taken together, these results show that commensal microbiomes regulate neutrophilic inflammation upon PM exposure, which is likely to be mediated by Tgd17 cells.

Tgd17 Cells Promote PM-Induced Acute Pulmonary Neutrophilic Inflammation
To obtain direct evidence that Tgd17 cells are associated with the pathogenesis of PM-induced airway inflammation, we investigated and compared the severity of neutrophilic inflammation between B6 wild-type (WT) and TCRd-deficient (Tcrd -/-) mice 24 hours after PM administration. We found that Tcrd -/mice showed significantly decreased neutrophilia ( Figures 7A, B) without affecting the frequencies of IL-17-producing MAIT and iNKT cells compared to those of WT mice (Figures 7C-E). We and others have previously shown that MAIT cells expand in the absence of NKT or gd T cells in the thymus and skin (21,36). Consistent with these findings, the number of MAIT17 cells increased three times in lung of Tcrd -/mice. However, they could not compensate the absence of gd T cells and there was an average 6.7-fold reduction of IL-17producing cells in the lung after PM exposure ( Figure 7F). We also confirmed that neutrophilic inflammation was significantly relieved in Il17a/f-double knockout mice (Supplementary Figure 12). Collectively, these findings indicate that Tgd17 cells play a major role in acute neutrophilia induced by PM exposure. We further analyzed the effect of gd T cells in a chronic allergic asthma model induced by HDM and PM ( Figure 8A). Previous reports showed that diesel dust converted allergic asthma from a Th2 to Th17-dominant inflammatory model (18,19), and we also found that co-administration of HDM and PM induced the dominant expansion of RORgt + CD4 T cells ( Figure 8B). In Tcrd -/mice, however, there was no decreased infiltration of neutrophils or other immune cells (Figures 8C, D), indicating that gd T cells do not influence the chronic model of Th17-dominant inflammation.

DISCUSSION
In this study, we found that the commensal microbiome mainly regulates the peripheral homeostasis of Tgd17 and thymic Tgd2 cells. We categorized gd T cells in the thymus and peripheral tissues according to transcription factors and surface marker expression, as TgdN, Tgd1, Tgd2, and Tgd17 cells. By using 6 different anti-TCRg antibodies, we analyzed TCRg usage in each subset and compared them between SPF and GF mice. In 3week-old GF mice, we found that the proportion of Vg1 usage increased whereas Vg6 usage decreased ( Figure 2B). Unlike the previous notion that gd T cells reside in tissues, we found that gd T cells have different residential/circulating phenotypes for each subset and their localization. In particular, TgdN cells exhibit a circulating phenotype, while Tgd1 and Tgd17 cells reside in tissues, especially in lungs and siLP, where they constantly encounter environmental components and microbial antigens. These results suggest that tissue-resident Tgd17 cells can rapidly induce an immune response in inflammatory conditions. Given the importance of IL-1/IL-1 receptor (IL-1R) signaling in the activation of Tgd17 cells (32,41,42), we showed that type 17 innate T cells and ILC3s express higher levels of IL-1R than conventional CD4 T cells in the lung (Figures 4A, B). Among IL-1R-expressing cells, pulmonary Tgd17 cells were the majority and expressed higher levels of IL-1R than those in other tissues ( Figure 4C). These features suggest that Tgd17 cells produce IL-17 most effectively in response to IL-1 signaling in lungs compared to those in other tissues. To define the pathological role of pulmonary Tgd17 cells, we used a mouse model of PM-induced acute airway inflammation and HDM/PM- induced chronic allergic asthma. As previously described (18,19), we showed that PM significantly aggravate neutrophilic inflammation in the airways and induce the production of IL-1b ( Figures 5A-E) signaling to lung-resident IL-1R + Tgd17 cells.
In mice deficient for gd T cells (Tcrd -/mice) and IL-17 (Il17a/f -/mice), acute neutrophilic inflammation was significantly relieved (Figure 7 and Supplementary Figure 12). However, there were no noticeable differences in allergic immune responses between WT and Tcrd -/mice under chronic allergic conditions ( Figure  8). We speculate that this might be due to the efficient development of Th17 CD4 T cells that replace the requirement of Tgd17 cells in the chronic phase. Previous report showed that TLR ligands driven from microbiome can stimulate the production of IL-1b, leading to proliferation and activation of lung-resident gd T cells thereby further augment inflammatory responses (8). Other studies also suggested that commensal microbiomes are required to maintain IL-1R1 + Tgd17 cells (11,32). Thus, these findings suggest that the commensal microbiota can orchestrate the maintenance of peripheral gd T cells by stimulating TLR ligands and IL-1b production.
Although the relationship between the commensal microbiome and immune system has been extensively studied, there are only a few studies on the effect of microbiota on the development of gd T cells (6,7). Nonetheless, the use of multiple Tgd subsets with microbiome-related variations has not been addressed. Here, we identified that even though the commensal microbiota regulates the development of gd T cells, there are not much different Vg TCR repertoires between SPF and GF mice, except for the 3-week-old GF mice. We have previously observed that Vg1 + cells expand in SPF Vg4/6 KO mice with undefined mechanism (21). Based on this, we speculate that the expanded Vg1 + cells in 3-week-old GF mice might be due to the defective development of Tgd17 cells with TCR Vg4 or Vg6. However, further investigation is required to define the molecular and cellular mechanisms of Vg TCR plasticity.
We unexpectedly found that intestinal TgdN and Tgd1 cells have unique properties that they have fewer Vg1 + cells compared to those of other tissues (Supplementary Figures 3A, B). In addition, TgdN cells in only siLP showed tissue-resident property ( Figures 3C, D), suggesting that specialized gut environments, such as microbial community or metabolite dynamics, might influence their tissue residency. Interestingly, we found that the Vg TCR usage of TgdN exhibited similar patterns to that of peripheral Tgd1 cells, which is mainly composed of Vg1 + and Vg4 + cells except in siLP (Supplementary Figure 3A-B). These findings suggest the possibility that circulating TgdN cells differentiate into Tgd1 cells in the tissue.
Unlike MAIT cells, we showed that later exposure of microbial stimulation is sufficient for peripheral expansion and maintenance of gd T cells. Thymic development of gd T cells, except Tgd2 cells, was not affected by the microbiota, whereas mature MAIT cells are absent in the GF thymus (36,46). It is possible that there is a specific time window for thymic development of MAIT cells and later colonization is not sufficient to restore it. In contrast, iNKT cells were not affected at all in the thymus and periphery of GF mice (47,48), suggesting that innate T cells recognize different types of antigens for their thymic development and peripheral expansion, which requires further investigation. Unlike previously report (11), we observed only marginal difference of hepatic Tgd17 cells between SPF and GF mice (P = 0.051). Since hepatic gd T cells are dependent on gut microbiota, we speculate that this difference is due to the different gut microbiomes of different animal facilities.
In this study, we used ERM-CZ-100 and ERM-CZ120 as clinically relevant air pollutants (Sigma, PM10-like, i.e., < 10 um median aerodynamic diameter). Although the composition of PM varies from source to source, our study is consistent with previous reports on PM-induced neutrophilic inflammation (13,18,19). Here we show that PM induces acute airway inflammation by recruiting IL-1b-producing neutrophils, which activate IL-17-producing IL-1R + Tgd17 cells. This is consistent with previous report (49) and we speculate that Tgd17 cells secrete IL-17, which recruits additional neutrophils to the site of inflammation, thus providing more IL-1b by feedforward circuit.
In conclusion, our study has identified a crosstalk between the commensal microbiota and lung-resident Tgd17 cells, and provided a mechanistic insight into PM-induced acute neutrophilia. These findings suggest that targeting gd T cells could be a new therapeutic strategy for acute lung injury dominated by neutrophilic inflammation.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation, to any qualified researcher.

ETHICS STATEMENT
The animal study was reviewed and approved by Institutional Animal Care and Use Committee of POSTECH.

AUTHOR CONTRIBUTIONS
CY designed and performed experiments. D-IK performed parabiosis surgery. MK performed immunofluorescence. S-HI provided research interpretation. CY and YL analyzed data and wrote the manuscript. YL conceptualized the research. All authors contributed to the article and approved the submitted version.