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* Department of Molecular Biology and Pharmacology, Washington University, St. Louis, MO 63110; and
Center for Experimental Medicine, Institute of Medical Science, University of Tokyo, Tokyo, Japan
| Abstract |
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β and 
TCR+ T cells with a CD4+CD8– or CD4–CD8– phenotype. These cells concentrated within the CNS within 3 days of adoptive transfer, and appeared to play a role in EAE induction as adoptive transfer of Th1 lines derived from wild-type mice into IL-17-deficient mice induced reduced EAE clinical outcomes. This study demonstrates that an encephalitogenic Th1 cell line induces recruitment of host IL-17-producing T cells to the CNS during the initiation of EAE and that these cells contribute to the incidence and severity of disease. | Introduction |
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(1, 2). Further work demonstrated that skewing of these Th1 infiltrating cells to a Th2 type cytokine profile was sufficient to ameliorate EAE clinical symptoms (3, 4, 5, 6). A role for Th1 cells in EAE seemed clearly defined when adoptive transfer of Th1 polarized myelin Ag-specific T cells was shown to be sufficient to induce EAE (4, 6, 7). Finally, studies done with mice deficient in T-bet, a regulatory element required for Th1 responses, demonstrated the necessity of T-bet expression to the induction of EAE (8, 9).
Despite clear indications of encephalitogenic potential in myelin peptide-specific Th1 polarized T cell lines, the actual role of the iconic Th1 cytokine IFN-
in EAE is controversial. Significant evidence indicates that IFN-
could have either ameliorating or exacerbating effects on EAE clinical disease severity, with the effects observed largely depending on the clinical outcome tested and model system used (10, 11, 12, 13, 14, 15, 16, 17, 18). Studies performed in IFN-
-deficient mice revealed a significant exacerbation of disease. These effects were seen both as increases in EAE disease severity in classic EAE model systems and susceptibility to EAE in normally resistant mouse strains (13, 14, 18). As such, while there still remains some doubt as to the ultimate contribution(s) of IFN-
to EAE initiation and clinical severity, it is very clear that IFN-
is not absolutely required for disease induction, leading some to question whether Th1 cells are truly involved in EAE induction (19).
Recently, a great deal of excitement was generated by work that suggested a vital role for IL-17 in the induction of several autoimmune diseases, including EAE (19, 20, 21). Initial studies demonstrated a profound defect in the initiation of EAE in IL23 gene-knockout (KO) mice, associated with the loss of IL-17-producing, but not IFN-
-producing, T cells (19). These findings were recapitulated in IL1 receptor-deficient mice, with resistance to EAE induction again associated with a lack of IL-17-producing T cells (22). Examination of IL-17-deficient models revealed delays in EAE induction as well as significant decreases in EAE clinical scores (23, 24). Additionally, studies have demonstrated the capacity of T cells skewed to produce IL-17, termed Th17 cells, to induce EAE upon adoptive transfer (19). Indeed, following adoptive transfer, Th17 cells were found to induce significantly greater encephalitogenic responses on a cell-to-cell basis when compared with Th1 cells in the same experiment (19). Further examination of the T cells found within the CNS after active induction of EAE revealed both IL-17 and IFN-
producing cell populations. At least one study demonstrated cell populations that produced both cytokines simultaneously, with the dual nature of the cytokine production only inhibited by long term in vitro culture (25).
The finding that loss of a Th17 T cell population correlated with resistance to active induction of EAE was surprising given the apparent antagonism between Th17 and Th1 lineage development and the number of studies that have demonstrated EAE induction following adoptive transfer of myelin Ag-specific T cells cultured under Th1 biasing conditions. Initial studies demonstrated extremely small numbers of IL-17-producing T cells present within in vitro Th1 skewed encephalitogenic CD4+ T cell lines (22, 25). The apparent importance of IL-17-producing CD4+ T cells to EAE induction has led some to suggest that the small numbers of IL-17-producing cells present within a "Th1 line" play a vital role in a lines encephalitogenicity (19, 22). These findings are particularly interesting in light of previous studies that demonstrated the entry of very small numbers of adoptively transferred cells into the CNS shortly following transfer, with the majority of the transferred cells infiltrating at later time points (26, 27, 28). These small numbers of early entry "pioneer" cells have been suggested to play a vital role in EAE induction by conditioning the blood-brain barrier and CNS to allow the later entry of large numbers of encephalitogenic T cells (29).
In this study, we tested the hypothesis that the IL-17-producing T cells present within the adoptively transferred T cell population comprised the "pioneer" population of adoptively transferred cells and, therefore, would preferentially accumulate in the CNS relatively early following adoptive transfer. To address this hypothesis, we examined the cytokine production capabilities of T cells present within the CNS at various time points following adoptive transfer. Localization of IL-17 and IFN-
producing cell populations following adoptive transfer of Th1 polarized encephalitogenic cell lines demonstrated no significant accumulation of IL-17-producing transferred cells early after adoptive transfer or at any other time point tested. Instead, this study found that a host IL-17-producing T cell population was recruited to the CNS following invasion of encephalitogenic Th1 cells.
| Materials and Methods |
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C57BL/6J and congenic B6.PL-Thy1a/CyJ mice were purchased from The Jackson Laboratory. IL-17-deficient mice have been previously described (30). IL-17-deficient mice were bred and maintained in house in the Washington University Animal Care Facility. All experiments were approved by the Washington University Animal Care Committee and all mice were housed in an Association for Assessment and Accreditation of Laboratory Animal Care approved facility.
T cell lines
To generate myelin oligodendrocyte glycoprotein (MOG)- and OVA-specific cell lines, CD90.1 congenic B6.PL-Thy1a/CyJ mice were immunized s.c. with either 50 µg of MOG35–55 peptide (Sigma Genosys) or 900 µg of whole OVA (Sigma-Aldrich), respectively, emulsified in IFA and supplemented with 500 µg/ml Mycobacterium tuberculosis. CD4+ cells were isolated from the spleen as previously described (31). Cells were then stimulated at a concentration of 1 x 106 cells/ml in the presence of 5 x 106 cells/ml irradiated C57BL/6 splenocytes, 10 µg/ml MOG35–55 peptide, 10 units/ml IL-12, and 10 units/ml IL-2, in 10% FCS containing RPMI 1640 complete media. Cells were isolated after 7 days by Histopaque 1.077 (Sigma-Aldrich), then restimulated under the conditions outlined above, with the only exception being an absence of IL-12. Cells underwent two rounds of stimulation before being stored at –70°C in 10% DMSO containing FCS. Before usage, cells were thawed and restimulated for 7 days with MOG peptide and IL-2. Immediately before injection, cells were restimulated for 4 days with MOG peptide and IL-2, separated by Histopaque 1.077, then washed and resuspended in HBSS. To induce EAE, 2.5–5 x 106 cells in 300 µl of HBSS were transferred to 6–8-wk-old mice by retro-orbitial injection.
Flow cytometric analysis
Single-cell suspensions of mononuclear cells present with the CNS were produced as previously reported (32). Briefly, mice were perfused with 35 ml of ice-cold PBS via cardiac puncture. The brain and spinal cords of mice were removed and mechanically disrupted through a 40-µm filter. The resultant slurry was incubated for 1 h at room temperature with continual agitation in the presence of 100 µg/ml of type 1 collagenase, 10 µg/ml of the protease inhibitor tosyl-L-lysine chloromethyl ketone, and 1 µg/ml of DNase. Single mononuclear cell suspensions were then separated from the slurry via a percoll density gradient involving centrifugation and collection of cells accumulated at the interface of stacked 70 and 37% percoll solutions.
In all flow cytometric experiments, single-cell suspensions were stained for surface markers on ice for 20 min with continual agitation. Abs against CD3, CD4, CD8, CD25, CD44, CD45.2, CD62L, CD69, CD90.1, CD90.2, TCR β, and TCR 
were all obtained from eBioscience. Staining with anti-CD90.1 and anti-CD90.2 Abs was used to distinguish adoptively transferred and host T cells, respectively.
Intracellular cytokine staining
Intracellular staining was performed, with some modification, as previously described (33). In brief, 1 x 106 cells were restimulated on plates precoated with anti-CD3 (10 µg/ml PBS) (eBioscience) or by incubation with 50 ng/ml PMA and 1 µM ionomycin (Sigma-Aldrich), with concurrent blockade of cytokine secretion by treatment with 1 µg/ml brefeldin A (Sigma-Aldrich) for 2–4 h at 37°C. Cells were stained for cell surface molecules then fixed with 5% buffered formalin phosphate (5% paraformaldehyde in PBS) for 20 min at room temperature, washed with PBS, and permeabilized with PBS supplemented with 0.5% BSA, 0.1% sodium azide, and 0.1% saponin for 10 min at room temperature. After washing, the cells were stained with 5 µg/ml IL-17-PE or allophycocyanin and/or IFN-
-FITC or isotype controls for 5 min at room temperature with continual agitation. After washing four times, cells were immediately collected using a FACSCalibur (BD Immunocytometry Systems) and were analyzed using CellQuest software (BD Immunocytometry Systems).
Soluble factor activation
Single-cell suspensions of mononuclear cells present with the CNS were produced as described above. The transwell system was established by insertion of a .4-µm cell culture insert into the well of a 6-well plate. A total of 1 x 106 cells were placed in the upper chamber or lower chamber of the transwell as noted. The lower chambers were precoated with 10 µg/ml anti-CD3 in PBS or left uncoated. All lower chambers were blocked with 2% BSA containing PBS for 2 h at 37°C before addition of test cells. Following addition of cells to the transwell system the plate was incubated for 4 h at 37°C with concurrent blockade of cytokine secretion by treatment with 1 µg/ml brefeldin A, as described above. Cells were recovered from individual chambers and prepared for flow cytometric evaluation of intracellular cytokine production as outlined above.
Statistics
All statistics were performed using GraphPad Prism 4 software. Individual tests used are stated in figure legends. A value of p < 0.05 was considered significant.
| Results |
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Given the recent information regarding the necessity for IL-17 production in initiation of EAE, we decided to examine the capacity of previously characterized encephalitogenic MOG35–55-specific cell lines produced under Th1 polarizing conditions to produce IL-17. Cell lines were examined for IL-17 production 4 days after Ag-specific stimulation, immediately before adoptive transfer. As previously reported, following stimulation with either activating plate-bound anti-CD3 Ab (data not shown) or PMA and ionomycin, very few cells were found to produce IL-17 (Fig. 1A). In contrast, large numbers of the encephalitogenic cells were capable of producing IFN-
following stimulation (Fig. 1A).
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1 day of adoptive transfer (Fig. 1B). These cells were found to principally (64%) contain IFN-
-producing cells. As such, in this model, IL-17-producing, adoptively transferred cells are not preferentially recruited to the CNS at early time points. Further examination revealed that while IL-17-producing adoptively transferred cells were observed at later times points, the number of IL-17-producing adoptively transferred cells never increased over the number of IL-17-producing cells found in vitro, demonstrating a lack of preferential accumulation during EAE initiation, peak, and resolution of acute disease (Fig. 1B). Although adoptively transferred cells remained largely incapable of producing IL-17, within the CNS a host CD90.2+ T cell population was found to produce IL-17 after stimulation with either plate bound anti-CD3 (data not shown) or PMA and ionomycin (Fig. 2). At the peak of EAE clinical symptoms, host CNS IL-17-producing T cell populations were found to be increased greater than 3-fold over populations found in naive CNS (Fig. 2). Increases in the IL-17-producing host cells (IL17Hc) appeared early in EAE induction and remained throughout the peak and resolution of clinical disease (Fig. 2).
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It was possible that the IL-17-producing T cells might represent contaminating cells from the blood introduced during CNS harvesting. To address this issue we compared the numbers of IL17Hc in both the blood and CNS of control and EAE mice. Although the percentage of IL17Hc was similar in control CNS and the blood from both control and EAE mice, CNS isolates from EAE mice contained
3-fold greater numbers (Fig. 3). As such, the IL17Hc found in control animals may represent contaminating cells introduced during the CNS harvest, but the IL17Hc in EAE mice represent a significant concentration in cell number over cells found within the blood, and as such must be actively recruited to or produced within the CNS. These findings demonstrate the specific recruitment and/or production of IL-17-producing host cells within the CNS during EAE induction.
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TCR+ cells comprising
60% of the total IL17Hc population. Indeed, a large number (
40%) of the total 
TCR+ cells T cells found in the CNS during EAE were found to be IL17Hc. The remaining subset of the IL17Hc population was found to be positive for the TCR β-chain (Fig. 5). The majority of IL17Hc were also found to have a CD25–CD44midCD62L–CD69+ phenotype, suggesting recent activation of the cells (Fig. 5). All IL17Hc were found to be NK1.1–, suggesting that NKT cells play no significant role in host cell IL-17 production (Fig. 5). Additional examination found almost all of the IL-17-producing host T cells to be CD8– but a proportion (
13%) of IL17Hc to be CD4+ (Fig. 5). Together these data demonstrate the existence of a heterogeneous population of CD4+ and CD4–CD8– host IL-17-producing T cells within the CNS of mice with EAE.
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| Discussion |
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and
β TCR expressing cells that were enriched within the CNS during initiation and maintenance of EAE. These cells were found to be either CD4+ or CD4–/CD8–, and over all demonstrated a recently activated phenotype (CD69+CD25–). These data suggest that host T cells may play an important role in EAE induction by supplying necessary inflammatory cytokines that are not produced in large quantities by initiating encephalitogenic T cells. The knowledge that induction of EAE via adoptive transfer of Th1 skewed T cell lines results in recruitment of large numbers of host cells and/or activated T cells into the CNS has long been available, as early studies clearly demonstrated the phenomena (37, 38). Despite their presence, the role for CNS invading host T cells in initializing EAE pathogenesis has been trivialized due to findings that demonstrate the capacity of adoptively transferred T cell lines to induce disease in T cell-deficient lymphopenic mice (39, 40). However, recent studies have highlighted the pleiotropic effects that transfer into a lymphopenic system has on T cells, suggesting that the unique activating nature of the lymphopenic system could mask a loss of pathogenic capacity associated with lack of host T cell invasion (41, 42). Further, several studies have demonstrated exacerbation of EAE associated with cotransfer of non-CNS Ag-specific cells and encephalitogenic cells (37, 43). These studies demonstrate that host cells could play a role of great importance in EAE initiation and pathogenesis in lymphoid sufficient environments.
In this study, we examined the capacity of both resident host and adoptively transferred T cell populations to produce IFN-
and/or IL-17 following a short-term stimulation with an anti-CD3 stimulation or a phorbal ester/calcium ionophore combination. The small numbers of IL-17-producing cells we report here within the Th1 skewed cell line come as little surprise given the suggested role for IFN-
in negative regulation of the Th17 phenotype (44, 45). Indeed, the low incidence of IL-17-producing cells reported here was found without exception in multiple Th1 skewed cell lines specific for distinct Ags (Lees and Russell, unpublished data), suggesting that the methods used to isolate these lines result in low numbers of IL-17-producing cells without regard for antigenic specificity.
It has been suggested that, despite their low numbers, the IL-17-producing cells present within "Th1" skewed cell lines may play a vital role in the induction of EAE (19, 25). This idea is particularly attractive in light of models that predict a vital role for small numbers of "pioneer" cells that rapidly infiltrate and condition the CNS to allow the full scale T cell infiltration that proceeds EAE clinical manifestation (29). Further, the necessity for cells that constitute a small fraction of the total transferred T cell pool would explain much about the large number of transferred cells required to induce adoptively transferred EAE in many models. As such, the original hypothesis of this study was that the IL-17-producing population within our encephalitogenic T cell lines was acting as a "pioneering" T cell population for the induction of EAE. The data shown here demonstrate that the low numbers of IL-17 cells present within the cell line do not increase in number relative to the classic Th1 cells present within the cell line at any time point examined. These data demonstrate that no specific recruitment, proliferation, or induction of IL-17-producing adoptively transferred myelin-specific T cells occurs throughout EAE induction. As such, these cells are most likely not involved in early conditioning of the CNS for EAE development.
Instead, it is clear from our data that the increase in IL-17-producing cells that occurs within the CNS during adoptively transferred EAE comes almost entirely from host T cells recruited to the CNS following transferred cell invasion. Further, this study demonstrates that host T cell production of IL-17 plays a role in EAE pathogenesis. The IL17Hc are fairly heterogeneous, but the largest population consisted of 
TCR+ cells. A high percentage of 
TCR+ cells present within the CNS were found to produce IL-17 upon stimulation. The 
TCR+ IL17Hc reported here share many similarities with recently described 
TCR+ IL-17-producing cells involved in inflammation in inflammatory arthritis and chronic granulomatous disease (46, 47). These findings are particularly interesting when applied to earlier studies that reported significant amelioration of EAE in 
T cell-deficient mice (48, 49). The possibility that the defect in EAE induction associated with 
T cell deficiency may stem from a reduction in IL-17 contribution from these cells is very interesting, but will require more investigation to thoroughly test.
Although EAE conditions are clearly sufficient to allow accumulation of IL-17-producing T cells within the CNS, the mechanisms used in this process remain unknown. Indeed, it remains undetermined whether the increases in IL-17-producing cells observed reflect changes in trafficking pattern, cell retention, cell survival, proliferation, or some combination of these factors. Enhancement or inhibition of any of these factors could result in the accumulation observed.
Active recruitment of the IL-17-producing host cells through production of chemotactic factors could play a role in the accumulation observed. Several groups have demonstrated the modulation of chemokine expression within the CNS during inflammation (50). However, the concentration of IL-17-producing cells in relation to other host T cells infiltrating the CNS during EAE would necessitate chemotactic activation specific to the IL-17-producing cells. Thus, if increases in chemotactic factors do play a role, we would expect expression of chemokine receptors in IL17Hc not widely observed in other activated T cell populations.
The phenotypic markers of activation observed in experiment 5 suggest that the IL17Hc may be receiving an antigenic signal within the CNS. It is possible then that the invading adoptively transferred T cells are enhancing presentation of self Ag(s) to IL-17-producing T cells within the CNS. This could then modulate T cell survival and proliferation with resultant changes in cell number.
Ultimately, this study demonstrates the functional capacity of host cells to provide IL-17 during initiation and pathogenesis of adoptively transferred EAE. Further, this work demonstrates the participation of host IL-17 in EAE pathogenesis. Obviously, additional work will need to be performed to determine precisely what role the host IL-17-producing cells play in the induction of adoptively transferred EAE. In particular, it will be interesting to dissect any differences in a requirement for host cell IL-17 production under both normal and lymphopenic conditions. Further work will also be required to determine the antigenic specificities, and activation mechanisms that regulate host IL-17 production. These studies may provide great assistance in determining the exact mechanisms responsible for the IL-17-producing cell accumulation within the CNS itself, and the contributions of different T cell subsets to the initiation and maintenance of autoimmunity in the CNS.
| Acknowledgments |
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| Disclosures |
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| Footnotes |
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1 This work was supported in part by a National Multiple Sclerosis Society Fellowship, Grants RG 3314, RG 3732, and CA 1012 from the National Multiple Sclerosis Society, and a W. M. Keck Fellowship in Molecular Medicine at St. Louis. ![]()
2 Address correspondence and reprint requests to Dr. John H. Russell, Washington University, 660 South Euclid Avenue, St. Louis, MO 63110. E-mail address: jrussell{at}wustl.edu ![]()
3 Abbreviations used in this paper: EAE, experimental autoimmune encephalomyelitis; MS, multiple sclerosis; KO, knockout; WT, wild type; MOG, myelin oligodendrocyte glycoprotein. ![]()
Received for publication September 21, 2007. Accepted for publication April 10, 2008.
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