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T Cells Regulate the Extent and Duration of Inflammation in the Central Nervous System by a Fas Ligand-Dependent Mechanism1
Blood Research Institute, Blood Center of S.E. Wisconsin, Milwaukee, WI 53201
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Abstract |
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T cells have been shown to regulate immune responses associated with inflammation, but the mechanism of this regulation is largely unknown. Using the experimental autoimmune encephalomyelitis (EAE) model of the human CNS autoimmune disease multiple sclerosis, we demonstrate that T cells are important regulators of CNS inflammation. This was shown using T cell-deficient mice that were unable to recover from EAE. The chronic disease was accompanied by a prolonged presence of both macrophages and lymphocytes in the CNS. This extended inflammatory response was due to alterations in both cell proliferation and death. In mice lacking T cells, proliferation of encephalitogenic T cells was 3-fold higher, and caspase activity, indicating apoptosis, was 2-fold lower compared with those in control mice recovering from EAE. T cell-deficient mice reconstituted with wild-type T cells recovered from EAE and resolved inflammation in the CNS, whereas mice reconstituted with Fas ligand-dysfunctional T cells did not. Thus, T cells regulate both inflammation in the CNS and disease recovery via Fas/Fas ligand-induced apoptosis of encephalitogenic T cells, and a quick resolution of inflammation in the CNS is essential to prevent permanent damage to the CNS resulting in chronic disease. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionDisclosuresReferences |
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T cells have been shown to regulate a variety of immune responses, including those associated with autoimmunity (1, 2, 3). The prevailing view is that T cells bridge the innate and adaptive immune responses, primarily by exerting specific functions, which are determined by the tissue and local microenvironment in which they reside (3). Thus, they have been shown to perform multiple functions during the immune response, with the ability to both reduce and exacerbate inflammation (1, 2, 3). T cells constitute a small proportion of circulating lymphocytes, and during infectious disease or autoimmunity they have been shown to migrate into the injured site and are thought to regulate the nature of the inflammatory response (1, 2, 3).
Mechanisms of immunoregulatory functions by T cells include the production of chemokines and cytokines and cytotoxicity (2, 3). One of the best-studied cytotoxicity mechanisms in T cells is the induction of the Fas (CD95) apoptotic pathway in target cells. The engagement of Fas with its counterreceptor, Fas ligand (FasL),3 leads to cell death via apoptosis of the Fas-expressing target (4, 5). T cells have been shown to kill a variety of target cells via Fas/FasL. T cells are known to express FasL at sites of inflammation (6, 7, 8), including the CNS (9). Much of what is known about the functions of Fas/FasL have been learned using lpr and gld mice, which carry spontaneous mutations in the Fas and FasL genes, respectively (10). Mice carrying either mutation are susceptible to dysregulation of homeostasis within the immune system that can result in autoimmunity (10, 11).
In the animal model of the human autoimmune CNS demyelinating disease multiple sclerosis (MS) (12), experimental autoimmune encephalomyelitis (EAE), a role for both Fas and FasL has been indicated in the pathogenesis of disease (13, 14, 15, 16, 17, 18, 19). EAE is an inflammatory autoimmune disease of the CNS that is associated with an ascending paralysis, demyelination, and accumulation of cellular infiltrates containing primarily macrophages and 
T cells as well as B cells and T cells. EAE is induced by the priming of Th1 T cells to myelin self-Ags by immunization or by the adoptive transfer of myelin-specific encephalitogenic Th1 T cells (13). In EAE, using the adoptive transfer model, a role for Fas expression in the CNS has been shown to be important for the development of EAE (13, 14, 15). Several studies, including our own, also demonstrated a role for FasL expression in mice in the resolution of EAE disease symptoms (13, 14, 15). EAE disease symptoms induced by adoptive transfer in gld mice were more severe and were associated with the sustained presence of infiltrating cells during the late/chronic stage of disease, suggesting that a FasL-expressing cell in the host is required for the effective elimination of the infiltrating encephalitogenic T cells (13, 14, 15). The phenotype of the FasL-expressing cell could not be determined by the above studies.
A role for T cells in EAE has been suggested, but the mechanism of their regulation has not been elucidated because contradictory results have been reported when EAE was induced in T cell-deficient rodents. In B10.PL mice deficient in T cells, we have found that IFN- expression in the CNS is reduced during early EAE, and this correlated with an inability to recover from EAE (20). This result is consistent with several studies that also observed aggravation of the severity of EAE in T cell-deficient mice (21, 22); however, a reduction in the severity of EAE has also been observed (23, 24).
In the present study we show a careful analysis of the nature of the CNS inflammation associated with EAE in B10.PL mice deficient in T cells compared with control mice. In our past studies using the EAE model in B10.PL mice, we showed that EAE disease in B10.PL mice is associated with complete recovery from disease (13). However, in this study, when B10.PL mice were rendered deficient in T cells, the mice were unable to recover from EAE, exhibiting a long term chronic disease course that was accompanied by a prolonged presence of inflammatory cells in the CNS. In determining the mechanism of the extended inflammation, we found that the presence of higher numbers of inflammatory cells in the CNS of T cell-deficient mice was due to both enhanced cell proliferation and increased survival of encephalitogenic T cells. Reconstitution of T cell-deficient mice with wild-type (wt) T cells by bone marrow (BM) transplantation reversed both the chronic disease course and the sustained presence of inflammatory infiltrates in the CNS. In contrast, reconstitution with T cells from gld donor mice had no effect on either EAE disease parameter. Thus, these data suggest that T cells promote the resolution of inflammation in the CNS by inducing apoptosis of encephalitogenic T cells through a FasL-dependent mechanism. Furthermore, these data show the importance of a quick resolution of inflammation in the CNS, because a prolongation of only 10 days resulted in chronic disease, suggesting that permanent damage to the nervous system had occurred.
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Materials and Methods |
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B10.PL (H-2u) and B6.129P2-Tcrdtm1Mom mice were purchased from The Jackson Laboratory. The myelin basic protein (MBP)-TCR transgenic mice expressing a TCR transgene specific for the acetylated NH2-terminal peptide of MBP (Ac1–11) were generated as previously described (13). B10.PL-TCR–/– (TCR–/–) mice were produced in our breeding colony by backcrossing B6.129P2-Tcrdtm1Mom mice onto B10.PL for three generations, then intercrossing to generate homozygous mice carrying the indicated gene disruption. The B10.PL-gld mice were generated as previously described (13).
Peptides and Abs
The MBP Ac1–11 peptide (Ac-ASQKRPSQRSK) was generated as previously described (13). The anti-mouse Abs CD11b-PE, CD4-FITC, CD45-FITC, TCR-PE, CD25-PE, and CD69-PE and streptavidin (SA)-CyChrome were purchased from eBioscience. The anti-mouse Abs TCR-FITC, anti-V8.2-biotin, anti-FasL (Armenian hamster IgG1), anti-Fas, and anti-Armenian hamster IgG1-biotin were purchased from Pharmingen. Anti-mouse F4/80-biotin and SA-PE were purchased from Caltag Laboratories. Anti-mouse CD11b-PE-Cy5 and Armenian hamster IgG1 isotype control were purchased from Biolegend. Clone 2.4G2 was purchased from American Tissue Culture Collection.
EAE induction
EAE was induced by the adoptive transfer of MBP-specific encephalitogenic T cells generated as previously described (13). Briefly, 1 x 106 activated MBP-TCR T cells were i.v. injected into sublethally irradiated (360 rad) 5- to 8-wk-old B10.PL and TCR–/– mice. Individual animals were assessed daily for symptoms of EAE and scored using a scale from 1 to 5 as follows: 0, no disease; 1, limp tail and/or hind limb ataxia; 2, hind limb paresis; 3, hind limb paralysis; 4, hind and fore limb paralysis; and 5, death.
Mononuclear cell isolation and flow cytometry
Mononuclear cells were isolated from the CNS of B10.PL or TCR–/– mice with EAE on days 0, 7, 10, 15, 25, and 40 from mice perfused with 25–30 ml of cold PBS. The brains and spinal cords were homogenized, cell suspensions were incubated with 0.5 mg/ml collagenase type II (Sigma-Aldrich) at 37°C for 30 min, and mononuclear cells were isolated using 40/70% discontinuous Percoll gradients. Total cell numbers were determined by counting on a hemocytometer, and viability was assessed by trypan blue exclusion. For the purification of T cells, total mononuclear cells were isolated on day 18 after EAE induction and stained with anti- TCR-FITC and CD11b-CyChrome, and the CD11b– TCR+ cells were sorted using a FACSAria (BD Biosciences). Two-color flow cytometry using anti-CD45-FITC and anti-CD11b-PE or anti-CD4-FITC and anti-TCR-PE with anti-CD4-FITC was conducted using total mononuclear cell preparations. Three-color flow cytometry using anti-CD45-FITC, anti-CD11b-PE-Cy5, and anti-F4/80-biotin combined with SA-PE, CD11b-PE-Cy5 and TCR-FITC, TCR-FITC and anti-CD25-PE, or anti-CD69-PE were conducted using total mononuclear cell preparations. FasL expression was assessed using a three-step process as follows: 1) anti-TCR-FITC or anti- TCR-FITC and anti-CD11b-PE-Cy5 and anti-FasL, 2) anti-Armenian hamster IgG1-biotin, and 3) SA-PE. For all Ab stainings, FcR were first blocked with anti-mouse FcR (2.4G2). Ab incubations were conducted on ice, and the cells were fixed in 1% paraformaldehyde and analyzed using a FACScan or LSR II (BD Biosciences).
BrdU labeling
For labeling of proliferating cells in vivo, groups of four or five B10.PL or TCR–/– mice on days 10, 15, and 21 after EAE induction by adoptive transfer were injected i.p. with 1 mg of BrdU (Sigma-Aldrich) 14 h before isolation of brain mononuclear cells. Freshly isolated mononuclear cells from each experimental group were pooled and stained with anti-CD4-PE and anti-V8.2-biotin combined with SA-CyChrome. Subsequently BrdU incorporation into the cellular DNA was detected using the BrdU flow kit (BD Biosciences) according to the manufacturer’s instructions. Briefly, cells were fixed, permeabilized, treated with DNase, and incubated with anti-BrdU-FITC. The samples were kept on ice and immediately analyzed by three-color flow cytometry.
Caspase apoptosis assay
Analysis of apoptosis of V8.2+ T cells isolated from the CNS was performed using the caspase 3-specific fluorogenic substrate PhiPhilux-G2D2 purchased from OncoImmune. Freshly isolated CNS mononuclear cells from four or five B10.PL or TCR–/– mice on days 10, 15, and 21 after EAE induction by adoptive transfer were pooled and incubated with PhiPhilux-G2D2 dissolved in RPMI 1640 medium containing 10% FBS at 37°C for 1 h according to the manufacturer’s instructions. The cells were washed twice with ice-cold buffer containing 2% FCS and 0.05% sodium azide. FcR were blocked for 15 min on ice before staining with anti-V8.2-FITC. After washing, the cells were kept on ice and immediately analyzed by three-color flow cytometry. Necrotic and dead cells were excluded from the light scatter gate using propidium iodide. Manipulations for mononuclear cell isolation were performed on ice within 3–4 h using 40/70% discontinuous Percoll gradients without collagenase treatment. For the apoptosis of encephalitogenic T cells, MBP-TCR T cells used for EAE induction were labeled with 0.3 µM SNARF-1 (Molecular Probes) dissolved in PBS at 37°C for 30 min. After three washes, 20 x 103 cells were placed in round-bottom microtiter plates in the presence of medium alone, anti-Fas mAb (20 µg/ml), or 100 x 103 T cells isolated from the CNS of either B10.PL or B10.PL-gld mice 18 days after EAE induction. After culture for 4.5 h at 37°C and 5% CO2, the cells were labeled with PhiPhilux-G1D2 as described above, and SNARF-1+ target cells were gated and analyzed for fluorescence.
BM chimeras
Mixed BM chimeras were generated by transferring 4 x 106 total BM cells from B10.PL, TCR–/–, or B10.PL-gld mice into sublethally irradiated (360 rad) TCR–/– or B10.PL mice and were allowed to reconstitute for 6 wk.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionDisclosuresReferences |
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T cells exhibit a chronic EAE disease course
To assess the role of T cells in the EAE disease course of B10.PL mice (H-2u), we generated B10.PL mice deficient in T cells (TCR–/–). EAE was induced in B10.PL and TCR–/– mice by the adoptive transfer of Ac1–11-specific CD4 T cells generated from MBP-TCR transgenic mice (13). B10.PL and TCR–/– mice exhibited an identical early disease course, with 100% of the mice in each group succumbing to disease with an average day of onset on day 10 (Fig. 1 and Table I). After a 12-day disease course, including onset and the effector phase, B10.PL mice spontaneously recovered, with full recovery observed before day 40 (Fig. 1). In contrast, TCR–/– mice did not undergo any signs of recovery (Fig. 1). This lack of recovery is reflected in the average day 40 disease score of 1.8 for the TCR–/– mice, whereas the B10.PL mice had a score of 0.4 (p < 0.0001; Table I). Likewise, the cumulative disease score for TCR–/– mice was significantly higher (p < 0.0001) than that for B10.PL mice (58 vs 35, respectively; Table I). These data clearly demonstrate a potential regulatory role for T cells in the recovery from EAE.
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–/– mice during EAE
In addition to ascending paralysis, EAE is characterized by accumulation of inflammatory cells in discrete lesions in the CNS. In B10.PL mice with acute disease, the inflammatory infiltrate increases steadily throughout the effector phase of disease and then gradually disappears during the recovery phase. To determine whether chronic disease in TCR–/– mice was associated with increased cell numbers in the CNS, we isolated total mononuclear cells from the CNS of B10.PL and TCR–/– mice throughout the EAE disease course and quantitated the absolute number of infiltrating cells, macrophages, lymphocytes, and CD4 T cells. The cells were analyzed for the expression of CD11b, CD45, CD4, and TCR, and macrophages (CD11b+CD45high) were distinguished from lymphocytes (CD11b–CD45high) and resident microglial cells (CD11b+CD45low) by the expression level of CD11b and CD45. In normal control mice, very few macrophages (Fig. 2B) and lymphocytes (Fig. 2C) were present in the CNS before the induction of EAE. To confirm that a CD45high expression pattern detected macrophages, we directly compared CD11b+ cells for the expression of CD45 (Fig. 2E) or F4/80 (Fig. 2F) and found them to be identical. Of the few lymphocytes present, about one-half are CD4 T cells (Fig. 2D). Upon EAE induction in the B10.PL mouse, the absolute number of infiltrating cells in the CNS paralleled the EAE disease course (Fig. 1) and increased steadily until the peak of disease was reached on day 15 (Fig. 2A). As the mice recovered from disease symptoms, the number of infiltrating cells also declined, as seen on day 25, and cell numbers remained slightly elevated on day 40, even though disease symptoms had subsided (Fig. 1). TCR–/– mice also had a steady increase in the number of infiltrating cells, which reached maximum at the peak of disease on day 15 and was similar in number to that in B10.PL mice (Fig. 2A). However, the cellular infiltrate was sustained through day 25, and a statistically significantly larger number of cells remained in the CNS (p < 0.008) compared with B10.PL mice (Fig. 2A). In addition, the sustained infiltration was accompanied by sustained disease symptoms (Fig. 1). The cellular infiltrate in the CNS of TCR–/– mice eventually subsided and was not significantly different from that in control mice on day 40 (Fig. 2A). These data suggest that T cells play an important role in regulating the extent of the inflammatory response in the CNS. In addition to regulating inflammation during recovery, T cells may play a role early in disease before disease onset, as indicated by a reduced number of infiltrating cells in the CNS of TCR–/– mice on day 7, just before disease onset, compared with that in B10.PL mice (p = 0.05).
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T cells differentially regulated the extent of lymphocyte and macrophage populations in the CNS during EAE, we quantitated the number of each cell population and found that both macrophage (Fig. 2B) and lymphocyte (Fig. 2C) populations remained elevated on day 25 in TCR–/– mice compared with control mice, with p values of 0.05 and 0.02, respectively. To further differentiate the lymphocyte population, we quantitated the number of CD4 T cells in the CNS that were also retained in the CNS on day 25 at a statistically significant higher level compared with that in control mice (p = 0.004; Fig. 2D). We also confirmed that T cells were present in the CNS during EAE and exhibited a similar kinetic pattern of emergence and decline, with 18 ± 6, 40 ± 7, 20 ± 6, and 4 ± 2 x 103 T cells present in the CNS on days 7, 15, 25, and 40, respectively. T cells were not detectable in the CNS before EAE induction by flow cytometry; however, they were detectable by PCR (20).
Proliferation of encephalitogenic T cells in TCR–/– mice is sustained during the late stages of EAE
The sustained presence of T cells in the CNS of TCR–/– mice suggests that T cells function to regulate the extent and/or duration of the inflammatory response. We reasoned that the prolonged presence of T cells in the CNS could occur by two mechanisms, either enhanced or sustained cell proliferation and/or a delay or inhibition of cell death. To determine whether cell proliferation was altered, we compared T cell proliferation in the CNS in B10.PL control mice and TCR–/– mice by measuring BrdU incorporation. Mice were i.p. injected with BrdU 14 h before the isolation of total mononuclear cells from the CNS, and BrdU incorporation was measured by flow cytometry. We examined BrdU incorporation in the two major populations of T cells in the CNS: CD4+V8.2+ encephalitogenic T cells (Fig. 3, A–F) and CD4+V8.2– nonencephalitogenic T cells (Fig. 3, G–L). V8.2 is the TCR -chain expressed by the MBP-TCR transgenic T cells used to induce EAE. For V8.2+ T cells, the number of dividing cells was equal 10 days after EAE induction in B10.PL and TCR–/– mice (Fig. 3, A and D, respectively). In wt V8.2+ cells, the number of dividing cells was decreased by >50% on days 15 and 21 (Fig. 3, B and C, respectively), whereas the number did not substantially decrease in the TCR–/– mice (Fig. 3, E and F, respectively). This analysis was performed three times, and the average cumulative data are presented in Fig. 3M, showing that the decreased number of proliferating B10.PL V8.2+ T cells vs the same cell population in TCR–/– mice during the day 15 and 21 time points is a consistent observation. The reduction in T cell proliferation in B10.PL mice during recovery from EAE (Fig. 3M) is consistent with the loss of total CD4 T cells from the CNS during this time period (Fig. 2D). Likewise, the prolonged presence of CD4 T cells in the CNS on day 25 (Fig. 2D) is consistent with the sustained level of proliferation of encephalitogenic T cells in TCR–/– mice (Fig. 3M).
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T cells also regulated the proliferation of nonencephalitogenic T cells in the CNS, we analyzed BrdU incorporation in CD4+V8.2– T cells in the same mononuclear cell preparations. In both B10.PL and TCR–/– mice on day 10, nonencephalitogenic T cells underwent proliferation, but at a rate 
50% lower than encephalitogenic T cells (Fig. 3G). As with encephalitogenic T cells, the percentage of BrdU+ cells decreased during the EAE time course in both types of mice (Fig. 3, G–I and J–L). However, there was no difference in the percentage of proliferating cells in B10.PL compared with TCR–/– mice at any time point examined (Fig. 3, G–L). For the V8.2– T cells, the average of three experiments is shown in Fig. 3N and is consistent with the data shown in the contour plots. BrdU incorporation of CD4+ cells was not different in the spleens of the same B10.PL and TCR–/– mice (data not shown), indicating that T cells do not regulate bystander proliferation of nonencephalitogenic T cells, but, rather, specifically regulate the proliferation of the Ag-specific encephalitogenic T cells in the CNS.
Survival of encephalitogenic T cells in TCR–/– mice is enhanced during EAE
In addition to a higher level of encephalitogenic T cell proliferation, the sustained levels of CD4 T cells in the CNS of TCR–/– mice could also be due to increased cell survival. This possibility was examined by measuring the level of caspase activity in encephalitogenic T cells during the EAE disease course. The presence of caspase activity indicates that an apoptotic pathway has been activated, and the cell is in the process of undergoing cell death. Caspase activity was examined by flow cytometry using the caspase 3-specific substrate PhiPhilulx-G2D2, which becomes fluorescent upon cleavage by activated caspase 3 (25). The negative control consisted of unstained cells without the addition of PhiPhilulx-G2D2 as shown for the day 10 point in B10.PL and TCR–/– mice (Fig. 5, B and F, respectively). When caspase activity was examined on day 10 after EAE induction, >50% more encephalitogenic T cells in B10.PL wt mice compared with TCR–/– mice were positive for caspase activity (Fig. 4A), a difference that is statistically significant (p = 0.03). A less pronounced difference was observed on day 15 (Fig. 4A). However, again on day 21, there was a statistically significance difference in caspase activity (p = 0.04), with 25% of the encephalitogenic T cells in B10.PL mice undergoing apoptosis and 14% in TCR–/– mice (Fig. 4A). A representative experiment of three is shown in Fig. 4, B–I, demonstrating the fluorescence intensity of the cleaved caspase substrate in V8.2+ encephalitogenic T cells on day 10 (C and G), day 15 (D and H), and day 21 (E and I) in B10.PL and TCR–/– mice after EAE induction, respectively.
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8.2+ cells in the CNS are also CD4+, making any contribution of CD8 T cells to the caspase analysis minimal. In addition, no difference was observed in caspase activity in CD4 T cells in the spleens of B10.PL and TCR–/– mice with EAE (data not shown). Thus, these data suggest that the sustained presence of T cells in the CNS of TCR–/– mice during the timeframe of recovery for B10.PL mice is a combination of higher cell proliferation (Fig. 3M) and lower rate of cell death (Fig. 4A) compared with the same cells in B10.PL mice.
Recovery from EAE and resolution of cellular infiltrates in CNS requires expression of FasL by T cells
The reduction in caspase activity in TCR–/– mice on day 21 after the induction of EAE indicates that the elimination of encephalitogenic T cells in the CNS occurs by apoptosis. However, the mechanism of apoptotic cell death could not be determined because caspase is activated by a variety of signals leading to apoptosis, including Fas/FasL. To determine whether T cells could be mediating the apoptosis of encephalitogenic T cells via a Fas/FasL mechanism, we reconstituted TCR–/– mice with T cells that were able to express either functional or dysfunctional FasL (gld). This was accomplished by generating mixed BM chimera mice by transplanting sublethally irradiated TCR–/– mice with BM from either wt B10.PL (wt
TCR–/–) or B10.PL-gld (gldTCR–/–) mice. In these chimera mice, the emerging T cell populations will express either wt FasL (B10.PL donor BM) or a nonfunctional FasL (gld donor BM), whereas the T cells will be mixed, with 50% of the cells being recipient in origin (data not shown). T cell reconstitution was evident 4 wk post-transplant, as indicated by the presence of T cells in the intestinal intraepithelial lymphocytes cell population (data not shown).
First, we determined whether the reconstitution of T cells in TCR–/– mice would revert their chronic EAE disease course (Fig. 1). As shown in Fig. 5A, TCR–/– mice reconstituted with BM from B10.PL wt mice (wtTCR–/–) were able to recover from disease symptoms and exhibited a similar disease course compared with the control chimeras, where B10.PL mice were transplanted with B10.PL BM (wtwt). In contrast, TCR–/– mice reconstituted with BM from gld mice were unable to resolve EAE disease symptoms and exhibited a chronic disease course similar to that observed in TCR–/– mice reconstituted with BM from TCR–/– (TCR–/–TCR–/–; Fig. 5A). These data show that recovery from EAE requires the expression of FasL by T cells. We next examined whether T cells in the CNS of mice with EAE expressed FasL, and found that T cells isolated from the CNS of mice with EAE during the recovery phase of disease expressed detectable FasL (Fig. 5E) at a level slightly higher than that expressed by T cells in the CNS of the same mice (Fig. 5B). Furthermore, we found that the number of T cells expressing CD25 (Fig. 5C) was 2-fold greater than that of T cells (Fig. 5F), whereas the level of CD69 was identical in the two cell populations (Fig. 5, D and G, respectively). These results are consistent with those of the Brosan laboratory (9).
To determine whether FasL expression by T cells is also required for the resolution of inflammatory infiltrates in the CNS during EAE, we generated identical BM chimeras as those shown in Fig. 5, isolated mononuclear cells from the CNS of chimera mice 25 days after EAE induction, and quantitated and phenotypically characterized the cellular infiltrate. We choose the day 25 point, because it is at this point in the EAE disease course that a statistically significantly greater number of mononuclear cells remain in the CNS of TCR–/– mice compared with control mice (Fig. 2A). As shown in Table II, only chimera mice, which either lacked T cells (TCR–/–TCR–/–) or were reconstituted with FasL dysfunctional T cells (gldTCR–/–) did not show signs of recovery, as indicated by the identical disease score of 2 ± 0.1. In contrast, the control chimeras, in which B10.PL mice were transplanted with either B10.PL BM (wtwt) or gld BM (gldwt), showed signs of recovery, with day 25 disease scores of 1 ± 0.2 and 1.25 ± 0.25, respectively (Table II). The gldwt chimeras are a control for the mixed populations of T cells in the chimeras generated by transplantation with gld BM, resulting in 50% of the cells being from the gld donor BM. Because these chimeras are able to recover, the lack of recovery in the gldTCR–/– chimeras is not due to a loss of FasL function in the endogenous T cells or to technical aspects of chimera generation. In addition, TCR–/– mice transplanted with wt BM (wtTCR–/–) were recovering and had a disease score of 1.1 ± 0.3 (Table II).
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T cells or reconstituted with gld T cells had similar numbers of all the characterized cell types in the CNS (Table II). Similarly, all control chimeras that exhibited recovery also had similar numbers of the characterized cell populations in the CNS (Table II). In a direct comparison, the recovering chimeras had 2.4- to 2.9-fold fewer total mononuclear cells, 2.9- to 4.2-fold fewer macrophages, 2- to 4-fold fewer lymphocytes, and 2.2- to 4.2-fold fewer CD4 T cells in the CNS on day 25 after EAE induction compared with TCR–/– mice reconstituted with gld BM (Table II). When we examined encephalitogenic T cells (CD4+V8.2+) in the chimeras, we found a 2.7- to 5.4-fold reduction in cell numbers in the control chimeras compared with the gldTCR–/– chimera mice (Table II). In addition, there was a statistically significant difference in all parameters examined between the gldTCR–/– and wt TCR–/– chimeras (Table II). The increased number of encephalitogenic T cells in the CNS of gldTCR–/– chimera mice on day 25 is consistent with the reduced apoptosis of these cells observed on day 21 (Fig. 4A).
We next confirmed that T cells isolated from the CNS of mice with EAE have the capacity to induce the apoptosis of encephalitogenic T cells. By measuring caspase activity, we determined that in vitro activated encephalitogenic T cells alone had a 3% background level of apoptosis (Fig. 5H) and that incubation with anti-Fas increased apoptosis to 24% (Fig. 5I). The addition of wt CNS T cells resulted in 14% of the encephalitogenic T cells undergoing apoptosis (Fig. 5J). This was reduced to 7% when T cells from gld mice were added (Fig. 5K). The 4% apoptosis above background with the addition of gld T cells (Fig. 5K) is probably due to other known T cell killing mechanisms, such as lymphotoxin or perforin, which are intact in gld T cells (2). These cumulative data demonstrate that T cells express a functional FasL that is required for recovery from EAE and for timely resolution of the cellular infiltrate in the CNS of mice with EAE.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionDisclosuresReferences |
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T cells could regulate inflammation associated with an immune response in the absence of an infectious pathogen. We observed that T cells regulated the number of inflammatory cells in the CNS both early and late in the EAE disease course. Early in disease, mice lacking T cells had both reduced numbers of mononuclear cell infiltrates and apoptotic cells. Our data also suggest that T cells regulate the resolution of inflammation late in disease, because mice deficient in T cells had a prolonged presence of mononuclear cell infiltrates in the CNS and were unable to recover from EAE. The inability to down-regulate the inflammatory infiltrate was found to be due to decreased apoptotic cell death of encephalitogenic T cells. The higher survival rate of the encephalitogenic T cells was accompanied by an increased percentage of proliferating cells. Finally, we show that the timely resolution of inflammation in the CNS during EAE is dependent upon expression of a functional FasL by T cells. Thus, T cells regulate inflammation in the CNS during EAE by promoting the death of encephalitogenic T cells via the Fas/FasL apoptotic pathway.
For our study examining the role of T cells in regulation of inflammation, we choose EAE in B10.PL mice as our model system because of the predictable and well-characterized inflammatory response that occurs in the CNS. In addition, by inducing EAE by the adoptive transfer of encephalitogenic T cells primed in vitro, we were able to eliminate any response or effect of the Mycobacterium tuberculosis in CFA on either or T cells. We believed that this was important because T cells have been shown to recognize and respond to mycobacterial heat shock proteins (1, 3, 26). In addition, TCR–/– mice infected with a low dose of M. tuberculosis exhibited an altered disease, manifested as a pyogenic form of the granulomatous response instead of the lymphocytic response seen in controls (27). Also, cross-talk between and T cells is indicated in vivo (28), with each having the capacity to regulate the responses of the other. Thus, the presence of CFA could nonspecifically activate and mobilize a population of regulatory T cells that could influence both T cell priming and inflammation.
One common theme among many studies examining the roles of T cells in a variety of infectious disease models is the ability of T cells to regulate the nature of cellular infiltration into the site of infection. This includes infection with Mycobacterium (27) or Listeria (29, 30), where lesions associated with disease were altered in the absence of T cells. Also, in a study examining neurocysticercosis, the number of infiltrating mononuclear cells was reduced in the brains of TCR–/– mice (31). These data are consistent with our observation that the absolute numbers of infiltrating lymphocytes and macrophages were reduced early in the EAE disease course in TCR–/– mice (Fig. 2). In addition, in a study by Rajan et al. (32), lower levels of leukocytes in the CNS were also observed early in the EAE disease course, although the phenotype of the cells was not examined. Collectively, these observations suggest that T cells are functionally able to specifically regulate the migration of inflammatory cells into the site of tissue injury. Presumably, if fewer cells were able to infiltrate the CNS, then a less severe EAE disease course could be the outcome, as was observed in several studies (24, 32). However, in our study using adoptive transfer in the B10.PL TCR–/– mouse, we saw no alteration in the early EAE disease course parameters of day of onset and peak disease score (Table I), even though reduced numbers of mononuclear cells were observed in the CNS (Fig. 2). The difference between our study and others using adoptive transfer is the source and potency of the encephalitogenic T cells. Our encephalitogenic T cells are sufficiently potent to induce disease with 5–15 and 50 times fewer cells than the studies by Spahn et al. (24) and Rajan et al. (32), respectively. Thus, it is likely that we observed a normal early EAE disease course in TCR–/– mice, because our model requires fewer encephalitogenic T cells to induce disease.
During EAE onset, the absolute number (Fig. 2) and percentage of proliferating T cells (Fig. 3) in the CNS are similar in TCR–/– and wt mice. In contrast, the percentage of apoptosing T cells was 50% less in TCR–/– mice (Fig. 4). One possible mechanism for how similar numbers of T cells are present in the CNS when fewer cells are undergoing cell death in the TCR–/– mouse is a decrease in the level of infiltration into the CNS. It has been shown in a model of neurocysticercosis that T cells may promote the infiltration of lymphocytes and macrophages into the CNS by secretion of cytokines and chemokines (31), which enhance the migration of immune cells from the periphery into the CNS. One important candidate cytokine that may regulate immune cell migration into the CNS and apoptosis of T cells is IFN-. We have found that the level of IFN- mRNA was significantly reduced in the CNS of TCR–/– mice during EAE onset (20). IFN- induces the expression of the chemokine IFN-inducible protein-10, a cytokine known to be up-regulated in the CNS in both EAE and MS (33, 34). A reduced migration of encephalitogenic T cells into the CNS of T cell-deficient mice could explain why in some models of EAE induction, disease was either reduced or suppressed (24).
A reduction in the percentage of apoptosing encephalitogenic T cells in TCR–/– mice compared with control mice was consistent throughout the EAE disease course, even during recovery on day 21 (Fig. 4). In addition, the level of proliferation of the encephalitogenic T cells on day 21 remained at a higher level than that observed in control mice (Fig. 3). The higher level of proliferation is probably due to the greater number of macrophages found in the CNS serving as APC. The combined effect of decreased cell death and a higher level of proliferation is consistent with the 7-fold greater number of CD4 T cells present in the CNS on day 25. These data suggest that T cells promote the apoptosis of encephalitogenic T cells in the CNS during the recovery phase of EAE. One possible mechanism that we explored by which T cells could induce the apoptosis of encephalitogenic T cells is through Fas/FasL. This possibility was consistent with our previous finding that EAE was more severe in mice expressing a dysfunctional FasL (gld) than in wt mice (13). In our previous study we were not able to identify the FasL-expressing cell required for a normal EAE disease course. However, by reconstituting TCR–/– mice with T cells from either wt or gld mice, we were able to demonstrate that both the recovery from EAE (Fig. 5) and the timely resolution of cellular lesions (Table II) were dependent upon the expression of a functional FasL by T cells. Our data demonstrating a role for the Fas/FasL pathway in recovery from autoimmunity are consistent with the role of T cell FasL-induced apoptosis of target cells expressing Fas in Borrelia-induced Lyme arthritis in humans (35) and in a mouse model of myocarditis caused by viral infection (6).
In our model, the persistence of neurological clinical symptoms in TCR–/– mice was correlated with a sustained presence of both macrophages and lymphocytes in the CNS on day 25. However, even though T cell-deficient mice were able to eventually resolve the inflammation (Fig. 2), they remained clinically sick (Fig. 1). The lack of clinical recovery may be explained by two mechanisms: 1) neuronal tissues suffer nonrepairable damage during the sustained inflammation; or 2) T cells promote the survival and repair of neuronal tissues during EAE. In addition, T cells seem to exert regulation of inflammation specifically in the CNS, because we did not find a difference in the rate of apoptosis and proliferation of CD4 T cells in spleen of TCR–/– mice with EAE (data not shown). Thus, the regulation of inflammation in the CNS is complex and may involve the interaction of T cells with a variety of cells from both the immune and nervous systems.
Our data suggest that T cells have the capacity to regulate inflammation at multiple levels during the EAE disease course. Early in disease, T cells may regulate the infiltration of cells into the CNS. Late in the disease course, they regulate the duration of the inflammatory response, which when dysregulated in the TCR–/– mice results in chronic disease. This regulation seems to be an innate function of T cells, because similar functions have now been observed under a variety of inflammatory conditions, including as detailed in this study autoimmunity. Because the control of inflammation in the CNS of MS patients is often a therapeutic strategy, and many T cells do not recognize ligands in a similar manner as the pathogenic CD4 T cells, treatment modalities directly targeting T cells may not only be feasible but also effective therapies for MS.
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1 This work was supported in part by a grant from the Wadsworth Foundation and Grant RG 3299-A-2 from the National Multiple Sclerosis Society. 
2 Address correspondence and reprint requests to Dr. Bonnie N. Dittel, Blood Research Institute, Blood Center of S.E. Wisconsin, P.O. Box 2178, 8727 Watertown Plank Road, Milwaukee, WI 53201-2178. E-mail address: bdittel{at}bcsew.edu
3 Abbreviations used in this paper: FasL, Fas ligand; BM, bone marrow; EAE, experimental autoimmune encephalomyelitis; MBP, myelin basic protein; MS, multiple sclerosis; MBP, myelin basic protein; SA, streptavidin; wt, wild type.
Received for publication July 8, 2004. Accepted for publication February 1, 2005.
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) and 20 TCR
) and TCR
) mice as described in Fig. 1
), gld