Invariant NKT Cells Regulate Experimental Autoimmune Encephalomyelitis and Infiltrate the Central Nervous System in a CD1d-Independent Manner

Lennart T. Mars, Anne-Sophie Gautron, Jan Novak, Lucie Beaudoin, Julien Diana, Roland S. Liblau and Agnès Lehuen
J Immunol August 15, 2008, 181 (4) 2321-2329; DOI: https://doi.org/10.4049/jimmunol.181.4.2321

Abstract

Invariant NKT cells are CD1d-restricted T cells specific for glycolipid Ags. Their activation or transgenic enrichment abrogates the development of experimental autoimmune encephalomyelitis (EAE). Herein, we demonstrate that in NKT-enriched mice the protection from EAE is associated with the infiltration of NKT cells in the CNS and the local expression of CD1d. This indicates that the CNS acquires the potential for local glycolipid presentation when exposed to inflammatory stress, permitting the triggering of NKT cells. To address the importance of CD1d-mediated Ag presentation, we used transgenic mice that express CD1d solely in the thymus. Interestingly, enrichment of NKT cells in these mice also conferred resistance to EAE, with an efficacy indistinguishable from that of NKT-enriched CD1d-sufficient mice. This protection was due to an abrogation of the encephalitogenic Th1 and Th17 response in the spleen, revealing that endogenous glycolipid presentation is dispensable for the regulatory function of NKT cells in EAE. Moreover, abrogating extrathymic CD1d expression failed to affect both the recruitment of NKT cells and their effector phenotype. CNS-infiltrating NKT cells were characterized by a cytotoxic IFN-γhighIL-4lowIL-10lowgranzyme Bhigh profile, irrespective of the local expression of CD1d. Glycolipid Ag presentation is therefore dispensable for the control of autoimmune demyelination by NKT cells, underlining the importance of alternative cognate and/or soluble factors in the control of NKT cell function.

Type I NKT cells are nonconventional T cells that recognize self as well as exogenous glycolipids contained within the hydrophobic groove of CD1d. NKT cells recognize the CD1d/lipid complex by a semiinvariant TCR. This TCR comprises a canonical Vα24-Jα18 rearranged TCR α-chain combined with a Vβ11 TCR β-chain in humans, and a Vα14-Jα18 α-chain paired with a restricted set of β-chains in mice (1). NKT cells share their thymic origin but differ developmentally, phenotypically, and functionally from conventional MHC-restricted T cells. NKT cells arise from common thymic precursors due to the stochastic expression of their invariant TCR, which instructs their commitment to the NKT lineage (1). Presentation of self ligands, such as isoglobotrihexosylceramide (iGb3) or other glycolipids (2, 3, 4), by double-positive (DP)5 thymocytes in the cortex triggers NKT cell selection and differentiation, independent of thymic epithelial cells and dendritic cells (DCs) (5, 6, 7). Committed CD4+ single-positive (SP) NKT precursors undergo intrathymic proliferation and CD4 down-modulation before intrathymic maturation into bona fide NKT cells expressing the characteristic effector/memory phenotype (8, 9, 10, 11). As a result, 30–50% of NKT cells are CD8−CD4− double negative (DN), while the remaining NKT cells retain CD4 expression (1). The acquisition of NK lineage receptors occurs in peripheral tissues following thymic egress (11, 12). Interestingly, maturation of NKT cells is crucially dependent on CD1d expression by DP thymocytes. This restricted CD1d expression is sufficient for NKT cell development, peripheral persistence, which requires IL-15, as well as their acquisition of NK markers and their effector/memory phenotype (5, 6, 13). Functionally, NKT cells contribute to microbial and tumor immune surveillance, as well as immune regulation, due to their capacity to interact with both innate and adaptive immune cells.

Multiple sclerosis (MS) is a disabling disease of the CNS characterized by multifocal white matter lesions associating a prominent inflammatory infiltrate with demyelination, substantial axonal damage, and glial scar formation (14). These neuropathological features are reproduced in animal models of experimental autoimmune encephalomyelitis (EAE) induced by deliberate immunization with myelin proteins such as myelin basic protein (MBP), proteolipid protein (PLP), or myelin oligodendrocyte glycoprotein (MOG) (14). However, in MS, T cell and Ab reactivity have equally been shown to lipids, which make up 70% of the myelin sheath (15, 16, 17, 18, 19, 20, 21). Lipid Ags are commonly presented to T cells in the context of CD1 (22). White matter lesions in MS induce CD1 expression; CD1a is observed on infiltrating APCs, CD1b is expressed on microglia, while CD1d is up-regulated on microglia and reactive astrocytes (23, 24, 25). Unlike the other isoforms, CD1d interacts preferentially with NKT cells. MS patients generally exhibit a reduction in the frequency of circulating NKT cells, yet transient increases in NKT cell numbers are observed during clinical relapses in the cerebrospinal fluid and occasionally in the peripheral blood (26, 27, 28). The potential antiinflammatory benefit of these cells in MS is suggested by a higher IL-4/IFN-γ ratio among CD4+ NKT cells during the following remission (28). Animal models support a beneficial role for NKT cells during CNS autoimmunity. Their CD1d-dependent activation using exogenous ligands, such as α-galactosylceramide (α-GalCer) or OCH, delays the onset and reduces the severity of EAE (29, 30, 31, 32). Alternatively, EAE can be prevented by the transgenic enrichment of the NKT cell population (33). Although both approaches attenuate the encephalitogenic T cell response, their effector mechanisms differ. Treatment with exogenous glycolipids is CD1d-dependent and relies on myelin-specific immune deviation mediated by IL-4 and IL-10, with an additional role for IFN-γ (34). In mice enriched in NKT cells the regulation of the encephalitogenic T cell response occurs after T cell priming and does not involve immune deviation or IL-4 (33). Whether this regulation requires CD1d-mediated triggering of NKT cells is currently unknown.

Herein we demonstrate that the protection of EAE in NKT-enriched mice is associated with NKT cell infiltration of the CNS and concomitant local expression of CD1d. Using mice that lack extrathymic CD1d, we addressed the necessity for NKT cell reactivation by lipid-Ag presentation. Our data indicate that the protection from EAE afforded by the enrichment of NKT cells is independent of extrathymic CD1d expression.

Materials and Methods

Mice

The transgenic Vα14-Jα18 nonobese diabetic (NOD) mice have already been described (35). Transgenic NOD mice that express CD1d exclusively on DP thymocytes were generated by expressing CD1d under the control of the proximal Lck (pLck) promoter and crossing these mice with CD1d−/− NOD mice obtained by >15 backcrosses with NOD (6, 36, 37). Five transgenic pLck-CD1d lines were selected and crossed with Vα14-Jα18 transgenic (Tg) NOD mice to generate Vα14-Jα18 Tg pLck-CD1d CD1d−/− NOD mice (named Vα14 pLck). Two lines, Vα14 pLck436 and Vα14 pLck473, were selected for the present study. All mice used in this study were raised and housed under specific pathogen-free conditions. All experimental protocols were approved by the local ethics committee on animal experimentation and are in compliance with European Union guidelines.

Induction and clinical evaluation of EAE

EAE was induced by the immunization of 6- to 12-wk-old mice s.c. with 100 μg of MOG35–55 peptide (MEVGWYRSPFSRVVHLYRNGK) (Mimotopes) emulsified in CFA (Difco) and supplemented with 5 mg/ml Mycobacterium tuberculosis (strain H37RA) (Difco). Pertussis toxin (List Biological Laboratories) was injected i.v. at day 0 (200 ng) and day 2 (400 ng) postimmunization. Disease severity was scored daily on a 5 point scale: 1, tail atony; 2, hind limb weakness; 3, hind limb paralysis; 4, quadriplegia; 5, moribund or death.

CNS-infiltrating mononuclear cell purification

Brain and spinal cord were dissected from mice perfused transcardially with PBS. Brain tissue was dissociated by passages over three consecutive cell strainers of 150, 100, and 40 μm. Single-cell suspensions were resuspended in 30% Percoll and deposited on 70% Percoll. The interface was collected after a 20 min centrifugation at 3000 rpm and washed twice in medium.

RNA isolation

Isolated cells were resuspended in RNAprotect cell reagent (Qiagen) and kept at −80°C until use. RNA was extracted using a RNeasy Plus mini kit and QIAshredders according to the manufacturer’s instructions (Qiagen). The quality of the RNA was assessed by capillary electrophoresis (Agilent 2100 bioanalyzer, Agilent Technologies) and by spectrometry (λ260/λ280) before analysis. Then, 0.1 microgram of total RNA was converted into a representative cDNA pool using the SuperScript III first-strand synthesis system and random hexamer primers, according to the manufacturer’s instructions (Invitrogen). cDNAs were stored at −80°C until use.

Quantitative PCR primers and gene expression analysis

The following published primer pairs were used: granzyme B (GrB), forward 5′-CGATCAAGGATCAGCAGCCT-3′, reverse 5′-CTTGCTGGGTCTTCTCCTGTTCT-3′; hypoxanthine phosphoribosyltransferase (HPRT), forward 5′-TGACACTGGTAAAACAATGCAAACT-3′, reverse 5′-AACAAAGTCTGGCCTGTATCCAA-3′; IFN-γμ, forward 5′-TCAAGTGGCATAGATGTGGAAGAA-3′, reverse 5′-TGGCTCTGCAGGATTTTCATG-3′; IL-4, forward 5′-ACAGGAGAAGGGACGCCAT-3′, reverse 5′-GAAGCCCTACAGACGAGCTCA-3′; IL-10, forward 5′-GGTTGCCAAGCCTTATCGGA-3′, reverse 5′-CTGTCATCGATTTCTCCC-3′; IL-17, forward 5′-CCACGTCACCCTGGAACTCTC-3′, reverse 5′-CTCCGCATTGACACAGCG-3′ (38, 39, 40). The housekeeping gene HPRT was used as an endogenous control for each sample to normalize for differences in cDNA content or quality. Quantitative PCR was performed using the ABI Prism 7000 sequence detection system (Applied Biosystems). Each reaction was performed in 25 μl volume in a 96-well plate (Applied Biosystems) containing 1 μl of cDNA, 12.5 μl of SYBR Green I master mix 2× (Eurogentec), 30 nM of yeast tRNA (Sigma-Aldrich), and 300 nM each of the forward and reverse primer (Invitrogen). Conditions for the PCR were 2 min at 95°C, and then 40 cycles each consisting of 15 s at 95°C and 1 min at 60–62°C.

The ΔΔCt calculation for the relative transcript quantification was applied as follows: ΔΔCt = (Cttarget gene − CtHPRT)x − (Cttarget gene − CtHPRT)y, where x is the sample to analyze and y is an arbitrarily chosen control sample. Results for each sample were expressed as fold changes in target gene copies, with the amount of target transcripts = 2−ΔΔCt. Two independent experiments were conducted for each gene and sample. In each experiment, each sample was tested in duplicate wells.

Flow cytometry

After Fcγ receptor blockade with the 2.4G2 mAb, surface staining was performed with the following mAbs: anti-αβ TCR (H57-597), anti-CD4 (RM4-5), anti-CD11c (HL3), anti-CD11b (Mac 1), anti-CD19 (1D3), anti-CD1d (1B1), Thy1.2 (53-2.1), MHC-II (39-10.8, crossreacts with I-Ag7), and CD45 (30-F11). Biotinylated CD1d:α-GalCer tetramers were prepared as previously described (41). Second-step stainings were performed with streptavidin-allophycocyanin, streptavidin-PerCP, and streptavidin-PE-Cy7. Cells were acquired with a FACSCalibur or FACSAria flow cytometer (BD Biosciences) and analyzed using CellQuest or FlowJo software. For cell sorting, cells were passed through a FACSAria flow cytometer (BD Biosciences) using FACSDiva software.

Analysis of NKT cell function in transgenic lines

To analyze NKT cell function in Vα14 pLck lines (Vα14-Jα18 Tg pLck-CD1d CD1d−/− NOD mice), splenocytes were cultured in 96-well plates (5 × 105cells/well) in RPMI 1640 containing 10% FCS, 2-ME, and penicillin-streptomycin. Cells were stimulated with Con A (4 μg/ml) (ICN Immunobiologicals) or with α-GalCer (100 ng/ml). When indicated, irradiated splenocytes (105/well) from CD1d+/+Cα−/− NOD mice were added. After 48 h of culture, supernatants were harvested and IL-4 and IFN-γ were measured by sandwich ELISA as previously described (35). Recombinant mouse IL-4 and IFN-γ were from R&D Systems.

Assessment of the MOG35–55 recall response of CD4+ T cells in vitro

To analyze the anti-MOG response, CD4+ T cells were purified in three steps. First, splenocytes were depleted of macrophages, granulocytes, and CD8+ T cells by incubation with the following Abs: anti-CD11b (Mac-1), anti-Gr1 (8C5), and anti-CD8 (Lyt-2) and subsequently with magnetic beads coupled to anti-rat Ig (Dynal Biotech, Invitrogen). Second, B cells were depleted using anti-mouse Ig (Dynal Biotech). Finally, the suspensions were incubated with anti-CD8 (Lyt-2) mAb and anti-CD5 mAb (53-7.3) and sorted with a FACSVantage cell sorter (BD Biosciences) as CD8−CD5+ cells. The suspension contained 97–99% of CD4+ T cells. Purified CD4+ splenocytes (1.5 × 105/well) plus irradiated (3000 rad) syngeneic splenocytes (7.5 × 105/well) were stimulated with Con A (4 μg/ml) or MOG35–55 (100 μg/ml). After 48 h of culture, supernatants were harvested and IFN-γ and IL-4 were measured by ELISA (35). Proliferation was assessed after 48 h of the culture by incorporation of [3H]thymidine (1 μCi/well). In a second set of experiments, the supernatants were tested for 10 independent cytokines (IFN-γ, TNF-α, CXCL10, IL-2, IL-4, IL-5, IL-6, IL-10, IL-13, IL-17) using LINCOplex multiplex assays acquired on Luminex instrumentation (Millipore).

Statistical analysis

Data were analyzed using the Mann-Whitney U test with the exception of cytokine production, which was compared using the unpaired Student t test, and the kinetics of EAE development, which were compared using the log-rank test. ∗, p < 0.05; ∗∗, p < 0.01; ∗∗∗, p < 0.001.

Results

NKT cells accumulate in the CNS during CNS inflammation

MOG35–55-immunized NOD mice develop a relapsing-remitting EAE driven by an I-Ag7-restricted CD4+ T cell response. We have previously shown that the enrichment of NKT cells in Vα14 Tg mice renders NOD mice resistant to the induction of EAE, despite generating a limited inflammatory infiltrate in the CNS (33). To further characterize this infiltrate, we purified the CNS-infiltrating mononuclear cells 15 days after immunization (Fig. 1⇓), corresponding to the first peak of clinical disability in wild-type NOD mice. At this time point, the Vα14 Tg mice exhibited occasionally a very mild clinical deficit (Fig. 1⇓A). In NOD mice, we observed a CD4+ T cell infiltrate (Fig. 1⇓B), representing on average 8.9% of infiltrating mononuclear cells, amounting to 1.1 × 105 cells per inflamed CNS (Fig. 1⇓C). In the protected Vα14 Tg mice, the frequency of infiltrating CD4+ T cells was significantly reduced (4.8%), but it still represented on average 0.3 × 105 CD4+ T cells per CNS. When assessing the infiltrating NKT cell compartment, we observed a significant increase in the frequency of CD1d:α-GalCer tetramer-positive NKT cells in Vα14 Tg vs NOD mice (3.7 vs 0.4%, respectively, Fig. 1⇓B), corresponding to 12 × 103 NKT cells in the protected Vα14 Tg CNS and 2 × 103 NKT cells in the diseased wild-type CNS (Fig. 1⇓, B and D). These data indicate that the composition of the inflammatory infiltrate in the protected CNS of Vα14 Tg mice comprises CD4+ T cells and is enriched in NKT cells. This is of interest, as CNS-resident APC populations are known to express CD1d under inflammatory conditions. CD1d has been identified on microglia during inflammatory demyelination in the rat, while in human MS lesions, CD1d expression was found on microglia and reactive astrocytes (24, 42).

FIGURE 1.
FIGURE 1.

CNS accumulation of CD4+ T cells and enrichment in NKT cells in protected Vα14 Tg mice. A, Clinical score of NOD (n = 22) and Vα14 Tg mice (n = 21) 15 days after EAE induction. B, Flow cytometry analysis of CNS-infiltrating mononuclear cells 15 days after immunization. NKT cell frequency (top) was assessed using CD1d:α-GalCer tetramers combined with a TCRβ-specific mAb on Thy1.2-gated T cells. The mean frequency of NKT cells ± SD was calculated from 10 individual experiments containing 2–7 pooled mice/observation. The frequency of infiltrating conventional CD4+ T cells (bottom) was defined as the percentage of CD4+ CD1d:α-GalCer tetramer-negative cells among total mononuclear cells. The mean frequency of CD4+ cells ± SD was calculated from 10 individual experiments containing 2–7 pooled mice/observation. C, Absolute number of CNS-infiltrating CD4+ T cells (CD1d:α-GalCer tetramer-negative) per CNS. The average number of CD4+ cells ± SEM was calculated from eight individual experiments containing two to seven pooled mice/observation. D, Absolute number of NKT cells (CD1d:α-GalCer tetramer-positive) per CNS. The average number of NKT cells ± SEM was calculated from seven individual experiments containing two to seven pooled mice/observation.

CD1d expression and NKT development in Vα14 pLck mice

To address the role of lipid-Ag presentation in the activation of regulatory NKT cells in EAE, we used transgenic mouse lines that express CD1d under the control of the proximal Lck promoter, which targets gene expression selectively to DP thymocytes. These transgenic pLck-CD1d mice were then crossed twice onto CD1d−/− mice to eliminate endogenous CD1d alleles, thereby restricting the expression of CD1d on DP thymocytes. This was sufficient to ensure the development and persistence of NKT cells (6). Introduction of the TCR Vα14-Jα18 transgene in these mice lacking extrathymic CD1d allowed the enrichment in NKT cells. Flow cytometric analysis illustrated that both selected transgenic lines (Vα14 pLck436 and 473) express CD1d in the thymus at levels similar to, or higher than, wild-type NOD mice (Fig. 2⇓A). In the Vα14 pLck473 line, the CD1d expression profiles on splenic αβ TCR+, CD19+, CD11b+, and CD11c+ cells were identical to those of CD1d−/− mice (Fig. 2⇓B). In the Vα14 pLck 436 line, the staining for CD1d was similar to that in CD1d−/− mice on splenocytes of CD19+, CD11b+, or CD11c+ cells, whereas minute CD1d expression was detected on αβ TCR+ (Fig. 2⇓B). Next, we analyzed the frequency and functional status of NKT cells in the Vα14 pLck mouse lines. Fig. 2⇓C shows that both lines exhibit a similar enrichment in α-GalCer:CD1d tetramer+ αβ TCR+ cells in the spleen as do Vα14 Tg NOD mice (7–9.3%), compared with 0.6% in wild-type NOD mice. We next assessed whether NKT cells from Vα14 pLck mice also exhibit the unique IL-4 and IFN-γ cytokine profile upon TCR triggering. To this end, splenocytes from Vα14 pLck mice and CD1d-sufficient Vα14 control mice were restimulated in vitro with α-GalCer. As shown in Fig. 2⇓D, α-GalCer alone failed to stimulate the NKT cells from Vα14 pLck mice, confirming their extrathymic CD1d deficiency. However, upon addition of CD1d+/+ APCs together with α-GalCer, NKT cells in the Vα14 pLck mice produced IL-4 and IFN-γ in similar quantities as those of CD1d-sufficient Vα14 Tg mice. Consequently, NKT cells from the pLck lines have retained their capacity to recognize and respond to a specific agonist if properly presented. Collectively, these experiments support the observation that thymic CD1d expression is sufficient for the generation of functionally mature NKT cells without affecting their long-term survival in peripheral lymphoid tissues (6).

FIGURE 2.
FIGURE 2.

Thymic CD1d expression suffices for the maturation and persistence of NKT cells in Vα14 pLck mice. Lymphoid organs of 6- to 8-wk-old mice were analyzed by flow cytometry. CD1d expression on (A) thymocytes (top) and splenocytes (bottom) and (B) splenic CD11b+ (myeloid cells), CD11c+ (DCs), TCRαβ+ (T cells), and CD19+ (B cells) of the different mouse lines (open histograms) in comparison to NOD mice (filled histograms). Columns from left to right (open histograms): CD1d−/−, Vα14 Tg, Vα14 pLck436, and Vα14 pLck473; the MFI of each histogram is indicated in the top right-hand corner. Data are representative of three mice per group. To determine NKT cell frequency and function, the spleens of the indicated mice were analyzed at the age of 6–8 wk. C, Flow cytometry analysis depicting the frequency of NKT cells among splenocytes using TCRβ-specific mAbs and CD1d:α-GalCer tetramers. D, Ag-specific activation of NKT cells. Splenocytes (5 × 105/well) were incubated in medium alone (striped bars), Con A (open bars), α-GalCer (gray bars), or α-GalCer plus irradiated CD1d+/+ APCs (black bars). After 48 h of culture, IFN-γ (left panel) and IL-4 (right panel) release was measured in the culture supernatant by ELISA. Indicated values represent the mean concentration ± SD from three individual mice. Similar data were obtained in two independent experiments.

CD1d expression in the inflamed CNS

To assess the potential for local glycolipid presentation in the CNS, we analyzed the expression of CD1d on CNS-resident APC populations at the first peak of disease, 15 days after immunization. Using a flow cytometry approach, we identified CD45highCD11chigh DCs, CD45highCD11bhighCD11clow macrophages, and CD45lowCD11bhigh microglial cells (Fig. 3⇓A) and measured their expression of CD1d and MHC-II. In wild-type mice, a clear MHC-II expression was found only on DCs (Fig. 3⇓B). The expression pattern of CD1d was much broader, including DCs, macrophages, and some microglial cells (Fig. 3⇓B). APC populations in the protected CNS of Vα14 Tg mice revealed a very similar expression pattern with strong CD1d expression on DCs and macrophages (Mφ) although at a slightly lower intensity (NOD: DCs mean fluorescence intensity (MFI)CD1d = 285, Mφ MFICD1d = 190; Vα14 Tg DCs MFICD1d = 195, Mφ MFICD1d = 156). To confirm the absence of CD1d expression in the mice lacking extrathymic CD1d expression, we used pLck mice that did not express the Vα14-Jα18 transgene to ensure local CNS inflammation. In the diseased pLck mice, CD1d staining was identical to that seen in CD1d−/− mice on CNS DCs, Mφ, and T cells (Fig. 3⇓B and data not shown). Therefore, even strong local inflammatory conditions cannot unveil CD1d expression in the pLck mice, emphasizing the validity of this model.

FIGURE 3.
FIGURE 3.

CNS DCs and macrophages from wild-type NOD and Vα14 Tg mice express CD1d during inflammation, while no CD1d expression is detectable in the pLck mice. CNS mononuclear cells were purified 15 days after EAE induction in 6- to 12-wk-old NOD (n = 3), Vα14 Tg (n = 3), CD1d−/− (n = 3), and pLck473 mice (n = 2). A flow cytometry analysis was performed using mAbs directed against CD45, CD11c, CD11b, CD1d, MHC-II, and CD4. A, Microglia were identified as CD45lowCD11b+ cells. CD45high cells comprise multiple subpopulations (CD11b+CD11c− macrophages (Mφ), CD11c+ DCs), while the CD11b−CD11c− subpopulation contains mostly T and B cells. B, MHC-II and CD1d expression was analyzed on DCs, macrophages, and microglia (top to bottom) from CD1d−/−, NOD, Vα14 Tg, and pLck473 mice (columns from left to right). MHC-II expression is observed on activated DCs in all four genotypes, while CD1d expression is observed on DCs, macrophages, and some microglia only in NOD and Vα14 Tg mice.

NKT cell enrichment prevents EAE even in the absence of extrathymic CD1d

The Vα14 pLck mice permitted us to assess the necessity for Ag presentation in the activation of regulatory NKT cells in the context of EAE. Based on the analogy with conventional T cells, one would expect that the peripheral absence of CD1d would abrogate the regulatory function of NKT cells by preventing their activation. Alternatively, if CD1d is dispensable for the regulatory function of NKT cells, the pLck mice would prove resistant to EAE. To test these hypotheses, we immunized the three groups of mice (NOD, Vα14 Tg, and Vα14 pLck) with MOG35–55. All immunized NOD mice developed a typical relapsing-remitting disease (Fig. 4⇓A and Table I⇓), whereas Vα14 Tg mice were mostly protected. Vα14 pLck mice also developed a significantly delayed and milder EAE than did NOD mice. To assess whether this inhibition of EAE could be merely ascribed to the expression of the Vα14-Jα18 transgene by conventional T cells and thereby perturb their encephalitogenic potential, we performed control experiments using Vα14 Tg mice on a CD1d-deficient background. These Vα14 Tg CD1d−/− mice, which express the Vα14-Jα18 TCR transgene but lack NKT cells, developed an EAE indistinguishable from wild-type NOD mice, while their CD1d-expressing Vα14 Tg counterparts were resistant (Fig. 4⇓B). These data demonstrate that the encephalitogenic potential of the αβ T cells in the CD1d-expressing Vα14 Tg mice is unaltered by the transgene expression. These data reinforce the implication of NKT cells in the observed alleviation of EAE and indicate that this regulatory function of NKT cells can be induced in the absence of CD1d-dependent TCR triggering. In fact, given the very similar disease evolution in the Vα14 Tg and Vα14 pLck mice, it appears that NKT cell regulation is as efficacious in a CD1d-sufficient as in a CD1d-deficient environment.

FIGURE 4.
FIGURE 4.

Enrichment of NKT cells prevents EAE irrespective of extrathymic CD1d expression. A, EAE was induced in 6- to 12-wk-old NOD (open circles, n = 13), Vα14 Tg (black circles, n = 9), and Vα14 pLck mice (gray circles, n = 11: for line 436, n = 5; for line 473, n = 6) in two independent experiments. B, EAE was induced in 6- to 12-wk-old NOD (open circles, n = 10), Vα14 Tg CD1d−/− (filled diamonds, n = 9), and Vα14 Tg CD1d+/+ mice (filled circles, n = 5) in two independent or a single (Vα14 Tg CD1d+/+) experiment. The clinical severity of EAE is presented as the mean clinical score over time.

View this table:
Table I.

Clinical severity of EAEa

NKT cells suppress the encephalitogenic T cell response in a CD1d-independent manner

We have previously shown that, in the Vα14 Tg mice, the priming of the MOG35–55-specific T cell response is unaffected in draining lymph nodes but is reduced in the spleen (30), where NKT cells are more frequent (1). To assess the role of CD1d in the NKT cell-mediated regulation, we analyzed the intensity and cytokine profile of the MOG35–55-specific CD4+ T cells in the different mice. CD4+ T cells were isolated from the spleen 12 days after immunization and restimulated in vitro in the presence of irradiated I-Ag7+CD1d−/−TCRCα−/− splenocytes (Fig. 5⇓). CD4+ T cells from the NOD mice proliferated readily to MOG35–55 restimulation, whereas CD4+ T cells from either the Vα14 Tg or Vα14 pLck mice failed to expand in response to MOG35–55 stimulation, suggesting that the encephalitogenic T cells are either absent or anergized. This process was indiscriminate between Th1 and Th17 responses, as the MOG35–55-specific production of both IFN-γ (Fig. 5⇓B) and IL-17 (Fig. 5⇓C) was severely impaired. Collectively, these results show a similar inhibition of the encephalitogenic T cell response in Vα14 Tg mice and Vα14 pLck mice, and they argue that CD1d is dispensable in this regulatory process.

FIGURE 5.
FIGURE 5.

NKT cell enrichment similarly inhibits the encephalitogenic T cell response in the presence or absence of CD1d. The MOG35–55-specific CD4+ T cell response was assessed in the spleens of NOD (n = 3), Vα14 Tg (n = 3), and Vα14 pLck 473 mice (n = 3) 12 days after immunization. Purified CD4+ T cells were cocultured with irradiated splenocytes and stimulated with MOG35–55 (100 μg/ml) or Con A (4 μg/ml). After 48 h of culture (A) the proliferation was measured by thymidine incorporation, or the supernatants were harvested and (B) IFN-γ was measured by ELISA. Data represent the mean proliferation or cytokine concentration ± SD and are representative of two independent experiments. C, IL-17 production was assessed under similar culture conditions by 10-plex Luminex analysis. The pooled data of two independent experiments are presented as the mean cytokine concentration ± SD of NOD (n = 8), Vα14 Tg (n = 7), and Vα14 pLck mice (n = 7). In the same experiments, the drastic reduction in IFN-γ production in the Vα14 Tg and Vα14 pLck mice was confirmed by Luminex analysis.

CNS-infiltrating NKT cells are predominantly double negative and exhibit a cytotoxic profile generated independently of CD1d expression

The Ag specificity of conventional αβ T cells and the presence of their restriction element are essential for their recruitment to or retention within the CNS, as well as their local reactivation (43, 44, 45, 46). To determine whether CD1d Ag presentation exerts a similar selective pressure on CNS-infiltrating NKT cells, we assessed their frequency, subset composition, and effector phenotype in the different mouse lines 15 days after immunization (Fig. 6⇓). Interestingly, the absence of CD1d had no effect on the accumulation of NKT cells in the CNS, as Vα14 pLck mice exhibited a similar NKT cell infiltration (5–6% of αβ T cells) as did CD1d-sufficient Vα14 Tg mice (Fig. 6⇓B). Since previous studies have suggested that various NKT cell subsets (DN vs CD4+, or NK1.1+ vs NK.1−) could be localized in different tissues and exert specialized functions, we have analyzed the phenotype and cytokine production of NKT cells present in the CNS and the spleen (47, 48). In all genotypes, the CNS-infiltrating NKT cells were predominantly of the DN rather than the CD4+ subset (Fig. 6⇓A), excluding a role for CD1d in subset selection. To analyze whether the local CNS expression of CD1d influenced the differentiation of CNS-infiltrating NKT cells, we FACS purified NKT cells from the CNS and the spleen (Fig. 6⇓C) to permit mRNA content analysis by real-time PCR. Splenic NKT cells purified from NOD mice produced predominantly IFN-γ, but no detectable IL-4 or IL-10. By contrast, splenic NKT cells from both Vα14 Tg and Vα14 pLck mice produced a similar level of IFN-γ transcripts but also IL-4 and IL-10 (Fig. 6⇓D). In the CNS, rather than a regulatory cytokine profile, the infiltrating NKT cells expressed cytotoxic and inflammatory molecules such as IFN-γ, IL-17, and granzyme B, while IL-4 and IL-10 were undetectable (Fig. 6⇓E). Again, no significant differences were observed between NKT cells from CD1d-sufficient Vα14 Tg mice and CD1d-deficient Vα14 pLck mice. This implies that in the spleen of NKT cell-enriched mice, the attenuation of the encephalitogenic T cell response coincides with the presence of antiinflammatory NKT cells producing IL-4, IL-10, and IFN-γ. In the same mice the protected CNS is enriched in NKT cells producing IFN-γ, IL-17, and granzyme B, indicative of a proinflammatory, cytotoxic phenotype. Lastly, the generation of the splenic antiinflammatory NKT cells, the infiltration and accumulation of NKT cells in the CNS, as well as their phenotype are all independent of the expression of CD1d in these organs.

FIGURE 6.
FIGURE 6.

Cell-surface phenotype and cytokine profile of splenic and CNS-infiltrating NKT cells 15 days after immunization. A, Cell-surface phenotype gated on CD1d:α-GalCer+TCRβ+ NKT cells, assessing the frequency of CD4+ vs DN NKT cells in the spleen (top) and CNS (bottom) in wild-type NOD (n = 8), Vα14 Tg (n = 6), and Vα14 pLck 473 mice (n = 4) 15 days after immunization. The mean percentages of CD4+ NKT cells ± SD from three independent experiments are added. B, Frequency of CD1d:α-GalCer+TCRβ+ NKT cells among CNS (left) and splenic (right) T cells of NOD (open bars), Vα14 Tg (black bars), or Vα14 pLck 473 mice (gray bars) 15 days after immunization. C, Flow cytometry analysis to illustrate the purification procedure of CD1d:α-GalCer+TCRβ+ NKT cells. D, Real-time PCR analysis of mRNA from purified NKT cells from the spleen of NOD (open bars), Vα14 Tg (black bars), or Vα14 pLck 473 mice (gray bars) 15 days after immunization. E, Real-time PCR analysis of mRNA transcripts from purified CNS-infiltrating NKT cells from NOD (open bars), Vα14 Tg (black bars), or Vα14 pLck 473 mice (gray bars) 15 days after immunization. For D, Three to five individual spleens per group; for E, two groups of three pooled CNS were analyzed for each genotype. Data represent the mean fold increase ± SEM. ND, not detected.

Discussion

The relapsing-remitting EAE induced by MOG35–55 in NOD mice can be controlled by the transgenic enrichment in invariant NKT cells (33). We have previously shown that T cell priming in the draining lymph nodes was unaffected by the enrichment in NKT cells. However, after their egress from the lymph nodes the encephalitogenic T cells failed to accumulate in the spleen. This regulation is not absolute, as occasionally a very mild EAE is observed among the MOG35–55-immunized Vα14 Tg mice. Assessing the composition of the CNS-infiltrating cells indeed confirmed the presence of a sizeable population of CD4+ T cells in the protected mice. This protection from autoimmune demyelination is associated with the CNS enrichment in NKT cells (predominantly DN), which coincides with the local induction of CD1d expression. As such, CD1d Ag presentation might activate infiltrating NKT cells, thereby contributing to CNS protection from autoimmune demyelination. Such a potential for local Ag presentation in the context of CD1d equally arises in MS lesions where transcripts for both the invariant TCR α-chain of NKT cells (Vα24-Jα18) and CD1d can be simultaneously detected (Ref. 24 and L. T. Mars, unpublished observations).

To assess the importance of CD1d-mediated Ag presentation by glial and hematopoietic cells, we developed mice that are deficient in extrathymic CD1d expression. In fact, the expression of CD1d on DP thymocytes sufficed for the development and long-term persistence of bona fide invariant Vα14-Jα18 NKT cells (5, 6). Transgenic enrichment of NKT cells in these mice not only afforded a strong resistance to MOG35–55-induced EAE, but it conveyed a protection identical to that observed in the CD1d-sufficient Vα14 Tg mice. This implies that the enrichment of NKT cells prevents EAE irrespective of extrathymic CD1d expression. We assessed the mechanisms involved in the protection by studying both the autoreactive CD4+ T cells and the NKT cells in Vα14 Tg and Vα14 pLck mice. Unexpectedly, the enrichment of NKT cells interfered with the encephalitogenic T cell response in a similar manner irrespective of the presence of CD1d. In both instances, the MOG35–55-induced T cell proliferation was inhibited, abrogating indiscriminately the encephalitogenic Th1 and Th17 responses. Moreover, in both genotypes the splenic NKT cells produced increased levels of IL-4 and IL-10 when compared with wild-type NOD mice. The precise mechanism remains unclear, but some insight has been obtained. A role for IL-4 in the regulatory mechanism was ruled out using IL-4 knockout mice and is supported by an absence of immune deviation as assessed by MOG35–55-specific T cells responses (33). The inhibition of MOG35–55-specific CD4+ T cell proliferation in the NKT-enriched mice, demonstrated in this study, suggested a mechanism involving anergy, elimination, and/or an altered migratory behavior of encephalitogenic T cells. The absence of cytotoxic molecules, such as granzyme B, by the splenic NKT cells would argue against the elimination of the encephalitogenic T cell responses favoring a nondeletional mechanism. A similar mechanism by which NKT cells restrained mature autoagressive T cells has been described in diabetes. Indeed, transferring naive CD4+ T cells with a known diabetogenic potential into mice enriched in NKT cells permitted their initial activation and expansion, followed by an inhibition of IL-2 and IFN-γ production leading to a state of T cell anergy (49).

The deficiency in CD1d failed to alter the NKT cell infiltration in the CNS. This is in agreement with previous observations regarding the migration of NKT cells to inflammatory sites (50), but stands in strong contrast with conventional T lymphocytes for which local Ag presentation is essential for their recruitment, retention, and reactivation in the CNS (43, 44, 45, 46). The CNS-infiltrating NKT cell population had acquired a cytokine profile radically different from that observed in the spleen, producing IFN-γ, granzyme B, and IL-17, while IL-4 and IL-10 were undetectable. This could be indicative of a selective recruitment of NKT cell subpopulations to, or their differentiation locally in, the CNS independent of CD1d stimulation (51, 52). At the steady-state, NK1.1− NKT cells have been identified to preferentially produce IL-17, suggesting that this preexisting NKT subpopulation might be recruited to the CNS (48).

The regulation of the encephalitogenic T cell response by NKT cells is evident in the spleen. To what extent the regulation by NKT cells also occurs in the CNS could not be addressed selectively. However, the phenotype of the CNS-infiltrating NKT cells is compatible with them having immunoregulatory properties. Indeed, the CNS-infiltrating NKT cells are reminiscent of NK cells, both in terms of their cytokine profile and their capacity to lyse their targets irrespective of CD1d expression (53). A beneficial antiinflammatory effect has been ascribed to NK cells in EAE, as their depletion resulted in an aggravated disease evolution (54). Further studies proposed that this beneficial function of NK cells in EAE is mediated by the cytotoxic killing of encephalitogenic T cells (55). It is therefore tempting to speculate that CNS NKT cells could contribute to the protection from EAE by lysing either infiltrating T cells or local APCs.

The control of NKT cell function is currently thought to arise from a combination of signals mediated by the TCR, local cytokines, and inhibitory receptors (1). The necessity and impact of TCR triggering might vary dependent on the inflammatory context and the pathogen encountered. During antimicrobial immune responses, exogenous and/or endogenous lipid-Ag presentation permits the activation of NKT cells (56, 57, 58). Even at the steady-state, the constitutive expression of CD1d on DCs and B cells permits NKT cell stimulation (59, 60). However, high doses of LPS activate NKT cells independent of lipid-Ag presentation (61). A similar capacity has been attributed to cytokine mixtures; for example, a combination of IL-12 and IL-18 can elicit proliferation, IFN-γ production, and cytotoxicity in a TCR-independent manner (62). These cytotoxic NKT cells are functional, as NKT-mediated killing can occur independent of CD1d (53). CD1d-independent activation can even promote different effector phenotypes. For instance, IL-18, in the absence of IL-12, drives a Th2-biased NKT cell response (63). Taken together, this would suggest that when cytokines are limited or absent, such as during the steady-state or during the initiation of an immune response, CD1d-Ag presentation is essential, but under conditions where cytokines are abundant, the role of CD1d might be reduced if not dispensable. In this study of NKT immune regulation in the EAE model, we might well fall in the latter category. Indeed, NKT cells did not interfere with the initial T cell priming in the draining lymph nodes, but they controlled the response after lymph node egress, when the cytokine response has been established.

The potential of driving NKT cell regulation to the inflamed CNS independent of lipid-Ag stimulation might endow NKT cells with a potential for bystander therapeutic benefit. In this perspective of cytokine-mediated activation, it is interesting to find that IFN-β, currently the most prescribed treatment for MS, drives the expansion of circulating NKT cells (64), an effect that might combine the IFN-β-mediated maturation of DCs and its capacity to induce IL-15 essential for the homeostasis of NKT cells (13, 64, 65, 66). Further understanding of the mechanisms driving NKT cell activation in the absence of exogenous ligands should provide novel targets for immune intervention in MS.

Acknowledgments

We are grateful to Drs A. Saoudi and D. Dunia for helpful discussions. We thank the staff of the Institut National de la Santé et de la Recherche Médicale U561 mouse facility for animal care, and Kirin Brewery for kindly providing KRN7000 (α-GalCer). The plasmid containing the CD1d and β2-microglobulin genes was kindly provided by Mitchel Kronenberg and Maria Leite-de-Moraes. We thank Arielle Estival and Aurore Desquesnes from the “Plateau d’Exploration Fonctionnelle” at the Genopole, IFR31 in Toulouse for the Luminex analysis.

Disclosures

The authors have no financial conflicts of interest.

Footnotes

  • The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

  • ↵1 This work was supported by the Institut National de la Santé et de la Recherche Médicale, the Association pour la Recherche sur la Sclérose en Plaques, the Agence Nationale de la Recherche (ANR-06-MIME), the European Union (Neuropromise: SHM-CT_2005-018637), and the region Midi-Pyrénées. L.T.M. was supported by a postdoctoral fellowship from the European Union (Neuropromise). A.S.G. and J.N. were supported by doctoral fellowships from the Ministère de l’Education Nationale, de la Recherche et Technique and the Foundation pour la Recherche Médicale, respectively.

  • ↵2 L.T.M. and A.-S.G. contributed equally to this work.

  • ↵3 R.S.L. and A.L. contributed equally to this work as principal investigators.

  • ↵4 Address correspondence and reprint requests to Dr. A. Lehuen, Institut National de la Santé et de la Recherche Médicale-U561, Hôpital Cochin/Saint Vincent de Paul, 82 avenue Denfert-Rochereau, 75014 Paris, France. E-mail address: lehuen{at}paris5.inserm.fr. or Dr. Roland S. Liblau, Institut National de la Santé et de la Recherche Médicale-U563, Centre de Physiopathologie de Toulouse Purpan, Centre Hospitalier Universitaire Purpan, BP 3028, 31024 Toulouse Cedex 3, France. E-mail address: rolandliblau{at}hotmail.com

  • ↵5 Abbreviations used in this paper: DP, double positive; α-GalCer, α-galactosylceramide; DC, dendritic cell; DN, double negative; EAE, experimental autoimmune encephalomyelitis; HPRT, hypoxanthine phosphoribosyltransferase; MBP, myelin basic protein; MFI, mean fluorescence intensity; MOG, myelin oligodendrocyte glycoprotein; Mφ, macrophage; MS, multiple sclerosis; pLck, proximal Lck; PLP, proteolipid protein; NOD, nonobese diabetic; SP, single positive; Tg, transgenic.

  • Received February 19, 2008.
  • Accepted June 9, 2008.

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