The interface between innate and acquired immunity: glycolipid antigen presentation by CD1d-expressing dendritic cells to NKT cells induces the differentiation of antigen-specific cytotoxic T lymphocytes

Takashi Nishimura1,2, Hidemitsu Kitamura2, Kenji Iwakabe2, Takashi Yahata2, Akio Ohta1,2, Marimo Sato2, Kazuyoshi Takeda3, Ko Okumura3, Luc Van Kaer4, Tetsu Kawano5, Masaru Taniguchi5, Minoru Nakui2, Masashi Sekimoto1,2 and Toshiaki Koda2

1 Division of Immunoregulation, Institute for Genetic Medicine, Hokkaido University, Sapporo 060-0815, Japan
2 Section of Genetic Engineering, Research Center for Genetic Engineering and Cell Transplantation, Tokai University School of Medicine, Bohseidai, Isehara 259-1193, Japan
3 Department of Immunology, Juntendo University School of Medicine, Tokyo 113-0033, Japan
4 Howard Hughes Medical Institute, Department of Microbiology and Immunology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA
5 CREST (Core Research for Evolutional Science and Technology) Project and Department of Molecular Immunology, Graduate School of Medicine, Chiba University, Chiba 260-8670, Japan

Correspondence to: T. Nishimura, Division of Immunoregulation, Institute for Genetic Medicine, Hokkaido University, Sapporo 060-0815, Japan


    Abstract
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 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
In vivo administration of NKT cell ligand, {alpha}-galactosylceramide ({alpha}-GalCer), caused the activation of NKT cells to induce a strong NK activity and cytokine production by CD1d-restricted mechanisms. Surprisingly, we also found that {alpha}-GalCer induced the activation of immunoregulatory cells involved in acquired immunity. Specifically, in vivo administration of {alpha}-GalCer resulted in the induction of the early activation marker CD69 on CD4+ T cells, CD8+ T cells and B cells in addition to macrophages and NKT cells. However, no significant induction of CD69 was observed on cells from CD1d- or V{alpha}14 NKT-deficient mice, indicating an essential role for the interaction between NKT cells and CD1d-expressing dendritic cells (DC) in the activation of acquired immunity in response to {alpha}-GalCer. Indeed, in vivo injection of {alpha}-GalCer resulted not only in the activation of NKT cells but also in the generation of CD69+CD8+ T cells possessing both cytotoxic T lymphocyte (CTL) activity and IFN-{gamma}-producing ability. Tumor-specific CTL generation was also accelerated by {alpha}-GalCer. The critical role of CD40–CD40 ligand (CD40L)-mediated NKT–DC interaction during the development of CD69+CD8+ CTL by {alpha}-GalCer was demonstrated by blocking experiments using anti-CD40L mAb. These findings provide direct evidence for a critical role of CD1d-restricted NKT cells and DC in bridging innate and acquired immunity.

Keywords: {alpha}-galactosylceramide, CD40, CD40 ligand, cytotoxic T lymphocyte, dendritic cell, NKT cells


    Introduction
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Immune systems are classified into two general systems, the innate or natural immune system and the acquired or adaptive immune system. These systems may have been selected by infectious agents during evolution (1). It is thought that the innate immune system initially keeps the infection under control, saving time for the adaptive immune system to develop an appropriate response. However, recent studies have suggested that the various components of the innate immune system trigger and augment the components of the adaptive immune system, including antigen-specific B and T lymphocytes (13). It is now clear that the innate and adaptive immune systems intimately interact with one another, and that adaptive immunity is controlled by the components of innate immunity (2,4).

Cells of the innate immune system, including macrophages and dendritic cells (DC), take up foreign antigens through pattern recognition receptors, combine peptide fragments of these antigens with MHC class I and class II molecules, and stimulate naive CD8+ and CD4+ T cells respectively (46). In addition, professional antigen-presenting cells (APC) communicate with T cells using positive and negative feedback systems, by producing cytokines such as IL-12, IL-18 and IL-10 (79). These interactions will influence the differentiation of naive CD4+ T cells into Th1 or Th2 lymphocytes that mediate cellular and humoral immunity respectively (1012). In order to understand host immune defenses against pathogenic microorganisms it is therefore important to study the molecular and cellular mechanisms that bridge innate and acquired immunity.

Recently, a novel lymphoid lineage, V{alpha}14 NKT cells, distinct from mainstream T cells, B cells and NK cells, has been identified (13,14). These cells are characterized by co-expression of NK cell receptors and semi-invariant TCR encoded by V{alpha}14 and J{alpha}281 gene segments. Upon activation, NKT cells produce high levels of IFN-{gamma} and IL-4, and can influence immune responses against autoantigens and tumors (15,16). These cells were therefore implicated as key effector cells in innate immune responses. However, the potential role of NKT cells in the development of adaptive immune responses remains unclear. Recently, it was demonstrated that NKT cells can be activated by the glycolipid {alpha}-galactosylceramide ({alpha}-GalCer) bound with the MHC class I-like molecule CD1d (17,18). In vivo administration of {alpha}-GalCer induced strong cytolytic activity and cytokine production in wild-type mice, but not CD1d- or NKT-deficient mice. Furthermore, using highly purified cell populations, we demonstrated that {alpha}-GalCer selectively activates NKT cells, and that this responses required CD11c+ DC and CD40–CD40 ligand (CD40L) interactions (19). This chemical therefore allowed us to directly test whether the interaction between NKT cells and DC early in an immune response affects the subsequent development of an adaptive immune response.

In the present paper we demonstrate that in vivo administration of the NKT cell ligand {alpha}-GalCer induces the expression for the early activation marker CD69 on conventional CD4+ and CD8+ T cells, B cells, macrophages and NKT cells. These effects of {alpha}-GalCer were absent in CD1d- and NKT cell-deficient mice. We further showed that activation of CD69 on CD8+ T cells resulted in the acquisition of a cytolytic phenotype and IFN-{gamma} production. Administration of anti-CD40L antibodies prior to treatment with {alpha}-GalCer blocked the activation of CD8+ T cells. These findings provide direct evidence that antigen presentation by CD1- and CD40-expressing DC to NKT cells is critically important for bridging innate and adaptive immunity.


    Methods
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 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Mice
C57BL/6 mice were purchased from Charles River Japan (Yokohama, Japan). V{alpha}14 NKT cell-deficient (J{alpha}281–/–) and CD1d–/– mice were established by specific deletion of the J{alpha}281 and CD1d gene segment respectively (17,20). All mice used in this study were 5–8 weeks old and were maintained in specific pathogen-free conditions.

{alpha}-GalCer
{alpha}-GalCer, [(2S,3S,4R)-1-O-({alpha}-D-galactopyranosyl)-2-(N-hexacosanoylamino)-1,3,4-octadecanetriol], used for this study was provided by Dr Y. Koezuka (Kirin Brewery, Gunma, Japan) (18). The stock solution of {alpha}-GalCer (220 µg/ml) was diluted in 0.5% polysorbate 20 (Nikko Chemical, Tokyo, Japan) in 0.9% NaCl solution. This stock solution was further diluted into an appropriate concentration with saline and used for the experiments. A vehicle control solution was prepared from a solution of 0.5% polysorbate 20 in 0.9% NaCl solution. The vehicle control was used in all experiments.

Isolation of lymphoid cell subsets by FACS
Spleen cells were incubated on nylon-wool columns for 45 min and the non-adherent cells were used for the isolation of CD4+ T cells and CD8+ T cells by cell sorting using a FACS Vantage instrument (Becton Dickinson, Mountain View, CA). All mAb used in these experiments [mAb against NK1.1 (PK136), CD3 (145-2C11), CD4 (GK1.5), CD8a (53-6.7), CD11b/Mac-1 (M1/70), CD11c (HL3), CD45/B220 (RA3-6B2) and CD69 (H1.2F3)] were purchased from PharMingen (San Diego, CA). The stained cells were isolated using FACS vantage. The purity of the sorted cells was >98%. The details of the staining and sorting have been described earlier (21).

Detection of cytokine activity
IL-4 or IFN-{gamma} activity in serum samples or culture supernatants was determined using the Biotrac mouse IL-4 or Biotrac mouse IFN-{gamma} ELISA system (Amersham, Little Chalfont, UK). Serum samples were obtained from C57BL/6 mice 24 h after injection of {alpha}-GalCer (2 µg/mouse) and cytokine levels were measured using ELISA kits (Amersham).

Cytotoxicity assay
The NK activity of spleen cells was determined by 4 h 51Cr-release assays using YAC-1 cells as target. Reverse antibody-dependent cellular cytotoxicity (R-ADCC) CTL activity was measured using Fc receptor-positive P815 mastocytoma cells in the presence of anti-CD3 mAb. Tumor antigen-specific CTL activity was detected using MBL-2 T lymphoma cells. One lytic unit was defined as the number of effector cells required to cause 25% lysis of 2500 target cells. The cytotoxicity (%) was calculated as described previously (13,22).

Quantitative RT-PCR assay for IL-12 receptor (IL-12R) mRNA measurement
C57BL/6, CD1d–/– and V{alpha}14 NKT cell-deficient mice were injected i.v. with {alpha}-GalCer (2 µg/mouse) or vehicle. At 4 h after treatment, mice were sacrificed and spleen cells were isolated. TaqMan real-time quantitative RT-PCR assay was carried out for the detection of IL-12R mRNA expression by these cells according to the method previously described (19). IL-12Rß1 and IL-12Rß2 mRNA expression was estimated from the ratio of fluorescence intensity as compared with GAPDH. IL-12R expression induced by {alpha}-GalCer was indicated in Table 1Go as an induction index, which was calculated by the following formula: induction index = (IL-12R expression of {alpha}-GalCer-stimulated sample/IL-12R expression of unstimulated sample) x100.


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Table 1. Serum cytokine production, killer activity and IL-12R up-regulation of C57BL/6, CD1d–/– and V{alpha}14 NKT-deficient mice injected with {alpha}-GalCer
 
TaqMan probes used for these analyses are as follows: IL-12Rß1 mRNA-605T: 5'-CGGATGCCCACAACGAATTGGA-3'; IL-12Rß2 mRNA-551T: 5'-AGCCACCTCAAAACATATCATGTGTCCAGG-3'; GAPDH-542T: 5'-CCTGGCCAAGGTCATCCATGACAACTTT-3'.

PCR primers used for these analyses are as follows: IL-12ß1 mRNA: forward primer (–563F) 5'-AATGTGTCTGAAGAGGCCGGT-3' and reverse primer (–657R) 5'-GAGTTAACCTGAGGTCCGCAGT-3'; IL-12Rß2 mRNA: forward primer (–529F) 5'-ATCTCAGTTGGTGTTGCTCCA-3' and reverse primer (–602R) 5'-GCCACAGTTCCATTTTCTCCT-3'; GAPDH: forward primer (–368F) 5'-CTTCACCACCATGGAGAAGGC-3' and reverse primer (–605R) 5'-GGCATGGACTGTGGTCATGAG-3'.

Immunopotentiating effect of {alpha}-GalCer in vivo
C57BL/6 mice were i.v. injected with {alpha}-GalCer (2 µg/mouse) and the mice were sacrificed 4–6 h after the treatment to examine serum cytokine levels, NK activity, IL-12R induction and acquisition of the early activation marker CD69 in spleen cell subpopulations. The generation of CD8+ CTL was investigated using the purified CD8+CD69+ T cells or CD8+CD69 T cells from the spleen of {alpha}-GalCer-injected mice. The acquisition of non-specific CTL activity was determined by measuring R-ADCC as described above. The ability of {alpha}-GalCer-activated CD4+ T cells and CD8+ T cells to produce IFN-{gamma} was determined after culture with IL-2 (20 U/ml; kind gift from Shionogi Pharmaceutical Institute, Osaka, Japan) plus IL-12 (20 U/ml; kindly donated by Genetics Institute, Cambridge, MA) for 2 days. IFN-{gamma} levels in culture supernatants were measured by ELISA.

The generation of tumor antigen-specific CTL by {alpha}-GalCer administration in vivo
C57BL/6 mice were hyperimmunized with mitomycin C-treated syngeneic MBL-2 T lymphoma cells twice with a 2 week interval. This protocol is sufficient to induce antitumor memory T cells, as described previously (23). Then, the immunized mice were i.v. injected with 2 µg/mouse of {alpha}-GalCer twice in 2 day intervals. Six hours after the final boosting with {alpha}-GalCer, CD8+ T cells, CD8+CD69 T cells and CD8+CD69+ T cells were isolated from the spleen by flow cytometry, and their ability to differentiate into tumor antigen-specific CTL was assessed after culture with IL-2 plus IL-12 for 2 days. The generation of tumor-antigen-specific CTL was determined by measuring their cytotoxicity against MBL-2 T lymphoma cells. As control target cells, syngeneic B16 melanoma cells were used.

Blocking of IL-12R induction by anti-CD40L mAb
Wild-type C57BL/6 mice were injected i.p. with 100 µg anti-CD154 (CD40L) mAb (MR1; PharMingen) at 0 and 1 days before priming with {alpha}-GalCer. As a control, the same amount of hamster IgG (PharMingen) was injected i.p. into control mice prior to injection of {alpha}-GalCer.


    Results
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Immunopotentiating effect of {alpha}-GalCer in vivo
Consistent with previous findings (1719), in vivo administration of {alpha}-GalCer caused the elevation of both IL-4 and IFN-{gamma} levels. {alpha}-GalCer administration also resulted in the activation of NK activity mediated by NKT cells. Moreover, {alpha}-GalCer administration caused the induction of IL-12R ß1 and ß2 mRNA in the spleen. These responses induced by {alpha}-GalCer were totally dependent on NKT cells, because these effects of {alpha}-GalCer were absent in NKT-deficient mice. Moreover, {alpha}-GalCer did not exhibit any immunostimulatory effect in CD1d–/– mice. These data are summarized in Table 1Go. Thus, {alpha}-GalCer exhibits a variety of immunopotentiating effects in a CD1d-dependent fashion.

{alpha}-GalCer activates immunoregulatory cells involved in both innate and acquired immunity
To assess whether {alpha}-GalCer, in addition to its effects on NKT cells, also influences the function of other immunoregulatory cells, we examined the effect of this agent on the acquisition of the early activation marker CD69 on NKT, macrophages, T and B cells. As shown in Fig. 1Go(A and F), CD69 antigen expression on NK1.1+ cells was greatly enhanced 4 h after the injection of {alpha}-GalCer in vivo. Further analysis by three-color flow cytometry demonstrated that >98% of NK1.1+TCR{alpha}ß+ NKT cells expressed CD69 antigen (data not shown). Surprisingly, we also found that {alpha}-GalCer stimulated the expression of CD69 antigen on CD4+ T cells, CD8+ T cells, B cells and macrophages within 4 h after {alpha}-GalCer treatment (Fig. 1B–JGo). To evaluate whether CD69 up-regulation on macrophages, T and B cells was directly induced by {alpha}-GalCer or indirectly induced via the activation of NKT cells, we examined the effect of {alpha}-GalCer administration on CD69 antigen expression in CD1d–/– and NKT-deficient mice. As illustrated in Fig. 2Go, neither CD1d–/– nor NKT-deficient mice revealed significant up-regulation of CD69 antigen expression on spleen cells. These data demonstrated that the acquisition of CD69 antigen on T cells, B cells and macrophages was not induced by direct stimulation with {alpha}-GalCer but was secondary to the activation of NKT cells.



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Fig. 1. The NKT cell ligand {alpha}-GalCer induces the up-regulation of the early activation antigen CD69 on macrophages, T cells and B cells, in addition to NKT cells. C57BL/6 mice were i.v. injected with {alpha}-GalCer (2 µg/mouse) (F and J) or control vehicle (A and E), and the mice were sacrificed 4 h after treatment to examine the expression of CD69 antigen on splenic NK1.1+ cells (A and F), Mac1+ macrophages (B and G), CD4+ T cells (C and H), CD8+ T cells (D and I) and B220+ B cells (E and J) by flow cytometry. The percentage of positive or negative cells is indicated.

 


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Fig. 2. {alpha}-GalCer does not up-regulate CD69 antigen expression on immunoregulatory cells from CD1d–/– or NKT-deficient mice. Wild-type C57BL/6 mice (A), CD1d–/– C57BL/6 mice (B) and NKT-deficient C57BL/6 mice (C) were i.v. injected with {alpha}-GalCer. The expression of CD69 antigen on spleen cell subpopulations was determined as described in the legend to Fig. 1Go. The bars represent mean ± SE of three mice. Hatched column, mice treated with vehicle; closed column, mice treated with {alpha}-GalCer. CD69 induction index was calculated using the following formula: induction index (%) = (percentage of CD69+ cells in each subpopulation/percentage of each subpopulation in total spleen cells)x100.

 
Bridging of innate and acquired immunity by {alpha}-GalCer-activated NKT cells
To investigate whether the acquisition of CD69 on T cells has functional implications, total CD8+, total CD4+, CD69CD8+, CD69CD4+, CD69+CD8+ or CD69+CD4+ T cells were isolated from the spleen of control or {alpha}-GalCer-injected mice by cell sorting, and their IFN-{gamma}-producing activity and cytotoxicity was compared. The IFN-{gamma}-producing ability of the cells was determined after culture with IL-2 and IL-12 for 2 days. The non-specifically activated CTL activity was determined by measuring R-ADCC CTL activity against P815 mastocytoma cells in the presence of anti-CD3 mAb. As shown in Fig. 3Go, {alpha}-GalCer administration caused a significant enhancement of IFN-{gamma} production by both CD8+ and CD4+ T cells, and these cells were markedly enriched in CD69+CD4+ T cells or CD69+CD8+ T cells. Moreover, R-ADCC CTL activity of CD8+ T cells but not CD4+ T cells was significantly enhanced by {alpha}-GalCer administration. This enhancement of CTL activity became more evident when CD69+CD8+ T cells were enriched by cell sorting. No significant R-ADCC was detected in CD69CD8+ T cells and CD4+ T cell subpopulations. Moreover, {alpha}-GalCer administration into mice that were hyperimmunized with syngeneic MBL-2 T lymphoma cells resulted in the development of CD69+CD8+ T cells which gave rise to IFN-{gamma}-producing and MBL-2-specific CTL after culture with IL-2 and IL-12 for 2 days (Fig. 4Go). Such CTL were not induced in total CD8+ T cells or CD69CD8+ T cell populations. Therefore, up-regulation of CD69 antigen is closely linked with the functional maturation of CD8+ CTL and CD4+ Th cells. From these results we concluded that NKT cells activated with {alpha}-GalCer presented by CD1d-expressing DC play a critical role in initiating T cell- and B cell-mediated acquired immunity.



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Fig. 3. In vivo administration of {alpha}-GalCer induces the development of CD69+CD8+ CTL with IFN-{gamma}-producing activity. CD8+ T cell subsets (CD8+, CD8+CD69 or CD8+CD69+ T cells) or CD4+ T cell subsets (CD4+, CD4+CD69 or CD4+CD69+ T cells) were isolated from the spleen of vehicle (open bars) or {alpha}-GalCer-injected (hatched bars) mice by cell sorting. (A) Then, their IFN-{gamma}-producing ability was determined after culture with IL-2 plus IL-12 for 2 days. The IFN-{gamma} activity in culture supernatants was detected by ELISA. The bars represent mean ± SE of triplicate samples. (B) The cytotoxicity of non-specifically activated CD8+ or CD4+ T cells was examined by R-ADCC CTL activity against P815 mastocytoma in the presence of anti-CD3 mAb. The cytotoxic activity is shown as lytic units (LU)/106 cells, calculated from the mean cytotoxicity of three samples.

 


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Fig. 4. In vivo administration of {alpha}-GalCer induces CD8+CD69+ T cells, which differentiate into tumor-specific CD8+ CTL. Mice hyperimmunized with MBL-2 T lymphoma cells were prepared as described in Methods. {alpha}-GalCer was i.p. injected into the MBL-2-immunized mice twice and 6 h later their spleen cells were prepared to isolate CD8+ T cell subsets (CD8+, CD8+CD69 or CD8+CD69+ T cells) (hatched bars). As a control, CD8+ T cells were isolated from vehicle-treated immunized mice. The isolated cells (2 x105/well) were cultured with IL-2 (20 U/ml) plus IL-12 (20 U/ml) for 2 days and their cytokine producing ability (A) or their ability to differentiate into CTL (B) was determined. The specific cytotoxicity was determined using 51Cr-labeled MBL-2 lymphoma cells (hatched bars). As a control target, B16 syngeneic melanoma cells were used (closed bars). IFN-{gamma} activity in culture supernatants was measured by ELISA. The bars represent mean ± SE of triplicate samples.

 
A critical role of CD40–CD40L interactions in bridging innate and acquired immunity by NKT cells
As reported previously (19), CD40–CD40L interaction between NKT cells and {alpha}-GalCer-bound CD1d+ DC is critically important for the induction of IFN-{gamma} production by NKT cells. To examine the role of CD40–CD40L interaction for the activation of CTL by {alpha}-GalCer, the effect of anti-CD40L injection was investigated. In vivo administration of anti-CD40L mAb caused almost complete inhibition of IFN-{gamma} production by NKT cells (Fig. 5AGo). In addition, anti-CD40L mAb strongly inhibited the final development of CD69+CD8+ CTL from CD69+CD8+ T cells induced by {alpha}-GalCer administration (Fig. 5BGo). These results strongly suggest an important role for CD40–CD40L interaction with sequential activation of NKT cells and CD69+CD8+ CTL by {alpha}-GalCer in vivo.



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Fig. 5. Induction of CD69+CD8+ CTL development upon administration of {alpha}-GalCer is blocked by anti-CD40L mAb. Anti-CD40L mAb (100 µg/mouse) was i.v. injected into the mice 12 h prior to {alpha}-GalCer injection. As a control, hamster IgG was injected by the same protocol. (A) IFN-{gamma} levels of serum samples prepared from the mice, which were treated with hamster IgG and {alpha}-GalCer or anti-CD40L mAb plus {alpha}-GalCer, were measured by ELISA. The data represent mean ± SE of triplicate samples. (B) CD69+CD8+ T cells were isolated from anti-CD40L mAb plus {alpha}-GalCer-treated mice or hamster IgG plus {alpha}-GalCer-treated mice. Then, their non-specific CTL activity against P815 was determined by R-ADCC in the presence of 2C11. The bars represent mean ± SE of triplicate samples.

 

    Discussion
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
The novel lymphoid lineage, NKT cells, expressing both NK receptors and TCR V{alpha}14, has been suggested to play an important role in immunoregulation (1317). NKT cells, which are classified as a part of the innate immune system, are stimulated by CD1d-bound glycolipid antigen and exhibit NK and cytokine-producing activity (1719, 24). Recent studies suggested that NKT cells regulate antigen-specific IgE production, suggesting that NKT cells may play a pivotal role in communication between innate immunity and acquired immunity (16,25). However, the finding that CD1d- and ß2-microglobulin-deficient mice, that lack NKT cells, exhibited normal levels of Th2 responses and IgE production made this issue controversial (26,27). Thus, direct evidence that NKT cells play an essential role in the bridging between innate and acquired immunity has not yet been available.

To evaluate this issue, we tested whether the NKT cell ligand {alpha}-GalCer could stimulate T cells and B cells involved in acquired immunity. Recently, Koezuka et al. (17,18) described a novel NKT cell-specific ligand, {alpha}-GalCer, which is a natural product isolated from marine sponges. {alpha}-GalCer can stimulate NK activity and cytokine production by NKT cells and exhibits potent antitumor activity in vivo (15). The finding that wild-type mice but not CD1d–/– or NKT-deficient mice respond to {alpha}-GalCer suggests that both NKT cells and CD1+ DC are important for the activation of immune responses to {alpha}-GalCer (17). Indeed, we and others clearly demonstrated that {alpha}-GalCer selectively stimulated CD4+NK1.1+ NKT cells to induce NK and cytokine production in the presence of CD1d+ DC in vitro (19,24). We also demonstrated that the immunostimulating effect of {alpha}-GalCer was initiated by CD40–CD40L-mediated NKT–DC interactions (19). Consistent with these findings, {alpha}-GalCer showed a variety of immunoregulatory functions in vivo and these effects were absent in both CD1d–/– and NKT-deficient mice (Table 1Go). Using the newly defined NKT-specific glycolipid antigen, {alpha}-GalCer, it became possible to investigate the role of NKT cells in triggering acquired immunity. Our results showed that NKT cells influence immune responses mediated by conventional CD4+ T cells, CD8+ T cells and B cells.

To test the activation status of lymphoid cells early after {alpha}-GalCer injection, we measured the expression of early-activation antigen CD69 on various lymphoid cells by flow cytometry. CD69 antigen is a type II integral membrane protein with a C-type lectin-binding domain and a member of the NK cell gene complex family of cell-surface receptors (28). CD69 has been demonstrated to be rapidly induced on the surface of activated lymphocytes as early as 3–4 h following stimulation and its expression is closely correlated with various immunological functions (29). As shown in Fig. 1Go, {alpha}-GalCer induced up-regulation of CD69 expression on conventional CD4+ T cells, CD8+ T cells and B cells in addition to NKT cells within 4 h after {alpha}-GalCer treatment. Two possibilities are considered to explain why T cells and B cells are stimulated by NKT cell-specific ligand. First, {alpha}-GalCer by itself may stimulate the expression of CD69 antigen on T cells and B cells in addition to NKT cells. Second, {alpha}-GalCer-activated NKT cells in the presence of CD1+ DC may act as immunoregulating cells to bridge innate and acquired immunity.

To address this issue, we investigated whether {alpha}-GalCer-induced up-regulation of CD69 antigen expression on T and B cells occurs in the absence of CD1+ DC or NKT cells using CD1d–/– or NKT-deficient mice. As shown in Fig. 2Go, neither CD1d–/– nor NKT-deficient mice revealed induction of CD69 antigen expression on T and B cells. Therefore, {alpha}-GalCer by itself cannot stimulate CD69 antigen expression on T and B cells directly. These data strongly suggested that NKT–DC interactions during the early phase of immune responses is critically important in the sequential activation of effector cells involved in acquired immunity.

As shown in Fig. 3Go, CD69+CD8+ T cells induced in vivo in response to {alpha}-GalCer-activated NKT cells showed strong R-ADCC CTL activity and higher levels of IFN-{gamma} production after culture with IL-2 and IL-12. However, CD69CD8+ T cells showed only low levels of cytokine production and R-ADCC CTL activity after culture with IL-2 and IL-12. Moreover, {alpha}-GalCer administration into mice hyperimmunized with syngeneic MBL-2 tumor cells induced the generation of antigen-specific CTL from CD69+CD8+ T cells, but not from CD69CD8+ T cells in the presence of IL-2 plus IL-12 (Fig. 4Go). Therefore, {alpha}-GalCer-induced acquisition of CD69 antigen directly correlated with the functional maturation of CD8+ CTL. These data provided direct evidence that {alpha}-GalCer-activated NKT cells can induce the development of CD69+CD8+ CTL.

Recently, it has been reported that conditioned DC are critically important for the interaction of naive CD4+ Th and CD8+ T cells (4,30). During these responses, CD40–CD40L molecules play a central role in cell–cell interactions which stimulate IL-12 production by DC and IFN-{gamma} production by naive Th cells. Consistent with this, CD40–CD40L appeared to be key molecules in bridging innate and acquired immunity by {alpha}-GalCer. Indeed, in vivo administration of anti-CD40L mAb greatly blocked the development of CD69+CD8+ T cells, which showed CTL activity and IFN-{gamma}-producing ability in response to {alpha}-GalCer (Fig. 5Go). Although anti-CD40L mAb strongly blocked CTL generation, it showed slight inhibition against CD69 antigen expression on CD8+ T cells (data not shown). Moreover, anti-IFN-{gamma} mAb also revealed partial blocking on {alpha}-GalCer-induced CD69 expression on CD8+ T cells, though it strongly inhibited CTL generation by {alpha}-GalCer (data not shown). Therefore, IFN-{gamma} produced by NKT cells through CD40–CD40L interaction with DC appeared to be a key factor for the final maturation of CD69+CD8+ T cells into CTL but not for initial CD69 expression on CD8+ T cells. Our preliminary experiment using anti-LFA-1 mAb indicated that the LFA-1/ICAM-dependent pathway may be important for initial activation of naive CD8+ T cells to express CD69 antigen by {alpha}-GalCer administration. During the preparation of this paper, Carnaud et al. also reported that {alpha}-GalCer administration induced CD69 expression on CD8+ T cells in addition to NKT, NK and B cells (31). Taken together, the present data and our previous in vitro analysis (19) suggest the following sequential immune responses early after {alpha}-GalCer administration: (i) {alpha}-GalCer binds with CD1d on DC; (ii) NKT cells interact with {alpha}-GalCer-bound DC; (iii) during NKT–DC interactions, DC produce IL-12 in a CD40–CD40L-dependent manner and NKT cells express IL-12R; (iv) NKT cells produce IFN-{gamma} in response to DC-derived IL-12; (v) during NKT–DC interactions, naive CD8+ T cells also interact with conditioned DC and express CD69 early activation antigen in an LFA-1-dependent, but not CD40–CD40L and IFN-{gamma}-dependent manner; (vi) IFN-{gamma} produced via NKT–{alpha}-GalCer-bound DC interactions induces the final maturation of CD69+CD8+ T cells into functional CTL.

In this paper, we have emphasized the effect of NKT cell activation on CD8+ CTL development. However, {alpha}-GalCer induced the production of both IFN-{gamma} and IL-4, and also activated B cells. Moreover, repeated administration of {alpha}-GalCer caused down-modulation of IFN-{gamma} production but not IL-4 production (3234). These studies suggest that {alpha}-GalCer may play an equally important role in bridging innate immunity mediated by NKT cells, and humoral immunity mediated by B cells and Th2 cells. A recent study has shown that NKT cells are involved in CD1d-restricted IgG formation to GPI-anchored antigens (25). Thus, our data presented here provided a new biological concept of NKT cells and suggests that immunodeviation through the control of NKT cell function may be a useful strategy for the modulation of acquired immunity involved in autoimmune diseases and cancer.


    Acknowledgments
 
We would like to thank to Dr Y. Koezuka (Kirin Brewery, Gunma, Japan) for providing {alpha}-GalCer. This work was supported in part by a Grant-in-Aid from The Science Frontier Program of MESSC, a Grant-in-Aid for Scientific Research on Priority Areas from MESSC, a Grant-in-Aid from the Ministry of Health and Welfare for Cancer Control, and a Grant-in-Aid for the IL-12 project of Tokai University School of Medicine.


    Abbreviations
 
{alpha}-GalCer {alpha}-galactosylceramide
APC antigen-presenting cells
CD40L CD40 ligand
CTL cytotoxic T lymphocyte
DC dendritic cell
IL-12R IL-12 receptor
NKT natural killer T
R-ADCC reverse antibody-dependent cellular cytotoxicity

    Notes
 
Transmitting editor: M. Miyasaka

Received 14 December 1999, accepted 7 March 2000.


    References
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 

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