CD28 controls the development of innate‐like CD8+ T cells by promoting the functional maturation of NKT cells

NK T cells(NKT cells) share functional characteristics and homing properties that are distinct from conventional T cells. In this study, we investigated the contribution of CD28 in the functional development of γδ NKT and αβ NKT cells in mice. We show that CD28 promotes the thymic maturation of promyelocytic leukemia zinc finger⁺ IL‐4⁺ NKT cells and upregulation of LFA‐1 expression on NKT cells. We demonstrate that the developmental defect of γδ NKT cells in CD28‐deficient mice is cell autonomous. Moreover, we show in both wild‐type C57BL/6 mice and in downstream of tyrosine kinase‐1 transgenic mice, a mouse model with increased numbers of γδ NKT cells, that CD28‐mediated regulation of thymic IL‐4⁺ NKT cells promotes the differentiation of eomesodermin⁺ CD44^high innate‐like CD8⁺ T cells. These findings reveal a previously unappreciated mechanism by which CD28 controls NKT‐cell homeostasis and the size of the innate‐like CD8⁺ T‐cell pool.     

H2‐M3‐restricted CD8+ T cells augment CD4+ T‐cell responses by promoting DC maturation

Infection with Listeria monocytogenes triggers the activation and expansion of nonconventional CD8⁺ T cells restricted by the MHC class Ib molecule, H2‐M3. H2‐M3‐restricted CD8⁺ T cells exhibit a memory phenotype, rapidly produce cytokines, and reach peak frequencies sooner than conventional MHC class Ia‐restricted CD8⁺ T cells. In this study, we found that simultaneous in vivo priming of H2‐M3‐restricted T cells and adoptively transferred OT‐II CD4⁺ T cells on the same DC enhances the survival of OT‐II cells. Stimulation of H2‐M3‐restricted T cells were found to induce DC maturation resulting in costimulatory molecule upregulation and production of T_H1‐type cytokines, which was dependent on both cell‐to‐cell contact and soluble factors, particularly TNF‐α, produced by activated H2‐M3‐restricted T cells. Interestingly, H2‐M3‐restricted T cells were more efficient than activated NK cells in inducing DC maturation. Furthermore, we found that OVA_323–339‐coated DC matured by coculturing with peptide‐stimulated H2‐M3‐restricted T cells were more efficient in stimulating the proliferation of Ag‐activated OT‐II cells. This study indicates that H2‐M3‐restricted T cells promote immune responses by CD4⁺ T cells by inducing DC maturation and suggests novel mechanisms for vaccine development.     

Inhibition of R5‐tropic HIV type‐1 replication in CD4+ natural killer T cells by γδ T lymphocytes

After the development of highly active anti‐retroviral therapy, it became clear that the majority of emergent HIV‐1 is macrophage‐tropic and infects CD4⁺, CCR5‐expressing cells (R5‐tropic). There are three distinct cell populations, R5‐tropic, HIV‐1‐susceptible CD4⁺ cells: (i) natural killer T (NKT) cells, (ii) dendritic cells and macrophages, and (iii) tissue‐associated T cells residing primarily at mucosal surfaces. We have confirmed that CD4⁺ NKT cells derived from peripheral blood mononuclear cells (PBMCs) predominantly express CCR5 rather than CXCR4, whereas the reverse is true for CD4⁺ T cells derived from circulating PBMCs, and that R5‐tropic HIV‐1 expands efficiently in the CD4⁺ NKT cells. Moreover, when PBMCs depleted of CD8α⁺ cells were stimulated in the presence of α‐galactosylceramide (α‐GalCer) and R5‐tropic HIV‐1 [NL(AD8)], the production of HIV‐1 virions was not suppressed, whereas, similar to the untreated PBMCs, depletion of CD8β⁺ cells from PBMCs significantly inhibited virion production. These findings suggest that CD8αα⁺ but not CD8αβ⁺ cells may have the ability to inhibit R5‐tropic HIV‐1 replication in CD4⁺ NKT cells. Here, we show that co‐culturing R5‐tropic HIV‐1‐infected CD4⁺ NKT cells with CD8αα⁺ γδ T cells, in particular Vγ1Vδ1 cells, but not with CD8αα⁺ NKT cells or CD8αα⁺ dendritic cells, inhibits HIV‐1 replication mainly by secreting chemokines, such as macrophage inflammatory proteins 1α and 1β and RANTES. Collectively, these results indicate the importance of CD8αα⁺ γδ T cells in the control of R5‐tropic HIV‐1 replication and persistence in CD4⁺ NKT cells.     

DC‐induced CD8+ T‐cell response is inhibited by MHC class II‐dependent DX5+CD4+ Treg

CD4⁺ T cells are important for CD8⁺ T‐cell priming by providing cognate signals for DC maturation. We analyzed the capacity of CD4⁺ T cells to influence CD8⁺ T‐cell responses induced by activated DC. Surprisingly, mice depleted for CD4⁺ cells were able to generate stronger antigen‐specific CD8⁺ T‐cell responses after DC vaccination than non‐depleted mice. The same observation was made when mice were vaccinated with MHC class II^?/? DC, indicating the presence of a MHC class II‐dependent CD4⁺ T‐cell population inhibiting CD8⁺ T‐cell responses. Recently we described the expansion of DX5⁺CD4⁺ T cells, a T‐cell population displaying immune regulatory properties, upon vaccination with DC. Intriguingly, we now observe an inverse correlation between CD8⁺ T‐cell induction and expansion of DX5⁺CD4⁺ T cells as the latter cells did not expand after vaccination with MHC class II^?/? DC. In vitro, DX5⁺CD4⁺ T cells were able to limit proliferation, modulate cytokine production and induce Foxp3⁺ expression in OVA‐specific CD8⁺ T cells. Together, our data show an inhibitory role of CD4⁺ T cells on the induction of CD8⁺ T‐cell responses by activated DC and indicate the involvement of DX5⁺CD4⁺, but not CD4⁺CD25⁺, T cells in this process.     

HIV gag protein is efficiently cross‐presented when targeted with an antibody towards the DEC‐205 receptor in Flt3 ligand‐mobilized murine DC

DC present exogenous proteins to MHC class I‐restricted CD8⁺ T cells. This function does not require endogenous antigen synthesis within DC, providing the potential to elicit CD8⁺ T‐cell responses to immune complexes, inactivated microbes, dying cells, and proteins such as OVA. In mice, the CD8⁺ or DEC‐205⁺ DC are specialized for cross‐presentation, and this subset can be increased 10‐fold in numbers following Fms‐like tyrosine kinase 3 ligand (Flt3L) treatment in vivo. Therefore, we studied cross‐presentation by abundant Flt3L DC using HIV gag protein. When enriched by positive selection with anti‐CD11c beads, cells from Flt3L mice are not only more abundant but are also more highly enriched in CD11c^high DC, particularly the DEC‐205⁺ subset. DC cross‐present HIV gag to primed CD8⁺ T cells, but when the antigen is delivered within an antibody to DEC‐205 receptor, cross‐presentation becomes 100‐fold more efficient than non‐targeted antigen. This finding requires gag to be engineered into anti‐DEC antibody, not just mixed with antibody. Flt3L DC are a valuable tool to study cross‐presentation, since their use overcomes the obstacle posed by the low number of cross‐presenting DC in the steady state. These findings support future experiments to use Flt3L to enhance presentation of DC‐targeted vaccines.     

Functional maturation of lamina propria dendritic cells by activation of NKT cells mediates the abrogation of oral tolerance

We previously showed that although systemic administration of α‐galactosylceramide (αGalCer) or agonistic anti‐CD40 induced functional maturation of dendritic cells (DC) in mesenteric lymph nodes, only the former treatment succeeded in breaking the induction of oral tolerance. In this study, we looked for the essential factor responsible for the disruption of oral tolerance. We found that lamina propria (LP)‐DC was responsible for the oral OVA presentation and that Peyer's patch was not essential for the induction of oral tolerance. Therefore, we investigated the role of LP‐DC. Treatment with αGalCer but not with anti‐CD40 induced the full maturation of LP‐DC at an early time point. This functional activation of LP‐DC was mediated by strong activation of NKT cells that reside abundantly in the small intestinal lamina propria (SI‐LP) and interferon‐γ partially contributed to the LP‐DC activation. LP‐DC isolated from αGalCer‐treated OVA‐fed mice induced the differentiation of naïve CD4⁺ T cells into Th1 and Th2 and was associated with the reduced Foxp3⁺ population. In contrast, LP‐DC isolated from anti‐CD40‐treated OVA‐fed mice failed to generate Th cell differentiation but induced more Foxp3⁺ CD4⁺ T cells. Our results demonstrate that triggered by NKT cells in SI‐LP, functional maturation of Ag‐capturing DC from SI‐LP is necessary for the abrogation of oral tolerance induction.     

In vivo activation of invariant natural killer T cells induces systemic and local alterations in T‐cell subsets prior to preterm birth

Preterm birth, the leading cause of neonatal morbidity and mortality worldwide, is frequently preceded by spontaneous preterm labour, a syndrome of multiple aetiologies. Pathological inflammation is causally linked to spontaneous preterm labour. Indeed, direct activation of invariant natural killer T (iNKT) cells via α‐galactosylceramide induces preterm labour/birth largely by initiating systemic and local (i.e. decidua and myometrium) innate immune responses. Herein, we investigated whether iNKT‐cell activation altered local and systemic T‐cell subsets. Administration of α‐galactosylceramide induced an expansion of activated CD1d‐restricted iNKT cells in the decidua and a reduction in the number of: (1) total T cells (conventional CD4⁺ and CD8⁺ T cells) through the down‐regulation of the CD3? molecule in the peripheral circulation, spleen, uterine‐draining lymph nodes (ULNs), decidua and/or myometrium; (2) CD4⁺ regulatory T cells in the spleen, ULNs and decidua; (3) T helper type 17 (Th17) cells in the ULNs but an increase in the number of decidual Th17 cells; (4) CD8⁺ regulatory T cells in the spleen and ULNs; and (5) CD4⁺ and CD8⁺ forkhead box protein 3 negative (Foxp3^–) responder T cells in the spleen and ULNs. As treatment with rosiglitazone prevents iNKT‐cell activation‐induced preterm labour/birth, we also explored whether the administration of this peroxisome proliferator‐activated receptor gamma (PPARγ) agonist would restore the number of T cells. Treating α‐galactosylceramide‐injected mice with rosiglitazone partially restored the number of T cells in the spleen but not in the decidua. In summary, iNKT‐cell activation altered the systemic and local T‐cell subsets prior to preterm labour/birth; however, treatment with rosiglitazone partially reversed such effects.     

In vivo depletion of DC impairs the anti‐tumor effect of agonistic anti‐CD137 mAb

Anti‐CD137 mAb are capable of inducing tumor rejection in several syngeneic murine tumor models and are undergoing clinical trials for cancer. The anti‐tumor effect involves co‐stimulation of tumor‐specific CD8⁺ T cells. Whether antigen cross‐presenting DC are required for the efficacy of anti‐CD137 mAb treatment has never been examined. Here we show that the administration of anti‐CD137 mAb eradicates EG7‐OVA tumors by a strictly CD8β⁺ T‐cell‐dependent mechanism that correlates with increased CTL activity. Ex vivo analyses to determine the identity of the draining lymph node cell type responsible for tumor antigen cross‐presentation revealed that CD11c⁺ cells, most likely DC, are the main players in this tumor model. A minute number of tumor cells, revealed by the presence of OVA cDNA, reach tumor‐draining lymph nodes. Direct antigen presentation by tumor cells themselves also participates in anti‐OVA CTL induction. Using CD11c diphtheria toxin receptor‐green fluorescent protein→C57BL/6 BM chimeric mice, which allow for sustained ablation of DC with diphtheria toxin, we confirmed the involvement of DC in tumor antigen cross‐presentation in CTL induction against OVA_257–264 epitope and in the antitumor efficacy induced by anti‐CD137 mAb.     

DC expressing transgene Foxp3 are regulatory APC

Tolerogenic DC and suppressive Foxp3⁺ Treg play important roles in preventing autoimmunity and allograft rejection. We report that (adenovirus mediated) ectopic expression of Foxp3 in human DC (i.e. DC.Foxp3) yields an APC that severely limits T‐cell proliferation and type‐1 immune responses from the naïve, but not memory, pool of responder T cells in vitro. In marked contrast, the frequencies of type‐2 and Treg responses were dramatically increased after stimulation of naïve T cells with DC.Foxp3 versus control DC. DC.Foxp3‐induced CD4⁺CD25⁺ Treg cells potently suppressed the proliferation of, and IFN‐γ production from, CD4⁺ and CD8⁺ responder T cells. Notably, the immunosuppressive biology of DC.Foxp3 was effectively normalized by addition of 1‐methyl‐tryptophan or neutralizing anti‐TGF‐β1 Ab during the period of T‐cell priming. These data suggest the potential utility of regulatory DC.Foxp3 and/or DC.Foxp3‐induced CD4⁺CD25⁺ Treg as translational agents for the amelioration or prevention of pathology in the setting of allograft transplantation and/or autoimmunity.     

Increased frequency of CD56Bright NK‐cells,CD3−CD16+CD56− NK‐cells and activated CD4+T‐cells or B‐cells in parallel with CD4+CDC25High T‐cells control potentially viremia in blood donors with HCV

A detailed phenotypic analysis of major and minor circulating lymphocyte subsets is described in potential blood donors with markers of hepatitis C virus (HCV), including non‐viremic and viremic groups. Although there were no changes in the hematological profile of either group, increased the levels of pre‐NK cells (CD3^?CD16⁺CD56^?) and a lower frequency of mature NK cells (CD3^?CD16⁺CD56⁺) characterized innate immunity in the non‐viremic group. Both non‐viremic and viremic groups displayed significantly increased levels of CD56^Bright NK cells. Furthermore, this subset was significantly elevated in the viremic subgroup with a low viral load. In addition, an increase in the NKT2 subset was observed only in this subgroup. An enhanced frequency of activated CD4⁺ T‐cells (CD4⁺HLA‐DR⁺) was a characteristic feature of the non‐viremic group, whereas elevated CD19⁺ B‐cells and CD19⁺CD86⁺ cell populations were the major phenotypic features of the viremic group, particularly in individuals with a low viral load. Although CD4⁺CD25^High T‐cells were significantly elevated in both the viremic and non‐viremic groups, it was particularly evident in the viremic low viral load subgroup. A parallel increase in CD4⁺CD25^High T‐cells, pre‐NK, and activated CD4⁺ T‐cells was observed in the non‐viremic group, whereas a parallel increase in CD4⁺CD25^High T‐cells and CD19⁺ B‐cells was characteristic of the low viral load subgroup. These findings suggest that CD56^Bright NK cells, together with pre‐NK cells and activated CD4⁺ T‐cells in combination with CD4⁺CD25^High T‐cells, might play an important role in controlling viremia. Elevated CD56^Bright NK cells, B‐cell responses and a T‐regulated immunological profile appeared to be associated with a low viral load. J. Med. Virol. 81:49–59, 2009. © 2008 Wiley‐Liss, Inc.     

Type I IFN regulate DC turnover in vivo

DC are the most potent antigen‐presenting cells that recognise signs of infection and serve as the main activators of naïve T cells. We have previously shown that type I IFN (IFN‐I) are produced by DC and can act in an autocrine manner to activate DC. In the present study, we have investigated the role of IFN‐I in regulating the turnover and lifespan of DC. We found that DC, especially the CD8α⁺ subset, from type I IFN receptor knock out (IFNAR KO) mice, display a reduced turnover rate when compared with DC from WT mice, as revealed by BrdU labelling kinetics. In vitro, IFNAR KO BM precursor cells cultured in the presence of GM‐CSF generated CD11c⁺ DC less efficiently than WT BM, and the IFNAR KO DC that arose displayed reduced migratory ability. Interestingly, splenic DC from IFNAR KO mice exhibited a higher survival rate in short‐term culture compared with control DC. Exposure to IFN‐I in vivo markedly increased the turnover rate of splenic DC, particularly CD8α⁺ DC, which was preceded by a transient induction of apoptosis. In accordance with this, IFN‐I stimulated the apoptosis of splenic DC in vitro. Overall, our data indicate that IFN‐I are important regulators of DC turnover in vivo and suggest that these cytokines may exert this function through the modulation of multiple processes involving DC apoptosis, proliferation and migration.     

Modulation of pulmonary DC function by vaccine‐encoded GM‐CSF enhances protective immunity against Mycobacterium tuberculosis infection

The rational design of new vaccines engineered to target key components of the host immune response is crucial to aid control of important infectious diseases such as tuberculosis. In this report, we determined whether modifying the function of pulmonary APC could improve protection against infection with Mycobacterium tuberculosis. Targeted delivery to the lung of the cytokine GM‐CSF, expressed by the Mycobacterium bovis BCG vaccine strain, increased pulmonary DC numbers and secretion of the immunoregulatory cytokine IL‐12, compared with parental BCG immunization. This impact on APC number by BCG:GM‐CSF resulted in accelerated priming of antigen‐specific CD4⁺ T cells in the mediastinal lymph nodes and increased migration of activated CD4⁺ T cells into the lung. i.n. administration of BCG:GM‐CSF resulted in significantly increased protection against M. tuberculosis infection compared with mice vaccinated with BCG alone. BCG:GM‐CSF exhibited an improved safety profile, as immunodeficient RAG1^?/? mice vaccinated i.n. with BCG:GM‐CSF survived significantly longer than control BCG‐vaccinated mice. These data demonstrate that manipulating immune cells in the lung by BCG‐based delivery of GM‐CSF can assist the development of protective mucosal immunity against pulmonary bacterial infection.     

NKT cells: Friend or foe during viral infections?

NKT cells are innate‐like T lymphocytes that are found in rodents and primates. They are non‐conventional T cells restricted by the CD1d molecule that presents self and exogenous glycolipids. NKT cells are unique in their ability to promptly secrete copious amounts of cytokines such as IFN‐γ and IL‐4. Once activated, NKT cells can provide maturation signals to downstream cells, including DC, NK cells, and lymphocytes, thereby contributing to both innate and acquired immunity. Accordingly, NKT cells can influence a wide array of immune responses, including tumor surveillance, maintenance of self‐tolerance and anti‐infectious defenses. Studies performed with NKT‐cell‐deficient mice have shown that these cells are critical for the clearance of various pathogens. During bacterial infections, NKT cells can be activated either indirectly by DC or directly by bacterial lipid antigens presented by CD1d. Although viruses do not contain lipid antigens, NKT cells have also been implicated in antiviral responses. The capacity of NKT cells to regulate viral immune‐surveillance, either constitutively or post‐activation, makes them an attractive clinical target. In this review, we summarize recent publications dealing with the functions and relevance of NKT cells in the context of viral infections, both in murine models and in humans.     

Regulation of IFN-γ production in granulocyte-macrophage colony-stimulating factor-deficient mice

Granulocyte-macrophage colony-stimulating factor-deficient (GM-CSF−/−) mice produce far lower serum levels of IFN-γ in response to LPS than GM-CSF+/+ mice. CD4⁺ and CD8⁺ T cells from LPS-injected GM-CSF−/− mice showed a deficiency in IFN-γ production and proliferative activity in response to IL-2 and IL-12, whereas IFN-γ production by NK cells was not compromised. These defects of T cells were reversed by administration of GM-CSF in vivo, but not by supplementation with GM-CSF in vitro. GM-CSF−/− mice do not have an intrinsic defect in IFN-γ production, because IL-12 injection induces the same high levels of IFN-γ in GM-CSF−/− and GM-CSF+/+ mice. To investigate the inhibitory effect of LPS on GM-CSF−/− T cells and the indirect restorative activity of GM-CSF, we tested the action of supernatants from cultured dendritic cells (DC). A factor or factors in the DC supernatant normalized serum IFN-γ levels and T cell responses in LPS-injected GM-CSF−/− mice. IL-18 reproduced some but not all of these in vivo and in vitro effects of DC supernatants. Our results indicate that GM-CSF is important in protecting T cells from inhibitory signals generated during immunization or exposure to LPS, and that this effect of GM-CSF is indirect and mediated by factors produced by DC.     

Innate and adaptive stimulation of murine diverse NKT cells result in distinct cellular responses

Natural killer T (NKT) cells recognize glycolipids presented on CD1d. They share features of adaptive T lymphocytes and innate NK cells, and mediate immunoregulatory functions via rapid production of cytokines. Invariant (iNKT) and diverse (dNKT) NKT cell subsets are defined by their TCR. The immunological role of dNKT cells, that do not express the invariant TCRα‐chain used by iNKT cells, is less well explored than that of iNKT cells. Here, we investigated signals driving Toll‐like receptor (TLR) ligand activation of TCR‐transgenic murine dNKT cells. IFN‐γ production by dNKT cells required dendritic cells (DC), cell‐to‐cell contact and presence of TLR ligands. TLR‐stimulated DC activated dNKT cells to secrete IFN‐γ in a CD1d‐, CD80/86‐ and type I IFN‐independent manner. In contrast, a requirement for IL‐12p40, and a TLR ligand‐selective dependence on IL‐18 or IL‐15 was observed. TLR ligand/DC stimulation provoked early secretion of pro‐inflammatory cytokines by both CD62L⁺ and CD62L^? dNKT cells. However, proliferation was limited. In contrast, TCR/co‐receptor‐mediated activation resulted in proliferation and delayed production of a broader cytokine spectrum preferentially in CD62L^? dNKT cells. Thus, innate (TLR ligand/DC) and adaptive (TCR/co‐receptor) stimulation of dNKT cells resulted in distinct cellular responses that may contribute differently to the formation of immune memory.     

IL‐33 promotes innate IFN‐γ production and modulates dendritic cell response in LCMV‐induced hepatitis in mice

Recent studies have revealed IL‐33 as a key factor in promoting antiviral T‐cell responses. However, it is less clear as to how IL‐33 regulates innate immunity. In this study, we infected wild‐type (WT) and IL‐33^?/? mice with lymphocytic choriomeningitis virus and demonstrated an essential role of infection‐induced IL‐33 expression for robust innate IFN‐γ production in the liver. We first show that IL‐33 deficiency resulted in a marked reduction in the number of IFN‐γ⁺ γδ T and NK cells, but an increase in that of IL‐17⁺ γδ T cells at 16 h postinfection. Recombinant IL‐33 (rIL‐33) treatment could reverse such deficiency via increasing IFN‐γ‐producing γδ T and NK cells, and inhibiting IL‐17⁺ γδ T cells. We also found that rIL‐33‐induced type 2 innate lymphoid cells were not involved in T‐cell responses and liver injury, since the adoptive transfer of type 2 innate lymphoid cells neither affected the IFN‐γ and TNF‐α production in T cells, nor liver transferase levels in lymphocytic choriomeningitis virus infected mice. Interestingly, we found that while IL‐33 was not required for costimulatory molecule expression, it was critical for DC proliferation and cytokine production. Together, this study highlights an essential role of IL‐33 in regulating innate IFN‐γ‐production and DC function during viral hepatitis.     

IL‐15 is critical for the maintenance and innate functions of self‐specific CD8+ T cells

IL‐15 is a pleiotropic cytokine involved in host defense as well as autoimmunity. IL‐15‐deficient mice show a decrease of memory phenotype (MP) CD8⁺ T cells, which develop naturally in naïve mice and whose origin is unclear. It has been shown that self‐specific CD8⁺ T cells developed in male H‐Y antigen‐specific TCR transgenic mice share many similarities with naturally occurring MP CD8⁺ T cells in normal mice. In this study, we found that H‐Y antigen‐specific CD8⁺ T cells in male but not female mice decreased when they were crossed with IL‐15‐deficient mice, mainly due to impaired peripheral maintenance. The self‐specific TCR transgenic CD8⁺ T cells developed in IL‐15‐deficient mice showed altered surface phenotypes and reduced effector functions ex vivo. Bystander activation of the self‐specific CD8⁺ T cells was induced in vivo during infection with Listeria monocytogenes, in which proliferation but not IFN‐γ production was IL‐15‐dependent. These results indicated important roles for IL‐15 in the maintenance and functions of self‐specific CD8⁺ T cells, which may be included in the naturally occurring MP CD8⁺ T‐cell population in naïve normal mice and participate in innate host defense responses.     

IFN-γ production downstream of NKT cell activation in mice infected with influenza virus enhances the cytolytic activities of both NK cells and viral antigen-specific CD8 T cells

Natural killer T (NKT) cell activation is responsible for eliminating pathogens. However, the biological functions of NKT cells against influenza virus are not fully understood. We therefore investigated the effects of NKT cells in viral infection using CD1d knockout (KO) mice. When CD1d KO or wild-type (WT) mice were infected with a sub-lethal dosage of the influenza virus, the survival rate of CD1d KO mice was significantly lower than for WT mice in association with delayed viral clearance in the lungs. Consistently, IFN-γ production in bronchoalveolar lavage fluid of CD1d KO mice was largely reduced compared to WT mice during infection. Moreover, the cytotoxic activities of NK cells and viral antigen-specific CD8⁺ T cells were impaired in CD1d KO mice. It was concluded that activated NKT cell-induced IFN-γ release enhances both NK-cell activity and antigen-specific CD8⁺ T cells to eliminate the influenza virus, thus leading to an enhanced survival.     

Depletion of CD4+CD25+ regulatory T cells enhances natural killer T cell‐mediated anti‐tumour immunity in a murine mammary breast cancer model

Both invariant natural killer T (NK T) cells and CD4⁺CD25⁺ T regulatory cells (T_regs) regulate the immune system to maintain homeostasis. In a tumour setting, NK T cells activated by α‐galactosylceramide (α‐GalCer) execute anti‐tumour activity by secreting cytokines. By contrast, T_regs intrinsically suppress antigen‐specific immune responses and are often found to be elevated in tumour patients. In this study, we have shown that T_regs regulate NK T cell function negatively in vitro, suggesting a direct interaction between these cell types. In a murine mammary tumour model, we demonstrated that administration of either α‐GalCer or anti‐CD25 antibody alone markedly suppressed tumour formation and pulmonary metastasis, and resulted in an increase in the survival rate up to 44% (from a baseline of 0%). When treatments were combined, depletion of T_regs boosted the anti‐tumour effect of α‐GalCer, and the survival rate jumped to 85%. Our results imply a potential application of combining T_reg cell depletion with α‐GalCer to stimulate NK T cells for cancer therapy.