The functions of type I and type II natural killer T cells in inflammatory bowel diseases.

CD1d-restricted natural killer T (NKT) cells are a distinct subset of T cells that rapidly produce an array of cytokines on activation and play a critical role in regulating various immune responses. NKT cells are classified into 2 groups based on differences in T-cell receptor usage. Type I NKT cells have an invariant T-cell receptor α-chain and are readily detectable by α-galactosylceramide (α-GalCer)-loaded CD1d tetramers. Type II NKT cells have a more diverse T-cell receptor repertoire and cannot be directly identified. Both types of NKT cells and multiple CD1d-expressing cell types are present in the intestine, and their interactions are likely to be modulated by pathogenic and commensal microbes, which in turn contribute to the intestinal immune responses in health and disease. Indeed, in several animal models of inflammatory bowel disease, type I NKT cells have been shown to make both protective and pathogenic contributions to disease. In contrast, in patients with ulcerative colitis, and a mouse model in which both CD1d expression and the frequency of type II NKT cells are increased, type II NKT cells seem to promote intestinal inflammation. In this review, we summarize the present knowledge on the antigen recognition, activation, and function of NKT cells with a particular focus on their role in inflammatory bowel disease and discuss factors that may influence the functional outcome of NKT cell responses in intestinal inflammation.

dysregulation of CD1d-restricted type ii natural killer T cells leads to spontaneous development of colitis in mice.

BACKGROUND & AIMS: CD1d-restricted natural killer (NK) T cells are a subset of immunoregulatory T cells that comprise type I (express the semi-invariant T-cell receptor [TCR] and can be detected using the α-galactosylceramide/CD1d tetramer) and type II (express diverse TCRs and cannot be directly identified). Studies in mouse models of inflammatory bowel disease revealed a complex role for type I NKT cells in the development of colitis. Type II NKT cells have been associated with intestinal inflammation in patients with ulcerative colitis. METHODS: To investigate whether dysregulation of type II NKT cells, caused by increased expression of CD1d, can contribute to colitis, we generated transgenic mice that express high levels of CD1d and a TCR from an autoreactive, type II NKT cell (CD1dTg/24αßTg mice). RESULTS: CD1dTg/24αßTg mice had reduced numbers of 24αß T cells compared with 24αßTg mice, indicating that negative selection increases among type II NKT cells engaged by abundant self-antigen. The residual 24αß T cells in CD1dTg/24αßTg mice had an altered surface phenotype and acquired a cytokine profile distinct from that of equivalent cells in 24αßTg mice. Interestingly, CD1dTg/24αßTg mice spontaneously developed colitis; adoptive transfer experiments confirmed that type II NKT cells that develop in the context of increased CD1d expression are pathogenic. CONCLUSIONS: Aberrant type II NKT cell responses directly contribute to intestinal inflammation in mice, indicating the importance of CD1d expression levels in the development and regulation of type II NKT cells.

CD1-restricted adaptive immune responses to Mycobacteria in human group 1 CD1 transgenic mice.

Group 1 CD1 (CD1a, CD1b, and CD1c)-restricted T cells recognize mycobacterial lipid antigens and are found at higher frequencies in Mycobacterium tuberculosis (Mtb)-infected individuals. However, their role and dynamics during infection remain unknown because of the lack of a suitable small animal model. We have generated human group 1 CD1 transgenic (hCD1Tg) mice that express all three human group 1 CD1 isoforms and support the development of group 1 CD1-restricted T cells with diverse T cell receptor usage. Both mycobacterial infection and immunization with Mtb lipids elicit group 1 CD1-restricted Mtb lipid-specific T cell responses in hCD1Tg mice. In contrast to CD1d-restricted NKT cells, which rapidly respond to initial stimulation but exhibit anergy upon reexposure, group 1 CD1-restricted T cells exhibit delayed primary responses and more rapid secondary responses, similar to conventional T cells. Collectively, our data demonstrate that group 1 CD1-restricted T cells participate in adaptive immune responses upon mycobacterial infection and could serve as targets for the development of novel Mtb vaccines.

Polymorphisms in CD1d affect antigen presentation and the activation of CD1d-restricted T cells.

CD1 proteins constitute a distinct lineage of antigen-presenting molecules specialized for the presentation of lipid antigens to T cells. In contrast to the extensive sequence polymorphism characteristic of classical MHC molecules, CD1 proteins exhibit limited sequence diversity. Here, we describe the identification and characterization of CD1d alleles in wild-derived mouse strains. We demonstrate that polymorphisms in CD1d affect the presentation of endogenous and exogenous ligands to CD1d-restricted T cells, including type I (Valpha14i) and type II (non-Valpha14i) natural killer T (NKT) cells. Using congenic mice, we found CD1d polymorphisms affect the thymic selection of type I NKT cells and induce allogeneic T cell responses. Collectively, results from these studies demonstrate a role for polymorphisms in influencing the development and function of CD1d-restricted T cells.

A cell-type specific CD1d expression program modulates invariant NKT cell development and function.

Invariant NK T (iNKT) cells are a distinct subset of T cells that rapidly produce an array of immunoregulatory cytokines upon activation. Cytokines produced by iNKT cells subsequently transactivate other leukocytes and elicit their respective effector functions. In this way, iNKT cells play a central role in coordinating the development of immune responses in a variety of settings. However, the mechanisms governing the quality of the iNKT cell response elicited remain poorly defined. To address whether changes in the CD1d expression pattern could regulate iNKT cell function, we generated a transgenic (Tg) mouse model in which thymocytes and peripheral T cells express high levels of CD1d (Lck-CD1d Tg+ mice). The expression of CD1d by T cells was sufficient to rescue development of iNKT cells in mice deficient of endogenous CD1d. However, the relative proportions of iNKT cell subsets in Lck-CD1d Tg+ mice were distinctly different from those in wild-type mice, suggesting an altered developmental program. Additionally, iNKT cells were hyporesponsive to antigenic stimulation in vivo. Interestingly, Lck-CD1d Tg+ mice develop liver pathology in the absence of any exogenous manipulation. The results of these studies suggest that changes to the CD1d expression program modulate iNKT cell development and function.

Selective inhibition of anthrax edema factor by adefovir, a drug for chronic hepatitis B virus infection.

Edema factor (EF), a key virulence factor in anthrax pathogenesis, has calmodulin (CaM)-activated adenylyl cyclase activity. We have found that adefovir dipivoxil, a drug approved to treat chronic infection of hepatitis B virus, effectively inhibits EF-induced cAMP accumulation and changes in cytokine production in mouse primary macrophages. Adefovir diphosphate (PMEApp), the active cellular metabolite of adefovir dipivoxil, inhibits the adenylyl cyclase activity of EF in vitro with high affinity (K(i) = 27 nM). A crystal structure of EF-CaM-PMEApp reveals that the catalytic site of EF forms better van der Waals contacts and more hydrogen bonds with PMEApp than with its endogenous substrate, ATP, providing an explanation for the approximately 10,000-fold higher affinity EF-CaM has for PMEApp versus ATP. Adefovir dipivoxil is a clinically approved drug that can block the action of an anthrax toxin. It can be used to address the role of EF in anthrax pathogenesis.

A role for class A scavenger receptor in dendritic cell nibbling from live cells.

Monocyte-derived dendritic cells (DC) possess the unique capacity to capture Ag from live cells through intimate cell contact, a process referred to as nibbling. We sought to define the receptor(s) mediating DC nibbling. Uptake of fluorescently labeled plasma membrane from live cells by DC was inhibited by protease treatment and by a panel of polyanionic ligands, implicating scavenger receptors (SR) in this process. Differential expression of SR on DC and macrophages correlated with the capacity to acquire membrane from live cells. Internalized membrane colocalized with SR ligand and entered the endosomal pathway. DC very efficiently acquired and internalized gp100 tumor Ag expressed at the surface of viable adenocarcinoma cells via recombinant adenoviral infection. Cross-presentation of gp100 by DC to MHC class I-restricted T cells was inhibited by polyanionic SR ligand and an Ab to type A SR (SR-A), whereas Ab to the class B SR CD36, which mediates uptake of apoptotic cells, induced no inhibition. DC capture of fluorescently labeled membrane from live cells was partially blocked by SR-A-specific Ab, suggesting that other SR may also be contributing to nibbling. DC maturation resulted in a switch in expression from type II SR-A (SR-AII) to the SR-AI splice variant. Finally, SR-A was identified on interdigitating DC isolated from monkey lymph nodes. These findings define a novel role for SR-A, and suggest that Ag uptake from live cells by DC may be important in the generation of immunity and in the maintenance of peripheral tolerance in vivo.

Disrupted homeostasis of Langerhans cells and interdigitating dendritic cells in monkeys with AIDS.

Langerhans cells (LCs) are immature dendritic cells (DCs) that capture antigen in peripheral tissues and migrate to draining lymph nodes, where they reside in the paracortex as interdigitating dendritic cells (IDCs). We studied the effects of simian immunodeficiency virus (SIV) on LCs and IDCs during different stages of infection in monkeys. LCs isolated from monkeys with acute SIV infection or acquired immunodeficiency syndrome (AIDS) underwent normal maturation in vitro, including a switch in chemokine receptor expression from CCR5 to CXCR4 and CCR7. LCs migrated normally from skin in response to contact sensitization in monkeys with acute SIV infection. In contrast, LC migration from skin was markedly impaired during AIDS, associated with a reduction in antigen-bearing DCs in draining lymph nodes. Lymph node IDCs were increased in proportion during acute SIV infection and had an activated phenotype, whereas during AIDS IDCs had significantly lower expression of CD40 and the activation marker CD83. IDCs from monkeys with AIDS were refractory to stimulation with CD40L, demonstrating a functional consequence of decreased CD40 expression. SIV-infected DCs were not identified in lymph nodes or skin of monkeys with AIDS, suggesting an indirect effect of infection on DC populations in vivo. These data indicate that DCs are mobilized to lymph nodes during acute SIV infection, but that during AIDS this process is suppressed, with LC migration and IDC activation being impaired. We conclude that disruption of DC homeostasis may play a role in immunopathology induced by human immunodeficiency virus and suggest that therapeutic strategies targeting DCs may have limited efficacy during AIDS.