The molecular biology of CD1

Clonal dynamics underlying the skewed CD4/CD8 ratio of mouse thymocytes revealed by TCR-independent barcoding

T cell differentiation in the thymus generates CD4⁺ helper and cytotoxic CD8⁺ cells as the two principal T cell lineages. Curiously, at the end of this complex selection process, CD4⁺ cells invariably outnumber CD8⁺ cells. Here, we examine the dynamics of repertoire formation and the emergence of the skewed CD4/CD8 ratio using high-resolution endogenous CRISPR/Cas9 barcoding that indelibly marks immature T cells at the DN2/DN3 pre-TCR stage. In wild-type mice, greater clone size of CD4⁺ cells and an intrinsically greater probability of Tcr β clonotypes for pMHCII interactions are major contributors to the skewed CD4/CD8 ratio. Clonal perturbations of thymocyte differentiation following the precocious expression of a rearranged iNKT invariant TCR α chain are due to loss of Tcr β clonotypes from the CD4 lineage-committed pre-selection repertoire. The present barcoding scheme offers a novel means to examine the clonal dynamics of lymphocyte differentiation orthogonal to that using TCR clonotypes.

CRSIPR-mediated barcoding of pre-TCR-expressing T cells is used to investigate the clonal dynamics of thymic T cells and reveals new fundamental understanding of how the skewed CD4/CD8 ratio arises during thymic selection.

Piperine for treating breast cancer: A review of molecular mechanisms, combination with anticancer drugs, and nanosystems

Piperine (PIP) is an alkaloid found primarily in Piper longum, and this natural compound has been shown to exert effects on proliferation and survival against various types of cancer. In particular, PIP has potent inhibitory effects on breast cancer (BC), the most prevalent type of cancer in women worldwide. PIP targets numerous signaling pathways associated with the therapy of BC cells through the following mechanisms: (a) induction of arrest of the cell cycle and apoptosis; (b) alteration of the signaling protein expression; (c) reduction in transcription factors; and (d) inhibition of tumor growth. BC cells have the ability to resist conventional drugs, so one of the strategies is the combination of PIP with other phytochemicals such as paclitaxel, thymoquinone, hesperidin, bee venom, tamoxifen, mitoxantrone, piperlongumin, and curcumin. Nanotechnology-based drug encapsulation systems are currently used to enhance the release of PIP. This includes polymer nanoparticles, carbon nanotubes, and liposomes. In the present review, the chemistry and bioavailability of PIP, its molecular targets in BC, and nanotechnological strategies are discussed. Future research directions are also discussed to further understand this promising natural product.

Type I Natural Killer T Cells as Key Regulators of the Immune Response to Infectious Diseases

The immune system must work in an orchestrated way to achieve an optimal response upon detection of antigens. The cells comprising the immune response are traditionally divided into two major subsets, innate and adaptive, with particular characteristics for each type. Type I natural killer T (iNKT) cells are defined as innate-like T cells sharing features with both traditional adaptive and innate cells, such as the expression of an invariant T cell receptor (TCR) and several NK receptors. The invariant TCR in iNKT cells interacts with CD1d, a major histocompatibility complex class I (MHC-I)-like molecule. CD1d can bind and present antigens of lipid nature and induce the activation of iNKT cells, leading to the secretion of various cytokines, such as gamma interferon (IFN-γ) and interleukin 4 (IL-4). These cytokines will aid in the activation of other immune cells following stimulation of iNKT cells. Several molecules with the capacity to bind to CD1d have been discovered, including α-galactosylceramide. Likewise, several molecules have been synthesized that are capable of polarizing iNKT cells into different profiles, either pro- or anti-inflammatory. This versatility allows NKT cells to either aid or impair the clearance of pathogens or to even control or increase the symptoms associated with pathogenic infections. Such diverse contributions of NKT cells to infectious diseases are supported by several publications showing either a beneficial or detrimental role of these cells during diseases. In this article, we discuss current data relative to iNKT cells and their features, with an emphasis on their driving role in diseases produced by pathogenic agents in an organ-oriented fashion.

Identification and Characterization of Adipose Tissue-Derived Human Antibodies With "Anti-self" Specificity

We have previously shown that the human obese adipose tissue (AT) contributes to increased secretion of adipocyte-specific IgG antibodies in individuals with obesity. This occurs without any exogenous stimulation, because the ongoing process of cell death in the obese AT leads to the release of "self" antigens able to induce chronic stimulation of B cells. We have identified several mechanisms responsible for the release of "self" antigens, such as hypoxia, cell cytotoxicity, and DNA damage. In this paper, we confirm and extend our initial observation on a different cohort of individuals, and we show that also the plasma of these individuals is enriched in IgG antibodies with specificities for adipocyte-derived antigens. Adipocyte-specific IgG secreted in the obese AT are significantly correlated with those present in plasma. Using immunoprecipitation and mass spectrometry, we have identified these antigenic specificities. The antigens are almost exclusively intracellular or cell-associated, usually not recognized as "self" antigens, but they are released by cells dying in the AT. We also show for the first time that the adipocytes in the obese AT contribute to the secretion of IgG autoimmune antibodies and this seems to be due to their expression of the antigen-presenting molecules CD1d and, to a much lesser extent, MHC class II, as our mechanistic experiments performed in mice have shown. These results may lead to the development of novel therapeutic strategies to control autoimmunity.

The Implication and Significance of Beta 2 Microglobulin: A Conservative Multifunctional Regulator

OBJECTIVE

This review focuses on the current knowledge on the implication and significance of beta 2 microglobulin (β2M), a conservative immune molecule in vertebrate.

DATA SOURCES

The data used in this review were obtained from PubMed up to October 2015. Terms of β2M, immune response, and infection were used in the search.

STUDY SELECTIONS

Articles related to β2M were retrieved and reviewed. Articles focusing on the characteristic and function of β2M were selected. The exclusion criteria of articles were that the studies on β2M-related molecules.

RESULTS

β2M is critical for the immune surveillance and modulation in vertebrate animals. The dysregulation of β2M is associated with multiple diseases, including endogenous and infectious diseases. β2M could directly participate in the development of cancer cells, and the level of β2M is deemed as a prognostic marker for several malignancies. It also involves in forming major histocompatibility complex (MHC class I or MHC I) or like heterodimers, covering from antigen presentation to immune homeostasis.

CONCLUSIONS

Based on the characteristic of β2M, it or its signaling pathway has been targeted as biomedical or therapeutic tools. Moreover, β2M is highly conserved among different species, and overall structures are virtually identical, implying the versatility of β2M on applications.

Interplay between carbohydrate and lipid in recognition of glycolipid antigens by natural killer T cells

Natural killer T (NKT) cells are a T cell subpopulation that were named originally based on coexpression of receptors found on natural killer (NK) cells, cells of the innate immune system, and by T lymphocytes. The maturation and activation of NKT cells requires presentation of glycolipid antigens by CD1d, a cell surface protein distantly related to the major histocompatibility complex (MHC)-encoded antigen presenting molecules. This specificity distinguishes NKT cells from most CD4(+) and CD8(+) T cells that recognize peptides presented by MHC class I and class II molecules. The rapid secretion of a large amount of both Th1 and Th2 cytokines by activated NKT cells endows them with the ability to play a vital role in the host immune defense against various microbial infections. In this review, we summarize progress on identifying the sources of microbe-derived glycolipid antigens recognized by NKT cells and the biochemical basis for their recognition.

Structural and functional aspects of lipid binding by CD1 molecules

Over the past ten years, investigators have shown that T lymphocytes can recognize not only peptides in the context of MHC class I and class II molecules but also foreign and self-lipids in association with the nonclassical MHC class I molecules the CD1 proteins. We describe the events that have led to the discovery of the role of CD1 molecules, their pattern of intracellular trafficking, and their ability to sample different intracellular compartments for self- and foreign lipids. Structural and functional aspects of lipid presentation by CD1 molecules are presented in the context of the function of CD1-restricted T cells in antimicrobial responses, antitumor immunity, and the regulation of the tolerance and autoimmunity immunoregulatory axis. Particular emphasis is on invariant NKT (iNKT) cells and their ability to modulate innate and adaptive immune responses.

Role of natural killer T cells in liver diseases

Natural killer T (NKT) cells are a unique subset of lymphocytes, which express both T and NK cell surface markers as well as functional characteristics. They connect innate and acquired immunity, and restrictedly recognize glycolipid antigens presented by CD1d molecule. NKT cells secrete a great quantity of cytokines rapidly which regulate human immune response after being activated. For there are abundant NKT cells in human liver, researchers have a great interest in their roles.

B cell receptor-mediated uptake of CD1d-restricted antigen augments antibody responses by recruiting invariant NKT cell help in vivo

Highly regulated activation of B cells is required for the production of specific antibodies necessary to provide protection from pathogen infection. This process is initiated by specific recognition of antigen through the B cell receptor (BCR), leading to early intracellular signaling followed by the late recruitment of T cell help. In this study we demonstrate that specific BCR uptake of CD1d-restricted antigens represents an effective means of enhancing invariant natural killer T (iNKT)-dependent B cell responses in vivo. This mechanism is effective over a wide range of antigen affinities but depends on exceeding a tightly regulated avidity threshold necessary for BCR-mediated internalization and CD1d-dependent presentation of particulate antigenic lipid. Subsequently, iNKT cells provide the help required for stimulating B cell proliferation and differentiation. iNKT-stimulated B cells develop within extrafollicular foci and mediate the production of high titers of specific IgM and early class-switched antibodies. Thus, we have demonstrated that in response to particulate antigenic lipids iNKT cells are recruited for the assistance of B cell activation, resulting in the enhancement of specific antibody responses. We propose that such a mechanism may operate to potentiate adaptive immune responses against pathogens in vivo.

IgG regulates the CD1 expression profile and lipid antigen-presenting function in human dendritic cells via FcgammaRIIa

Dendritic cells (DCs) process and present bacterial and endogenous lipid antigens in complex with CD1 molecules to T cells and invariant natural killer T (NKT) cells. However, different types of DCs, such as blood myeloid DCs and skin Langerhans cells, exhibit distinct patterns of CD1a, CD1b, CD1c, and CD1d expression. The regulation of such differences is incompletely understood. Here, we initially observed that monocyte-derived DCs cultured in an immunoglobulin-rich milieu expressed CD1d but not CD1a, CD1b, and CD1c, whereas DCs cultured in the presence of low levels of immunoglobulins had an opposite CD1 profile. Based on this, we tested the possibility that immunoglobulins play a central role in determining these differences. IgG depletion and intravenous immunoglobulin (IVIg) add-in experiments strongly supported a role for IgG in directing the CD1 expression profile. Blocking experiments indicated that this effect was mediated by FcgammaRIIa (CD32a), and quantitative polymerase chain reaction data demonstrated that regulation of the CD1 profile occurred at the gene expression level. Finally, the ability of DCs to activate CD1-restricted NKT cells and T cells was determined by this regulatory effect of IgG. Our data demonstrate an important role for FcgammaRIIa in regulating the CD1 antigen presentation machinery of human DCs.

Inhibition of groups 1 and 2 CD1 molecules on human dendritic cells by Leishmania species

Dendritic cells are potent immune-activating cells, which traditionally are thought of as presenters of protein antigen to lymphocytes to initiate an immune response. Recently, another mechanism of immune surveillance, the detection of lipid antigens, has been found to be mediated by the nonpolymorphic family of CD1 molecules. There are two different CD1 families, Group 1 consisting of CD1a, CD1b and CD1c, and Group 2 consisting only of CD1d. Leishmania donovani-infected dendritic cells have previously been demonstrated to exhibit decreased surface levels of Group 1 CD1 molecules and are no longer able to initiate a CD1b-restricted T cell response. In contrast to L. donovani, which disseminates to the visceral organs, L. major remains localized, forming a cutaneous lesion. We investigate here whether L. major, the aetiological agent of cutaneous leishmaniasis, also inhibits CD1 expression. We demonstrate that infection of human monocyte-derived dendritic cells with either L. major or L. donovani results in transcriptional down-regulation of both Groups 1 and 2 CD1 molecules. Furthermore, infection of monocytes during differentiation results in a cell phenotype similar to undifferentiated monocytes. Finally, we demonstrate that this down-regulation is not mediated by lipophosphoglycan or other phosphoglycans.

Evolutionary biology of CD1

The recognition more than a decade ago that lipids presented by CD1 could function as T cell antigens revealed a startling and previously unappreciated complexity to the adaptive immune system. The initial novelty of lipid antigen presentation by CD1 has since given way to a broader perspective of the immune system's capacity to sense and respond to a diverse array of macromolecules. Some immune recognition systems such as Toll-like receptors can trace their origins back into the deep history of sea urchins and arthropods. Others such as the major histocompatibility complex (MHC) appear relatively recently and interestingly, only in animals that also possess a jaw. The natural history of CD1 is thus part of the wider story of immune system evolution and should be considered in this context. Most evidence indicates that CD1 probably evolved from a classical MHC class I (MHC I) gene at some point during vertebrate evolution. This chapter reviews the evidence for this phylogenetic relationship and attempts to connect CD1 to existing models of MHC evolution. This endeavor is facilitated today by the recent availability of whole genome sequence data from a variety of species. Investigators have used these data to trace the ultimate origin of the MHC to a series of whole genome duplications that occurred roughly 500 million years ago. Sequence data have also revealed homologs of the mammalian MHC I and MHC II gene families in virtually all jawed vertebrates including sharks, bony fishes, reptiles, and birds. In contrast, CD1 genes have thus far been found only in a subset of these animal groups. This pattern of CD1 occurrence in the genomes of living species suggests the emergence of CD 1 in an early terrestrial vertebrate.

Activating Transcription Factor/cAMP Response Element Binding Protein Family Member Regulated Transcription of CD1A

CD1a has a unique expression pattern among Ag-presenting molecules, expressed specifically on cortical thymocytes and APCs. As autoimmune disease, infection, and tumors can all result in alteration of CD1a expression, we are attempting to characterize the transcriptional regulation, and thus shed some light on specific expression, of CD1A. In this study, we have identified a minimal proximal promoter region required for CD1A transcription. Computer searches within this region identified numerous potential binding sites for lymphoid-specific transcription factors, including the ETS transcription factors, C/EBP, GATA, and CREB. Deletion and site-specific mutant analysis revealed a critical role of a potential cAMP response element (CRE) 965 bp upstream of the CD1A translation start site. Two activating transcription factor (ATF)/CREB family members, CREB-1 and ATF-2, are able to bind this site in vitro and in vivo. Notably, activation of ATF/CREB family members decreases CD1A transcription, while decrease in ATF-2 expression results in increased CD1A RNA level. The fact that these factors also bind the CD1A promoter in human monocytes strongly suggests a role for ATF/CREB family members in regulation of CD1A expression.

Transcriptional regulation of CD1D1 by Ets family transcription factors

CD1 molecules are MHC class I-like glycoproteins specialized in presenting lipid/glycolipid Ags to T cells. The distinct cell-type specific expression of CD1D1 plays an important role in the development and function of NKT cells, a unique subset of immunoregulatory T cells. However, the mechanisms regulating CD1D1 expression are largely unknown. In this study, we have characterized the upstream region of the CD1D1 gene and identified a minimal promoter region within 200 bp from the translational start site of CD1D1 that exhibits cell-type specific promoter activity. Analysis of this region revealed an Ets binding site critical for CD1D1 promoter activity. Gel shift assays and chromatin immunoprecipitation experiments showed that Elf-1 and PU.1 bind to the CD1D1 promoter. Furthermore, we found that gene disruption of Elf-1 resulted in decreased CD1D1 expression on B cells but not other cell types, whereas conditional activation of PU.1 negatively regulated CD1D1 expression in PU.1-deficient myeloid cells. These findings are the first to demonstrate that Ets proteins are involved in the transcriptional regulation of CD1D1 and that they may function uniquely in different cell types.

The mouse CD1d cytoplasmic tail mediates CD1d trafficking and antigen presentation by adaptor protein 3-dependent and -independent mechanisms

The short cytoplasmic tail of mouse CD1d (mCD1d) is required for its endosomal localization, for the presentation of some glycolipid Ags, and for the development of Valpha14i NKT cells. This tail has a four-amino acid Tyr-containing motif, Tyr-Gln-Asp-Ile (YQDI), similar to those sequences known to be important for the interaction with adaptor protein complexes (AP) that mediate the endosomal localization of many different proteins. In fact, mCD1d has been shown previously to interact with the AP-3 adaptor complex. In the present study, we mutated each amino acid in the YQDI motif to determine the importance of the entire motif sequence in influencing mCD1d trafficking, its interaction with adaptors, and its intracellular localization. The results indicate that the Y, D, and I amino acids are significant functionally because mutations at each of these positions altered the intracellular distribution of mCD1d and reduced its ability to present glycosphingolipids to NKT cells. However, the three amino acids are not all acting in the same way because they differ with regard to how they influence the intracellular distribution of CD1d, its rate of internalization, and its ability to interact with the mu subunit of AP-3. Our results emphasize that multiple steps, including interactions with the adaptors AP-2 and AP-3, are required for normal trafficking of mCD1d and that these different steps are mediated by only a few cytoplasmic amino acids.

Expression of CD1d under the control of a MHC class Ia promoter skews the development of NKT cells, but not CD8+ T cells

Although CD1d and MHC class Ia share similar overall structure, they have distinct levels and patterns of surface expression. While the expression of CD1d1 is known to be essential for the development of NKT cells, the contribution of CD1d1 to the development of CD8(+) T cells appears to be inconsequential. To investigate whether CD1d tissue distribution and expression levels confer differential capacity in selecting these two T cell subsets, we analyzed CD8 and NKT cell compartments in K(b)-CD1d-transgenic mice that lack endogenous MHC class Ia and CD1d, respectively. We found that MHC class Ia-like expression pattern and tissue distribution are not sufficient for CD1d to rescue the development of CD8(+) T cells, suggesting that unique structural features of CD1d preclude its active participation in selection of CD8(+) T cells. Conversely, cell type-specific CD1d surface density is important for the selection of NKT cells, as the NKT cell compartment was only partially rescued by the K(b)-CD1d transgene. We have previously demonstrated that increased CD1d expression on dendritic cells enhanced negative selection of NKT cells. In this study, we show that cell type-specific expression levels of CD1d establish a narrow window between positive and negative selection, suggesting that the distinct CD1d expression pattern may be selected evolutionarily to ensure optimal output of NKT cells.

Lysosomal localization of murine CD1d mediated by AP-3 is necessary for NK T cell development

The presentation of lipid and glycolipid Ags to T cells is mediated through CD1 molecules. In the mouse and rat only a single isoform, CD1d, performs these functions, while humans and all other mammals studied have members of both group I (CD1a, -b, and -c) and group II (CD1d) isoforms. Murine CD1d contains a cytoplasmic tyrosine-based sorting motif that is similar to motifs recognized by adaptor protein complexes that sort transmembrane proteins. Here we show that the adaptor protein complex, AP-3, directly interacts with murine CD1d and controls its targeting to lysosomes. AP-3 deficiency results in a redistribution of CD1d from lysosomes to the cell surface of thymocytes, B cell-depleted splenocytes, and dendritic cells. The altered trafficking of CD1d in AP-3-deficient mice results in a significant reduction of NK1.1(+)TCR-beta(+) and CD1d tetramer-positive cells, consistent with a defect in CD1d self-Ag presentation and thymocyte-positive selection. The AP-3 complex has recently been shown to associate with the human CD1b isoform, which has an intracellular distribution pattern similar to that of murine CD1d. We propose that lysosomal sampling may be so critical for efficient host defense that mice have evolved mechanisms to target their single CD1 isoform to lysosomes for sampling lipid Ags. Here we show the dominant mechanism for this trafficking is mediated by AP-3.

The CD1 family and T cell recognition of lipid antigens

For many years it was thought that T lymphocytes recognized only peptide antigens presented by MHC class I or class II molecules. Recently, it has become clear that a wide variety of lipids and glycolipids are also targets of the T cell response. This novel form of cell-mediated immune recognition is mediated by a family of lipid binding and presenting molecules known as CD1. The CD1 proteins represent a small to moderate sized family of beta2-microglobulin-associated transmembrane proteins that are distantly related to MHC class I and class II molecules. They are conserved in most or all mammals, and control the development and function of T cell populations that participate in innate and adaptive immune responses through the recognition of self and foreign lipid antigens. Here we review the current state of our understanding of the structure and function of CD1 proteins, and the role of CD1-restricted T cell responses in the immune system.

CD1 tetramers: a powerful tool for the analysis of glycolipid-reactive T cells

CD1 proteins constitute a third class of antigen-presenting molecules. They bind lipids rather than peptides, and the T cells reactive to lipids presented by CD1 have been implicated in the protection against autoimmune diseases and infectious microorganisms and in the immune surveillance for tumors. Thus, the ability to identify, purify, and track the response of CD1-reactive cells is of paramount importance. Previously existing methods for identifying these T cells were not based on TCR specificity, and therefore the data obtained by these methods were in some cases difficult to interpret. The recent generation of tetramers of alpha-galactosyl ceramide (alpha-GalCer) with CD1d has already permitted significant insight into the biology of NKT cells. Tetramers constructed from other CD1 molecules also have been obtained during the previous year. Collectively, these new reagents promise to greatly expand knowledge of the functions of lipid-reactive T cells, with potential use in monitoring the response to lipid-based vaccines and other treatments and in the diagnosis of autoimmune diseases.

Lipid length controls antigen entry into endosomal and nonendosomal pathways for CD1b presentation

CD1 proteins present various glycolipid antigens to T cells, but the cellular mechanisms that control which particular glycolipids generate T cell responses are not understood. We show here that T cell recognition of glucose monomycolate antigens with long (C(80)) alkyl chains involves the delivery of CD1b proteins and antigens to late endosomes in a process that takes several hours. In contrast, analogs of the same antigen with shorter (C(32)) alkyl chains are rapidly, but inefficiently, presented by cell surface CD1b proteins. Dendritic cells (DCs) preferentially present long-chain glycolipids, which results, in part, from their rapid internalization and selective delivery of antigens to endosomal compartments. Nonprofessional antigen-presenting cells, however, preferentially present short-chain glycolipids because of their lack of prominent endosomal presentation pathways. Because long alkyl chain length distinguishes certain microbial glycolipids from common mammalian glycolipids, these findings suggest that DCs use a specialized endosomal-loading pathway to promote preferential recognition of glycolipids with a more intrinsically foreign structure.