The surprising diversity of lipid antigens for CD1-restricted T cells

Phosphatidylserine externalization as immune checkpoint in cancer

Cancer is the second leading cause of mortality worldwide. Despite recent advances in cancer treatment including immunotherapy with immune checkpoint inhibitors, new unconventional biomarkers and targets for the detection, prognosis, and treatment of cancer are still in high demand. Tumor cells are characterized by mutations that allow their unlimited growth, program their local microenvironment to support tumor growth, and spread towards distant sites. While a major focus has been on altered tumor genomes and proteomes, crucial signaling molecules such as lipids have been underappreciated. One of these molecules is the membrane phospholipid phosphatidylserine (PS) that is usually found at cytosolic surfaces of cellular membranes but can be rapidly and massively shuttled to the extracellular leaflet of the plasma membrane during apoptosis to serve as a limiting factor for immune responses. These immunosuppressive interactions are exploited by tumor cells to evade the immune system. In this review, we describe mechanisms of immune regulation in tumors, discuss if PS may constitute an inhibitory immune checkpoint, and describe current and future strategies for targeting PS to reactivate the tumor-associated immune system.

Infantile Rosai-Dorfman Disease With Isolated Brain Lesions Disseminated to the Parenchyma and Intraventricular Ependyma, Alteration of Leukocytes as a Promotion Factor in Immune Defense, and New Proposals: A Case Report and Literature Review

The patient is a one-year-old girl referred to the hospital for an enlarged head after a 1.5-month history of two falls, followed by polydipsia, polyuria, and slow movement and growth. Three subsequent magnetic resonance imaging (MRI) examinations of the brain revealed nodular lesions disseminated in the brain parenchyma and intraventricular ependyma, resulting in obstructive hydrocephalus. Thoracic and abdominopelvic sonography showed no additional lesions. The preliminary diagnosis was a primary or metastatic neoplasm or infection. A biopsy of a lesion in the right frontal lobe was taken. The histological examination revealed features of Rosai-Dorfman disease (RDD), consisting of limited perivascular lymphoplasma cell infiltration with intervening sheets of proliferated histiocytes, with some of the histiocytes showing endocytosis of a single intact lymphocyte (emperipolesis).

CD99 isoforms regulate CD1a expression in human monocyte-derived DCs through ATF-2/CREB-1 phosphorylation

CD1a expression is considered one of the major characteristics qualifying in vitro human dendritic cells (DCs) during their generation process. Here, we report that CD1A transcription is regulated by a mechanism involving the long and short isoforms of CD99. Using a lentiviral construct encoding for a CD99 short hairpin RNA, we were able to inhibit CD99 expression in human primary DCs. In such cells, CD1a membrane expression increased and CD1A transcripts were much higher in abundance compared to cells expressing CD99 long form (CD99LF). We also show that CD1A transcription is accompanied by a switch in expression from CD99LF to expression at comparable levels of both CD99 isoforms during immature DCs generation in vitro. We demonstrate that CD99LF maintains a lower level of CD1A transcription by up-regulating the phosphorylated form of the ATF-2 transcription factor and that CD99 short form (SF) is required to counteract this regulatory mechanism. Elucidation of the molecular mechanisms related to CD99 alternative splicing will be very helpful to better understand the transcriptional regulatory mechanism of CD1a molecules during DCs differentiation and its involvement in the immune response.

All hands on DE(T)C: Epithelial-resident γδ T cells respond to tissue injury

Immunology has traditionally focused on the lymphocytes circulating among primary lymphoid organs while the large reservoir of tissue-resident T cells have received relatively less attention. In epithelia, these populations are comprised of significant, and sometimes exclusive, subsets of γδ T cells that are highly specialized in promoting tissue homeostasis. As the epithelial layers of the skin and gut are permanently exposed to the environment, they are continually subject to injury and therefore require highly efficient repair processes to maintain barrier functions. Here, we review the role of γδ T cells in promoting wound healing, a critical and complex process occurring in the skin and other barrier sites.

CD1 Antigen Presentation and Autoreactivity in the Pregnant Human Uterus

PROBLEM

CD11c(HI) human decidual macrophages express several isoforms of CD1 molecules. Their expression pattern and function required investigation.

METHOD OF STUDY

CD11c(HI) macrophages were isolated from decidua. Expression of CD1 isoforms and their ability to present lipid antigens to T cells was studied.

RESULTS

CD1a, CD1c, and CD1d were all expressed on CD11c(HI) dMϕ, a pattern differing from those previously observed. Exposure of peripheral monocytes and dendritic cells to lipid isolates from decidua led to increased surface CD1a levels only. The CD1a and CD1c on dMϕ were able to present the appropriate lipid antigens to lipid antigen-specific T cells. Finally, autoreactivity of decidual T cells to CD1a was observed.

CONCLUSION

The unique pattern of expression of CD1 isoforms on CD11c(HI) dMϕ is consistent with organ-specific roles of CD1 in human T-cell responses. dMϕ are able to present lipid antigens to both peripheral and decidual T cells and are major antigen-presenting cells in human decidua.

Toward Understanding the Essence of Post-Translational Modifications for the Mycobacterium tuberculosis Immunoproteome

CD4⁺ T cells are prominent effector cells in controlling Mycobacterium tuberculosis (Mtb) infection but may also contribute to immunopathology. Studies probing the CD4⁺ T cell response from individuals latently infected with Mtb or patients with active tuberculosis using either small or proteome-wide antigen screens so far revealed a multi-antigenic, yet mostly invariable repertoire of immunogenic Mtb proteins. Recent developments in mass spectrometry-based proteomics have highlighted the occurrence of numerous types of post-translational modifications (PTMs) in proteomes of prokaryotes, including Mtb. The well-known PTMs in Mtb are glycosylation, lipidation, or phosphorylation, known regulators of protein function or compartmentalization. Other PTMs include methylation, acetylation, and pupylation, involved in protein stability. While all PTMs add variability to the Mtb proteome, relatively little is understood about their role in the anti-Mtb immune responses. Here, we review Mtb protein PTMs and methods to assess their role in protective immunity against Mtb.

Host-pathogen interactions during Mycobacterium tuberculosis infections

The intimate and persistent connection between Mycobacterium tuberculosis and its human host suggests that the pathogen has evolved extensive mechanisms to evade eradication by the immune system. In particular, the organism has adapted to replicate within phagocytic cells, especially macrophages, which are specialized to kill microbes. Over the past decade of M. tuberculosis research, the means to manipulate both the organism and the host has ushered in an exciting time that has uncovered some of the mechanisms of the innate macrophage-pathogen interactions that lie at the heart of M. tuberculosis pathogenesis, though many interactions likely still await discovery. In this chapter, we will delve into some of these advances, with an emphasis on the interactions that occur on the cellular level when M. tuberculosis cells encounter macrophages. In particular, we focus on two major aspects of M. tuberculosis biology regarding the proximal physical interface between the bacterium and host, namely the interactions with the phagosomal membrane as well as the distinctive mycobacterial cell wall. Importantly, some of the emerging paradigms in M. tuberculosis pathogenesis and host response represent common themes in bacterial pathogenesis, such as the role of host cell membrane perforation in intracellular survival and host response. However, the array of unique bacterial lipid mediators and their interaction with host cells highlights the unique biology of this persistent pathogen.

HIV-1 Vpu interference with innate cell-mediated immune mechanisms

The HIV-1 accessory protein Vpu is emerging as a viral factor with a range of activities devoted to counteracting host innate immunity. Here, we review recent findings concerning the role of Vpu in hampering activation of cellular immune responses mediated by CD1d-restricted invariant natural killer T (iNKT) cells and natural killer (NK) cells. The two key findings are that Vpu interferes with CD1d expression and antigen presentation, and also with expression of the NK cell activation ligand NK-T and B cell antigen (NTB-A). Both these activities are mechanistically distinct from CD4 and Tetherin (BST-2) down-modulation. We summarize the mechanistic insights gained into Vpu interference with CD1d and NTB-A, as well as important challenges going forward, and discuss these mechanisms in the context of the role that iNKT and NK cells play in HIV-1 immunity and immunopathogenesis.

Lipoproteins are major targets of the polyclonal human T cell response to Mycobacterium tuberculosis

Most vaccines and basic studies of T cell epitopes in Mycobacterium tuberculosis emphasize water-soluble proteins that are secreted into the extracellular space and presented in the context of MHC class II. Much less is known about the role of Ags retained within the cell wall. We used polyclonal T cells from infected humans to probe for responses to immunodominant Ags in the M. tuberculosis cell wall. We found that the magnitude of response to secreted or cell wall intrinsic compounds was similar among healthy controls, patients with latent tuberculosis, and patients with active tuberculosis. Individual responses to secreted Ags and cell wall extract were strongly correlated (r(2) = 0.495, p = 0.001), suggesting that T cells responding to cell wall and secreted Ags are present at similar frequency. Surprisingly, T cell stimulatory factors intrinsic to the cell wall partition into organic solvents; however, these responses are not explained by CD1-mediated presentation of lipids. Instead, we find that molecules soluble in organic solvents are dependent upon MHC class II and recognized by IFN-γ-secreting CD4(+) T cells. We reasoned that MHC class II-dependent Ags extracting into lipid mixtures might be found among triacylated lipoproteins present in mycobacteria. We used M. tuberculosis lacking prolipoprotein signal peptidase A (lspA), an enzyme required for lipoprotein synthesis, to demonstrate loss of polyclonal T cell responses. Our results demonstrate the use of bacterial genetics to identify lipoproteins as an unexpected and immunodominant class of cell wall-associated Ags targeted by the polyclonal human T cell response to M. tuberculosis.

Tuberculosis pathogenesis and immunity

Despite the development of potentially curative chemotherapy, tuberculosis (TB) continues to cause increasing worldwide morbidity and is a leading cause of human mortality in the developing world. Recent advances in bacterial molecular genetics, immunology, and human genetics have yielded insight into the molecular determinants of virulence, the immune responses that are essential for restricting progressive disease, and the determinants of immunopathology in TB. Despite these advances, a large knowledge gap still exists that limits the development and testing of new interventions, including novel drugs and efficacious vaccines. This review focuses on our current knowledge of TB pathogenesis and immunity that has been derived from in vitro and in vivo studies. In addition, it highlights topics that need to be better understood to provide improved means of controlling TB worldwide.

A naive-like population of human CD1d-restricted T cells expressing intermediate levels of promyelocytic leukemia zinc finger

Rare CD1d-α-galactosylceramide-specific T cells that do not express the invariant Vα24 chain of human NKT cells were recently identified after expansion in vitro with the lipid Ag, but their phenotype and frequency in vivo and lineage relationship with NKT cells could not be elucidated. By using a CD1d tetramer-based method to enrich these cells from fresh peripheral blood, we demonstrated their naive-like CD62L(high)CD45RO(-)CD4(+) phenotype and relatively high frequency of ∼10(-5) in several healthy individuals. Notably, these cells expressed the NKT lineage-specific transcription promyelocytic leukemia zinc finger (PLZF), indicating a developmental relationship with NKT cells and ruling out the possibility that they were conventional MHC-restricted T cells cross-reacting against CD1d-α-galactosylceramide. Although PLZF is known to direct the effector program of NKT cells, we show in this study that the naive-like cells expressed it at a significantly lower amount than NKT cells. Further, we present mouse studies demonstrating a sharp PLZF expression threshold requirement for induction of the effector phenotype. These findings directly demonstrate in vivo the existence of naive-like CD1d-restricted human T cells marked by intermediate levels of PLZF.

Cross-talk between cd1d-restricted nkt cells and γδ cells in t regulatory cell response

CD1d is a non-classical major histocompatibility class 1-like molecule which primarily presents either microbial or endogenous glycolipid antigens to T cells involved in innate immunity. Natural killer T (NKT) cells and a subpopulation of γδ T cells expressing the Vγ4 T cell receptor (TCR) recognize CD1d. NKT and Vγ4 T cells function in the innate immune response via rapid activation subsequent to infection and secrete large quantities of cytokines that both help control infection and modulate the developing adaptive immune response. T regulatory cells represent one cell population impacted by both NKT and Vγ4 T cells. This review discusses the evidence that NKT cells promote T regulatory cell activation both through direct interaction of NKT cell and dendritic cells and through NKT cell secretion of large amounts of TGFβ, IL-10 and IL-2. Recent studies have shown that CD1d-restricted Vγ4 T cells, in contrast to NKT cells, selectively kill T regulatory cells through a caspase-dependent mechanism. Vγ4 T cell elimination of the T regulatory cell population allows activation of autoimmune CD8+ effector cells leading to severe cardiac injury in a coxsackievirus B3 (CVB3) myocarditis model in mice. CD1d-restricted immunity can therefore lead to either immunosuppression or autoimmunity depending upon the type of innate effector dominating during the infection.

CD1a-autoreactive T cells are a normal component of the human αβ T cell repertoire

CD1 activates T cells, but the function and size of the possible human T cell repertoires that recognize each of the CD1 antigen-presenting molecules remain unknown. Using an experimental system that bypasses major histocompatibility complex (MHC) restriction and the requirement for defined antigens, we show that polyclonal T cells responded at higher rates to cells expressing CD1a than to those expressing CD1b, CD1c or CD1d. Unlike the repertoire of invariant natural killer T (NKT) cells, the CD1a-autoreactive repertoire contained diverse T cell antigen receptors (TCRs). Functionally, many CD1a-autoreactive T cells homed to skin, where they produced interleukin 22 (IL-22) in response to CD1a on Langerhans cells. The strong and frequent responses among genetically diverse donors define CD1a-autoreactive cells as a normal part of the human T cell repertoire and CD1a as a target of the T(H)22 subset of helper T cells.

General aspects and neuropathology of X-linked adrenoleukodystrophy

X-adrenoleukodystrophy (X-ALD) is a metabolic, peroxisomal disease affecting the nervous system, adrenal cortex and testis resulting from inactivating mutations in ABCD1 gene which encodes a peroxisomal membrane half-adenosine triphosphate (ATP)-binding cassette transporter, ABCD1 (or ALDP), whose defect is associated with impaired peroxisomal beta-oxidation and accumulation of saturated very long-chain fatty acids (VLCFA) in tissues and body fluids. Several phenotypes are recognized in male patients including cerebral ALD in childhood, adolescence or adulthood, adrenomyeloneuropathy (AMN), Addison's disease and, eventually, gonadal insufficiency. Female carriers might present with mild to severe myeloneuropathy that resembles AMN. There is a lack of phenotype-genotype correlations, as the same ABCD1 gene mutation may be associated with different phenotypes in the same family, suggesting that genetic, epigenetic, environmental and stochastic factors are probably contributory to the development and course of the disease. Degenerative changes, like those seen in pure AMN without cerebral demyelination, are characterized by loss of axons and secondary myelin in the long tracts of the spinal cord, possibly related to the impaired lipid metabolism of VLCFAs and the associated alterations (ie, oxidative damage). Similar lesions are encountered following inactivation of ABCD1 in mice (ABCD1(-)). A different and more aggressive phenotype is secondary to cerebral demyelination, very often accompanied by inflammatory changes in the white matter of the brain and associated with activation of T lymphocytes, CD1 presentation and increased levels of cytokines, gamma-interferon, interleukin (IL)-1alpha, IL-2 and IL-6, Granulocyte macrophage colony-stimulating factor (GM-CSF), tumor necrosis factor-alpha, chemokines and chemokine receptors.

Fatty acyl structures of mycobacterium tuberculosis sulfoglycolipid govern T cell response

CD1b-restricted T lymphocytes recognize a large diversity of mycobacterial lipids, which differ in their hydrophilic heads and the structure of their acyl appendages. Both moieties participate in the antigenicity of lipid Ags, but the structural constraints governing binding to CD1b and generation of antigenic CD1b:lipid Ag complexes are still poorly understood. Here, we investigated the structural requirements conferring antigenicity to Mycobacterium tuberculosis sulfoglycolipid Ags using a combination of CD1b:lipid binding and T cell activation assays with both living dendritic cells and plate-bound recombinant soluble CD1b. Comparison of the antigenicity of a panel of synthetic analogs, sharing the same trehalose-sulfate polar head, but differing in the structure of their acyl tails, shows that the number of C-methyl substituents on the fatty acid, the configuration of the chiral centers, and the respective localization of the two different acyl chains on the sugar moiety govern TCR recognition and T lymphocyte activation. These studies have major implications for the design of sulfoglycolipid analogs with potential use as tuberculosis subunit vaccines.

CD1c bypasses lysosomes to present a lipopeptide antigen with 12 amino acids

The recent discovery of dideoxymycobactin (DDM) as a ligand for CD1a demonstrates how a nonribosomal lipopeptide antigen is presented to T cells. DDM contains an unusual acylation motif and a peptide sequence present only in mycobacteria, but its discovery raises the possibility that ribosomally produced viral or mammalian proteins that commonly undergo lipidation might also function as antigens. To test this, we measured T cell responses to synthetic acylpeptides that mimic lipoproteins produced by cells and viruses. CD1c presented an N-acyl glycine dodecamer peptide (lipo-12) to human T cells, and the response was specific for the acyl linkage as well as the peptide length and sequence. Thus, CD1c represents the second member of the CD1 family to present lipopeptides. lipo-12 was efficiently recognized when presented by intact cells, and unlike DDM, it was inactivated by proteases and augmented by protease inhibitors. Although lysosomes often promote antigen presentation by CD1, rerouting CD1c to lysosomes by mutating CD1 tail sequences caused reduction in lipo-12 presentation. Thus, although certain antigens require antigen processing in lysosomes, others are destroyed there, providing a hypothesis for the evolutionary conservation of large CD1 families containing isoforms that survey early endosomal pathways.

CD1a and MHC class I follow a similar endocytic recycling pathway

CD1 proteins are a family of major histocompatibility complex (MHC) class I-like antigen-presenting molecules that present lipids to T cells. The cytoplasmic tails (CTs) of all human CD1 isoforms, with the exception of CD1a, contain tyrosine-based sorting motifs, responsible for the internalization of proteins by the clathrin-mediated pathway. The role of the CD1a CT, which does not possess any sorting motifs, as well as its mode of internalization are not known. We investigated the internalization and recycling pathways followed by CD1a and the role of its CT. We found that CD1a can be internalized by a clathrin- and dynamin-independent pathway and that it follows a Rab22a- and ADP ribosylation factor (ARF)6-dependent recycling pathway, similar to other cargo internalized independent of clathrin. We also found that the CD1a CT is S-acylated. However, this posttranslational modification does not determine the rate of internalization or recycling of the protein or its localization to detergent-resistant membrane microdomains (DRMs) where we found CD1a to be enriched. We also show that plasma membrane DRMs are essential for efficient CD1a-mediated antigen presentation. These findings place CD1a closer to MHC class I in its trafficking and potential antigen-loading compartments among CD1 isoforms. Furthermore, we identify CD1a as a new marker for the clathrin- and dynamin-independent and DRM-dependent pathway of internalization as well as the Rab22a- and ARF6-dependent recycling pathway.

Carbohydrate Moieties as Vaccine Candidates: meeting summary

In September 2007, a meeting entitled 'Carbohydrate Moieties as Vaccine Candidates' was held at the National Institutes of Health (Bethesda, MD). This meeting brought together scientists from a number of disciplines to address issues concerning carbohydrate moieties as targets for vaccines for a variety of pathogens and tumors. In addition, the meeting participants addressed fundamental topics of glycoimmunology including the recognition of glycotopes by B and T lymphocytes, the ontogeny of anti-carbohydrate immune responses, peptide mimicry, carbohydrate antigen processing pathways and adjuvants. One session reported progress in the development of new tools such as computational algorithms, glycan arrays and oligosaccharide synthesis and their application to carbohydrate vaccine research. The session titles were: (1) immune response to bacterial carbohydrate antigens; (2) immune response to glycolipids; (3) immune response to carbohydrate antigens on other microbes and on tumors; (4) novel vaccine approaches; (5) novel tools in carbohydrate vaccine research; (6) bench to bedside: carbohydrate moieties as vaccine immunopotentiators.

Application of nine-color flow cytometry for detailed studies of the phenotypic complexity and functional heterogeneity of human lymphocyte subsets

Innate and adaptive cellular immunity is initiated, directed and regulated by a vast array of cell surface receptors. Attempts to harness the cellular immune system in translational settings such as immunotherapy and vaccine development require tools to accurately describe and isolate lymphocytes with specific characteristics. One such tool, flow cytometry, is undergoing a revolution in instrumentation and reagents, providing opportunities for high resolution phenotypic and functional analysis of lymphocytes. Here, we demonstrate how nine-color flow cytometry can be adapted, optimized and applied to investigate the phenotypic complexity and functional heterogeneity of human lymphocyte subsets. We provide examples of studies of adaptive T cell responses against viruses, as well as the assessment of CD1d-restricted NKT cells and NK cells. We discuss the importance of this technology for detailed investigations of lymphocyte subsets in studies of infectious diseases and cancer.