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

Functional and biological implications of clonotypic diversity among human donor-unrestricted T cells

T cells express a T-cell receptor (TCR) heterodimer that is the product of germline rearrangement and junctional editing resulting in immense clonotypic diversity. The generation of diverse TCR repertoires enables the recognition of pathogen-derived peptide antigens presented by polymorphic major histocompatibility complex (MHC) molecules. However, T cells also recognize nonpeptide antigens through nearly monomorphic antigen-presenting systems, such as cluster of differentiation 1 (CD1), MHC-related protein 1 (MR1) and butyrophilins (BTNs). This potential for shared immune responses across genetically diverse populations led to their designation as donor-unrestricted T cells (DURTs). As might be expected, some CD1-, MR1- and BTN-restricted T cells express a TCR that is conserved across unrelated individuals. However, several recent studies have reported unexpected diversity among DURT TCRs, and increasing evidence suggests that this diversity has functional consequences. Recent reports also challenge the dogma that immune cells are either innate or adaptive and suggest that DURT TCRs may act in both capacities. Here, we review this evidence and propose an expanded view of the role for clonotypic diversity among DURTs in humans, including new perspectives on how DURT TCRs may integrate their adaptive and innate immune functions.

CD1a and skin T cells: a pathway for therapeutic intervention

The CD1 and MR1 protein families present lipid antigens and small molecules to T cells, complementing well-studied major histocompatibility complex-peptide mechanisms. The CD1a subtype is highly and continuously expressed within the skin, most notably on Langerhans cells, and has been demonstrated to present self and foreign lipids to T cells, highlighting its cutaneous sentinel role. Alteration of CD1a-dependent T-cell responses has recently been discovered to contribute to the pathogenesis of several inflammatory skin diseases. In this review, we overview the structure and role of CD1a and outline the current evidence implicating CD1a in the development of psoriasis, atopic dermatitis and allergic contact dermatitis.

Human Skin T Cells Express Conserved T-Cell Receptors that Cross-React with Staphylococcal Superantigens and CD1a

Human Langerhans cells highly express CD1a antigen-presenting molecules. To understand the functions of CD1a in human skin, we used CD1a tetramers to capture T cells and determine their effector functions and TCR patterns. Skin T cells from all donors showed CD1a tetramer staining, which in three cases exceeded 10% of skin T cells. CD1a tetramer-positive T cells produced diverse cytokines, including IL-2, IL-4, IL-5, IL-9, IL-17, IL-22, and IFN-γ. Conserved TCRs often recognize nonpolymorphic antigen-presenting molecules, but no TCR motifs are known for CD1a. We detected highly conserved TCRs that used TRAV34 and TRBV28 variable genes, which is a known motif for recognition of staphylococcal enterotoxin B, a superantigen associated with atopic dermatitis. We found that these conserved TCRs did not respond to superantigen presented by CD1a, but instead showed a cross-reactive response with two targets: CD1a and staphylococcal enterotoxin B presented by classical major histocompatibility complex II. These studies identify a conserved human TCR motif for CD1a-reactive T cells. Furthermore, the demonstrated cross-reaction of T cells with two common skin-specific stimuli suggests a candidate mechanism by which CD1a and skin flora could synergize during natural immune response and in Staphylococcus-associated skin diseases.

Keratinocyte-derived small extracellular vesicles supply antigens for CD1a-resticted T cells and promote their type 2 bias in the context of filaggrin insufficiency

Introduction

Exosome-enriched small extracellular vesicles (sEVs) are nanosized organelles known to participate in long distance communication between cells, including in the skin. Atopic dermatitis (AD) is a chronic inflammatory skin disease for which filaggrin (FLG) gene mutations are the strongest genetic risk factor. Filaggrin insufficiency affects multiple cellular function, but it is unclear if sEV-mediated cellular communication originating from the affected keratinocytes is also altered, and if this influences peptide and lipid antigen presentation to T cells in the skin.

Methods

Available mRNA and protein expression datasets from filaggrin-insufficient keratinocytes (shFLG), organotypic models and AD skin were used for gene ontology analysis with FunRich tool. sEVs secreted by shFLG and control shC cells were isolated from conditioned media by differential centrifugation. Mass spectrometry was carried out for lipidomic and proteomic profiling of the cells and sEVs. T cell responses to protein, peptide, CD1a lipid antigens, as well as phospholipase A2-digested or intact sEVs were measured by ELISpot and ELISA.

Results

Data analysis revealed extensive remodeling of the sEV compartment in filaggrin insufficient keratinocytes, 3D models and the AD skin. Lipidomic profiles of shFLGsEV showed a reduction in the long chain (LCFAs) and polyunsaturated fatty acids (PUFAs; permissive CD1a ligands) and increased content of the bulky headgroup sphingolipids (non-permissive ligands). This resulted in a reduction of CD1a-mediated interferon-γ T cell responses to the lipids liberated from shFLG-generated sEVs in comparison to those induced by sEVs from control cells, and an increase in interleukin 13 secretion. The altered sEV lipidome reflected a generalized alteration in the cellular lipidome in filaggrin-insufficient cells and the skin of AD patients, resulting from a downregulation of key enzymes implicated in fatty acid elongation and desaturation, i.e., enzymes of the ACSL, ELOVL and FADS family.

Discussion

We determined that sEVs constitute a source of antigens suitable for CD1a-mediated presentation to T cells. Lipids enclosed within the sEVs secreted on the background of filaggrin insufficiency contribute to allergic inflammation by reducing type 1 responses and inducing a type 2 bias from CD1a-restricted T cells, thus likely perpetuating allergic inflammation in the skin.

Disseminated thymoma in pleural fluid: An unusual case

Thymic epithelial tumours show characteristic cytological features on fine-needle aspiration cytology, however the cytological features of thymoma in fluid cytology are not well described. We present the case of a 77 year-old-woman with known pleural dissemination of type B2/B3 thymoma presenting with shortness of breath and orthopnoea due to a pleural effusion. Cytological evaluation of the pleural fluid showed cellular smears composed of numerous small lymphocytes with small numbers of admixed mesothelial cells. There was no epithelial component. On immunohistochemical (IHC) staining the lymphocytes were T cells which were positive for CD3. CD1a and terminal deoxynucleotide transferase (TdT) were also positive, consistent with immature lymphocytes of thymic origin. Despite the lack of an epithelial component, this case was diagnosed as suspicious for recurrent/ metastatic thymoma. This is only the second published case of thymoma identified on pleural fluid cytology, and to our knowledge the first case describing thymoma in pleural fluid with no epithelial component, a potential pitfall with the more common differential diagnosis of a reactive lymphocytic effusion.

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).

CD1 lipidomes reveal lipid-binding motifs and size-based antigen-display mechanisms

The CD1 system binds lipid antigens for display to T cells. Here, we solved lipidomes for the four human CD1 antigen-presenting molecules, providing a map of self-lipid display. Answering a basic question, the detection of >2,000 CD1-lipid complexes demonstrates broad presentation of self-sphingolipids and phospholipids. Whereas peptide antigens are chemically processed, many lipids are presented in an unaltered form. However, each type of CD1 protein differentially edits the self-lipidome to show distinct capture motifs based on lipid length and chemical composition, suggesting general antigen display mechanisms. For CD1a and CD1d, lipid size matches the CD1 cleft volume. CD1c cleft size is more variable, and CD1b is the outlier, where ligands and clefts show an extreme size mismatch that is explained by uniformly seating two small lipids in one cleft. Furthermore, the list of compounds that comprise the integrated CD1 lipidome supports the ongoing discovery of lipid blockers and antigens for T cells.

CD1a and bound lipids drive T-cell responses in human skin disease

In addition to serving as the main physical barrier with the outside world, human skin is abundantly infiltrated with resident αβ T cells that respond differently to self, infectious, microbiome, and noxious stimuli.  To study skin T cells during infection and inflammation, experimental biologists track T-cell surface phenotypes and effector functions, which are often interpreted with the untested assumption that MHC proteins and peptide antigens drive measured responses.  However, a broader perspective is that CD1 proteins also activate human T cells, and in skin, Langerhans cells (LCs) are abundant antigen presenting cells that express extremely high levels of CD1a.  The emergence of new experimental tools, including CD1a tetramers carrying endogenous lipids, now show that CD1a-reactive T cells comprise a large population of resident T cells in human skin.  Here, we review studies showing that skin-derived αβ T cells directly recognize CD1a proteins, and certain bound lipids, such as contact dermatitis allergens, trigger T-cell responses. Other natural skin lipids inhibit CD1a-mediated T-cell responses, providing an entry point for the development of therapeutic lipids that block T-cell responses. Increasing evidence points to a distinct role of CD1a in type 2 and 22 T-cell responses, providing new insights into psoriasis, contact dermatitis, and other T-cell-mediated skin diseases.

Promiscuous recognition of MR1 drives self-reactive mucosal-associated invariant T cell responses

Mucosal-associated invariant T (MAIT) cells use canonical semi-invariant T cell receptors (TCR) to recognize microbial riboflavin precursors displayed by the antigen-presenting molecule MR1. The extent of MAIT TCR crossreactivity toward physiological, microbially unrelated antigens remains underexplored. We describe MAIT TCRs endowed with MR1-dependent reactivity to tumor and healthy cells in the absence of microbial metabolites. MAIT cells bearing TCRs crossreactive toward self are rare but commonly found within healthy donors and display T-helper-like functions in vitro. Experiments with MR1-tetramers loaded with distinct ligands revealed significant crossreactivity among MAIT TCRs both ex vivo and upon in vitro expansion. A canonical MAIT TCR was selected on the basis of extremely promiscuous MR1 recognition. Structural and molecular dynamic analyses associated promiscuity to unique TCRβ-chain features that were enriched within self-reactive MAIT cells of healthy individuals. Thus, self-reactive recognition of MR1 represents a functionally relevant indication of MAIT TCR crossreactivity, suggesting a potentially broader role of MAIT cells in immune homeostasis and diseases, beyond microbial immunosurveillance.

CD1 displays its own negative regulators

After two decades of the study of lipid antigens that activate CD1-restricted T cells, new studies show how autoreactive αβ T-cell receptors (TCRs) can directly recognize the outer surface of CD1 proteins in ways that are lipid-agnostic. Most recently, this lipid agnosticism has turned to negativity, with the discovery of natural CD1 ligands that dominantly negatively block autoreactive αβ TCR binding to CD1a and CD1d. This review highlights the basic differences between positive and negative regulation of cellular systems. We outline strategies to discover lipid inhibitors of CD1-reactive T cells, whose roles in vivo are becoming clear, especially in CD1-mediated skin disease.

MAITs and their mates: "Innate-like" behaviors in conventional and unconventional T cells

Most CD4 and CD8 T cells are restricted by conventional major histocompatibility complex (MHC) molecules and mount TCR-dependent adaptive immune responses. In contrast, MAIT, iNKT, and certain γδ TCR bearing cells are characterized by their abilities to recognize antigens presented by unconventional antigen-presenting molecules and to mount cytokine-mediated TCR-independent responses in an "innate-like" manner. In addition, several more diverse T-cell subsets have been described that in a similar manner are restricted by unconventional antigen-presenting molecules but mainly depend on their TCRs for activation. Vice versa, innate-like behaviour was reported in defined subpopulations of conventional T cells, particularly in barrier sites, showing that these two features are not necessarily linked. The abilities to recognize antigens presented by unconventional antigen-presenting molecules or to mount TCR-independent responses creates unique niches for these T cells and is linked to wide range of functional capabilities. This is especially exemplified by unconventional and innate-like T cells present at barrier sites where they are involved in pathogen defense, tissue homeostasis as well as in pathologic processes.

Group A Streptococcus induces CD1a-autoreactive T cells and promotes psoriatic inflammation

Group A Streptococcus (GAS) infection is associated with multiple clinical sequelae, including different subtypes of psoriasis. Such post-streptococcal disorders have been long known but are largely unexplained. CD1a is expressed at constitutively high levels by Langerhans cells and presents lipid antigens to T cells, but the potential relevance to GAS infection has not been studied. Here, we investigated whether GAS-responsive CD1a-restricted T cells contribute to the pathogenesis of psoriasis. Healthy individuals had high frequencies of circulating and cutaneous GAS-responsive CD4+ and CD8+ T cells with rapid effector functions, including the production of interleukin-22 (IL-22). Human skin and blood single-cell CITE-seq analyses of IL-22-producing T cells showed a type 17 signature with proliferative potential, whereas IFN-γ-producing T cells displayed cytotoxic T lymphocyte characteristics. Furthermore, individuals with psoriasis had significantly higher frequencies of circulating GAS-reactive T cells, enriched for markers of activation, cytolytic potential, and tissue association. In addition to responding to GAS, subsets of expanded GAS-reactive T cell clones/lines were found to be autoreactive, which included the recognition of the self-lipid antigen lysophosphatidylcholine. CD8+ T cell clones/lines produced cytolytic mediators and lysed infected CD1a-expressing cells. Furthermore, we established cutaneous models of GAS infection in a humanized CD1a transgenic mouse model and identified enhanced and prolonged local and systemic inflammation, with resolution through a psoriasis-like phenotype. Together, these findings link GAS infection to the CD1a pathway and show that GAS infection promotes the proliferation and activation of CD1a-autoreactive T cells, with relevance to post-streptococcal disease, including the pathogenesis and treatment of psoriasis.

Comparative transcriptomes reveal pro-survival and cytotoxic programs of mucosal-associated invariant T cells upon Bacillus Calmette-Guérin stimulation

Mucosal-associated invariant T (MAIT) cells are protective against tuberculous and non-tuberculous mycobacterial infections with poorly understood mechanisms. Despite an innate-like nature, MAIT cell responses remain heterogeneous in bacterial infections. To comprehensively characterize MAIT activation programs responding to different bacteria, we stimulated MAIT cells with E. coli to compare with Bacillus Calmette-Guérin (BCG), which remains the only licensed vaccine and a feasible tool for investigating anti-mycobacterial immunity in humans. Upon sequencing mRNA from the activated and inactivated CD8+ MAIT cells, results demonstrated the altered MAIT cell gene profiles by each bacterium with upregulated expression of activation markers, transcription factors, cytokines, and cytolytic mediators crucial in anti-mycobacterial responses. Compared with E. coli, BCG altered more MAIT cell genes to enhance cell survival and cytolysis. Flow cytometry analyses similarly displayed a more upregulated protein expression of B-cell lymphoma 2 and T-box transcription factor Eomesodermin in BCG compared to E.coli stimulations. Thus, the transcriptomic program and protein expression of MAIT cells together displayed enhanced pro-survival and cytotoxic programs in response to BCG stimulation, supporting BCG induces cell-mediated effector responses of MAIT cells to fight mycobacterial infections.

Staphylococcal phosphatidylglycerol antigens activate human T cells via CD1a

Expressed on epidermal Langerhans cells, CD1a presents a range of self-lipid antigens found within the skin; however, the extent to which CD1a presents microbial ligands from bacteria colonizing the skin is unclear. Here we identified CD1a-dependent T cell responses to phosphatidylglycerol (PG), a ubiquitous bacterial membrane phospholipid, as well as to lysylPG, a modified PG, present in several Gram-positive bacteria and highly abundant in Staphylococcus aureus. The crystal structure of the CD1a-PG complex showed that the acyl chains were buried within the A'- and F'-pockets of CD1a, while the phosphoglycerol headgroup remained solvent exposed in the F'-portal and was available for T cell receptor contact. Using lysylPG and PG-loaded CD1a tetramers, we identified T cells in peripheral blood and in skin that respond to these lipids in a dose-dependent manner. Tetramer+CD4+ T cell lines secreted type 2 helper T cell cytokines in response to phosphatidylglycerols as well as to co-cultures of CD1a+ dendritic cells and Staphylococcus bacteria. The expansion in patients with atopic dermatitis of CD4+ CD1a-(lysyl)PG tetramer+ T cells suggests a response to lipids made by bacteria associated with atopic dermatitis and provides a link supporting involvement of PG-based lipid-activated T cells in atopic dermatitis pathogenesis.

CD1a promotes systemic manifestations of skin inflammation

Inflammatory skin conditions are increasingly recognised as being associated with systemic inflammation. The mechanisms connecting the cutaneous and systemic disease are not well understood. CD1a is a virtually monomorphic major histocompatibility complex (MHC) class I-like molecule, highly expressed by skin and mucosal Langerhans cells, and presents lipid antigens to T-cells. Here we show an important role for CD1a in linking cutaneous and systemic inflammation in two experimental disease models. In human CD1a transgenic mice, the toll-like receptor (TLR)7 agonist imiquimod induces more pronounced splenomegaly, expansion of the peripheral blood and spleen T cell compartments, and enhanced neutrophil and eosinophil responses compared to the wild-type, accompanied by elevated skin and plasma cytokine levels, including IL-23, IL-1α, IL-1β, MCP-1 and IL-17A. Similar systemic escalation is shown in MC903-induced skin inflammation. The exacerbated inflammation could be counter-acted by CD1a-blocking antibodies, developed and screened in our laboratories. The beneficial effect is epitope dependent, and we further characterise the five best-performing antibodies for their capacity to modulate CD1a-expressing cells and ameliorate CD1a-dependent systemic inflammatory responses. In summary, we show that a therapeutically targetable CD1a-dependent pathway may play a role in the systemic spread of cutaneous inflammation.

Characterization of Mycobacterium tuberculosis-Specific Th22 Cells and the Effect of Tuberculosis Disease and HIV Coinfection

The development of a highly effective tuberculosis (TB) vaccine is likely dependent on our understanding of what constitutes a protective immune response to TB. Accumulating evidence suggests that CD4+ T cells producing IL-22, a distinct subset termed "Th22" cells, may contribute to protective immunity to TB. Thus, we characterized Mycobacterium tuberculosis-specific Th22 (and Th1 and Th17) cells in 72 people with latent TB infection or TB disease, with and without HIV-1 infection. We investigated the functional properties (IFN-γ, IL-22, and IL-17 production), memory differentiation (CD45RA, CD27, and CCR7), and activation profile (HLA-DR) of M. tuberculosis-specific CD4+ T cells. In HIV-uninfected individuals with latent TB infection, we detected abundant circulating IFN-γ-producing CD4+ T cells (median, 0.93%) and IL-22-producing CD4+ T cells (median, 0.46%) in response to M. tuberculosis The frequency of IL-17-producing CD4+ T cells was much lower, at a median of 0.06%. Consistent with previous studies, IL-22 was produced by a distinct subset of CD4+ T cells and not coexpressed with IL-17. M. tuberculosis-specific IL-22 responses were markedly reduced (median, 0.08%) in individuals with TB disease and HIV coinfection compared with IFN-γ responses. M. tuberculosis-specific Th22 cells exhibited a distinct memory and activation phenotype compared with Th1 and Th17 cells. Furthermore, M. tuberculosis-specific IL-22 was produced by conventional CD4+ T cells that required TCR engagement. In conclusion, we confirm that Th22 cells are a component of the human immune response to TB. Depletion of M. tuberculosis-specific Th22 cells during HIV coinfection may contribute to increased risk of TB disease.

Phospholipase activity of acyloxyacyl hydrolase induces IL-22-producing CD1a-autoreactive T cells in individuals with psoriasis

Psoriasis is a chronic inflammatory skin disease characterized by Th17 responses. Recent evidence has identified Langerhans cells to have a key role in disease pathogenesis, with constitutive high expression of CD1a and capacity to present lipid antigens to T cells. Phospholipase A2 enzymes generate neolipid antigens for recognition by CD1a-reactive T cells; however, the broader enzymatic pathways of CD1a lipid ligand generation have not been thoroughly investigated. In this study, we used immunofluorescence of skin and ELISpot analyses of CD1a-reactive T cells to investigate the role of the lipase acyloxyacyl hydrolase (AOAH) in CD1a ligand generation with relevance to the pathogenesis of psoriasis. We found that the PLA2 activity of rAOAH leads to the activation of circulating CD1a auto-reactive T cells, leading to the production of IFN-γ and IL-22. Circulating AOAH-responsive CD1a-reactive T cells from patients with psoriasis showed elevated IL-22 production. We observed that AOAH is highly expressed in psoriatic lesions compared to healthy skin. Overall, these data present a role for AOAH in generating antigens that activate circulating lipid-specific CD1a-restricted T cells and, thus, contribute to psoriatic inflammation. These findings suggest that inhibition of PLA2 activity of AOAH may have therapeutic potential for individuals with psoriasis.

CD1-Restricted T Cells in Inflammatory Skin Diseases

Autoimmunity results from the breaking of immune tolerance, leading to inflammation and pathology. Although well studied in the conventional T-cell field, the role of nonconventional T cells in autoimmunity is less understood. CD1-restricted T cells recognize lipid antigens rather than peptide antigens and have been implicated in various autoimmune skin conditions, including psoriasis and atopic dermatitis. In this review, we will discuss the self-lipids that CD1-restricted T cells recognize and how these T cells become aberrantly regulated in pathogenic skin conditions.

Atopic Dermatitis: The Fate of the Fat

Atopic dermatitis (AD) is a chronic and relapsing inflammatory skin disease in which dry and itchy skin may develop into skin lesions. AD has a strong genetic component, as children from parents with AD have a two-fold increased chance of developing the disease. Genetic risk loci and epigenetic modifications reported in AD mainly locate to genes involved in the immune response and epidermal barrier function. However, AD pathogenesis cannot be fully explained by (epi)genetic factors since environmental triggers such as stress, pollution, microbiota, climate, and allergens also play a crucial role. Alterations of the epidermal barrier in AD, observed at all stages of the disease and which precede the development of overt skin inflammation, manifest as: dry skin; epidermal ultrastructural abnormalities, notably anomalies of the lamellar body cargo system; and abnormal epidermal lipid composition, including shorter fatty acid moieties in several lipid classes, such as ceramides and free fatty acids. Thus, a compelling question is whether AD is primarily a lipid disorder evolving into a chronic inflammatory disease due to genetic susceptibility loci in immunogenic genes. In this review, we focus on lipid abnormalities observed in the epidermis and blood of AD patients and evaluate their primary role in eliciting an inflammatory response.

CD4 and CD8 co-receptors modulate functional avidity of CD1b-restricted T cells

T cells recognize mycobacterial glycolipid (mycolipid) antigens presented by CD1b molecules, but the role of CD4 and CD8 co-receptors in mycolipid recognition is unknown. Here we show CD1b-mycolipid tetramers reveal a hierarchy in which circulating T cells expressing CD4 or CD8 co-receptor stain with a higher tetramer mean fluorescence intensity than CD4-CD8- T cells. CD4+ primary T cells transduced with mycolipid-specific T cell receptors bind CD1b-mycolipid tetramer with a higher fluorescence intensity than CD8+ primary T cells. The presence of either CD4 or CD8 also decreases the threshold for interferon-γ secretion. Co-receptor expression increases surface expression of CD3ε, suggesting a mechanism for increased tetramer binding and activation. Targeted transcriptional profiling of mycolipid-specific T cells from individuals with active tuberculosis reveals canonical markers associated with cytotoxicity among CD8+ compared to CD4+ T cells. Thus, expression of co-receptors modulates T cell receptor avidity for mycobacterial lipids, leading to in vivo functional diversity during tuberculosis disease.

In Vitro Monitoring of Human T Cell Responses to Skin Sensitizing Chemicals-A Systematic Review

Background: Chemical allergies are T cell-mediated diseases that often manifest in the skin as allergic contact dermatitis (ACD). To prevent ACD on a public health scale and avoid elicitation reactions at the individual patient level, predictive and diagnostic tests, respectively, are indispensable. Currently, there is no validated in vitro T cell assay available. The main bottlenecks concern the inefficient generation of T cell epitopes and the detection of rare antigen-specific T cells. Methods: Here, we systematically review original experimental research papers describing T cell activation to chemical skin sensitizers. We focus our search on studies published in the PubMed and Scopus databases on non-metallic allergens in the last 20 years. Results: We identified 37 papers, among them 32 (86%) describing antigen-specific human T cell activation to 31 different chemical allergens. The remaining studies measured the general effects of chemical allergens on T cell function (five studies, 14%). Most antigen-specific studies used peripheral blood mononuclear cells (PBMC) as antigen-presenting cells (APC, 75%) and interrogated the blood T cell pool (91%). Depending on the individual chemical properties, T cell epitopes were generated either by direct administration into the culture medium (72%), separate modification of autologous APC (29%) or by use of hapten-modified model proteins (13%). Read-outs were mainly based on proliferation (91%), often combined with cytokine secretion (53%). The analysis of T cell clones offers additional opportunities to elucidate the mechanisms of epitope formation and cross-reactivity (13%). The best researched allergen was p-phenylenediamine (PPD, 12 studies, 38%). For this and some other allergens, stronger immune responses were observed in some allergic patients (15/31 chemicals, 48%), illustrating the in vivo relevance of the identified T cells while detection limits remain challenging in many cases. Interpretation: Our results illustrate current hardships and possible solutions to monitoring T cell responses to individual chemical skin sensitizers. The provided data can guide the further development of T cell assays to unfold their full predictive and diagnostic potential, including cross-reactivity assessments.

Does CD1a Expression Influence T Cell Function in Patients With Langerhans Cell Histiocytosis?

Langerhans cell histiocytosis lesions are characterized by CD1a+ myeloid lineage LCH cells and an inflammatory infiltrate of cytokines and immune cells, including T cells. T cells that recognize CD1a may be implicated in the pathology of many disease states including cancer and autoimmunity but have not been studied in the context of LCH despite the expression of CD1a by LCH cells. In this perspective article, we discuss the expression of CD1a by LCH cells, and we explore the potential for T cells that recognize CD1a to be involved in LCH pathogenesis.

A simple assay to quantify mycobacterial lipid antigen-specific T cell receptors in human tissues and blood

T cell receptors (TCRs) encode the history of antigenic challenge within an individual and have the potential to serve as molecular markers of infection. In addition to peptide antigens bound to highly polymorphic MHC molecules, T cells have also evolved to recognize bacterial lipids when bound to non-polymorphic CD1 molecules. One such subset, germline-encoded, mycolyl lipid-reactive (GEM) T cells, recognizes mycobacterial cell wall lipids and expresses a conserved TCR-ɑ chain that is shared among genetically unrelated individuals. We developed a quantitative PCR assay to determine expression of the GEM TCR-ɑ nucleotide sequence in human tissues and blood. This assay was validated on plasmids and T cell lines. We tested blood samples from South African subjects with or without tuberculin reactivity or with active tuberculosis disease. We were able to detect GEM TCR-ɑ above the limit of detection in 92% of donors but found no difference in GEM TCR-ɑ expression among the three groups after normalizing for total TCR-ɑ expression. In a cohort of leprosy patients from Nepal, we successfully detected GEM TCR-ɑ in 100% of skin biopsies with histologically confirmed tuberculoid and lepromatous leprosy. Thus, GEM T cells constitute part of the T cell repertoire in the skin. However, GEM TCR-ɑ expression was not different between leprosy patients and control subjects after normalization. Further, these results reveal the feasibility of developing a simple, field deployable molecular diagnostic based on mycobacterial lipid antigen-specific TCR sequences that are readily detectable in human tissues and blood independent of genetic background.

STAT3 Gain-of-Function Mutations Underlie Deficiency in Human Nonclassical CD16+ Monocytes and CD141+ Dendritic Cells

Genetic analysis of human inborn errors of immunity has defined the contribution of specific cell populations and molecular pathways in the host defense against infection. The STAT family of transcription factors orchestrate hematopoietic cell differentiation. Patients with de novo activating mutations of STAT3 present with multiorgan autoimmunity, lymphoproliferation, and recurrent infections. We conducted a detailed characterization of the blood monocyte and dendritic cell (DC) subsets in patients with gain-of-function (GOF) mutations across the gene. We found a selective deficiency in circulating nonclassical CD16⁺ and intermediate CD16⁺CD14⁺ monocytes and a significant increase in the percentage of classical CD14⁺ monocytes. This suggests a role for STAT3 in the transition of classical CD14⁺ monocytes into the CD16⁺ nonclassical subset. Developmentally, ex vivo-isolated STAT3^GOF CD14⁺ monocytes fail to differentiate into CD1a⁺ monocyte-derived DCs. Moreover, patients with STAT3^GOF mutations display reduced circulating CD34⁺ hematopoietic progenitors and frequency of myeloid DCs. Specifically, we observed a reduction in the CD141⁺ DC population, with no difference in the frequencies of CD1c⁺ and plasmacytoid DCs. CD34⁺ hematopoietic progenitor cells from patients were found to differentiate into CD1c⁺ DCs, but failed to differentiate into CD141⁺ DCs indicating an intrinsic role for STAT3 in this process. STAT3^GOF-differentiated DCs produced lower amounts of CCL22 than healthy DCs, which could further explain some of the patient pathological phenotypes. Thus, our findings provide evidence that, in humans, STAT3 serves to regulate development and differentiation of nonclassical CD16⁺ monocytes and a subset of myeloid DCs.

Transcriptomic Analysis Identifies A Tolerogenic Dendritic Cell Signature

Dendritic cells (DC) are central to regulating innate and adaptive immune responses. Strategies that modify DC function provide new therapeutic opportunities in autoimmune diseases and transplantation. Current pharmacological approaches can alter DC phenotype to induce tolerogenic DC (tolDC), a maturation-resistant DC subset capable of directing a regulatory immune response that are being explored in current clinical trials. The classical phenotypic characterization of tolDC is limited to cell-surface marker expression and anti-inflammatory cytokine production, although these are not specific. TolDC may be better defined using gene signatures, but there is no consensus definition regarding genotypic markers. We address this shortcoming by analyzing available transcriptomic data to yield an independent set of differentially expressed genes that characterize human tolDC. We validate this transcriptomic signature and also explore gene differences according to the method of tolDC generation. As well as establishing a novel characterization of tolDC, we interrogated its translational utility in vivo, demonstrating this geneset was enriched in the liver, a known tolerogenic organ. Our gene signature will potentially provide greater understanding regarding transcriptional regulators of tolerance and allow researchers to standardize identification of tolDC used for cellular therapy in clinical trials.

Diversity of T cells in the skin: Novel insights

T cells populate the skin to provide an effective immunosurveillance against external insults and to maintain tissue homeostasis. Most cutaneous T cells are αβ T cells, however, γδ T cells also exist although in much lower frequency. Different subsets of αβ T cells can be found in the skin, such as short-lived effector T cells, central memory T cells, effector memory T cells, and tissue-resident memory T cells. Their differential biology, function, and location provide an ample spectrum of immune responses in the skin. Foxp3+ memory regulatory T cells have a pivotal role in maintaining homeostasis in the skin and their dysregulation has been linked with different skin pathologies. The skin also contains populations of non-classical T cells, such as γδ T cells, NK T cells, and MR1-restricted T cells. Their role in skin homeostasis and response to pathogens has been well established in the past years, however, there is also growing evidence of their role in mediating allergic skin inflammation and promoting sensitization to allergens. In this review, we provide an updated overview on the different subsets of T cells that populate the skin with a specific focus on their role in allergic skin inflammation.

Diversification of CD1 Molecules Shapes Lipid Antigen Selectivity

Molecular studies of host-pathogen evolution have largely focused on the consequences of variation at protein-protein interaction surfaces. The potential for other microbe-associated macromolecules to promote arms race dynamics with host factors remains unclear. The cluster of differentiation 1 (CD1) family of vertebrate cell surface receptors plays a crucial role in adaptive immunity through binding and presentation of lipid antigens to T-cells. Although CD1 proteins present a variety of endogenous and microbial lipids to various T-cell types, they are less diverse within vertebrate populations than the related major histocompatibility complex (MHC) molecules. We discovered that CD1 genes exhibit a high level of divergence between simian primate species, altering predicted lipid-binding properties and T-cell receptor interactions. These findings suggest that lipid-protein conflicts have shaped CD1 genetic variation during primate evolution. Consistent with this hypothesis, multiple primate CD1 family proteins exhibit signatures of repeated positive selection at surfaces impacting antigen presentation, binding pocket morphology, and T-cell receptor accessibility. Using a molecular modeling approach, we observe that interspecies variation as well as single mutations at rapidly-evolving sites in CD1a drastically alter predicted lipid binding and structural features of the T-cell recognition surface. We further show that alterations in both endogenous and microbial lipid-binding affinities influence the ability of CD1a to undergo antigen swapping required for T-cell activation. Together these findings establish lipid-protein interactions as a critical force of host-pathogen conflict and inform potential strategies for lipid-based vaccine development.

Human T cells engineered with a leukemia lipid-specific TCR enables donor-unrestricted recognition of CD1c-expressing leukemia

Acute leukemia relapsing after chemotherapy plus allogeneic hematopoietic stem cell transplantation can be treated with donor-derived T cells, but this is hampered by the need for donor/recipient MHC-matching and often results in graft-versus-host disease, prompting the search for new donor-unrestricted strategies targeting malignant cells. Leukemia blasts express CD1c antigen-presenting molecules, which are identical in all individuals and expressed only by mature leukocytes, and are recognized by T cell clones specific for the CD1c-restricted leukemia-associated methyl-lysophosphatidic acid (mLPA) lipid antigen. Here, we show that human T cells engineered to express an mLPA-specific TCR, target diverse CD1c-expressing leukemia blasts in vitro and significantly delay the progression of three models of leukemia xenograft in NSG mice, an effect that is boosted by mLPA-cellular immunization. These results highlight a strategy to redirect T cells against leukemia via transfer of a lipid-specific TCR that could be used across MHC barriers with reduced risk of graft-versus-host disease.

Leukaemia therapy may benefit from the use of antigens that are less restricted to individual donors. Here the authors engineered T cells with a TCR specific for a CD1c restricted lipid leukaemia antigen and show that they can protect against disease progression in mouse leukaemia xenograft models.

CD1a selectively captures endogenous cellular lipids that broadly block T cell response

We optimized lipidomics methods to broadly detect endogenous lipids bound to cellular CD1a proteins. Whereas membrane phospholipids dominate in cells, CD1a preferentially captured sphingolipids, especially a C42, doubly unsaturated sphingomyelin (42:2 SM). The natural 42:2 SM but not the more common 34:1 SM blocked CD1a tetramer binding to T cells in all human subjects tested. Thus, cellular CD1a selectively captures a particular endogenous lipid that broadly blocks its binding to TCRs. Crystal structures show that the short cellular SMs stabilized a triad of surface residues to remain flush with CD1a, but the longer lipids forced the phosphocholine group to ride above the display platform to hinder TCR approach. Whereas nearly all models emphasize antigen-mediated T cell activation, we propose that the CD1a system has intrinsic autoreactivity and is negatively regulated by natural endogenous inhibitors selectively bound in its cleft. Further, the detailed chemical structures of natural blockers could guide future design of therapeutic blockers of CD1a response.

CD1a autoreactivity: When size does matter

CD1a-autoreactive T cells represent a significant proportion of circulating αβ T cells in humans and appear to be enriched in the skin. How their autoreactivity is regulated remains unclear. In this issue of JEM, Cotton et al. (2021. J. Exp. Med.https://doi.org/10.1084/jem.20202699) show that CD1a molecules do not randomly survey cellular lipids but instead capture certain lipid classes that broadly interfere with the binding of autoreactive T cell antigen receptors to the target CD1a. These findings provide new potential therapeutic avenues for manipulating CD1a autoreactive T cell responses.

Current Understanding of the Roles of CD1a-Restricted T Cells in the Immune System

Cluster of differentiation 1 (CD1) is a family of cell-surface glycoproteins that present lipid antigens to T cells. Humans have five CD1 isoforms. CD1a is distinguished by the small volume of its antigen-binding groove and its stunted A' pocket, its high and exclusive expression on Langerhans cells, and its localization in the early endosomal and recycling intracellular trafficking compartments. Its ligands originate from self or foreign sources. There are three modes by which the T-cell receptors of CD1a-restricted T cells interact with the CD1a:lipid complex: they bind to both the CD1a surface and the antigen or to only CD1a itself, which activates the T cell, or they are unable to bind because of bulky motifs protruding from the antigen-binding groove, which might inhibit autoreactive T-cell activation. Recently, several studies have shown that by producing TH2 or TH17 cytokines, CD1a-restricted T cells contribute to inflammatory skin disorders, including atopic dermatitis, psoriasis, allergic contact dermatitis, and wasp/bee venom allergy. They may also participate in other diseases, including pulmonary disorders and cancer, because CD1a-expressing dendritic cells are also located in non-skin tissues. In this mini-review, we discuss the current knowledge regarding the biology of CD1a-reactive T cells and their potential roles in disease.

Resident Memory T Cells in Autoimmune Skin Diseases

Tissue resident memory T cells (TRM) are a critical component of the immune system, providing the body with an immediate and highly specific response against pathogens re-infecting peripheral tissues. More recently, however, it has been demonstrated that TRM cells also form during autoimmunity. TRM mediated autoimmune diseases are particularly destructive, because unlike foreign antigens, the self-antigens are never cleared, continuously activating self-reactive TRM T cells. In this article, we will focus on how TRMs mediate disease in autoimmune skin conditions, specifically vitiligo, psoriasis, cutaneous lupus erythematosus, alopecia areata and frontal fibrosing alopecia.

Novel Molecular Insights into Human Lipid-Mediated T Cell Immunity

T cells represent a critical arm of our immune defense against pathogens. Over the past two decades, considerable inroads have been made in understanding the fundamental principles underpinning the molecular presentation of peptide-based antigens by the Major Histocompatibility Complex molecules (MHC-I and II), and their molecular recognition by specialized subsets of T cells. However, some T cells can recognize lipid-based antigens presented by MHC-I-like molecules that belong to the Cluster of Differentiation 1 (CD1) family. Here, we will review the advances that have been made in the last five years to understand the molecular mechanisms orchestrating the presentation of novel endogenous and exogenous lipid-based antigens by the CD1 glycoproteins and their recognition by specific populations of CD1-reactive T cells.

Exploiting CD1-restricted T cells for clinical benefit

CD1-restricted T cells were first described over 30 years ago along with the cloning of the CD1 family. Around the same time, invariant Natural Killer cells (iNKT) were identified based on invariant TCR-alpha chains with additional expression of natural killer (NK) cell markers. About 5 years later, iNKT were shown to react with CD1d. Since then, iNKT have been shown to be a major population of CD1d-restricted T cells in humans and many animals. Like NK cells, iNKT are innate lymphocytes with rapid and wide-ranging effector potential. These activities include cytotoxicity and an unusually broad and high-level cytokine production. The development of highly-specific methods of isolating, stimulating, expanding or depleting these relatively rare cells and controlling their potent activities has stimulated considerable interest in therapeutic targeting of iNKT cells. Potential applications include cancers, inflammatory, infectious and autoimmune among other diseases. To date, most trials have targeted various cancers, there are 2 published trials in viral hepatitis and one in sickle cell lung disease. Uniform safety, evidence of immunologic activity and increasingly clinical efficacy have been seen. Approaches to targeting iNKT cells in clinical development include highly specific natural glycolipid ligands presented by CD1d and chemical analogues thereof and monoclonal antibody-based targeting of iNKT cells. In the case of iNKT cell-based therapies, novel approaches include arming them with Chimeric Antigen Receptors (CARs) and recombinant TCRs (rTCR), gene editing and allogeneic use. Controlling the iTCR:CD1d molecular interaction and consequences is a unique and promising therapeutic technology.

T Cells Specific for a Mycobacterial Glycolipid Expand after Intravenous Bacillus Calmette-Guérin Vaccination

Intradermal vaccination with Mycobacterium bovis bacillus Calmette-Guérin (BCG) protects infants from disseminated tuberculosis, and i.v. BCG protects nonhuman primates (NHP) against pulmonary and extrapulmonary tuberculosis. In humans and NHP, protection is thought to be mediated by T cells, which typically recognize bacterial peptide Ags bound to MHC proteins. However, during vertebrate evolution, T cells acquired the capacity to recognize lipid Ags bound to CD1a, CD1b, and CD1c proteins expressed on APCs. It is unknown whether BCG induces T cell immunity to mycobacterial lipids and whether CD1-restricted T cells are resident in the lung. In this study, we developed and validated Macaca mulatta (Mamu) CD1b and CD1c tetramers to probe ex vivo phenotypes and functions of T cells specific for glucose monomycolate (GMM), an immunodominant mycobacterial lipid Ag. We discovered that CD1b and CD1c present GMM to T cells in both humans and NHP. We show that GMM-specific T cells are expanded in rhesus macaque blood 4 wk after i.v. BCG, which has been shown to protect NHP with near-sterilizing efficacy upon M. tuberculosis challenge. After vaccination, these T cells are detected at high frequency within bronchoalveolar fluid and express CD69 and CD103, markers associated with resident memory T cells. Thus, our data expand the repertoire of T cells known to be induced by whole cell mycobacterial vaccines, such as BCG, and show that lipid Ag-specific T cells are resident in the lungs, where they may contribute to protective immunity.

CD1a function in human skin disease

The high expression of CD1a on Langerhans cells in normal human skin suggests a central role for this lipid antigen presenting molecule in skin homeostasis and immunity. Although the lipid antigen presenting function of CD1a has been known for years, the physiological and pathological functions of the CD1a system in human skin remain incompletely understood. This review provides an overview of this active area of investigation, and discusses recent insights into the functions of CD1a, CD1a-restricted T cells, and lipid antigens in inflammatory and allergic skin disease. We include recent publications and work presented at the biennial CD1-MR1 EMBO workshop held in 2019 in Oxford, regarding lipids that increase and those that decrease T cell responses to CD1a.

Human skin is colonized by T cells that recognize CD1a independently of lipid

CD1a-autoreactive T cells contribute to skin disease, but the identity of immunodominant self-lipid antigens and their mode of recognition are not yet solved. In most models, MHC and CD1 proteins serve as display platforms for smaller antigens. Here, we showed that CD1a tetramers without added antigen stained large T cell pools in every subject tested, accounting for approximately 1% of skin T cells. The mechanism of tetramer binding to T cells did not require any defined antigen. Binding occurred with approximately 100 lipid ligands carried by CD1a proteins, but could be tuned upward or downward with certain natural self-lipids. TCR recognition mapped to the outer A' roof of CD1a at sites remote from the antigen exit portal, explaining how TCRs can bind CD1a rather than carried lipids. Thus, a major antigenic target of CD1a T cell autoreactivity in vivo is CD1a itself. Based on their high frequency and prevalence among donors, we conclude that CD1a-specific, lipid-independent T cells are a normal component of the human skin T cell repertoire. Bypassing the need to select antigens and effector molecules, CD1a tetramers represent a simple method to track such CD1a-specific T cells from tissues and in any clinical disease.

Role of Group 1 CD1-Restricted T Cells in Host Defense and Inflammatory Diseases

Group 1 CD1-restricted T cells are members of the unconventional T cell family that recognize lipid antigens presented by CD1a, CD1b, and CD1c molecules. Although they developmentally mirror invariant natural killer T cells, they have diverse antigen specificity and functional capacity, with both anti-microbial and autoreactive targets. The role of group 1 CD1-restricted T cells has been best established in Mycobacterium tuberculosis (Mtb) infection in which a wide variety of lipid antigens have been identified and their ability to confer protection against Mtb infection in a CD1 transgenic mouse model has been shown. Group 1 CD1-restricted T cells have also been implicated in other infections, inflammatory conditions, and malignancies. In particular, autoreactive group 1 CD1-restricted T cells have been shown to play a role in several skin inflammatory conditions. The prevalence of group 1 CD1 autoreactive T cells in healthy individuals suggests the presence of regulatory mechanisms to suppress autoreactivity in homeostasis. The more recent use of group 1 CD1 tetramers and mouse models has allowed for better characterization of their phenotype, functional capacity, and underlying mechanisms of antigen-specific and autoreactive activation. These discoveries may pave the way for the development of novel vaccines and immunotherapies that target group 1 CD1-restricted T cells.

Animal models for human group 1 CD1 protein function

The CD1 antigen presenting system is evolutionary conserved and found in mammals, birds and reptiles. Humans express five isoforms, of which CD1a, CD1b and CD1c represent the group 1 CD1-molecules. They are recognized by T cells that express diverse αβ-T cell receptors. Investigation of the role of group 1 CD1 function has been hampered by the fact that CD1a, CD1b and CD1c are not expressed by mice. However, other animals, such as guinea pigs or cattle, serve as alternative models and have established basic aspects of CD1-dependent, antimicrobial immune functions. Group 1 CD1 transgenic mouse models became available about ten years ago. In a series of seminal studies these mouse models coined the mechanistical understanding of the role of the corresponding CD1 restricted T cell responses. This review gives a short overview of available animal studies and the lessons that have been and still can be learned.

The Many Faces of CD4+ T Cells: Immunological and Structural Characteristics

As a major arm of the cellular immune response, CD4⁺ T cells are important in the control and clearance of infections. Primarily described as helpers, CD4⁺ T cells play an integral role in the development and activation of B cells and CD8⁺ T cells. CD4⁺ T cells are incredibly heterogeneous, and can be divided into six main lineages based on distinct profiles, namely T helper 1, 2, 17 and 22 (Th1, Th2, Th17, Th22), regulatory T cells (Treg) and T follicular helper cells (Tfh). Recent advances in structural biology have allowed for a detailed characterisation of the molecular mechanisms that drive CD4⁺ T cell recognition. In this review, we discuss the defining features of the main human CD4⁺ T cell lineages and their role in immunity, as well as their structural characteristics underlying their detection of pathogens.

Off-the-Shelf Allogeneic T Cell Therapies for Cancer: Opportunities and Challenges Using Naturally Occurring "Universal" Donor T Cells

Chimeric antigen receptor (CAR) engineered T cell therapies individually prepared for each patient with autologous T cells have recently changed clinical practice in the management of B cell malignancies. Even though CARs used to redirect polyclonal T cells to the tumor are not HLA restricted, CAR T cells are also characterized by their endogenous T cell receptor (TCR) repertoire. Tumor-antigen targeted TCR-based T cell therapies in clinical trials are thus far using “conventional” αβ-TCRs that recognize antigens presented as peptides in the context of the major histocompatibility complex. Thus, both CAR- and TCR-based adoptive T cell therapies (ACTs) are dictated by compatibility of the highly polymorphic HLA molecules between donors and recipients in order to avoid graft-versus-host disease and rejection. The development of third-party healthy donor derived well-characterized off-the-shelf cell therapy products that are readily available and broadly applicable is an intensive area of research. While genome engineering provides the tools to generate “universal” donor cells that can be redirected to cancers, we will focus our attention on third-party off-the-shelf strategies with T cells that are characterized by unique natural features and do not require genome editing for safe administration. Specifically, we will discuss the use of virus-specific T cells, lipid-restricted (CD1) T cells, MR1-restricted T cells, and γδ-TCR T cells. CD1- and MR1-restricted T cells are not HLA-restricted and have the potential to serve as a unique source of universal TCR sequences to be broadly applicable in TCR-based ACT as their targets are presented by the monomorphic CD1 or MR1 molecules on a wide variety of tumor types. For each cell type, we will summarize the stage of preclinical and clinical development and discuss opportunities and challenges to deliver off-the-shelf targeted cellular therapies against cancer.

Hypersensitivities following allergen antigen recognition by unconventional T cells

Conventional T cells recognise protein-derived antigens in the context of major histocompatibility complex (MHC) class Ia and class II molecules and provide anti-microbial and anti-tumour immunity. Conventional T cells have also been implicated in type IV (also termed delayed-type or T cell-mediated) hypersensitivity reactions in response to protein-derived allergen antigens. In addition to conventional T cells, subsets of unconventional T cells exist, which recognise non-protein antigens in the context of monomorphic MHC class I-like molecules. These include T cells that are restricted to the cluster of differentiation 1 (CD1) family members, known as CD1-restricted T cells, and mucosal-associated invariant T cells (MAIT cells) that are restricted to the MHC-related protein 1 (MR1). Compared with conventional T cells, much less is known about the immune functions of unconventional T cells and their role in hypersensitivities. Here, we review allergen antigen presentation by MHC-I-like molecules, their recognition by unconventional T cells, and the potential role of unconventional T cells in hypersensitivities. We also speculate on possible scenarios of allergen antigen presentation by MHC-I-like molecules to unconventional T cells, the hallmarks of such responses, and the expected frequencies of hypersensitivities within the human population.

Innate Lymphocyte Mechanisms in Skin Diseases

Innate lymphocyte populations are emerging as key effectors in tissue homeostasis, microbial defense, and inflammatory skin disease. The cells are evolutionarily ancient and carry conserved principles of function, which can be achieved through shared or unique specific mechanisms. Recent technological and treatment advances have provided insight into heterogeneity within and between individuals and species. Similar pathways can extend through to adaptive lymphocytes, which softens the margins with innate lymphocyte populations and allows investigation of nonredundant pathways of immunity and inflammation that might be amenable to therapeutic intervention. Here, we review advances in understanding of innate lymphocyte biology with a focus on skin disease and the roles of commensal and pathogen responses and tissue homeostasis.

Re-evaluation of human BDCA-2+ DC during acute sterile skin inflammation

Plasmacytoid dendritic cells (pDCs) produce type I interferon (IFN-I) and are traditionally defined as being BDCA-2+CD123+. pDCs are not readily detectable in healthy human skin, but have been suggested to accumulate in wounds. Here, we describe a CD1a-bearing BDCA-2+CD123int DC subset that rapidly infiltrates human skin wounds and comprises a major DC population. Using single-cell RNA sequencing, we show that these cells are largely activated DCs acquiring features compatible with lymph node homing and antigen presentation, but unexpectedly express both BDCA-2 and CD123, potentially mimicking pDCs. Furthermore, a third BDCA-2-expressing population, Axl+Siglec-6+ DCs (ASDC), was also found to infiltrate human skin during wounding. These data demonstrate early skin infiltration of a previously unrecognized CD123intBDCA-2+CD1a+ DC subset during acute sterile inflammation, and prompt a re-evaluation of previously ascribed pDC involvement in skin disease.

Human T cell response to CD1a and contact dermatitis allergens in botanical extracts and commercial skin care products

During industrialization, humans have been exposed to increasing numbers of foreign chemicals. Failure of the immune system to tolerate drugs, cosmetics, and other skin products causes allergic contact dermatitis, a T cell-mediated disease with rising prevalence. Models of αβ T cell response emphasize T cell receptor (TCR) contact with peptide-MHC complexes, but this model cannot readily explain activation by most contact dermatitis allergens, which are nonpeptidic molecules. We tested whether CD1a, an abundant MHC I-like protein in human skin, mediates contact allergen recognition. Using CD1a-autoreactive human αβ T cell clones to screen clinically important allergens present in skin patch testing kits, we identified responses to balsam of Peru, a tree oil widely used in cosmetics and toothpaste. Additional purification identified benzyl benzoate and benzyl cinnamate as antigenic compounds within balsam of Peru. Screening of structurally related compounds revealed additional stimulants of CD1a-restricted T cells, including farnesol and coenzyme Q2. Certain general chemical features controlled response: small size, extreme hydrophobicity, and chemical constraint from rings and unsaturations. Unlike lipid antigens that protrude to form epitopes and contact TCRs, the small size of farnesol allows sequestration deeply within CD1a, where it displaces self-lipids and unmasks the CD1a surface. These studies identify molecular connections between CD1a and hypersensitivity to consumer products, defining a mechanism that could plausibly explain the many known T cell responses to oily substances.

Genome-wide association study identifies CD1A associated with rate of increase in plasma neurofilament light in non-demented elders

As a marker of neuroaxonal injury, neurofilament light (NFL) in blood is robustly elevated in many neurodegenerative conditions. We aimed to discover single nucleotide polymorphisms (SNPs) associated with longitudinal changes in plasma NFL levels that affect the risk of developing neurodegenerative disease and clinical disease progression. 545 eligible non-Hispanic white participants from the Alzheimer's Disease Neuroimaging Initiative (ADNI) with longitudinal plasma NFL data were included. Three SNPs (rs16840041, p=4.50×10-8; rs2269714, p=4.50×10-8; rs2269715, p=4.83×10-8) in CD1A were in high linkage disequilibrium (LD) and significantly associated with the increase in plasma NFL levels. We demonstrate a promoting effect of rs16840041-A on clinical disease progression (p = 0.006). Moreover, the minor allele (A) of rs16840041 was significantly associated with accelerated decline in [18F] Fluorodeoxyglucose (FDG) (estimate -1.6% per year [95% CI -0.6 to -2.6], p=0.0024). CD1A is a gene involved in longitudinal changes in plasma NFL levels and AD-related phenotypes among non-demented elders. Given the potential effects of these variants, CD1A should be further investigated as a gene of interest in neurodegenerative diseases and as a potential target for monitoring disease trajectories and treating disease.

Lipid Antigen Presentation by CD1b and CD1d in Lysosomal Storage Disease Patients

The lysosome has a key role in the presentation of lipid antigens by CD1 molecules. While defects in lipid antigen presentation and in invariant Natural Killer T (iNKT) cell response were detected in several mouse models of lysosomal storage diseases (LSD), the impact of lysosomal engorgement in human lipid antigen presentation is poorly characterized. Here, we analyzed the capacity of monocyte-derived dendritic cells (Mo-DCs) from Fabry, Gaucher, Niemann Pick type C and Mucopolysaccharidosis type VI disease patients to present exogenous antigens to lipid-specific T cells. The CD1b- and CD1d-restricted presentation of lipid antigens by Mo-DCs revealed an ability of LSD patients to induce CD1-restricted T cell responses within the control range. Similarly, freshly isolated monocytes from Fabry and Gaucher disease patients had a normal ability to present α-Galactosylceramide (α-GalCer) antigen by CD1d. Gaucher disease patients' monocytes had an increased capacity to present α-Gal-(1-2)-αGalCer, an antigen that needs internalization and processing to become antigenic. In summary, our results show that Fabry, Gaucher, Niemann Pick type C, and Mucopolysaccharidosis type VI disease patients do not present a decreased capacity to present CD1d-restricted lipid antigens. These observations are in contrast to what was observed in mouse models of LSD. The percentage of total iNKT cells in the peripheral blood of these patients is also similar to control individuals. In addition, we show that the presentation of exogenous lipids that directly bind CD1b, the human CD1 isoform with an intracellular trafficking to the lysosome, is normal in these patients.

The contribution of IL-17 to the development of autoimmunity in psoriasis

Psoriasis is an (auto)immune-mediated disease that manifests as widespread desquamative erythema. The TNF-α/IL-23/IL-17A axis is crucial to its pathogenesis, which is demonstrated by its excellent therapeutic response to biologics that target this axis. There is a strong association between HLA-C*0602 and psoriasis, and researchers have identified autoantigens that are restricted to this major histocompatibility class I molecule. These auto-Ags include LL-37, A disintegrin and metalloprotease domain containing thrombospondin type 1 motif-like 5 (ADAMTSL5), and keratin 17. IL-17A-producing T cells have been identified in T cell populations that are reactive to these auto-Ags. In addition, lipid Ags have surfaced as candidate auto-Ags that activate IL-17A-producing T cells in a CD1a-restricted manner. In this article, we review the candidate auto-Ags that may contribute to the activation of the IL-17A-deviated immune response in psoriasis.

Harnessing donor unrestricted T-cells for new vaccines against tuberculosis

Mycobacterium bovis bacille Calmette-Guérin (BCG) prevents extrapulmonary tuberculosis (TB) and death among infants but fails to consistently and sufficiently prevent pulmonary TB in adults. Thus, TB remains the leading infectious cause of death worldwide, and new vaccine approaches are urgently needed. T-cells are important for protective immunity to Mycobacterium tuberculosis (Mtb), but the optimal T-cell antigens to be included in new vaccines are not established. T-cells are often thought of as responding mainly to peptide antigens presented by polymorphic major histocompatibility complex (MHC) I and II molecules. Over the past two decades, the number of non-peptidic Mtb derived antigens for αβ and γδ T-cells has expanded rapidly, creating broader perspectives about the types of molecules that could be targeted by T-cell-based vaccines against TB. Many of these non-peptide responsive T-cell subsets in humans are activated in a manner that is unrestricted by classical MHC-dependent antigen-presenting systems, but instead require essentially nonpolymorphic presentation systems. These systems are Cluster of differentiation 1 (CD1), MHC related protein 1 (MR1), butyrophilin 3A1, as well as the nonclassical MHC class Ib family member HLA-E. Thus, the resulting T-cell responses can be shared among a genetically diverse population, creating the concept of donor-unrestricted T-cells (DURTs). Here, we review evidence that DURTs are an abundant component of the human immune system and recognize many antigens expressed by Mtb, including antigens that are expressed in BCG and other candidate whole cell vaccines. Further, DURTs exhibit functional diversity and demonstrate the ability to control microbial infection in small animal models. Finally, we outline specific knowledge gaps and research priorities that must be addressed to realize the full potential of DURTs as part of new TB vaccines approaches.