Albert Bendelac

Exogenous and endogenous glycolipid antigens activate NKT cells during microbial infections

CD1d-restricted natural killer T (NKT) cells are innate-like lymphocytes that express a conserved T-cell receptor and contribute to host defence against various microbial pathogens. However, their target lipid antigens have remained elusive. Here we report evidence for microbial, antigen-specific activation of NKT cells against Gram-negative, lipopolysaccharide (LPS)-negative alpha-Proteobacteria such as Ehrlichia muris and Sphingomonas capsulata. We have identified glycosylceramides from the cell wall of Sphingomonas that serve as direct targets for mouse and human NKT cells, controlling both septic shock reaction and bacterial clearance in infected mice. In contrast, Gram-negative, LPS-positive Salmonella typhimurium activates NKT cells through the recognition of an endogenous lysosomal glycosphingolipid, iGb3, presented by LPS-activated dendritic cells. These findings identify two novel antigenic targets of NKT cells in antimicrobial defence, and show that glycosylceramides are an alternative to LPS for innate recognition of the Gram-negative, LPS-negative bacterial cell wall.

Lysosomal Glycosphingolipid Recognition by NKT Cells

NKT cells represent a distinct lineage of T cells that coexpress a conserved αβ T cell receptor (TCR) and natural killer (NK) receptors. Although the TCR of NKT cells is characteristically autoreactive to CD1d, a lipid-presenting molecule, endogenous ligands for these cells have not been identified. We show that a lysosomal glycosphingolipid of previously unknown function, isoglobotrihexosylceramide (iGb3), is recognized both by mouse and human NKT cells. Impaired generation of lysosomal iGb3 in mice lacking β-hexosaminidase b results in severe NKT cell deficiency, suggesting that this lipid also mediates development of NKT cells in the mouse. We suggest that expression of iGb3 in peripheral tissues may be involved in controlling NKT cell responses to infections and malignancy and in autoimmunity.

Distinct Functional Lineages of Human Vα24 Natural Killer T Cells

CD1d-restricted autoreactive natural killer (NK)T cells have been reported to regulate a range of disease conditions, including type I diabetes and immune rejection of cancer, through the secretion of either T helper (Th)2 or Th1 cytokines. However, mechanisms underlying Th2 versus Th1 cytokine secretion by these cells are not well understood. Since most healthy subjects express <1 NKT cell per 1,000 peripheral blood lymphocytes (PBLs), we devised a new method based on the combined used of T cell receptor (TCR)-specific reagents α-galactosylceramide (αGalCer) loaded CD1d-tetramers and anti-Vα24 monoclonal antibody, to specifically identify and characterize these rare cells in fresh PBLs. We report here that CD4+ and CD4−CD8− (double negative [DN]) NKT cell subsets represent functionally distinct lineages with marked differences in their profile of cytokine secretion and pattern of expression of chemokine receptors, integrins, and NK receptors. CD4+ NKT cells were the exclusive producers of interleukin (IL)-4 and IL-13 upon primary stimulation, whereas DN NKT cells had a strict Th1 profile and prominently expressed several NK lineage receptors. These findings may explain how NKT cells could promote Th2 responses in some conditions and Th1 in others, and should be taken into consideration for intervention in relevant diseases.

The Transcription Factor PLZF Directs the Effector Program of the NKT Cell Lineage

The transcriptional control of CD1d-restricted NKT cell development has remained elusive. We report that PLZF (promyelocytic leukemia zinc finger, Zbtb16), a member of the BTB/POZ-ZF family of transcription factors that includes the CD4-lineage-specific c-Krox (Th-POK), is exquisitely specific to CD1d-restricted NKT cells and human MR1-specific MAIT cells. PLZF was induced immediately after positive selection of NKT cell precursors, and PLZF-deficient NKT cells failed to undergo the intrathymic expansion and effector differentiation that characterize their lineage. Instead, they preserved a naive phenotype and were directed to lymph nodes. Conversely, transgenic expression of PLZF induced CD4(+) thymocytes to acquire effector differentiation and migrate to nonlymphoid tissues. We suggest that PLZF is a transcriptional signature of NKT cells that directs their innate-like effector differentiation during thymic development.

A committed precursor to innate lymphoid cells

Innate lymphoid cells (ILCs) specialize in the rapid secretion of polarized sets of cytokines and chemokines to combat infection and promote tissue repair at mucosal barriers. Their diversity and similarities with previously characterized natural killer (NK) cells and lymphoid tissue inducers (LTi) have prompted a provisional classification of all innate lymphocytes into groups 1, 2 and 3 solely on the basis of cytokine properties, but their developmental pathways and lineage relationships remain elusive. Here we identify and characterize a novel subset of lymphoid precursors in mouse fetal liver and adult bone marrow that transiently express high amounts of PLZF, a transcription factor previously associated with NK T cell development, by using lineage tracing and transfer studies. PLZFhigh cells were committed ILC progenitors with multiple ILC1, ILC2 and ILC3 potential at the clonal level. They excluded classical LTi and NK cells, but included a peculiar subset of NK1.1+DX5− ‘NK-like’ cells residing in the liver. Deletion of PLZF markedly altered the development of several ILC subsets, but not LTi or NK cells. PLZFhigh precursors also expressed high amounts of ID2 and GATA3, as well as TOX, a known regulator of PLZF-independent NK and LTi lineages. These findings establish novel lineage relationships between ILC, NK and LTi cells, and identify the common precursor to ILCs, termed ILCP. They also reveal the broad, defining role of PLZF in the differentiation of innate lymphocytes.

Testing the NKT cell hypothesis of human IDDM pathogenesis

Defects in IL-4-producing CD1d-autoreactive NKT cells have been implicated in numerous Th1-mediated autoimmune diseases, including diabetes, multiple sclerosis, rheumatoid arthritis, lupus, and systemic sclerosis. Particular attention has been focused on autoimmune insulin-dependent diabetes mellitus (IDDM) because nonobese diabetic (NOD) mice and humans with IDDM are both reported to express severe deficiencies in the frequency and Th2 functions of NKT cells. Furthermore, experimental manipulations of the NKT defect in the NOD mouse induced corresponding changes in disease. Taken together, these converging studies suggested a general role of NKT cells in natural protection against destructive autoimmunity. However, in previous reports the identification of NKT cells was based on indirect methods. We have now devised a direct, highly specific CD1d tetramer-based methodology to test whether humans with IDDM have associated NKT cell defects. Surprisingly, although we find marked and stable differences in NKT cells between individuals, our study of IDDM patients and healthy controls, including discordant twin pairs, demonstrates that NKT cell frequency and IL-4 production are conserved during the course of IDDM. These results contradict previous conclusions and refute the hypothesis that NKT cell defects underlie most autoimmune diseases.

Characterization of the early stages of thymic NKT cell development

Upon reaching the mature heat stable antigen (HSA)low thymic developmental stage, CD1d-restricted Vα14-Jα18 thymocytes undergo a well-characterized sequence of expansion and differentiation steps that lead to the peripheral interleukin-4/interferon-γ–producing NKT phenotype. However, their more immature HSAhigh precursors have remained elusive, and it has been difficult to determine unambiguously whether NKT cells originate from a CD4+CD8+ double-positive (DP) stage, and when the CD4+ and CD4−CD8− double-negative (DN) NKT subsets are formed. Here, we have used a CD1d tetramer-based enrichment strategy to physically identify HSAhigh precursors in thymuses of newborn mice, including an elusive DPlow stage and a CD4+ stage, which were present at a frequency of ∼10−6. These HSAhigh DP and CD4+ stages appeared to be nondividing, and already exhibited the same Vβ8 bias that characterizes mature NKT cells. This implied that the massive expansion of NKT cells is separated temporally from positive selection, but faithfully amplifies the selected TCR repertoire. Furthermore, we found that, unlike the DN γδ T cells, the DN NKT cells did not originate from a pTα-independent pathway bypassing the DP stage, but instead were produced during a short window of time from the conversion of a fraction of HSAlow NK1.1neg CD4 cells. These findings identify the HSAhigh CD4+ stage as a potential branchpoint between NKT and conventional T lineages and between the CD4 and DN NKT sublineages.

Liver Autoimmunity Triggered by Microbial Activation of Natural Killer T Cells

Summary Humans with primary biliary cirrhosis (PBC), a disease characterized by the destruction of small bile ducts, exhibit signature autoantibodies against mitochondrial Pyruvate Dehydrogenase Complex E2 (PDC-E2) that crossreact onto the homologous enzyme of Novosphingobium aromaticivorans, an ubiquitous alphaproteobacterium. Here, we show that infection of mice with N. aromaticivorans induced signature antibodies against microbial PDC-E2 and its mitochondrial counterpart but also triggered chronic T cell-mediated autoimmunity against small bile ducts. Disease induction required NKT cells, which specifically respond to N. aromaticivorans cell wall α-glycuronosylceramides presented by CD1d molecules. Combined with the natural liver tropism of NKT cells, the accumulation of N. aromaticivorans in the liver likely explains the liver specificity of destructive responses. Once established, liver disease could be adoptively transferred by T cells independently of NKT cells and microbes, illustrating the importance of early microbial activation of NKT cells in the initiation of autonomous, organ-specific autoimmunity.

TCR-inducible PLZF transcription factor required for innate phenotype of a subset of γδ T cells with restricted TCR diversity

Some γδ and αβ T lymphocytes exhibit an “innate” phenotype associated with rapid cytokine responses. The PLZF transcription factor is essential for the innate phenotype of NKT cells. This report shows that PLZF is likewise responsible for the innate, NKT-like phenotype of Vγ1+Vδ6.3/Vδ6.4+ cells. TCR cross-linking induced PLZF expression in all polyclonal immature γδ thymocytes, suggesting that agonist selection might be required for PLZF induction. Transgenic expression of Vγ1Vδ6.4 TCR was sufficient to support the development of large numbers of PLZF+ T cells, further supporting the importance of the TCR for PLZF induction. Interestingly, expression of this TCR transgene led to the development of spontaneous dermatitis.

Expansion and long-range differentiation of the NKT cell lineage in mice expressing CD1d exclusively on cortical thymocytes

Unlike conventional major histocompatibility complex–restricted T cells, Vα14-Jα18 NKT cell lineage precursors engage in cognate interactions with CD1d-expressing bone marrow–derived cells that are both necessary and sufficient for their thymic selection and differentiation, but the nature and sequence of these interactions remain partially understood. After positive selection mediated by CD1d-expressing cortical thymocytes, the mature NKT cell lineage undergoes a series of changes suggesting antigen priming by a professional antigen-presenting cell, including extensive cell division, acquisition of a memory phenotype, the ability to produce interleukin-4 and interferon-γ, and the expression of a panoply of NK receptors. By using a combined transgenic and chimeric approach to restrict CD1d expression to cortical thymocytes and to prevent expression on other hematopoietic cell types such as dendritic cells, macrophages, or B cells, we found that, to a large extent, expansion and differentiation events could be imparted by a single-cognate interaction with CD1d-expressing cortical thymocytes. These surprising findings suggest that, unlike thymic epithelial cells, cortical thymocytes can provide unexpected, cell type–specific signals leading to lineage expansion and NKT cell differentiation.