Recognized more than a decade ago, NKT cells differentiate from mainstream thymic precursors through instructive signals emanating during TCR engagement by CD1d-expressing cortical thymocytes. Their semi-invariant alphabeta TCRs recognize isoglobotrihexosylceramide, a mammalian glycosphingolipid, as well as microbial alpha-glycuronylceramides found in the cell wall of Gram-negative, lipopolysaccharide-negative bacteria. This dual recognition of self and microbial ligands underlies innate-like antimicrobial functions mediated by CD40L induction and massive Th1 and Th2 cytokine and chemokine release. Through reciprocal activation of NKT cells and dendritic cells, synthetic NKT ligands constitute promising new vaccine adjuvants. NKT cells also regulate a range of immunopathological conditions, but the mechanisms and the ligands involved remain unknown. NKT cell biology has emerged as a new field of research at the frontier between innate and adaptive immunity, providing a powerful model to study fundamental aspects of the cell and structural biology of glycolipid trafficking, processing, and recognition.

Figure 3 of this Letter contains an inadvertently duplicated panel: the PBS 30 panel is identical to the aGalCer panel (top right). The corrected panels are shown here. Our results and conclusions are unaffected by this oversight. CORRIGENDUM

Persistent viral infections are a major health concern. One obstacle inhibiting the clearance of persistent infections is functional inactivation of antiviral T cells. Although such immunosuppression occurs rapidly after infection, the mechanisms that induce the loss of T-cell activity and promote viral persistence are unknown. Herein we document that persistent viral infection in mice results in a significant upregulation of interleukin (IL)-10 by antigen-presenting cells, leading to impaired T-cell responses. Genetic removal of Il10 resulted in the maintenance of robust effector T-cell responses, the rapid elimination of virus and the development of antiviral memory T-cell responses. Therapeutic administration of an antibody that blocks the IL-10 receptor restored T-cell function and eliminated viral infection. Thus, we identify a single molecule that directly induces immunosuppression leading to viral persistence and demonstrate that a therapy to neutralize IL-10 results in T-cell recovery and the prevention of viral persistence.

NKT cells represent a distinct lineage of T cells that coexpress a conserved alphabeta 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 beta-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.

The central event in the cellular immune response to invading microorganisms is the specific recognition of foreign peptides bound to major histocompatibility complex (MHC) molecules by the alphabeta T cell receptor (TCR). The x-ray structure of the complete extracellular fragment of a glycosylated alphabeta TCR was determined at 2.5 angstroms, and its orientation bound to a class I MHC-peptide (pMHC) complex was elucidated from crystals of the TCR-pMHC complex. The TCR resembles an antibody in the variable Valpha and Vbeta domains but deviates in the constant Calpha domain and in the interdomain pairing of Calpha with Cbeta. Four of seven possible asparagine-linked glycosylation sites have ordered carbohydrate moieties, one of which lies in the Calpha-Cbeta interface. The TCR combining site is relatively flat except for a deep hydrophobic cavity between the hypervariable CDR3s (complementarity-determining regions) of the alpha and beta chains. The 2C TCR covers the class I MHC H-2Kb binding groove so that the Valpha CDRs 1 and 2 are positioned over the amino-terminal region of the bound dEV8 peptide, the Vbeta chain CDRs 1 and 2 are over the carboxyl-terminal region of the peptide, and the Valpha and Vbeta CDR3s straddle the peptide between the helices around the central position of the peptide.

Emerging evidence indicates that CD8+ and CD4+ T-cell immunity is differentially regulated. Here we have delineated differences and commonalities among antiviral T-cell responses by enumeration and functional profiling of eight specific CD8+ and CD4+ T-cell populations during primary, memory and recall responses. A high degree of coordinate regulation among all specific T-cell populations stood out against an approximately 20-fold lower peak expansion and prolonged contraction phase of specific CD4+ T-cell populations. Surprisingly, although CD8+ T-cell memory was stably maintained for life, levels of specific CD4+ memory T cells gradually declined. However, this decay, which seemed to result from less efficient rescue from apoptosis, did not affect functionality of surviving virus-specific CD4+ T cells. Our results indicate that CD4+ T-cell memory might become limiting under physiological conditions and that conditions precipitating CD4+ T-cell loss might compromise protective immunity even in the presence of unimpaired CD8+ T-cell responses.

The ice reservoir that served as the source for the meltwater pulse IA remains enigmatic and controversial. We show that each of the melting scenarios that have been proposed for the event produces a distinct variation, or fingerprint, in the global distribution of meltwater. We compare sea-level fingerprints associated with various melting scenarios to existing sea-level records from Barbados and the Sunda Shelf and conclude that the southern Laurentide Ice Sheet could not have been the sole source of the meltwater pulse, whereas a substantial contribution from the Antarctic Ice Sheet is consistent with these records.

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.