Alterations of the iNKT cell compartment in brain-injured patients.

BACKGROUND: Brain injury (BI) induces a state of immunodepression leading to pneumonia. We investigated the invariant natural killer T (iNKT) cell compartment. METHODS: This is an observational study in two surgical intensive care units (ICUs) of a single institution and a research laboratory. Clinical data and samples from a prospective cohort were extracted. Severe brain-injured patients (n = 33) and sex- and age-matched healthy donors (n = 40) were studied. RESULTS: We observed the presence of IL-10 in serum, a loss of IFN-γ and IL-13 production by peripheral blood mononuclear cells (PBMCs) following IL-2 stimulation, and downregulation of HLA-DR expression on both monocytes and B cells early after BI. Inversely, CD1d, the HLA class I-like molecule involved in antigen presentation to iNKT cells, was over-expressed on patients' monocytes and B cells. The antigen-presenting activity to iNKT cells of PBMCs was increased in the patients who developed pneumonia, but not in those who remained free of infection. Frequencies of iNKT cells among PBMCs were dramatically decreased in patients regardless of their infection status. Following amplification, an increased frequency of CD4+ iNKT cells producing IL-4 was noticed in the group of patients free of infection compared with those who became infected and with healthy donors. Finally, serum from BI patients inhibited the iNKT cells' specific response as well as the non-specific IL-2 stimulation of PBMCs, and the expression of the beta-2 adrenergic receptor was elevated at the surface of patients T lymphocytes. CONCLUSIONS: We observed severe alterations of the iNKT cell compartment, including the presence of inhibitory serum factors. We demonstrate for the first time that the decreased capacity to present antigens is not a generalized phenomenon because whereas the expression of HLA-DR molecules is decreased, the capacity for presenting glycolipids through CD1d expression is higher in patients.

CD1a, CD1b, and CD1c in immunity against mycobacteria.

The CD1 system is composed of five types of human CD1 proteins, CD1a, CD1b, CD1c, CD1d, and CD1e, and their mammalian orthologs. Each type of CD1 protein has a distinct antigen binding groove and shows differing patterns of expression within cells and in different tissues. Here we review the molecular mechanisms by which CD1a, CD1b, and CD1c capture distinct classes of self- and mycobacterial antigens. We discuss how CD1-restricted T cells participate in the immune response, emphasizing new evidence for mycobacterial recognition in vivo in human and non-human models.

Technical advance. Measurement of iNKT cell responses at the single-cell level against rare HIV-1-infected dendritic cells in a mixed culture.

iNKT cells recognize lipid antigens, such as α-GalCer, presented in complex with CD1d expressed by DCs. Exposure of DCs to HIV-1 can lead to productive infection, and it was demonstrated recently that HIV-1 inhibits CD1d surface expression in an apparent mode of immune evasion. However, studies of the interaction between T cells, including iNKT cells and HIV-infected DCs in vitro, are hampered by the low frequency of productive infection in DCs. Here, we demonstrate the utility of full-length HIV-1 modified to express eGFP to address this problem. This virus allowed identification of single, rare productively infected cells in a mixed DC population by fluorescence microscopy and enabled detailed studies of the interaction of such cells with individual iNKT cells. iNKT cell responses to α-GalCer presented by HIV-1-positive and -negative DCs were quantified by intracellular IFN-γ staining in iNKT cells forming conjugates with DCs. Whereas complex formation was observed between iNKT cells and uninfected and infected DCs, only iNKT cells in contact with uninfected DCs produced IFN-γ. This microscopy assay, based on full-length HIV-1 modified to express eGFP, thus allows detailed evaluation of HIV-1 immune-evasion mechanisms in rare virus-infected live DCs.

Lysosomal recycling terminates CD1d-mediated presentation of short and polyunsaturated variants of the NKT cell lipid antigen alphaGalCer.

Short or polyunsaturated lipid variants of the NKT cell antigen alpha-galactosylceramide (alphaGC) exhibit decreased potency and a Th2 bias in vivo despite conserved TCR contact residues and stable binding to CD1d at neutral and acidic pH. Using reagents to directly visualize lipids in their free or CD1d-bound form, we determined that, contrary to predictions, these lipids reached the lysosome better than alphaGC. However, in contrast with alphaGC, they loaded CD1d at the cell surface and underwent immediate pH-dependent dissociation upon recycling to the lysosome. In cell-free assays, ultrafast dissociation of preformed complexes could be induced at acidic pH only when free competitor lipids were added, suggesting active lipid displacement. These findings provide a common cell biological explanation for the decreased stimulatory properties of short and polyunsaturated alphaGC variants. They also suggest that direct lipid displacement is a potent mechanism underlying highly dynamic lipid exchange reactions in the lysosomal compartment that shape the repertoire of lipids associated with CD1d.

Liver natural killer and natural killer T cells: immunobiology and emerging roles in liver diseases.

Hepatic lymphocytes are enriched in NK and NKT cells that play important roles in antiviral and antitumor defenses and in the pathogenesis of chronic liver disease. In this review, we discuss the differential distribution of NK and NKT cells in mouse, rat, and human livers, the ultrastructural similarities and differences between liver NK and NKT cells, and the regulation of liver NK and NKT cells in a variety of murine liver injury models. We also summarize recent findings about the role of NK and NKT cells in liver injury, fibrosis, and repair. In general, NK and NKT cells accelerate liver injury by producing proinflammatory cytokines and killing hepatocytes. NK cells inhibit liver fibrosis via killing early-activated and senescent-activated stellate cells and producing IFN-gamma. In regulating liver fibrosis, NKT cells appear to be less important than NK cells as a result of hepatic NKT cell tolerance. NK cells inhibit liver regeneration by producing IFN-gamma and killing hepatocytes; however, the role of NK cells on the proliferation of liver progenitor cells and the role of NKT cells in liver regeneration have been controversial. The emerging roles of NK/NKT cells in chronic human liver disease will also be discussed.Understanding the role of NK and NKT cells in the pathogenesis of chronic liver disease may help us design better therapies to treat patients with this disease.