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Human innate lymphoid cells have been described to exist in different organs, with functional deregulation of these cells contributing to several disease states. Here, we performed the first detailed characterization of the phenotype, tissue-residency properties, and functionality of ILC1s, ILC2s, and ILC3s in the human adult and fetal liver. In addition, we investigated changes in the ILC compartment in liver fibrosis. A unique composition of tissue-resident ILCs was observed in nonfibrotic livers as compared with that in mucosal tissues, with NKp44− ILC3s accounting for the majority of total intrahepatic ILCs. The frequency of ILC2s, representing a small fraction of ILCs in nonfibrotic livers, increased in liver fibrosis and correlated directly with the severity of the disease. Notably, intrahepatic ILC2s secreted the profibrotic cytokine IL-13 when exposed to IL-33 and thymic stromal lymphopoetin (TSLP); these cytokines were produced by hepatocytes, hepatic stellate cells (HSCs), and Kupffer cells in response to TLR-3 stimulation. In summary, the present results provide the first detailed characterization of intrahepatic ILCs in human adult and fetal liver. The results indicate a role for ILC2s in human liver fibrosis, implying that targeting ILC2s might be a novel therapeutic strategy for its treatment. [ABSTRACT FROM AUTHOR]

Regulatory T cells (Tregs) are important for maintaining immune homeostasis, but many studies suggest that Tregs are functionally impaired in autoimmune and chronic inflammatory disorders. In addition, effector T cells may vary in sensitivity toward Treg suppression. Herein, we have studied the interplay between T effectors and Tregs in the rheumatic joint. Synovial Tregs demonstrated a high degree of FOXP3 demethylation and displayed only marginal IL-17 and virtually no IFN-γ production following in vitro stimulation, altogether indicating suppressive capacity. Still, the frequency of FOXP3 expression could not predict the degree of suppression. Instead, the inflammatory milieu in the joint, i.e. proliferative capacity of effector T cells and in situ levels of pro-inflammatory cytokines influenced Treg function. Indeed, blocking IL-6 or TNF increased the suppression by Tregs in co-cultures. Additionally, approximately 30% of the synovial FOXP3+ T cells were Ki67+ and hence actively dividing, but proliferation did not overlap with cytokine production, suggesting that these cells represent functional Tregs having met their cognate antigen and expanded in an attempt to alleviate joint inflammation. Overall, our data argue against a general functional deficit in joint-derived Tregs and instead emphasize the importance of the inflammatory milieu to set the threshold for immune regulation. [ABSTRACT FROM AUTHOR]

Natural killer (NK) cells are activated early during inflammatory events and contribute to the shaping of the ensuing adaptive immune response. To further understand the role for NK cells in inflammation, we investigated the phenotype and function of synovial fluid (SF) NK cells from patients with chronic joint inflammation, as well as from patients with transient inflammation of the knee following trauma. We confirm that synovial NK cells are similar to the well-characterized CD56bright peripheral blood (PB) NK-cell subset present in healthy individuals. However, compared to this PB subset the synovial NK cells express a higher degree of activation markers including CD69 and NKp44, the latter being up-regulated also on CD56bright NK cells in the PB of patients. Activated synovial NK cells produced interferon-γ and tumour necrosis factor, and the production was further up-regulated by antibody masking of CD94/NKG2A, and down-regulated by target cells expressing human leucocyte antigen-E in complex with peptides known to engage CD94/NKG2A. We conclude that synovial NK cells have an activated phenotype and that CD94/NKG2A is a key regulator of synovial NK-cell cytokine synthesis. [ABSTRACT FROM AUTHOR]

β2-Microglobulin (β2m) is non-covalently linked to the major histocompatibility (MHC) class I heavy chain and interacts with CD8 and Ly49 receptors. Murine MHC class I can bind human β2m (hβ2m) and such hybrid molecules are often used in structural and functional studies. The replacement of mouse β2m (mβ2m) by hβ2m has important functional consequences for MHC class I complex stability and specificity, but the structural basis for this is unknown. To investigate the impact of species-specific β2m subunits on MHC class I conformation, murine MHC class I H-2Db in complex with hβ2m and the peptide gp33 derived from lymphocytic choriomeningitis virus (LCMV) has been expressed, refolded in vitro and crystallized. Crystals containing two complexes per asymmetric unit and belonging to the space group P21, with unit-cell parameters a = 68.1, b = 65.2, c = 101.9 Å, β = 102.4°, were obtained. [ABSTRACT FROM AUTHOR]

A prediction from the "missing self" hypothesis is that down-regulation of MHC class I on resting hematopoietic cells should be sufficient to make them susceptible to NK cell killing. Using a method enabling kinetic and quantitative assessments of NK cell-mediated rejection responses in vivo, we here show that resting hematopoietic cells from β-microglobulin-deficient (βm) mice were rapidly rejected in unmanipulated C57BL/6 (B6) mice. In situations of allelic MHC class I mismatches rejection occurred but required longer time. βm donor cells pre-activated with concanavalin A were more efficiently eliminated compared to resting cells, as were MHC tumor cells. When recipient mice were pretreatedwith an IFN inducer to activate NK cells, rejection was also enhanced. The signaling adaptor KARAP/DAP12 was dispensable for rejection of βm cells (lacking MHC) but critical for rejection of BALB/c cells (mismatched MHC) in unmanipulated B6 recipients. In contrast, B6 recipients with pre-activated NK cells rejected BALB/c cells in a KARAP/DAP12-independent fashion. Loss or mismatch of MHC class I in resting cells was thus sufficient to convey susceptibility to NK cell rejection. However, activation of the effector or the target enhanced rejection and shifted the balance between different signaling pathways involved. [ABSTRACT FROM AUTHOR]

Natural killer (NK) cells have been thought to depend largely on perforin-mediated mechanisms for the induction of cell death in targets. However, this view has more recently been challenged. It is now clear that NK cells are capable of using death ligands like Fas ligand (FasL) or tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) to induce cytotoxicity. Still, relatively little is known about the control of these "perforin-independent" cell death eliciting reactions, for example, the regulation of FasL expression on NK cells. In the present study, we confirm the ability of NK cells to mediate target cytotoxicity in the absence of perforin, in vivo and in vitro. We show that the induction of perforin-independent NK cell-mediated cell death is prevented by inhibiting signals mediated by MHC class I recognition. Furthermore, we demonstrate in vitro that cross-linking of the activation receptor NK1.1 on NK cells leads to the up-regulation of FasL on the cell surface. However, simultaneous engagement of an MHC class I binding inhibitory receptor prevents the externalization of FasL. These results provide a mechanistic explanation for theMHC class I-dependent regulation of perforin-independent cytotoxicity. [ABSTRACT FROM AUTHOR]

Beta2-microglobulin (beta2m) is non-covalently linked to the major histocompatibility (MHC) class I heavy chain and interacts with CD8 and Ly49 receptors. Murine MHC class I can bind human beta2m (hbeta2m) and such hybrid molecules are often used in structural and functional studies. The replacement of mouse beta2m (mbeta2m) by hbeta2m has important functional consequences for MHC class I complex stability and specificity, but the structural basis for this is unknown. To investigate the impact of species-specific beta2m subunits on MHC class I conformation, murine MHC class I H-2Db in complex with hbeta2m and the peptide gp33 derived from lymphocytic choriomeningitis virus (LCMV) has been expressed, refolded in vitro and crystallized. Crystals containing two complexes per asymmetric unit and belonging to the space group P2(1), with unit-cell parameters a = 68.1, b = 65.2, c = 101.9 A, beta = 102.4 degrees, were obtained.