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Abstract: The CD8 T cell response to the HLA-A2-restricted epitope LLWNGPMAV (LLW) of the non-structural protein 4b of Yellow Fever Virus (YFV) is remarkably immunodominant, highly prevalent and powerful in YFV-vaccinated humans. Here we used a combinatorial peptide library screening in the context of an A2/LLW-specific CD8 T cell clone to identify a superagonist that features a methionine to isoleucine substitution at position 7. Based on in silico modeling, the functional enhancement of this LLW-7I mutation was associated with alterations in the structural dynamics of the peptide in the major histocompatibility complex (pMHC) binding with the T cell receptor (TCR). While the TCR off-rate of LLW-7I pMHC is comparable to the wild type peptide, the rigidity of the 7I peptide seems to confer less entropy loss upon TCR binding. This LLW-7I superagonist is an example of improved functionality in human CD8 T cells associated with optimized ligand rigidity for TCR binding and not with changes in TCR:pMHC off-rate kinetics.

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Abstract: The progression of acute myeloid leukemia (AML)—the most severe blood/bone marrow cancer—is determined by the ability of malignant cells to escape host immune surveillance. However, the systemic regulatory mechanisms underlying this phenomenon remain largely unknown. In this study, we discovered a fundamental systemic biochemical strategy that allows AML cells to employ physiological systems within the body to survive and escape immune attack. We found that AML cells use a crucial human adrenal cortex hormone (cortisol) to induce the expression of neuronal receptor latrophilin 1 (LPHN1), which facilitates exocytosis. This receptor interacts with the blood plasma protein fibronectin leucine rich transmembrane protein 3 (FLRT3) to cause secretion of the immune suppressor galectin-9, which impairs the anticancer activities of cytotoxic lymphoid cells. AML is a cancer of the blood and bone marrow that originates from self-renewing malignant immature myeloid cells and rapidly progresses into a systemic, and very often fatal, malignancy.1 AML cells employ physiological systems in the body to produce factors required for proliferation/disease progression.2,3 This includes the hijacking of stem cell factor (SCF), a major hematopoietic growth factor that controls AML progression and thus can become highly oncogenic.2,3 The expression and release of SCF can be triggered by AML cells via cytokines (e.g., interleukin-1β).2 Recent evidence clearly demonstrated that AML cells are also capable of impairing the activities of cytotoxic lymphoid cells (e.g., natural killer (NK) cells and cytotoxic T cells).4 One of the biochemical mechanisms underlying this phenomenon lies in the ability of AML cells to secrete the protein galectin-9. This tandem-type galectin binds the immune receptor Tim-3 and induces a variety of intracellular and cell-to-cell signaling events leading to the inactivation of NK cells, as well as the death of cytotoxic T cells.4,5 We recently reported that the process of galectin-9 secretion in AML cells is stimulated by the unique G protein-coupled receptor LPHN1, which normally functions in neurons to facilitate exocytosis.4,6 LPHN1 is also found in hematopoietic stem cells (HSCs), but its expression disappears at the early stages of their maturation.4,7 However, upon malignant transformation, AML cells preserve their abilities to express LPHN1 and to produce high levels of galectin-9 and Tim-3, in which the latter is involved in trafficking galectin-9 during the secretion process (HSCs express neither galectin-9 nor Tim-34). It is currently unknown which molecular mechanisms trigger elevated levels of LPHN1 expression in primary human AML cells, and in general, the mechanisms of upregulation of LPHN1 expression at the genomic level remain unclear. It is also unknown whether FLRT3, a natural LPHN1 ligand,4,8 is present in human blood plasma and in other tissues associated with AML. Unraveling these mechanisms is crucial to understanding the pathways that control the ability of AML cells to protect themselves against cytotoxic lymphoid cells and, thus, was the aim of the present study.

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Abstract: The Fc region of IgG antibodies (Cγ2 and Cγ3 domains) is responsible for effector functions such as antibody-dependent cell-mediated cytotoxicity and phagocytosis, through engagement with Fcγ receptors, although the ability to elicit these functions differs between the four human IgG subclasses. A key determinant of Fcγ receptor interactions is the FG loop in the Cγ2 domain. High resolution cryogenic IgG4-Fc crystal structures have revealed a unique conformation for this loop, which could contribute to the particular biological properties of this subclass. To further explore the conformation of the IgG4 Cγ2 FG loop at near-physiological temperature, we solved a 2.7 Å resolution room temperature structure of recombinant human IgG4-Fc from crystals analysed in situ. The Cγ2 FG loop in one chain differs from the cryogenic structure, and adopts the conserved conformation found in IgG1-Fc; however, this conformation participates in extensive crystal packing interactions. On the other hand, at room temperature, and free from any crystal packing interactions, the Cγ2 FG loop in the other chain adopts the conformation previously observed in the cryogenic IgG4-Fc structures, despite both conformations being accessible. The room temperature human IgG4-Fc structure thus provides a more complete and physiologically relevant description of the conformation of this functionally critical Cγ2 FG loop.

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Abstract: IgE antibodies play a central role in allergic disease. They recognize allergens via their Fab regions, whilst their effector functions are controlled through interactions of the Fc region with two principal cell surface receptors, FcɛRI and CD23. Crosslinking of FcɛRI-bound IgE on mast cells and basophils by allergen initiates an immediate inflammatory response, while the interaction of IgE with CD23 on B-cells regulates IgE production. We have determined the structures of the C-type lectin “head” domain of CD23 from seven crystal forms. The thirty-five independent structures reveal extensive conformational plasticity in two loops that are critical for IgE binding.

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Abstract: The structure of the complement-binding domain of Staphylococcus aureus protein Sbi (Sbi-IV) in complex with ligand C3d is presented. The 1.7 Å resolution structure reveals the molecular details of the recognition of thioester-containing fragment C3d of the central complement component C3, involving interactions between residues of Sbi-IV helix ?2 and the acidic concave surface of C3d. The complex provides a structural basis for the binding preference of Sbi for native C3 over C3b and explains how Sbi-IV inhibits the interaction between C3d and complement receptor 2. A second C3d binding site on Sbi-IV is identified in the crystal structure that is not observed in related S. aureus C3 inhibitors Efb-C and Ehp. This binding mode perhaps hints as to how Sbi-IV, as part of Sbi, forms a C3b–Sbi adduct and causes futile consumption of C3, an extraordinary aspect of Sbi function that is not shared by any other known Staphylococcal complement inhibitor.