Inflammatory response induced by microglia plays a critical role in the demise of neuronal populations in neuroinflammatory diseases. Although the role of toll-like receptor 4 (TLR4) in microglia's inflammatory response is fully acknowledged, little is known about endogenous ligands that trigger TLR4 activation. Here, we report that galectin-3 (Gal3) released by microglia acts as an endogenous paracrine TLR4 ligand. Gal3-TLR4 interaction was further confirmed in a murine neuroinflammatory model (intranigral lipopolysaccharide [LPS] injection) and in human stroke subjects. Depletion of Gal3 exerted neuroprotective and anti-inflammatory effects following global brain ischemia and in the neuroinflammatory LPS model. These results suggest that Gal3-dependent-TLR4 activation could contribute to sustained microglia activation, prolonging the inflammatory response in the brain.

Human histocompatibility leukocyte antigen (HLA)-E is a nonclassical major histocompatibility complex (MHC) class I molecule which presents a restricted set of nonameric peptides, derived mainly from the signal sequence of other MHC class I molecules. It interacts with CD94/NKG2 receptors expressed on the surface of natural killer (NK) cells and T cell subsets. Here we demonstrate that HLA-E also presents a peptide derived from the leader sequence of human heat shock protein 60 (hsp60). This peptide gains access to HLA-E intracellularly, resulting in up-regulated HLA-E/hsp60 signal peptide cell-surface levels on stressed cells. Notably, HLA-E molecules in complex with the hsp60 signal peptide are no longer recognized by CD94/NKG2A inhibitory receptors. Thus, during cellular stress an increased proportion of HLA-E molecules may bind the nonprotective hsp60 signal peptide, leading to a reduced capacity to inhibit a major NK cell population. Such stress induced peptide interference would gradually uncouple CD94/NKG2A inhibitory recognition and provide a mechanism for NK cells to detect stressed cells in a peptide-dependent manner.

The Ras-GAP SH3 domain–binding proteins (G3BP) are essential regulators of the formation of stress granules (SG), cytosolic aggregates of proteins and RNA that are induced upon cellular stress, such as virus infection. Many viruses, including Semliki Forest virus (SFV), block SG induction by targeting G3BP. In this work, we demonstrate that the G3BP-binding motif of SFV nsP3 consists of two FGDF motifs, in which both phenylalanine and the glycine residue are essential for binding. In addition, we show that binding of the cellular G3BP-binding partner USP10 is also mediated by an FGDF motif. Overexpression of wt USP10, but not a mutant lacking the FGDF-motif, blocks SG assembly. Further, we identified FGDF-mediated G3BP binding site in herpes simplex virus (HSV) protein ICP8, and show that ICP8 binding to G3BP also inhibits SG formation, which is a novel function of HSV ICP8. We present a model of the three-dimensional structure of G3BP bound to an FGDF-containing peptide, likely representing a binding mode shared by many proteins to target G3BP. Stress granules (SGs) are dynamic aggregates of proteins and translationally silenced mRNA that are formed in cells upon various stress conditions, such as virus infection. SGs are thought to be antiviral, and many viruses have hence evolved countermeasures to prevent their formation, often targeting the essential SG protein G3BP. Here, we show that several otherwise unrelated viral and cellular proteins all bind G3BP with the sequence motif FGDF, and thereby repress SG formation: the non-structural protein 3 (nsP3) of the Old World alphavirus Semliki Forest virus (a close relative of the emerging, highly pathogenic Chikungunya virus); the protein ICP8 of herpes simplex virus; and in addition, the cellular protein USP10 (an SG component and protein deubiquitinase that stabilises e.g. the tumor suppressor p53). In this work, we also present and validate a model of the three-dimensional structure of G3BP bound to an FGDF-containing peptide. The FGDF-mediated G3BP binding represents an attractive target for therapeutic interventions against a range of diverse viral infections, and may also regulate the p53-stabilising function of USP10 in cancers.

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease, which exhibits multiple B cell abnormalities including expanded populations of memory B cells and elevated levels of autoantibodies. Belimumab is a monoclonal antibody targeting the B cell cytokine BAFF (a.k.a. BLyS), approved for the treatment of SLE. In this prospective cohort study, B cells from peripheral blood of 23 SLE patients initiating belimumab treatment and followed longitudinally for up to three years, were assessed using mass cytometry. B cells decreased during the study period, with a rapid decrease of both naïve and CD11c⁺CD21^- B cells at the first follow-up visit, followed by a continuous reduction at subsequent follow-ups. In contrast, plasma cells and switched memory B cells remained stable throughout the study. The observed immunological changes correlated with early, but not late, clinical improvements. Moreover, high baseline B cell counts were predictive of failure to attain low disease activity. In summary, our data unveiled both rapid and gradual later therapy-associated alterations of both known and unforeseen B cell phenotypes. Our results suggest that evaluation of B cell counts might prove useful prior to initiation of belimumab treatment and that early treatment evaluation and discontinuation might underestimate delayed clinical improvements resultant of late B cell changes.

Antibodies to citrullinated proteins, common in Rheumatoid Arthritis (RA) patients, are strongly associated to a specific set of HLA-DR alleles including HLA-DRB1*04:01, *04:04 and *01:01. Here, we first demonstrate that autoantibody levels towards the dominant citrullinated B cell epitope from α-enolase are significantly elevated in HLA-DRB1*04:01-positive RA patients. Furthermore, we identified α-enolase-derived T cell epitopes and demonstrated that native and citrullinated versions of several peptides bind with different affinities to HLA-DRB1*04:01, *04:04 and *01:01. The citrulline residues in the eight identified peptides are distributed throughout the entire length of the presented epitopes and more specifically, localized at peptide positions p-2, p2, p4, p6, p7, p10 and p11. Importantly, in contrast to its native version peptide 26 (TSKGLFRAAVPSGAS), the HLA-DRB1*04:01-restricted citrullinated peptide Cit26 (TSKGLFCitAAVPSGAS) elicited significant functional T cell responses in primary cells from RA patients. Comparative analysis of the crystal structures of HLA-DRB1*04:01 in complex with peptide 26 or Cit26 demonstrated that the post-translational modification did not alter the conformation of the peptide. And since citrullination is the only structural difference between the two complexes this indicates that the neo-antigen Cit26 is recognized by T cells with high specificity to the citrulline residue.