Human and viral membrane-associated E3 ubiquitin ligases MARCH1 and MIR2 recognize different features of CD86 to downregulate surface expression.

Immune-stimulatory ligands, such as major histocompatibility complex molecules and the T-cell costimulatory ligand CD86, are central to productive immunity. Endogenous mammalian membrane-associated RING-CHs (MARCH) act on these and other targets to regulate antigen presentation and activation of adaptive immunity, whereas virus-encoded homologs target the same molecules to evade immune responses. Substrate specificity is encoded in or near the membrane-embedded domains of MARCHs and the proteins they regulate, but the exact sequences that distinguish substrates from nonsubstrates are poorly understood. Here, we examined the requirements for recognition of the costimulatory ligand CD86 by two different MARCH-family proteins, human MARCH1 and Kaposi's sarcoma herpesvirus modulator of immune recognition 2 (MIR2), using deep mutational scanning. We identified a highly specific recognition surface in the hydrophobic core of the CD86 transmembrane (TM) domain (TMD) that is required for recognition by MARCH1 and prominently features a proline at position 254. In contrast, MIR2 requires no specific sequences in the CD86 TMD but relies primarily on an aspartic acid at position 244 in the CD86 extracellular juxtamembrane region. Surprisingly, MIR2 recognized CD86 with a TMD composed entirely of valine, whereas many different single amino acid substitutions in the context of the native TM sequence conferred MIR2 resistance. These results show that the human and viral proteins evolved completely different recognition modes for the same substrate. That some TM sequences are incompatible with MIR2 activity, even when no specific recognition motif is required, suggests a more complicated mechanism of immune modulation via CD86 than was previously appreciated.

Cryptic association of B7-2 molecules and its implication for clustering.

T-cell co-stimulation through CD28/CTLA4:B7-1/B7-2 axis is one of the extensively studied pathways that resulted in the discovery of several FDA-approved drugs for autoimmunity and cancer. However, many aspects of the signaling mechanism remain elusive, including oligomeric association and clustering of B7-2 on the cell surface. Here, we describe the structure of the IgV domain of B7-2 and its cryptic association into 1D arrays that appear to represent the pre-signaling state of B7-2 on the cell membrane. Super-resolution microscopy experiments on heterologous cells expressing B7-2 and B7-1 suggest, B7-2 form relatively elongated and larger clusters compared to B7-1. The sequence and structural comparison of other B7 family members, B7-1:CTLA4 and B7-2:CTLA-4 complex structures, support our view that the observed B7-2 1D zipper array is physiologically important. This observed 1D zipper-like array also provides an explanation for its clustering, and upright orientation on the cell surface, and avoidance of spurious signaling.

Prediction of CD28-CD86 protein complex structure using different level of resolution approach.

Immune system plays essential role in functioning of higher organisms. Its hyperactivity can lead to autoimmune diseases or even anaphylactic shock while hypoactivity leads to proneness to infections or even cancer. T-cells play crucial role in immunity mechanisms and their activation and inhibition is strictly controlled by the regulatory proteins, such as CD28 and CTLA-4. Activity of these proteins is controlled by a pair of ligands, named CD80 and CD86, which can non-covalently bound to their receptors. While structure of human CTLA-4-CD86 complex in known, there is still no available structure for the CD28-CD86 system. To obtain the reliable structure of CD28-CD86 complex we first validated our methodology on the CTLA-4-CD86 system. Then coarse-grained UNRES-dock molecular docking simulation was performed followed by all-atom molecular dynamics simulations. As a result, we obtained a complete CD28-CD86 complex structure on atomistic level, in which interaction interface is consistent with available data. We also determined the kinetic properties for CTLA4-CD86 and CD28-CD86 complexes with use of coarse-grained model and determined the key residues for complex formation with use of Robetta, PPCheck and HawkDock servers. Our results not only verify high accuracy of the UNRES-dock method, but also provide a highly reliable model of the CD28-CD86 complex, which can be used in further studies and drug design.

The preliminary efficacy evaluation of the CTLA-4-Ig treatment against Lupus nephritis through in-silico analyses.

The humanized cytotoxic T lymphocyte-associated antigen 4 immunoglobulin (CTLA-4-Ig) has been used to treat Lupus nephritis (LN) based on CTLA-4s negative regulation of T-cell activation through competent to binding with CD80/CD86, the inherent genetic factors influencing the CTLA-4-Ig treatment efficacy are widely unknown. Here, 62 nonsynonymous single nucleotide variants (nsSNVs) of CTLA-4 gene, 184 of CD80 and 201 of CD86 were identified and validated within both EMBL-EBI and dbSNP databases. Next, the nsSNVs rs1466152724 in CTLA-4, rs1196816748, rs765515058, rs1157880125, rs1022857991, and rs142547094 in CD80 and rs1203132714 in CD86 were consistently suggested to be deleterious by SIFT, PolyPhen-2, PROVEAN and meta LR. Based on the 3D structure stability analysis, the variant rs765515058 causing G167V in CD80 was found to reduce the protein's stability through changing the characters of constructed structure of complete CD80 apo form and stabilizing amino acid residues of CD80 holo form in a great degree. Furthermore, the interaction energy analysis results suggested that rs1022857991 causing C50F may reduce the binding energy of CTLA-4 with CD80. Along with the increasing variants, these nsSNVs' effects on the interaction of CTLA-4 with CD80/CD86 will increase, and thus influence the CTLA-4-Ig treatment efficacy against LN.

Docosahexaenoic acid induces changes in microglia/macrophage polarization after spinal cord injury in rats.

Docosahexaenoic acid (DHA, 22:6 (n-3)) leads to recovery of locomotor functions observed of spinal cord injury (SCI) in rats. In present study, we characterized the expression of iba-1, CD86, CD163 in microglia/macrophages, to assess activation state and M1 (pro-inflammatory)/M2 (anti-inflammatory) phenotypes respectively, in the rostral, central and caudal segment of the spinal cord on 7 and 35 days after SCI. We found that DHA treatment leads to: (1) an increased activation and proliferation of microglial cells; (2) an alteration in the dynamics between M1 and M2 microglia/macrophages phenotypes (3) and increased production of an antioxidant enzymes. Overall, our data demonstrates that DHA has a complex effect in post-traumatic process within the central nervous system, and supports the therapeutic potential of DHA-based drugs.

Superantigens hyperinduce inflammatory cytokines by enhancing the B7-2/CD28 costimulatory receptor interaction.

Full T-cell activation requires interaction between the costimulatory receptors B7-2 and CD28. By binding CD28, bacterial superantigens elicit harmful inflammatory cytokine overexpression through an unknown mechanism. We show that, by engaging not only CD28 but also its coligand B7-2 directly, superantigens potently enhance the avidity between B7-2 and CD28, inducing thereby T-cell hyperactivation. Using the same 12-aa ß-strand-hinge-α-helix domain, superantigens engage both B7-2 and CD28 at their homodimer interfaces, areas remote from where these coreceptors interact, implying that inflammatory signaling can be controlled through the receptor homodimer interfaces. Short B7-2 dimer interface mimetic peptides bind diverse superantigens, prevent superantigen binding to cell-surface B7-2 or CD28, attenuate inflammatory cytokine overexpression, and protect mice from lethal superantigen challenge. Thus, superantigens induce a cytokine storm not only by mediating the interaction between MHC-II molecule and T-cell receptor but also, critically, by promoting B7-2/CD28 coreceptor engagement, forcing the principal costimulatory axis to signal excessively. Our results reveal a role for B7-2 as obligatory receptor for superantigens. B7-2 homodimer interface mimotopes prevent superantigen lethality by blocking the superantigen-host costimulatory receptor interaction.

Prohibitins and the cytoplasmic domain of CD86 cooperate to mediate CD86 signaling in B lymphocytes.

CD86 engagement on a CD40L/IL-4-primed murine B cell activates signaling intermediates that promote NF-κB activation to increase Oct-2 and mature IgG1 mRNA and protein expression, as well as the rate of IgG1 transcription, without affecting class switch recombination. One of the most proximal signaling intermediates identified is phospholipase Cγ2, a protein reported to bind tyrosine residues, which are absent in the cytoplasmic domain of CD86. Using a proteomics-based identification approach, we show that the tyrosine-containing transmembrane adaptor proteins prohibitin (Phb)1 and Phb2 bind to CD86. The basal expression of Phb1/2 and association with CD86 was low in resting B cells, whereas the level of expression and association increased primarily after priming with CD40. The CD86-induced increase in Oct-2 and IgG1 was less when either Phb1/2 expression was reduced by short hairpin RNA or the cytoplasmic domain of CD86 was truncated or mutated at serine/threonine protein kinase C phosphorylation sites, which did not affect Phb1/2 binding to CD86. Using this approach, we also show that Phb1/2 and the CD86 cytoplasmic domain are required for the CD86-induced phosphorylation of IκBα, which we previously reported leads to NF-κB p50/p65 activation, whereas only Phb1/2 was required for the CD86-induced phosphorylation of phospholipase Cγ2 and protein kinase Cα/ß(II), which we have previously reported leads to NF-κB (p65) phosphorylation and subsequent nuclear translocation. Taken together, these findings suggest that Phb1/2 and the CD86 cytoplasmic domain cooperate to mediate CD86 signaling in a B cell through differential phosphorylation of distal signaling intermediates required to increase IgG1.

Heterologous expression of human costimulatory molecule B7-2 and construction of B7-2 immobilized polyhydroxyalkanoate nanoparticles for use as an immune activation agent.

BACKGROUND: Costimulation of T cells via costimulatory molecules such as B7 is important for eliciting cell-mediated antitumor immunity. Presenting costimulation molecules by immobilizing recombinant B7 on the surface of nanovectors is a novel strategy for complementary therapy. Polyhydroxyalkanoates (PHAs) are a family of biodegradable, non-toxic, biocompatible polyesters, which can be used as a nonspecific immobilizing matrix for protein presentation. Recombinant protein fusion with PHA granule binding protein phasin (PhaP) can be easily immobilized on the surface of PHA nanoparticles through hydrophobic interactions between PhaP and PHA, and therefore provides a low-cost protein presenting strategy. RESULTS: In this study, the extracellular domain of the B7-2 molecule (also named as CD86) was fused with PhaP at its N-terminal and heterogeneously expressed in recombinant Escherichia coli strain BL21 (DE3). The purified B7-2-PhaP protein was immobilized on the surface of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx)-based nanoparticles. Loading of 240 µg (3.2 pMol) of B7-2-PhaP protein per mg nanoparticles was achieved. Immobilized B7-2-PhaP on PHBHHx nanoparticles induced T cell activation and proliferation in vitro. CONCLUSIONS: A PHA nanoparticle-based B7-2 costimulation molecule-presenting system was constructed. The PHA-based B7 presenting nanosystem provided costimulation signals to induce T cell activation and expansion in vitro. The B7-2-PhaP immobilized PHA nanosystem is a novel strategy for costimulation molecule presentation and may be used for costimulatory molecule complementary therapy.

The intertransmembrane region of Kaposi's sarcoma-associated herpesvirus modulator of immune recognition 2 contributes to B7-2 downregulation.

Kaposi's sarcoma-associated herpesvirus (KSHV), a human tumor virus, encodes two homologous membrane-associated E3 ubiquitin ligases, modulator of immune recognition 1 (MIR1) and MIR2, to evade host immunity. Both MIR1 and MIR2 downregulate the surface expression of major histocompatibility complex class I (MHC I) molecules through ubiquitin-mediated endocytosis followed by lysosomal degradation. Since MIR2 additionally downregulates a costimulatory molecule (B7-2) and an integrin ligand (intercellular adhesion molecule 1 [ICAM-1]), MIR2 is thought to be a more important molecule for immune evasion than MIR1; however, the molecular basis of the MIR2 substrate specificity remains unclear. To address this issue, we determined which regions of B7-2 and MIR2 are required for MIR2-mediated B7-2 downregulation. Experiments with chimeras made by swapping domains between human B7-2 and CD8α, a non-MIR2 substrate, and between MIR1 and MIR2 demonstrated a significant contribution of the juxtamembrane (JM) region of B7-2 and the intertransmembrane (ITM) region of MIR2 to MIR2-mediated downregulation. Structure prediction and mutagenesis analyses indicate that Phe119 and Ser120 in the MIR2 ITM region and Asp244 in the B7-2 JM region contribute to the recognition of B7-2 by MIR2. This finding provides new insight into the molecular basis of substrate recognition by MIR family members.

Different cell surface oligomeric states of B7-1 and B7-2: implications for signaling.

The costimulatory ligands B7-1 and B7-2 are expressed on the surface of antigen-presenting cells and interact with the costimulatory receptors CD28 and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) expressed on T cells. Although B7-1 and B7-2 are homologous ligands having common receptors, they exhibit distinct biochemical features and roles in immune regulation. Several biochemical and structural studies have indicated differences in the oligomeric state of B7-1 and B7-2. However, the organization of B7 ligands on the cell surface has not been examined. By using photobleaching-based FRET (pbFRET), we demonstrate that B7-1 and B7-2 adopt different oligomeric states on the cell surface. Our study shows that B7-2 exists as a monomer on the cell surface whereas B7-1 exists predominantly as dimers on the cell surface. A series of mutations in B7-1 result in the expression of a predominantly monomeric species on the cell surface and validate the dimer interface proposed by prior crystallographic analysis. The difference in the oligomeric states of B7-1 and B7-2 provides insight into the geometric organization of the costimulatory receptor-ligand complexes in the immunological synapse and suggests constraints on signal transduction mechanisms involved in T cell activation.