Inhibition of TREM-2 Markedly Suppresses Joint Inflammation and Damage in Experimental Arthritis.

The triggering receptors expressed on myeloid cells (TREMs) are a family of activating immune receptors that regulate the inflammatory response. TREM-1, which is expressed on monocytes and/or macrophages and neutrophils, functions as an inflammation amplifier and plays a role in the pathogenesis of rheumatoid arthritis (RA). Unlike TREM-1, the role in RA of TREM-2, which is expressed on macrophages, immature monocyte-derived dendritic cells, osteoclasts, and microglia, remains unclear and controversial. TREM-2 ligands are still unknown, adding further uncertainty to our understanding of TREM-2 function. Previously, we demonstrated that TREM-1 blockade, using a ligand-independent TREM-1 inhibitory peptide sequence GF9 rationally designed by our signaling chain homooligomerization (SCHOOL) model of cell signaling, ameliorates collagen-induced arthritis (CIA) severity in mice. Here, we designed a TREM-2 inhibitory peptide sequence IA9 and tested it in the therapeutic CIA model, either as a free 9-mer peptide IA9, or as a part of a 31-mer peptide IA31 incorporated into lipopeptide complexes (IA31-LPC), for targeted delivery. We demonstrated that administration of IA9, but not a control peptide, after induction of arthritis diminished release of proinflammatory cytokines and dramatically suppressed joint inflammation and damage, suggesting that targeting TREM-2 may be a promising approach for the treatment of RA.

Bulk and Single-Cell Profiling of Breast Tumors Identifies TREM-1 as a Dominant Immune Suppressive Marker Associated With Poor Outcomes.

BACKGROUND: Triggering receptor expressed on myeloid cells (TREM)-1 is a key mediator of innate immunity previously associated with the severity of inflammatory disorders, and more recently, the inferior survival of lung and liver cancer patients. Here, we investigated the prognostic impact and immunological correlates of TREM1 expression in breast tumors. METHODS: Breast tumor microarray and RNAseq expression profiles (n=4,364 tumors) were analyzed for associations between gene expression, tumor immune subtypes, distant metastasis-free survival (DMFS) and clinical response to neoadjuvant chemotherapy (NAC). Single-cell (sc)RNAseq was performed using the 10X Genomics platform. Statistical associations were assessed by logistic regression, Cox regression, Kaplan-Meier analysis, Spearman correlation, Student's t-test and Chi-square test. RESULTS: In pre-treatment biopsies, TREM1 and known TREM-1 inducible cytokines (IL1B, IL8) were discovered by a statistical ranking procedure as top genes for which high expression was associated with reduced response to NAC, but only in the context of immunologically "hot" tumors otherwise associated with a high NAC response rate. In surgical specimens, TREM1 expression varied among tumor molecular subtypes, with highest expression in the more aggressive subtypes (Basal-like, HER2-E). High TREM1 significantly and reproducibly associated with inferior distant metastasis-free survival (DMFS), independent of conventional prognostic markers. Notably, the association between high TREM1 and inferior DMFS was most prominent in the subset of immunogenic tumors that exhibited the immunologically hot phenotype and otherwise associated with superior DMFS. Further observations from bulk and single-cell RNAseq analyses indicated that TREM1 expression was significantly enriched in polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) and M2-like macrophages, and correlated with downstream transcriptional targets of TREM-1 (IL8, IL-1B, IL6, MCP-1, SPP1, IL1RN, INHBA) which have been previously associated with pro-tumorigenic and immunosuppressive functions. CONCLUSIONS: Together, these findings indicate that increased TREM1 expression is prognostic of inferior breast cancer outcomes and may contribute to myeloid-mediated breast cancer progression and immune suppression.

Triggering Receptor Expressed on Myeloid Cells-1 (TREM-1) Contributes to Bordetella pertussis Inflammatory Pathology.

Whooping cough (pertussis) is a severe pulmonary infectious disease caused by the bacteria Bordetella pertussis. Pertussis infects an estimated 24 million people annually, resulting in >150,000 deaths. The NIH placed pertussis on the list of emerging pathogens in 2015. Antibiotics are ineffective unless administered before the onset of the disease characteristic cough. Therefore, there is an urgent need for novel pertussis therapeutics. We have shown that sphingosine-1-phosphate receptor (S1PR) agonists reduce pertussis inflammation without increasing bacterial burden. Transcriptomic studies were performed to identify this mechanism and allow for the development of pertussis therapeutics that specifically target problematic inflammation without sacrificing bacterial control. These data suggested a role for triggering receptor expressed on myeloid cells-1 (TREM-1). TREM-1 cell surface receptor functions as an amplifier of inflammatory responses. Expression of TREM-1 is increased in response to bacterial infection of mucosal surfaces. In mice, B. pertussis infection results in Toll-like receptor 9 (TLR9)-dependent increased expression of TREM-1 and its associated cytokines. Interestingly, S1PR agonists dampen pulmonary inflammation and TREM-1 expression. Mice challenged intranasally with B. pertussis and treated with ligand-dependent (LP17) and ligand-independent (GF9) TREM-1 inhibitors showed no differences in bacterial burden and significantly reduced tumor necrosis factor-α (TNF-α) and C-C motif chemokine ligand 2 (CCL-2) expression compared to controls. Mice receiving TREM-1 inhibitors showed reduced pulmonary inflammation compared to controls, indicating that TREM-1 promotes inflammatory pathology, but not bacterial control, during pertussis infection. This implicates TREM-1 as a potential therapeutic target for the treatment of pertussis.

SCHOOL of nature: ligand-independent immunomodulatory peptides.

Groundbreaking studies in protein biophysics have identified the mechanisms of transmembrane signaling at the level of druggable protein-protein interactions (PPIs). This resulted in the development of the signaling chain homooligomerization (SCHOOL) strategy to modulate cell responses using receptor-specific peptides. Inspired by nature, these short peptides use ligand-independent mechanisms of receptor inhibition and demonstrate potent efficacy in vitro and in vivo. The SCHOOL strategy is especially important when receptor ligands are unknown. An example is the triggering receptor expressed on myeloid cells-1 (TREM-1) receptor, an emerging therapeutic target involved in the pathogenesis of most inflammatory diseases. Here, I discuss advances in the field with a focus on TREM-1 inhibitory SCHOOL peptides that offer new hope for a 'magic bullet' cure for cancer, arthritis, sepsis, retinopathy, and other medical challenges.

Inhibition of Triggering Receptor Expressed on Myeloid Cells 1 Ameliorates Inflammation and Macrophage and Neutrophil Activation in Alcoholic Liver Disease in Mice.

Alcoholic liver disease (ALD) is characterized by macrophage and neutrophil leukocyte recruitment and activation in the liver. Damage- and pathogen-associated molecular patterns contribute to a self-perpetuating proinflammatory state in ALD. Triggering receptor expressed on myeloid cells 1 (TREM-1) is a surface receptor that amplifies inflammation induced by toll-like receptors (TLRs) and is expressed on neutrophils and monocytes/macrophages. We hypothesized that TREM-1 signaling contributes to proinflammatory pathway activation in ALD. Using an in vivo ALD model in mice, we tested the effects of ligand-independent TREM-1 inhibitory peptides that were formulated into human high-density lipoprotein (HDL)-mimicking complexes GF9-HDL and GA/E31-HDL. As revealed in vitro, macrophages endocytosed these rationally designed complexes through scavenger receptors. A 5-week alcohol feeding with the Lieber-DeCarli diet in mice resulted in increased serum alanine aminotransferase (ALT), liver steatosis, and increased proinflammatory cytokines in the liver. TREM-1 messenger RNA (mRNA) expression was significantly increased in alcohol-fed mice, and TREM-1 inhibitors significantly reduced this increase. TREM-1 inhibition significantly attenuated alcohol-induced spleen tyrosine kinase (SYK) activation, an early event in both TLR4 and TREM-1 signaling. The TREM-1 inhibitors significantly inhibited macrophage (epidermal growth factor-like module-containing mucin-like hormone receptor-like 1 [F4/80], clusters of differentiation [CD]68) and neutrophil (lymphocyte antigen 6 complex, locus G [Ly6G] and myeloperoxidase [MPO]) markers and proinflammatory cytokines (monocyte chemoattractant protein 1 [MCP-1], tumor necrosis factor α [TNF-α], interleukin-1ß [IL-1ß], macrophage inflammatory protein 1α [MIP-1α]) at the mRNA level compared to the HDL vehicle. Administration of TREM-1 inhibitors ameliorated liver steatosis and early fibrosis markers (α-smooth muscle actin [αSMA] and procollagen1α [Pro-Col1α]) at the mRNA level in alcohol-fed mice. However, the HDL vehicle also reduced serum ALT and some cytokine protein levels in alcohol-fed mice, indicating HDL-related effects. Conclusion: HDL-delivered novel TREM-1 peptide inhibitors ameliorate early phases of inflammation and neutrophil and macrophage recruitment and activation in the liver and attenuate hepatocyte damage and liver steatosis. TREM-1 inhibition represents a promising therapeutic approach for further investigations in ALD.

Blockade of TREM-1 prevents vitreoretinal neovascularization in mice with oxygen-induced retinopathy.

In pathological retinal neovascularization (RNV) disorders, the retina is infiltrated by activated leukocytes and macrophages. Triggering receptor expressed on myeloid cells 1 (TREM-1), an inflammation amplifier, activates monocytes and macrophages and plays an important role in cancer, autoimmune and other inflammation-associated disorders. Hypoxia-inducible TREM-1 is involved in cancer angiogenesis but its role in RNV remains unclear. Here, to close this gap, we evaluated the role of TREM-1 in RNV using a mouse model of oxygen-induced retinopathy (OIR). We found that hypoxia induced overexpression of TREM-1 in the OIR retinas compared to that of the room air group. TREM-1 was observed specifically in areas of pathological RNV, largely colocalizing with macrophage colony-stimulating factor (M-CSF) and CD45- and Iba-1-positive cells. TREM-1 blockade using systemically administered first-in-class ligand-independent TREM-1 inhibitory peptides rationally designed using the signaling chain homooligomerization (SCHOOL) strategy significantly (up to 95%) reduced vitreoretinal neovascularization. The peptides were well-tolerated when formulated into lipopeptide complexes for peptide half-life extension and targeted delivery. TREM-1 inhibition substantially downregulated retinal protein levels of TREM-1 and M-CSF suggesting that TREM-1-dependent suppression of pathological angiogenesis involves M-CSF. Targeting TREM-1 using TREM-1-specific SCHOOL peptide inhibitors represents a novel strategy to treat retinal diseases that are accompanied by neovascularization including retinopathy of prematurity.

Targeting Intramembrane Protein-Protein Interactions: Novel Therapeutic Strategy of Millions Years Old.

Intramembrane protein-protein interactions (PPIs) are involved in transmembrane signal transduction mediated by cell surface receptors and play an important role in health and disease. Recently, receptor-specific modulatory peptides rationally designed using a general platform of transmembrane signaling, the signaling chain homooligomerization (SCHOOL) model, have been proposed to therapeutically target these interactions in a variety of serious diseases with unmet needs including cancer, sepsis, arthritis, retinopathy, and thrombosis. These peptide drug candidates use ligand-independent mechanisms of action (SCHOOL mechanisms) and demonstrate potent efficacy in vitro and in vivo. Recent studies surprisingly revealed that in order to modify and/or escape the host immune response, human viruses use similar mechanisms and modulate cell surface receptors by targeting intramembrane PPIs in a ligand-independent manner. Here, I review these intriguing mechanistic similarities and discuss how the viral strategies optimized over a billion years of the coevolution of viruses and their hosts can help to revolutionize drug discovery science and develop new, disruptive therapies. Examples are given.

Novel TREM-1 Inhibitors Attenuate Tumor Growth and Prolong Survival in Experimental Pancreatic Cancer.

Pancreatic cancer (PC) is a highly lethal cancer with an urgent need to expand the limited treatment options for patients. Tumor-associated macrophages (TAMs) promote tumor aggressiveness and metastasis. High expression of triggering receptor expressed on myeloid cells 1 (TREM-1) on TAMs directly correlates with poor survival in patients with non-small cell lung cancer (NSCLC). We have previously hypothesized that blockade of TREM-1 could be a promising therapeutic strategy to treat cancer and shown that the novel, ligand-independent TREM-1 inhibitory peptides rationally designed using the signaling chain homooligomerization (SCHOOL) strategy suppress NSCLC growth in vivo. Here, we evaluated the therapeutic potential of these inhibitors in three human PC xenograft mouse models. Administration of SCHOOL peptides resulted in a strong antitumor effect achieving an optimal treatment/control (T/C) value of 19% depending on the xenograft and formulation used and persisting even after treatment was halted. The effect correlated significantly with increased survival and suppressed TAM infiltration. The peptides were well-tolerated when deployed either in free form or formulated into lipopeptide complexes for peptide half-life extension and targeted delivery. Finally, blockade of TREM-1 significantly reduced serum levels of interleukin (IL)-1α, IL-6, and macrophage colony-stimulating factor (M-CSF), but not vascular endothelial growth factor, suggesting M-CSF-dependent antitumor mechanisms. Collectively, these promising data suggest that SCHOOL TREM-1-specific peptide inhibitors have a cancer type independent, therapeutically beneficial antitumor activity and can be potentially used as a stand-alone therapy or as a component of combinational therapy for PC, NSCLC, and other solid tumors.

Rationally designed ligand-independent peptide inhibitors of TREM-1 ameliorate collagen-induced arthritis.

Triggering receptor expressed on myeloid cells 1 (TREM-1) is critically involved in the pathogenesis of rheumatoid arthritis (RA). In contrast to cytokine blockers, therapeutic blockade of TREM-1 can blunt excessive inflammation while preserving the capacity for microbial control. However, the nature of the TREM-1 ligand(s) and mechanisms of TREM-1 signalling are still not yet well understood, impeding the development of clinically relevant inhibitors of TREM-1. The aim of this study was to evaluate the anti-arthritic activity of a novel, ligand-independent TREM-1 inhibitory nonapeptide GF9 that was rationally designed using the signalling chain homo oligomerization (SCHOOL) model of cell signalling. Free GF9 and GF9 bound to macrophage-targeted nanoparticles that mimic human high-density lipoproteins (GF9-HDL) were used to treat collagen-induced arthritis (CIA). We also tested if 31-mer peptides with sequences from GF9 and helices 4 (GE31) and 6 (GA31) of the major HDL protein, apolipoprotein A-I, are able to perform three functions: assist in the self-assembly of GA/E31-HDL, target these particles to macrophages and block TREM-1 signalling. We showed that GF9, but not control peptide, ameliorated CIA and protected against bone and cartilage damage. The therapeutic effect of GF9 was accompanied by a reduction in the plasma levels of macrophage colony-stimulating factor and pro-inflammatory cytokines such as tumour necrosis factor-α, interleukin (IL)-1 and IL-6. Incorporation of GF9 alone or as a part of GE31 and GA31 peptides into HDL significantly increased its therapeutic efficacy. Collectively, our findings suggest that TREM-1 inhibitory SCHOOL sequences may be promising alternatives for the treatment of RA.

SARS Coronavirus Fusion Peptide-Derived Sequence Suppresses Collagen-Induced Arthritis in DBA/1J Mice.

During the co-evolution of viruses and their hosts, the viruses have evolved numerous strategies to counter and evade host antiviral immune responses in order to establish a successful infection, replicate and persist in the host. Recently, based on our model of immune signaling, the Signaling Chain HOmoOLigomerization (SCHOOL) model, we suggested specific molecular mechanisms used by different viruses such as severe acute respiratory syndrome coronavirus (SARS-CoV) to modulate the host immune response mediated by members of the family of multichain immune recognition receptors (MIRRs). This family includes T cell receptor (TCR) that is critically involved in immune diseases such as autoimmune arthritis. In the present study, we provide compelling experimental in vivo evidence in support of our hypothesis. Using the SCHOOL approach and the SARS-CoV fusion peptide sequence, we rationally designed a novel immunomodulatory peptide that targets TCR. We showed that this peptide ameliorates collagen-induced arthritis in DBA/1J mice and protects against bone and cartilage damage. Incorporation of the peptide into self-assembling lipopeptide nanoparticles that mimic native human high density lipoproteins significantly increases peptide dosage efficacy. Together, our data further confirm that viral immune evasion strategies that target MIRRs can be transferred to therapeutic strategies that require similar functionalities.

Diagnostic Magnetic Resonance Imaging of Atherosclerosis in Apolipoprotein E Knockout Mouse Model Using Macrophage-Targeted Gadolinium-Containing Synthetic Lipopeptide Nanoparticles.

Cardiovascular disease is the leading cause of death in Western cultures. The vast majority of cardiovascular events, including stroke and myocardial infarction, result from the rupture of vulnerable atherosclerotic plaques, which are characterized by high and active macrophage content. Current imaging modalities including magnetic resonance imaging (MRI) aim to characterize anatomic and structural features of plaques rather than their content. Previously, we reported that macrophage-targeted delivery of gadolinium (Gd)-based contrast agent (GBCA-HDL) using high density lipoproteins (HDL)-like particles significantly enhances the detection of plaques in an apolipoprotein (apo) E knockout (KO) mouse model, with an atherosclerotic wall/muscle normalized enhancement ratio (NER) of 120% achieved. These particles are comprised of lipids and synthetic peptide fragments of the major protein of HDL, apo A-I, that contain a naturally occurring modification which targets the particles to macrophages. Targeted delivery minimizes the Gd dose and thus reduces the adverse effects of Gd. The aims of the current study were to test whether varying the GBCA-HDL particle shape and composition can further enhance atherosclerotic plaque MRI and control organ clearance of these agents. We show that the optimized GBCA-HDL particles are efficiently delivered intracellularly to and uptaken by both J774 macrophages in vitro and more importantly, by intraplaque macrophages in vivo, as evidenced by NER up to 160% and higher. This suggests high diagnostic power of our GBCA-HDL particles in the detection of vulnerable atherosclerotic plaques. Further, in contrast to discoidal, spherical GBCA-HDL exhibit hepatic clearance, which could further diminish adverse renal effects of Gd. Finally, activated macrophages are reliable indicators of any inflamed tissues and are implicated in other areas of unmet clinical need such as rheumatoid arthritis, sepsis and cancer, suggesting the expanded diagnostic and prognostic use of this method.

A novel ligand-independent peptide inhibitor of TREM-1 suppresses tumor growth in human lung cancer xenografts and prolongs survival of mice with lipopolysaccharide-induced septic shock.

Triggering receptor expressed on myeloid cells-1 (TREM-1) amplifies the inflammatory response and plays a role in cancer and sepsis. Inhibition of TREM-1 by short hairpin RNA (shRNA) in macrophages suppresses cancer cell invasion in vitro. In the clinical setting, high levels of TREM-1 expression on tumor-associated macrophages are associated with cancer recurrence and poor survival of patients with non-small cell lung cancer (NSCLC). TREM-1 upregulation on peritoneal neutrophils has been found in human sepsis patients and in mice with experimental lipopolysaccharide (LPS)-induced septic shock. However, the precise function of TREM-1 and the nature of its ligand are not yet known. In this study, we used the signaling chain homooligomerization (SCHOOL) model of immune signaling to design a novel, ligand-independent peptide-based TREM-1 inhibitor and demonstrated that this peptide specifically silences TREM-1 signaling in vitro and in vivo. Utilizing two human lung tumor xenograft nude mouse models (H292 and A549) and mice with LPS-induced sepsis, we show for the first time that blockade of TREM-1 function using non-toxic and non-immunogenic SCHOOL peptide inhibitors: 1) delays tumor growth in xenograft models of human NSCLC, 2) prolongs survival of mice with LPS-induced septic shock, and 3) substantially decreases cytokine production in vitro and in vivo. In addition, targeted delivery of SCHOOL peptides to macrophages utilizing lipoprotein-mimicking nanoparticles significantly increased peptide half-life and dosage efficacy. Together, the results suggest that ligand-independent modulation of TREM-1 function using small synthetic peptides might be a suitable treatment for sepsis and NSCLC and possibly other types of inflammation-associated disorders.

Nature-inspired nanoformulations for contrast-enhanced in vivo MR imaging of macrophages.

Magnetic resonance imaging (MRI) of macrophages in atherosclerosis requires the use of contrast-enhancing agents. Reconstituted lipoprotein particles that mimic native high-density lipoproteins (HDL) are a versatile delivery platform for Gd-based contrast agents (GBCA) but require targeting moieties to direct the particles to macrophages. In this study, a naturally occurring methionine oxidation in the major HDL protein, apolipoprotein (apo) A-I, was exploited as a novel way to target HDL to macrophages. We also tested if fully functional GBCA-HDL can be generated using synthetic apo A-I peptides. The fluorescence and MRI studies reveal that specific oxidation of apo A-I or its peptides increases the in vitro macrophage uptake of GBCA-HDL by 2-3 times. The in vivo imaging studies using an apo E-deficient mouse model of atherosclerosis and a 3.0 T MRI system demonstrate that this modification significantly improves atherosclerotic plaque detection using GBCA-HDL. At 24 h post-injection of 0.05 mmol Gd kg(-1) GBCA-HDL containing oxidized apo A-I or its peptides, the atherosclerotic wall/muscle normalized enhancement ratios were 90 and 120%, respectively, while those of GBCA-HDL containing their unmodified counterparts were 35 and 45%, respectively. Confocal fluorescence microscopy confirms the accumulation of GBCA-HDL containing oxidized apo A-I or its peptides in intraplaque macrophages. Together, the results of this study confirm the hypothesis that specific oxidation of apo A-I targets GBCA-HDL to macrophages in vitro and in vivo. Furthermore, our observation that synthetic peptides can functionally replace the native apo A-I protein in HDL further encourages the development of these contrast agents for macrophage imaging.

Bi-specific MHC heterodimers for characterization of cross-reactive T cells.

T cell cross-reactivity describes the phenomenon whereby a single T cell can recognize two or more different peptide antigens presented in complex with MHC proteins. Cross-reactive T cells have previously been characterized at the population level by cytokine secretion and MHC tetramer staining assays, but single-cell analysis is difficult or impossible using these methods. In this study, we describe development of a novel peptide-MHC heterodimer specific for cross-reactive T cells. MHC-peptide monomers were independently conjugated to hydrazide or aldehyde-containing cross-linkers using thiol-maleimide coupling at cysteine residues introduced into recombinant MHC heavy chain proteins. Hydrazone formation provided bi-specific MHC heterodimers carrying two different peptides. Using this approach we prepared heterodimers of the murine class I MHC protein H-2K(b) carrying peptides from lymphocytic choriomeningitis virus and vaccinia virus, and used these to identify cross-reactive CD8+ T cells recognizing both lymphocytic choriomeningitis virus and vaccinia virus antigens. A similar strategy could be used to develop reagents to analyze cross-reactive T cell responses in humans.

Pseudo-merohedral twinning and noncrystallographic symmetry in orthorhombic crystals of SIVmac239 Nef core domain bound to different-length TCRzeta fragments.

HIV/SIV Nef mediates many cellular processes through interactions with various cytoplasmic and membrane-associated host proteins, including the signalling zeta subunit of the T-cell receptor (TCRzeta). Here, the crystallization strategy, methods and refinement procedures used to solve the structures of the core domain of the SIVmac239 isolate of Nef (Nef(core)) in complex with two different TCRzeta fragments are described. The structure of SIVmac239 Nef(core) bound to the longer TCRzeta polypeptide (Leu51-Asp93) was determined to 3.7 A resolution (R(work) = 28.7%) in the tetragonal space group P4(3)2(1)2. The structure of SIVmac239 Nef(core) in complex with the shorter TCRzeta polypeptide (Ala63-Arg80) was determined to 2.05 A resolution (R(work) = 17.0%), but only after the detection of nearly perfect pseudo-merohedral crystal twinning and proper assignment of the orthorhombic space group P2(1)2(1)2(1). The reduction in crystal space-group symmetry induced by the truncated TCRzeta polypeptide appears to be caused by the rearrangement of crystal-contact hydrogen-bonding networks and the substitution of crystallographic symmetry operations by similar noncrystallographic symmetry (NCS) operations. The combination of NCS rotations that were nearly parallel to the twin operation (k, h, -l) and a and b unit-cell parameters that were nearly identical predisposed the P2(1)2(1)2(1) crystal form to pseudo-merohedral twinning.

The SCHOOL of nature: III. From mechanistic understanding to novel therapies.

Protein-protein interactions play a central role in biological processes and thus represent an appealing target for innovative drug design and development. They can be targeted by small molecule inhibitors, modulatory peptides and peptidomimetics, which represent a superior alternative to protein therapeutics that carry many disadvantages. Considering that transmembrane signal transduction is an attractive process to therapeutically control multiple diseases, it is fundamentally and clinically important to mechanistically understand how signal transduction occurs. Uncovering specific protein-protein interactions critical for signal transduction, a general platform for receptor-mediated signaling, the signaling chain homooligomerization (SCHOOL) platform, suggests these interactions as universal therapeutic targets. Within the platform, the general principles of signaling are similar for a variety of functionally unrelated receptors. This suggests that global therapeutic strategies targeting key protein-protein interactions involved in receptor triggering and transmembrane signal transduction may be used to treat a diverse set of diseases. This also assumes that clinical knowledge and therapeutic strategies can be transferred between seemingly disparate disorders, such as T cell-mediated skin diseases and platelet disorders or combined to develop novel pharmacological approaches. Intriguingly, human viruses use the SCHOOL-like strategies to modulate and/or escape the host immune response. These viral mechanisms are highly optimized over the millennia, and the lessons learned from viral pathogenesis can be used practically for rational drug design. Proof of the SCHOOL concept in the development of novel therapies for atopic dermatitis, rheumatoid arthritis, cancer, platelet disorders and other multiple indications with unmet needs opens new horizons in therapeutics.

Signaling chain homooligomerization (SCHOOL) model.

Multichain immune recognitionreceptors (MIRRs) represent a family of surfacereceptors expressed on different cells of the hematopoietic system and function to transduce signals leading to a variety of biologic responses. The most intriguing and distinct structural feature ofMIRR family members is that extracellular recognition domains and intracellular signaling domains are located on separate subunits. The biochemical cascades triggered by MIRRs are understood in significant detail, however, the mechanism by which extracellular ligand binding initiates intracellular signal transduction processes is not clear and no model fully explains how MIRR signaling commences. In this Chapter, I describe a novel mechanistic model of MIRR-mediated signal transduction, the signaling chain homooligomerization (SCHOOL) model. The basic concept of this model assumes that the structural similarity of the MIRRs provides the basis for the similarity in the mechanisms ofMIRR-mediated transmembrane signaling. Within the SCHOOL model, MIRR triggering is considered to be a result of the ligand-induced interplay between (1) intrareceptor transmembrane interactions between MIRR recognition and signaling subunits that stabilize and maintain receptor integrity and (2) interreceptor homointeractions between MIRR signaling subunits that lead to the formation of oligomeric signaling structures, thus triggering the receptors and initiating the signaling cascade. Thus, the SCHOOL model is based on specific protein-protein interactions--biochemical processes that can be influenced and controlled. In this context, this plausible and easily testable model is fundamentally different from those previously suggested for particular MIRRs and has several important advantages. The basic principles oftransmembrane signaling learned from the SCHOOL model may be used in different fields of immunology and cell biology to describe, explain and predict immunological phenomena and processes mediated by structurally related but functionally different membrane receptors. Important applications of the SCHOOL model in clinical immunology, molecular pharmacology and virology are described in the Chapters 20 and 22 of this book.

SCHOOL model and new targeting strategies.

Protein-protein interactions play a central role in biological processes and thus are an appealing target for innovative drug design a nd development. They can be targeted bysmall molecule inhibitors, peptides and peptidomimetics, which represent an alternative to protein therapeutics that carry many disadvantages. In this chapter, I describe specific protein-protein interactions suggested by a novel model of immune signaling, the Signaling Chain HOmoOLigomerization (SCHOOL) model, to be critical for cell activation mediated by multichain immune recognition receptors (MIRRs) expressed on different cells of the hematopoietic system. Unraveling a long-standing mystery of MIRR triggering and transmembrane signaling, the SCHOOL model reveals the intrareceptor transmembrane interactions and interreceptor cytoplasmic homointeractions as universal therapeutic targets for a diverse variety of disorders mediated by immune cells. Further, assuming that the general principles underlying MIRR-mediated transmembrane signaling mechanisms are similar, the SCHOOL model can be applied to any particular receptor of the MIRR family. Thus, an important application of the SCHOOL model is that global therapeutic strategies targeting key protein-protein interactions involved in MIRR triggering and transmembrane signal transduction may be used to treat a diverse set of immune-mediated diseases. This assumes that clinical knowledge and therapeutic strategies can be transferred between seemingly disparate disorders, such as T-cell-mediated skin diseases and platelet disorders, or combined to develop novel pharmacological approaches. Intriguingly, the SCHOOL model unravels the molecular mechanisms underlying ability of different human viruses such as human immunodeficiency virus, cytomegalovirus and severe acute respiratory syndrome coronavirus to modulate and/or escape the host immune response. It also demonstrates how the lessons learned from viral pathogenesis can be used practically for rational drug design. Application of this model to platelet collagen receptor signaling has already led to the development of a novel concept of platelet inhibition and the invention of new platelet inhibitors, thus proving the suggested hypothesis and highlighting the importance and broad perspectives of the SCHOOL model in the development of new targeting strategies.