Peptide derived from SLAMF1 prevents TLR4-mediated inflammation in vitro and in vivo

Inflammation plays a crucial role in the development and progression of many diseases, and is often caused by dysregulation of signalling from pattern recognition receptors, such as TLRs. Inhibition of key protein-protein interactions is an attractive target for treating inflammation. Recently, we demonstrated that the signalling lymphocyte activation molecule family 1 (SLAMF1) positively regulates signalling downstream of TLR4 and identified the interaction interface between SLAMF1 and the TLR4 adaptor protein TRIF-related adapter molecule (TRAM). Based on these findings, we developed a SLAMF1-derived peptide, P7, which is linked to a cell-penetrating peptide for intracellular delivery. We found that P7 peptide inhibits the expression and secretion of IFNβ and pro-inflammatory cytokines (TNF, IL-1β, IL-6) induced by TLR4, and prevents death in mice subjected to LPS shock. The mechanism of action of P7 peptide is based on interference with several intracellular protein-protein interactions, including TRAM-SLAMF1, TRAM-Rab11FIP2, and TIRAP-MyD88 interactions. Overall, P7 peptide has a unique mode of action and demonstrates high efficacy in inhibiting TLR4-mediated signalling in vitro and in vivo.

C1q and MBL, components of the innate immune system, influence monocyte cytokine expression

It has recently been recognized that the innate immune response, the powerful first response to infection, has significant influence in determining the nature of the subsequent adaptive immune response. C1q, mannose-binding lectin (MBL), and other members of the defense collagen family of proteins are pattern recognition molecules, able to enhance the phagocytosis of pathogens, cellular debris, and apoptotic cells in vitro and in vivo. Humans deficient in C1q inevitably develop a lupus-like autoimmune disorder, and studies in C1q knockout mice demonstrate a deficiency in the clearance of apoptotic cells with a propensity for autoimmune responses. The data presented here show that under conditions in which phagocytosis is enhanced, C1q and MBL modulate cytokine production at the mRNA and protein levels. Specifically, these recognition molecules of the innate immune system contribute signals to human peripheral blood mononuclear cells, leading to the suppression of lipopolysaccharide-induced proinflammatory cytokines, interleukin (IL)-1alpha and IL-1beta, and an increase in the secretion of cytokines IL-10, IL-1 receptor antagonist, monocyte chemoattractant protein-1, and IL-6. These data support the hypothesis that defense collagen-mediated suppression of a proinflammatory response may be an important step in the avoidance of autoimmunity during the clearance of apoptotic cells.

Role of Complement Component C1q in the IgG-Independent Opsonophagocytosis of Group B Streptococcus

We investigated the role of complement component C1q in the IgG-independent opsonophagocytosis of type III group B Streptococcus (GBS) by peripheral blood leukocytes. We report that C1q binds to type III GBS both in normal human serum deficient in IgG specific for type III capsular polysaccharide and in a low-ionic strength buffer. The dissociation constant Kd ranged from 2.0 to 5.5 nM, and the number of binding sites Bmax ranged from 630 to 1360 molecules of C1q per bacterium (CFU). An acapsular mutant strain of GBS bound C1q even better than the wild type, indicating that the polysaccharide capsule is not the receptor for C1q. In serum, binding of C1q to GBS was associated with activation of the classical complement pathway. However, normal human serum retained significant opsonic activity after complete depletion of C1q, suggesting that the serum contains a molecule that is able to replace C1q in opsonization and/or complement activation. Mannan-binding lectin, known to share some functions with C1q, appeared not to be involved, since its depletion from serum had little effect on opsonic activity. Excess soluble C1q or its collagen-like fragment inhibited phagocytosis mediated by normal human serum, suggesting that C1q may compete with other opsonins for binding to receptor(s) on phagocytes. We conclude that, although C1q binds directly to GBS, C1q binding is neither necessary nor sufficient for IgG-independent opsonophagocytosis. The results raise the possibility that additional unknown serum factor(s) may contribute to opsonization of GBS directly or via a novel mechanism of complement activation.

C1qRP Is a Heavily O-Glycosylated Cell Surface Protein Involved in the Regulation of Phagocytic Activity

C1q, mannose-binding lectin (MBL), and pulmonary surfactant protein A (SPA) interact with human monocytes and macrophages, resulting in the enhancement of phagocytosis of suboptimally opsonized targets. mAbs that recognize a cell surface molecule of 126,000 Mr, designated C1qRP, have been shown to inhibit C1q- and MBL-mediated enhancement of phagocytosis. Similar inhibition of the SPA-mediated enhancement of phagocytosis by these mAbs now suggests that C1qRP is a common component of a receptor for these macromolecules. Ligation of human monocytes with immobilized R3, a IgM mAb recognizing C1qRP, also triggers enhanced phagocytic capacity of these cells in the absence of ligand, verifying the direct involvement of this polypeptide in the regulation of phagocytosis. While the cDNA for C1qRP encodes a 631 amino acid membrane protein, Chinese hamster ovary cells transfected with the cDNA of the C1qRP coding region express a surface glycoprotein with the identical 126,000 Mr in SDS-PAGE as the native C1qRP. Use of glycosylation inhibitors, cleavage of the mature C1qRP with specific glycosidases, and in vitro translation of C1qRP cDNA demonstrated that both posttranslational glycosylation and the nature of the amino acid sequence of the protein contribute to the difference between its predicted m.w. and its migration on SDS-PAGE. These results verify that the cDNA cloned codes for the mature C1qRP, that C1qRP contains a relatively high degree of O-linked glycoslyation, and that C1qRP cross-linked directly by monoclonal anti-C1qRP or engaged as a result of cell surface ligation of SPA, as well as C1q and MBL, enhances phagocytosis.

Requirement for the Alternative Pathway as Well as C4 and C2 in Complement-Dependent Hemolysis Via the Lectin Pathway

Mannan-binding lectin (MBL) is a C1q-like molecule opsonic for several micro-organisms. MBL can activate C4, C2, and later acting complement components in the presence of serine proteases similar to but distinct from C1r and C1s via the lectin pathway of complement activation. We report here that mannan-coated MBL-sensitized erythrocytes are lysed via the lectin pathway in human serum-Mg-EGTA. The surprising occurrence of MBL-initiated lysis in the absence of calcium contrasts with the calcium requirement for C1q-initiated activation of C4 and C2. C2 is required, and lysis is significantly enhanced when indicator cells presensitized with C4 and then coated with mannan (EAC4-M) are used. The alternative pathway also is required, since lysis is lost when either factor D or factor B is removed and is restored upon reconstitution with the purified protein. Even though MBL is a C-type lectin, it is retained on mannan-coated erythrocytes in the absence of calcium. This contrasts with the absence of calcium-independent retention on mannan immobilized on polystyrene plates or beads, and helps explain the MBL-initiated hemolysis in Mg-EGTA. These investigations show that the alternative pathway as well as C4 and C2 of the classical pathway are required for complement-dependent hemolysis via the lectin pathway and provide a method for assay of lectin pathway-mediated complement activity in human serum that should be useful in unraveling the molecular interactions of this pathway.