FliC's Hypervariable D3 Domain Is Required for Robust Anti-Flagellin Primary Antibody Responses

Bacterial flagellin is a well-known agonist of the innate immune system that induces proinflammatory responses through the TLR5 and Naip5/6 recognition pathways. Several clinical trials investigating flagellin fusion proteins have demonstrated promising results for inducing protective immunity toward influenza virus, which has been largely attributed to flagellin's ability to activate TLR5. Our laboratory previously demonstrated that the Salmonella enterica serovar Typhimurium flagellin protein, FliC, induces Ab responses in mice through a third pathway that is independent of TLR5, Casp1/11, and MyD88. In this study, we further define the structural features of FliC that contribute to this unknown third pathway. By destroying the Naip5/6 and TLR5 recognition sites, we demonstrate that neither were required for the TLR5-, inflammasome- and MyD88-independent Ab responses toward FliC. In contrast, deletion of FliC's D3 or D0/D1 domains eliminated primary anti-flagellin Ab responses. For optimal primary and secondary anti-flagellin Ab responses we show that TLR5, inflammasome recognition, and the D3 domain of FliC are essential for flagellin's robust immunogenicity. Our data demonstrate that the D3 domain of FliC influences immunogenicity independent of the known innate recognition sites in the D0/D1 domains to augment Ab production. Our results suggest full-length FliC is critical for optimal immunogenicity and Ab responses in flagellin-based vaccines.

Genetic control of isotype specific antibody responses to flagellin in mice (IRC4P.450)

Adjuvants can improve the efficacy of vaccines by inducing class-switched immune responses. Host factors controlling the magnitude of class-switched antibody responses are poorly understood. We have observed flagellin-immunized mouse strains differ in the magnitude of isotype specific antibody responses. Some strains are high responders (A/J, Swiss Webster) and other strains are low responders (C57BL6, Balb/c). All strains had comparable primary anti-flagellin IgM responses, suggesting common genetic variations influence isotype class switching. We hypothesized different genetic differences between A/J and C57BL/6 mice determine magnitude of responder phenotype. We bred C57BL/6 and A/J mice and tested F1 and F2 offspring for isotype specific antibody responses. Mice were immunized with a FliC protein mutated in the Naip5/6 recognition domain to overcome known differences in Naip5-dependent inflammasome activation between these mouse strains. We demonstrate anti-flagellin IgG1, IgG2a/c and IgA responses are inherited as a quantitative trait. Using consomic mice, we show the high responder phenotype is not controlled by the IgH locus on A/J chromosome 12. Using mutant flagellin proteins, we demonstrate the high responder phenotype is primarily controlled by the FliC D2/D3 domain. Understanding the host factors contributing to FliC D2/D3 induced antibody responses aid in the rationale design of vaccines, and help establish principles for future adjuvant design and development.

The flagellin D2/D3 domain induces TLR5- and inflammasome- independent antibody responses (INM2P.357)

The detailed understanding of the molecular mechanisms underlying the biological activity adjuvants is critical for rational design of more effective vaccines. Bacterial flagellin, a potent immunogen, is being developed as a new adjuvant and platform for human vaccines. We have demonstrated that the conserved D0 and D1 domains of flagellin promote antibody responses through activation of their respective innate immune receptors, but are not required for Salmonella flagellin (FliC) induced antibody responses. We hypothesized that additional flagellin domains also influence specific aspects of flagellin-induced B cell responses. Mice were immunized with a mixture of FliC mutants proteins and ovalbumin, and sera were collected for quantification of anti-flagellin and anti-ovalbumin specific antibody responses. The results from our studies demonstrate that the D2/D3 domain of the FliC is also required for flagellin-induced primary antibody responses and robust secondary responses. D2/D3 domain is capable of inducing antibody responses independent of TLR5 and the inflammasome. Our data indicate the flagellin D2/D3 domain synergizes with the TLR5 (D1) and inflammasome (D0) activating domains to induce potent antibody responses, and that the D2/D3 domain also functions as an adjuvant to promote antibody responses against co-administered antigens. Future studies will elucidate the contribution of the FliC D2/D3 domain to B cell responses and its utility in vaccines.

Flagellin induces antibody responses through a TLR5- and inflammasome-independent pathway

Flagellin is a potent immunogen that activates the innate immune system via TLR5 and Naip5/6, and generates strong T and B cell responses. The adaptor protein MyD88 is critical for signaling by TLR5, as well as IL-1Rs and IL-18Rs, major downstream mediators of the Naip5/6 Nlrc4-inflammasome. In this study, we define roles of known flagellin receptors and MyD88 in Ab responses generated toward flagellin. We used mice genetically deficient in flagellin recognition pathways to characterize innate immune components that regulate isotype-specific Ab responses. Using purified flagellin from Salmonella, we dissected the contribution of innate flagellin recognition pathways to promote Ab responses toward flagellin and coadministered OVA in C57BL/6 mice. We demonstrate IgG2c responses toward flagellin were TLR5 and inflammasome dependent; IgG1 was the dominant isotype and partially TLR5 and inflammasome dependent. Our data indicate a substantial flagellin-specific IgG1 response was induced through a TLR5-, inflammasome-, and MyD88-independent pathway. IgA anti-FliC responses were TLR5 and MyD88 dependent and caspase-1 independent. Unlike C57BL/6 mice, flagellin-immunized A/J mice induced codominant IgG1 and IgG2a responses. Furthermore, MyD88-independent, flagellin-induced Ab responses were even more pronounced in A/J MyD88(-/-) mice, and IgA anti-FliC responses were suppressed by MyD88. Flagellin also worked as an adjuvant toward coadministered OVA, but it only promoted IgG1 anti-OVA responses. Our results demonstrate that a novel pathway for flagellin recognition contributes to Ab production. Characterization of this pathway will be useful for understanding immunity to flagellin and the rationale design of flagellin-based vaccines.

Reconstitution of high affinity leukaemia inhibitory factor (LIF) receptors in haemopoietic cells transfected with the cloned human LIF receptor

cDNA clones encoding the human leukaemia inhibitory factor (hLIF) receptor were isolated by screening a placental cDNA expression library in COS-7 cells with 125I-hLIF. The cloned LIF receptor is a member of the haemopoietin receptor family and comprises a signal sequence (44 amino acids), an extracellular region of two haemopoietin receptor domains and three fibronectin type III domains (789 amino acids), a transmembrane domain (26 amino acids) and a cytoplasmic domain (238 amino acids). The LIF receptor is expressed in COS-7 cells as a 190 kDa glycoprotein that specifically binds human LIF with low affinity, but does not bind mouse LIF. Clones encoding a soluble form of the homologous mouse LIF receptor have been isolated, suggesting complex interactions between the various forms of LIF ligand and receptor in vivo. The LIF receptor is most related to the gp130 signal-transducing component of the IL-6 receptor, a feature that may provide a molecular basis for the intertwined biologies of LIF and IL-6 in the absence of obvious structural similarly between the ligands. Mouse B9 plasmacytoma cells transfected with the human LIF receptor display novel high affinity LIF receptors that are presumed to consist of transfected receptors in association with endogenous mouse high affinity-converting subunits. Unlike the low affinity human LIF receptor, the mixed species high affinity receptor is capable of binding mouse LIF.

UL16-binding proteins, novel MHC class I-related proteins, bind to NKG2D and activate multiple signaling pathways in primary NK cells

The UL16-binding proteins (ULBPs) are a novel family of MHC class I-related molecules that were identified as targets of the human CMV glycoprotein, UL16. We have previously shown that ULBP expression renders a relatively resistant target cell sensitive to NK cytotoxicity, presumably by engaging NKG2D, an activating receptor expressed by NK and other immune effector cells. In this study we show that NKG2D is the ULBP counterstructure on primary NK cells and that its expression is up-regulated by IL-15 stimulation. Soluble forms of ULBPs induce marked protein tyrosine phosphorylation, and activation of the Janus kinase 2, STAT5, extracellular signal-regulated kinase, mitogen-activated protein kinase, and phosphatidylinositol 3-kinase (PI 3-kinase)/Akt signal transduction pathways. ULBP-induced activation of Akt and extracellular signal-regulated kinase and ULBP-induced IFN-gamma production are blocked by inhibitors of PI 3-kinase, consistent with the known binding of PI 3-kinase to DAP10, the membrane-bound signal-transducing subunit of the NKG2D receptor. While all three ULBPs activate the same signaling pathways, ULBP3 was found to bind weakly and to induce the weakest signal. In summary, we have shown that NKG2D is the ULBP counterstructure on primary NK cells and for the first time have identified signaling pathways that are activated by NKG2D ligands. These results increase our understanding of the mechanisms by which NKG2D activates immune effector cells and may have implications for immune surveillance against pathogens and tumors.

A poxvirus protein that binds to and inactivates IL-18, and inhibits NK cell response

IL-18 induces IFN-gamma and NK cell cytotoxicity, making it a logical target for viral antagonism of host defense. We demonstrate that the ectromelia poxvirus p13 protein, bearing homology to the mammalian IL-18 binding protein, binds IL-18, and inhibits its activity in vitro. Binding of IL-18 to the viral p13 protein was compared with binding to the cellular IL-18R. The dissociation constant of p13 for murine IL-18 is 5 nM, compared with 0.2 nM for the cellular receptor heterodimer. Mice infected with a p13 deletion mutant of ectromelia virus had elevated cytotoxicity for YAC-1 tumor cell targets compared with control animals. Additionally, the p13 deletion mutant virus exhibited decreased levels of infectivity. Our data suggest that inactivation of IL-18, and subsequent impairment of NK cell cytotoxicity, may be one mechanism by which ectromelia evades the host immune response.

Blood circulation as source for osteopontin in acellular extrinsic fiber cementum and other mineralizing tissues

Osteopontin (OPN) is one of the major non-collagenous proteins in root cementum and other mineralized tissues. Although most of this mineral-seeking protein is thought to be produced by local tissue cells, some of it might enter the mineralizing matrix from the blood. To test this hypothesis, we followed the distribution of a single dose of purified porcine or rat 125I-labeled OPN injected i.v. in rats, in mineralizing and non-mineralizing tissues and in subcutaneously implanted collagenous implants. The animals were killed 30 or 48 hrs after injection. Tissues (calvaria, tibia, lower and upper jaws) were harvested and processed for radioautography and biochemical analysis. Tissues as well as calcifying collagenous implants proved to have taken up radiolabel. In EDTA extracts of long bones, the majority of the radiolabel was demonstrated to be associated with intact OPN. The iodinated protein was also found in the acellular extrinsic fiber cementum (acellular cementum) layer investing the continuously growing incisors, in laminae limitantes, cement lines, and in forming bone near the mineralization front. Further, the label was present in the circumpulpal dentin of the incisors, and some of it appeared to have been incorporated into developing enamel. It is concluded that OPN in acellular cementum and other mineralizing tissues may-at least partially-originate from sources outside the direct environment following its transportation via serum.

Root development in mice lacking functional tissue non-specific alkaline phosphatase gene: inhibition of acellular cementum formation

Tissue non-specific alkaline phosphatase (TNAP) is richly present in developing teeth including the cells of the periodontal ligament. Here, we investigated tooth and root development in mice lacking the TNAP gene. Heterozygous mutants were obtained from The Jackson Laboratory, Animal Resources (Bar Harbor, ME, USA) and bred. TNAP-deficient mice and their littermates were killed from 6 to 25 days after birth and their molar blocks processed for light and electron microscopy. It was observed that the eruption of the incisors into the oral cavity was delayed for 2 to 3 days. Also, the onset of mineralization of the mantle dentin in the roots of the developing molars was delayed for 2 to 3 days. Yet, dentin and enamel formation in the homozygous mutants showed a more or less normal pattern, with the exception of localized enamel hypoplasias. The most conspicuous finding was the defective formation of acellular cementum along the molar roots. Instead of a continuous layer, the cementum was deposited as very thin and irregularly shaped patches around the bases of the periodontal ligament fibers. Sharpey's fibers were short and poorly developed. In contrast, the development of the alveolar bone, the periodontal ligament, and the cellular cementum was seemingly unaffected. It is concluded that TNAP represents an essential factor in mantle dentin mineralization and in the formation of acellular cementum.

A poxvirus-encoded semaphorin induces cytokine production from monocytes and binds to a novel cellular semaphorin receptor, VESPR

The vaccinia virus A39R protein is a member of the semaphorin family. A39R.Fc protein was used to affinity purify an A39R receptor from a human B cell line. Tandem mass spectrometry of receptor peptides yielded partial amino acid sequences that allowed the identification of corresponding cDNA clones. Sequence analysis of this receptor indicated that it is a novel member of the plexin family and identified a semaphorin-like domain within this family, thus suggesting an evolutionary relationship between receptor and ligand. A39R up-regulated ICAM-1 on, and induced cytokine production from, human monocytes. These data, then, describe a receptor for an immunologically active semaphorin and suggest that it may serve as a prototype for other plexin-semaphorin binding pairs.

Molecular characterization of the human interleukin (IL)-17 receptor

Human interleukin 17 (hIL-17) is a T-cell derived cytokine that exhibits 63% amino acid sequence identity to mouse IL-17 (mIL-17) and 57% identity to a viral protein encoded by the herpesvirus saimiri (HSV) gene 13 (HVS13). The IL-17 family of proteins binds to a unique mouse receptor (mIL-17R). Using nucleic acid hybridization techniques, a cDNA encoding a human homologue of the mIL-17R (hIL-17R) was isolated from a human T cell library. The predicted amino acid sequence of the hIL-17R is 69% identical to the mIL-17R, shares no homology with previously identified cytokine receptor families, and exhibits a broad tissue distribution. The hIL-17R gene was localized to chromosome 22. Monoclonal antibodies (mAbs) generated against the hIL-17R were able to block the IL-17-induced production of cytokine from human foreskin fibroblast (HFF) cells. Binding studies suggest that recombinant hIL-17 binds to the hIL-17R with low affinity.

LERK-7: a ligand of the Eph-related kinases is developmentally regulated in the brain

The eph family is the largest subfamily of receptor tyrosine kinases (RTKs). Members of this subfamily display specific expression in the developing and adult brain. Recently, cDNAs encoding membrane bound ligands for these receptors have been identified which we have termed LERKs (ligand for eph-related kinases). We report here the isolation of LERK-7 from a human fetal brain cDNA library. LERK-7 encodes a protein of 228 amino acids and is anchored to the membrane by glycosyl-phosphatidylinositol (GPI) linkage. When transfected into CV1/EBNA cells, LERK-7 binds soluble forms of both hek and elk. In addition, a soluble form of LERK-7 will induce phosphorylation of eck expressed in a human duodenum adenocarcinoma cell line. LERK-7 expressed multiple transcripts (7.5-kb, 6.0-kb, and 3.5-kb) with the highest levels in human adult brain, heart, spleen, and ovary and human fetal brain, lung, and kidney. Similar to the other ligands in this family, LERK-7 is developmentally regulated in the brain. LERK-7 is identical to the recently described AL-1.

Cell-cell adhesion mediated by binding of membrane-anchored ligand LERK-2 to the EPH-related receptor human embryonal kinase 2 promotes tyrosine kinase activity

Human embryonal kinase 2 (HEK2) is a protein-tyrosine kinase that is a member of the EPH family of receptors. Transcripts for HEK2 have a wide tissue distribution. Recently, a still growing family of ligands, which we have named LERKs, for ligands of the eph-related kinases, has been isolated. In order to analyze functional effects between the LERKs and the HEK2 receptor, we expressed HEK2 cDNA in an interleukin-3-dependent progenitor cell line 32D that grows as single cells in culture. Within the group of LERKs, LERK-2 and -5 were shown to bind to HEK2. Membrane-bound and soluble forms of LERK-2 were demonstrated to signal through HEK2 as judged by receptor phosphorylation. Coincubation of HEK2 and LERK-2 expressing cells induced cell-cell adhesion and formation of cell aggregates. This interaction could be inhibited by preincubation of HEK2 expressing cells with soluble LERK-2. Coexpression of HEK2 and LERK-2 in 32D cells showed reduced kinase activity and autophosphorylation of HEK2 compared with the juxtacrine stimulation, which seems to be due to a reduced sensitivity of the receptor.

Ligands for EPH-related tyrosine kinase receptors are developmentally regulated in the CNS

Elk is a member of the eph family of receptor-like tyrosine kinases. Although its function is unknown, elk is postulated to play a role in nervous system development. Using Northern analysis, we examined the developmental regulation of RNAs encoding elk, and several ligands for the eph family of RTKs, the LERKs. Expression of elk, LERK-1, and LERK-2 RNAs is high in all regions examined in the embryonic and postnatal rat brain and decreases to low levels with age. One exception is the adult olfactory bulb which continues to express a moderate level of LERK-2. In contrast, moderate LERK-4 expression was limited to the developing hippocampus and cerebral cortex. These data indicate that elk and some of the LERKs may play a role in nervous system development, maintenance, and/or regeneration.

Ligands for the receptor tyrosine kinases hek and elk: isolation of cDNAs encoding a family of proteins

Hek is a member of the eph subfamily of receptor tyrosine kinases whose members include elk, hek2, sek, eph and eck among others. Using a soluble form of hek consisting of the extracellular region of the receptor fused to the Fc domain of human IgG1 and an expression cloning strategy, we have isolated two different but related cDNAs from the human T-lymphoma line HSB-2 that encode ligands for hek. The cDNAs encode proteins of 238 and 201 amino acids (44% amino acid identity) that are anchored to the membrane by glycosylphosphatidylinositol (GPI)-linkage. The proteins encoded by these cDNAs are bound by hek with affinity constants of 2 x 10(8) M-1. These proteins also bind the elk tyrosine kinase receptor. These cDNAs are related to other cDNAs that we have recently isolated from a human placental library that encode ligands for both hek and elk and define an emerging family of ligands for eph-related kinases (LERKs).

Molecular and biological characterization of a ligand for CD27 defines a new family of cytokines with homology to tumor necrosis factor

CD27 is a surface antigen found on T and B cells that has homology to a family of molecules including the receptors for tumor necrosis factor (TNF) and nerve growth factor. A cDNA encoding a ligand for CD27 was isolated by a direct-expression cloning strategy using a fusion protein composed of the extracellular domain of CD27 linked to the constant domain of a human immunoglobulin G1 molecule as a probe. The predicted protein product is a type II transmembrane protein whose gene maps to 19p13 and that shows homology to TNF and the ligand for CD40. Biological characterization indicates that the cloned ligand induces the proliferation of costimulated T cells and enhances the generation of cytolytic T cells.

Recombinant soluble IgA Fc receptor: generation, biochemical characterization, and functional analysis of the recombinant protein

We previously described the cloning of a human myeloid cell surface receptor for the Fc region of immunoglobulin A (Fc alpha R). In the present study, a soluble version of the Fc alpha R (solFc alpha R) was generated by removing the transmembrane and cytoplasmic coding regions from full-length Fc alpha R cDNA and ligating into a mammalian expression vector. COS-7 cells transfected with the solFc alpha R plasmid secreted a protein that inhibited both immunoglobulin A (IgA) and anti-Fc alpha R monoclonal antibody (mAb) binding to Fc alpha R+ U937 cells. Furthermore, the solFc alpha R bound specifically to and could be eluted from an anti-Fc alpha R mAb-immunoaffinity column, retaining biological activity. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) demonstrated that the recombinant full-length Fc alpha R migrates over a molecular mass range of approximately 40-60 kd, consistent with the reported size and heterogeneity of the naturally occurring myeloid cell surface Fc alpha R. The solFc alpha R ran on SDS-PAGE as a smaller band (37-55 kd) that reduced to two bands of 23 and 25 kd following N-glycanase treatment, indicating that the Fc alpha R is a heavily glycosylated protein. The biochemical data, coupled with flow cytometry studies showing that the recombinant Fc alpha Rs bind to five different anti-Fc alpha R mAbs, clearly demonstrate that the cloned Fc alpha R corresponds directly to the major Fc alpha R species expressed on human monocytes, neutrophils, and myeloid cell lines. The generation of soluble receptor protein will permit investigations of the role of Fc alpha R in IgA-mediated immunoregulation, effector functions, and disease.

Molecular characterization of the interleukin-8 receptor

Recently a rabbit cDNA (F3R) was characterized as binding and causing calcium mobilization induced by the formyl-methionine-leucine-phenylalanine peptide (fMLP). In the study reported here, cloned DNAs were isolated from rabbit genomic DNA by PCR based on the sequence of F3R. The cloned DNAs have several differences in the DNA sequence compared to the reported F3R sequence that alter the predicted protein sequence. COS-7 cells transfected with these clones in a mammalian expression vector bind human IL-8 with high affinity, but do not bind fMLP. We therefore believe that the cDNAs isolated encode the rabbit IL-8 receptor.

Characterization and pharmacokinetic parameters of recombinant soluble interleukin-4 receptor

The interleukin-4 receptor (IL-4R) is expressed as a 140-Kd membrane glycoprotein that binds IL-4 with high affinity. Recently, cDNA clones for the murine IL-4R have been isolated. One clone encodes an integral membrane protein, while another encodes a protein in which translation is terminated before the transmembrane region, thus producing a soluble form of the IL-4R (sIL-4R). HeLa cell clones overexpressing sIL-4R were isolated using a novel filter-overlay and 125I-IL-4 ligand binding technique. Quantitative analysis demonstrated that the kinetics and affinity of IL-4 binding to the recombinant sIL-4R were similar to the native membrane-bound IL-4R. As low doses of sIL-4R specifically inhibited IL-4-induced proliferative responses in vitro, sIL-4R biodistribution and elimination parameters were evaluated to assess the pharmacokinetic potential of sIL-4R as a therapeutic agent. Pharmacokinetic studies demonstrated that radiolabeled sIL-4R had a distribution half-life of 9 minutes and an elimination half-life of 2.3 hours following intravenous (IV) administration. When administered by intraperitoneal or subcutaneous (SC) injection, the elimination half-lives were prolonged to 4.2 hours and 6.2 hours, respectively. Although the initial blood level of sIL-4R was reduced if administered by SC injection, the bioavailability was comparable with IV administration. The main sites of sIL-4R elimination were the liver and kidney.

A soluble form of the interleukin 4 receptor in biological fluids

Murine biological fluids and murine cell culture supernatants were analyzed for the presence of soluble murine interleukin 4 receptor (sIL4R) with the use of two monoclonal antibodies directed against the receptor. Mouse urine, serum, ascitic fluid, and cell culture supernatants contained varying levels of immunoreactive protein. All of the immunoreactive protein possessed interleukin 4 (IL 4) binding activity. Following partial purification of ascitic fluid a protein was isolated that binds IL 4 with high affinity. This data is consistent with the fact that murine biological fluids contain a soluble version of the murine IL 4 receptor that arises via secretion of the soluble receptor and/or via shedding of the extracellular portion of the full-length receptor from the cell surface.

The murine interleukin-4 receptor: molecular cloning and characterization of secreted and membrane bound forms

Receptors for interleukin-4 (IL-4) are expressed at low levels on a wide variety of primary cells and cultured cell lines. Fluorescence-activated sorting of CTLL-2 cells resulted in the isolation of a subclone, CTLL 19.4, which expressed 10(6) IL-4 receptors per cell. These cells were used for the purification of IL-4 receptor protein and to prepare a hybrid-subtracted cDNA probe for isolation of cDNA clones. Three classes of IL-4 receptor cDNA were identified. The first encoded a 140 kd membrane bound IL-4 receptor containing extracellular, transmembrane, and cytoplasmic domains. The second class lacked the cytoplasmic region, and the third encoded a secreted form of the receptor. All cDNA clones expressed in COS-7 cells had IL-4 binding properties comparable to the native IL-4 receptor. The soluble form of the IL-4 receptor blocked the ability of IL-4 to induce CTLL cell proliferation and may represent a regulatory molecule specific for IL-4-dependent immune responses.