Truncation of the constant domain drives amyloid formation by immunoglobulin light chains

AL amyloidosis is a life-threatening disease caused by deposition of immunoglobulin light chains. While the mechanisms underlying light chains amyloidogenesis in vivo remain unclear, several studies have highlighted the role that tissue environment and structural amyloidogenicity of individual light chains have in the disease pathogenesis. AL natural deposits contain both full-length light chains and fragments encompassing the variable domain (VL) as well as different length segments of the constant region (CL), thus highlighting the relevance that proteolysis may have in the fibrillogenesis pathway. Here, we investigate the role of major truncated species of the disease-associated AL55 light chain that were previously identified in natural deposits. Specifically, we study structure, molecular dynamics, thermal stability, and capacity to form fibrils of a fragment containing both the VL and part of the CL (133-AL55), in comparison with the full-length protein and its variable domain alone, under shear stress and physiological conditions. Whereas the full-length light chain forms exclusively amorphous aggregates, both fragments generate fibrils, although, with different kinetics, aggregate structure, and interplay with the unfragmented protein. More specifically, the VL-CL 133-AL55 fragment entirely converts into amyloid fibrils microscopically and spectroscopically similar to their ex vivo counterpart and increases the amorphous aggregation of full-length AL55. Overall, our data support the idea that light chain structure and proteolysis are both relevant for amyloidogenesis in vivo and provide a novel biocompatible model of light chain fibrillogenesis suitable for future mechanistic studies.

Engineered antibody CH2 domains binding to nucleolin: Isolation, characterization and improvement of aggregation

Smaller recombinant antibody fragments are now emerging as alternatives of conventional antibodies. Especially, immunoglobulin (Ig) constant CH2 domain and engineered CH2 with improved stability are promising as scaffolds for selection of specific binders to various antigens. We constructed a yeast display library based on an engineered human IgG1 CH2 scaffold with diversified loop regions. A group of CH2 binders were isolated from this yeast display library by panning against nucleolin, which is a tumor-associated antigen involved in cell proliferation, tumor cell growth and angiogenesis. Out of 20 mutants, we selected 3 clones exhibiting relatively high affinities to nucleolin on yeasts. However, recombinant CH2 mutants aggregated when they were expressed. To find the mechanism of the aggregation, we employed computational prediction approaches through structural homology models of CH2 binders. The analysis of potential aggregation prone regions (APRs) and solvent accessible surface areas (ASAs) indicated two hydrophobic residues, Val264 and Leu309, in the β-sheet, in which replacement of both charged residues led to significant decrease of the protein aggregation. The newly identified CH2 binders could be improved to use as candidate therapeutics or research reagents in the future.

The constant region contributes to the antigenic specificity and renal pathogenicity of murine anti-DNA antibodies

Affinity for DNA and cross-reactivity with renal antigens are associated with enhanced renal pathogenicity of lupus autoantibodies. In addition, certain IgG subclasses are enriched in nephritic kidneys, suggesting that isotype may determine the outcome of antibody binding to renal antigens. To investigate if the isotype of DNA antibodies affects renal pathogenicity by influencing antigen binding, we derived IgM, IgG1, IgG2b and IgG2a forms of the PL9-11 antibody (IgG3 anti-DNA) by in vitro class switching or PCR cloning. The affinity and specificity of PL9-11 antibodies for nuclear and renal antigens were analyzed using ELISA, Western blotting, surface plasmon resonance (SPR), binding to mesangial cells, and glomerular proteome arrays. Renal deposition and pathogenicity were assayed in mice injected with PL9-11 hybridomas. We found that PL9-11 and its isotype-switched variants had differential binding to DNA and chromatin (IgG3>IgG2a>IgG1>IgG2b>IgM) by direct and competition ELISA, and SPR. In contrast, in binding to laminin and collagen IV the IgG2a isotype actually had the highest affinity. Differences in affinity of PL9-11 antibodies for renal antigens were mirrored in analysis of specificity for glomeruli, and were associated with significant differences in renal pathogenicity in vivo and survival. Our novel findings indicate that the constant region plays an important role in the nephritogenicity of antibodies to DNA by affecting immunoglobulin affinity and specificity. Increased binding to multiple glomerular and/or nuclear antigens may contribute to the renal pathogenicity of anti-DNA antibodies of the IgG2a and IgG3 isotype. Finally, class switch recombination may be another mechanism by which B cell autoreactivity is generated.

Characterization of the IgD binding site of encapsulated Haemophilus influenzae serotype b

Encapsulated Haemophilus influenzae is a causative agent of invasive disease, such as meningitis and septicemia. Several interactions exist between H. influenzae and the human host. H. influenzae has been reported to bind IgD in a nonimmune manner, but the responsible protein has not yet been identified. To define the binding site on IgD for H. influenzae, full-length IgD and four chimeric IgDs with interspersed IgG sequences and Ag specificity for dansyl chloride were expressed in stably transfected Chinese hamster ovary cells. The binding of recombinant IgD to a panel of encapsulated H. influenzae serotype b (Hib) and nontypeable strains were investigated using a whole cell ELISA and flow cytometry. IgD binding was detected in 50% of the encapsulated Hib strains examined, whereas nontypeable H. influenzae did not interact with IgD. Finally, mapping experiments using the chimeric IgD/IgG indicated that IgD CH1 aa 198-224 were involved in the interaction between IgD and H. influenzae. Thus, by using recombinant IgD and chimeras with defined Ag specificity, we have confirmed that Hib specifically binds IgD, and that this binding involves the IgD CH1 region.

Human IgG1 Cgamma1 domain is crucial for the bioactivity of the engineered anti-CD20 antibodies

In this study, we discussed the necessity of human IgG1 Cgamma1 domain for recombinant antibody using computer-aided homology modeling method and experimental studies. The heavy (VH) and light (VL) chain variable regions of 1-28, a murine IgM-type anti-CD20 mAb, were ligated by linker peptide (Gly4Ser)3 to form the single-chain Fv fragment (scFv). Then, the engineered antibody (LH1-3) was generated by fusing scFv with the entire IgG1 heavy constant regions. The 3-D structure of LH1-3 was modeled using computer-aided homology modeling method and the binding activity of LH1-3 was evaluated theoretically. Compared to the 3-D structure of the Fv fragment of the parent antibody, the conformation of the active pocket of LH1-3 was remained because of the rigid support of Cgamma1. Further experimental results of flow cytometry showed that the engineered anti-CD20 antibody possessed specifically binding activity to CD20-expressing target cells. The anti-CD20 antibody fragments could also mediate complement-dependent cytotoxicity (CDC) of human B-lymphoid cell lines. Our study highlights some interests and advantages of a methodology based on the homology modeling and analysis of molecular structural properties.

One Step Generation of Fully Chimeric Antibodies Using Cγ1- and Cκ Mutant Mice

Humanized antibodies (Abs) are effective drugs against a variety of diseases such as cancer, autoimmune diseases, transplant rejection and others. The most powerful technology to develop humanized Abs is the use of mice that produce humanized Abs. By modifying the genetic background of F004 mice a new mouse substrain was developed for optimized "one step" generation of chimeric humanized monoclonal Abs. The new mice (F004-Jen) demonstrated improved fertility still expressing the human locus at the same level as the parental F004 mouse. The value of these mice for the generation of chimeric Abs was exemplified for a panel of chimeric Abs against the human neural cell adhesion molecule (NCAM): The fully chimeric human IgG1/kappa Ab Ch.MK1 bound to NCAM expressing cells with a K(D)=4.3-8.7 x 10(-8) M and was functionally active as demonstrated by depleting NCAM expressing cells. We also demonstrated that chimeric IgG1/kappa Abs can be induced by hybridoma class switching of IgM producing hybridoma cells, providing an alternative way to chimeric Abs. The present data highlight F004-Jen mice as an efficient tool for "one step" generation of chimeric Abs.

[A research on Ig heavy chain constant region of five Acipenseridae]

Analyzed the immunoglobulin (Ig) heavy chain constant regions of Russian sturgeon (A. gueldenstaedtii); Sterlte sturgeon (A. ruthenus); Amur sturgeon (A. schrenckii); Chinese sturgeon (A. sinensis) and Great sturgeon (Huso huso) with molecular biology and bioinformatics methods. We cloned IgH nucleic acid sequences by RT-PCR using the specific primer, then determined the characteristics and functions of the amino acid sequences and plotted out the sequence into four sub-regions (CH1-CH4), of which CH4 sub-regions one another had the highest identity. According to the analysis of the variation expectation value (Kaa), species differentiation time (T) in the CH4 sub-region phylogenetic tree, we found that Chinese sturgeon and the theoretical common ancestor of the other five sturgeon form the first bifurcation of the tree, while, among the five left, Amur sturgeon and Huso sturgeon, Russian sturgeon and Siberian sturgeon, Siberian sturgeon (sequence from GenBank), Sterlte itself form another three bifurcations respectively. This result can clearly explain the relations of taxonomic status, geographical distribution and evolution among the species studied.

Differential expression of immunoglobulin kappa chain constant region in human abdominal aortic aneurysm

BACKGROUND

A number of the research into the pathogenesis of the abdominal aortic aneurysm (AAA) have focused on the alteration of gene expression. The current technique for elucidating alterations of gene expression has a setback in that many artifact complementary DNA (cDNA) products present abnormal polymerase chain reaction (PCR) amplification. Our study was designed to identify differentially expressed genes in AAA using the annealing control primer (ACP) system, which was recently developed to identify only authentic genes.

MATERIALS AND METHODS

The tissues of the human abdominal aorta were obtained from the patients of AAA and aortic occlusive disease (AOD), and normal abdominal aorta (NA) from brain death donors. Total RNAs were isolated from three groups of human abdominal aorta (10 AAA, five NA, three AOD) and then reverse transcribed into complementary DNA (cDNA). The ACP method was done to screen the difference in the expression pattern of the mRNA (mRNA).

RESULTS

One differentially expressed cDNA band was detected in AAA but not in NA and AOD. This cDNA was sequenced and computer searching against the GenBank revealed that the cDNA had more than 90% identity with the immunoglobulin kappa chain constant region (Ig kappa-C).

DISCUSSION

Our finding suggests that differentially expressed Ig kappa-C gene only in AAA is a candidate gene that may play a pivotal role in the pathogenesis of AAA formation. The correlation of mRNA level and protein level is, however, not clear. Thus, to directly identify the role of Ig light chains in the pathogenic event of AAA, the further study comparing the level and kinds of expressed protein with the corresponding Ig kappa-C gene will be required.

Neonatal Fc receptors discriminates and monitors the pathway of native and modified immunoglobulin G in placental endothelial cells

In the placenta, immunoglobulin G (IgG) is selectively transported from mother to fetus by a highly regulated transcellular mechanism aimed to achieve fetal humoral immunity. We questioned the role of neonatal Fc receptors (FcRn) in the traffic of IgG in human placental endothelial cells (HPEC). Cells were cultured in a double-chamber system and further exposed to IgG or Fc or to diethylpyrocarbonate-modified IgG or Fc in which the receptor recognition domain of the molecule (CH2-CH3) was altered. We provide evidence that the native IgG/Fc probes are transcytosed or recycled by HPEC, whereas the probes with the altered receptor recognition domain (which do not bind to FcRn) massively accumulate into the endocytic/lysosomal compartments. The results indicate that FcRn distinguishes between the intact and modified IgG and control their cellular traffic: native IgG is salvaged and released out of the cells, whereas modified IgG is retained and sorted to a degradative pathway. The data advance the understanding of the basic mechanism for IgG traffic in human endothelial cells, which may be exploited for the specific transport of antibodies in various immune disorders.

Germline transcription and switch recombination of a transgene containing the entire H chain constant region locus: effect of a mutation in a STAT6 binding site in the gamma 1 promoter

The switch (S) in H chain class is preceded by germline transcription and then mediated by a DNA recombination event. One of the impediments toward understanding the mechanism is the lack of a system in which a recombinant DNA molecule undergoes cytokine-regulated class S recombination. To study class S recombination, we used transgenic mice with a 230-kb bacterial artificial chromosome that included a rearranged VDJ gene and the entire murine H chain constant region locus. We found that both germline transcription and S recombination to the transgenic gamma1 H chain gene were regulated by IL-4 like that of the endogenous genes. In mice with two or more copies of the H chain locus transgene, both germline transcripts and S recombination took place at levels comparable to those from the endogenous loci. We also prepared a version of the transgene with a 4-bp mutation in a STAT6 binding site in the gamma1 promoter region. On the average, this mutation reduced germline transcription by 80%, but did not change the amount of S recombination in vitro. Among both the wild-type and mutant transgenes, we found no significant correlation between the amount of germline transcripts and the amount of S recombination. We infer that the physiologic level of germline transcription of the gamma1 gene is in excess over the amount required for efficient S recombination.

CpG DNA induces IgG class switch DNA recombination by activating human B cells through an innate pathway that requires TLR9 and cooperates with IL-10

TLRs are pattern recognition receptors that initiate innate immune responses. TLR9 detects microbial DNA with hypomethylated CpG motifs and in humans is preferentially expressed by IFN-alpha-producing plasmacytoid dendritic cells and B cells. In addition to favoring IFN-alpha release, TLR9 signals B cell activation, proliferation, and IgM production. Recent findings suggest that CpG DNA-TLR9 interaction plays a key role in systemic lupus erythematosus and rheumatoid arthritis, two autoimmune disorders characterized by dysregulated production of DNA-reactive IgG. We show that CpG DNA initiates germline C(gamma)1, C(gamma)2, and C(gamma)3 gene transcription by activating B cells through a TLR9-mediated NF-kappaB-Rel-dependent innate pathway that cooperates with IL-10 through STAT proteins and IFN-responsive factors. This pathway is inhibited by chloroquine, a drug that attenuates the clinical manifestations of IgG-mediated autoimmune disorders. Germline C(gamma) gene transcription is associated with up-regulation of activation-induced cytidine deaminase, a key element of the B cell class switch-inducing machinery, and is followed by class switch DNA recombination from C(micro) to C(gamma)1, C(gamma)2, and C(gamma)3. Subsequent IgG production requires additional signals from BCR and a B cell-activating factor of the TNF family (BAFF), produced by dendritic cells upon exposure to IFN-alpha. Our findings suggest that CpG DNA-TLR9 interaction may be important to initiate or amplify early T cell-independent IgG responses against pathogens. This implies that CpG DNA released during infections may exacerbate autoimmunity by stimulating autoreactive B cells to switch from an IgM to a more pathogenic IgG isotype.

Nonpeptidic, noncovalent inhibitors of the cysteine protease cathepsin S

The first nonpeptidic, noncovalent inhibitors of the cysteine protease cathepsin S (CatS) are described. Electronic database searching using the program DOCK generated a screening set of potential CatS inhibitors from which two lead structures were identified as promising starting points for a drug discovery effort. Lead optimization afforded potent (IC(50) < 50 nM) and selective inhibitors of CatS demonstrating cellular activity and reversibility of enzyme inhibition.

Interaction of murine precursor B cell receptor with stroma cells is controlled by the unique tail of lambda 5 and stroma cell-associated heparan sulfate

Efficient clonal expansion of early precursor B (pre-B) cells requires signals delivered by an Ig-like integral membrane complex, the so-called pre-B cell receptor (pre-BCR). A pre-BCR consists of two membrane micro H chains, two covalently associated surrogate L chains, and the heterodimeric signaling transducer Igalphabeta. In contrast to a conventional Ig L chain, the surrogate L chain is a heterodimer composed of the invariant polypeptides VpreB and lambda5. Although it is still unclear how pre-BCR signals are initiated, two recent findings support a ligand-dependent initiation of pre-BCR signals: 1) a pre-BCR/galectin-1 interaction is required to induce phosphorylation of Igalphabeta in a human precursor B line, and 2) soluble murine as well as human pre-BCR molecules bind to stroma and other adherent cells. In this study, we show that efficient binding of a soluble murine pre-BCR to stroma cells requires the non-Ig-like unique tail of lambda5. Surprisingly however, a murine pre-BCR, in contrast to its human counterpart, does not interact with galectin-1, as revealed by lactose blocking, RNA interference, and immunoprecipitation assays. Finally, the binding of a murine pre-BCR to stroma cells can be blocked either with heparin or by pretreatment of stroma cells with heparitinase or a sulfation inhibitor. Hence, efficient binding of a murine pre-BCR to stroma cells requires the unique tail of lambda5 and stroma cell-associated heparan sulfate. These findings not only identified heparan sulfate as potential pre-BCR ligands, but will also facilitate the development of appropriate animal models to determine whether a pre-BCR/heparan sulfate interaction is involved in early B cell maturation.

The porcine Ig delta gene: unique chimeric splicing of the first constant region domain in its heavy chain transcripts

The pig delta gene is located approximately 3.4 kb downstream of the second transmembrane exon of the micro gene and shows a similar genomic structure to its counterpart in cow with three exons encoding the CH1, CH2, and CH3 domains. The porcine genomic deltaCH1 exon has been replaced by a recent duplication of the micro CH1 and its flanking sequences, a genetic event that also led to the formation of a short switch delta region, immediately upstream of the delta gene. The deltaCH1 exhibits a 98.7% similarity (314 of 318 bp) to the micro CH1 at the DNA level, whereas the homologies between the deltaCH2 and micro CH3, and the deltaCH3 and micro CH4 are only 33.3 and 35.8%, respectively. Either of the two CH1 exons ( micro and delta) could be observed in the expressed porcine IgD H chain cDNA sequences VDJ- micro CH1-H-deltaCH2-deltaCH3 or VDJ-deltaCH1-H-deltaCH2-deltaCH3, showing a pattern that has not been observed previously in vertebrates. In addition, transfection of a human B cell line, using artificial constructs resembling the porcine C micro -Cdelta locus, also generated both VDJ- micro CH1-deltaCH1-H1-deltaCH2 and VDJ -deltaCH1-H1-deltaCH2 transcripts. An examination of the pig delta genomic sequence shows a putative, second hinge region-encoding exon. Due to the lack of a normal branchpoint sequence for RNA splicing, this exon is not present in the normal pig delta cDNA. However, the exon could be spliced into most of the expressed transcripts in vitro in cell transfection experiments after introduction of a single T nucleotide to restore the branchpoint sequence upstream of the putative H2 exon.

Cutting edge: link between innate and adaptive immunity: Toll-like receptor 2 internalizes antigen for presentation to CD4+ T cells and could be an efficient vaccine target

An ideal vaccine for induction of CD4(+) T cell responses should induce local inflammation, maturation of APC, and peptide loading of MHC class II molecules. Ligation of Toll-like receptor (TLR) 2 provides the first two of these three criteria. We have studied whether targeting of TLR2 results in loading of MHC class II molecules and enhancement of CD4(+) T cell responses. To dissociate MHC class II presentation from APC maturation, we have used an antagonistic, mouse anti-human TLR2 mAb (TL2.1) as ligand and measured proliferation of a mouse Ckappa-specific human CD4(+) T cell clone. TL2.1 mAb was 100-1000 times more efficiently presented by APC compared with isotype-matched control mAb. Moreover, TL2.1 mAb was internalized into endosomes and processed by the conventional MHC class II pathway. This novel function of TLR2 represents a link between innate and adaptive immunity and indicates that TLR2 could be a promising target for vaccines.

Fusion of the CH1 domain of IgG1 to epidermal growth factor (EGF) prolongs its retention in the blood but does not increase tumor uptake

An expression vector (pJW4) for a human epidermal growth factor (hEGF)-CH1 fusion protein was constructed by fusing the gene for hEGF with the gene for CH1 of murine IgG1 with/without a peptide linker sequence [(GGGGS)3] and inserting the recombinant gene into vector pGEX2T. Expression vector pGEX2T was transfected into E. coli (BL-21) and hEGF-CH1 expressed by induction of the lac Iq promotor with 50 microM isopropyl beta-D-thiogalactopyranoside (IPTG). hEGF- CH1 fused to glutathione S-transferase (GST) was isolated and purified by affinity chromatography. GST was cleaved using thrombin. SDS-PAGE demonstrated a protein with the expected M(r) (18 kDa) positive for hEGF by Western blot. hEGF-linker-CH1 exhibited preserved binding to A431 (2-3 x 10(6) EGFR/cell) and MDA-MB-468 breast cancer cells (1-2 x 10(6) EGFR/cell). hEGF-CH1 without the linker exhibited poor receptor binding. hEGF-linker-CH1 also exhibited strong binding to soluble EGFR equivalent to that of hEGF. The tumor and normal tissue distribution of hEGF-linker-CH1 labeled with 123I was compared with 123 I-hEGF at 24 h after i.v. injection to mice implanted with s.c. MDA-MB-468 xenografts. Fusion of hEGF with CH1 increased its retention in the blood 14-fold but did not significantly increase tumor uptake. Tumor/blood ratios were higher for hEGF than for hEGF-linker-CH1. We conclude that hEGF is more attractive than hEGF-linker-CH1 for imaging EGFR-positive tumors.

High frequency of matrix attachment regions and cut-like protein x/CCAAT-displacement protein and B cell regulator of IgH transcription binding sites flanking Ig V region genes

A major component in controlling V(D)J recombination is differential accessibility through localized changes in chromatin structure. Attachment of DNA to the nuclear matrix via matrix attachment region (MAR) sequences, and interaction with MAR-binding proteins have been shown to alter chromatin conformation, promote histone acetylation, and influence gene transcription. In this study, the flanking regions of several human and mouse Ig V(H) and Ig Vkappa genes were analyzed extensively for the presence of MARs by in vitro matrix-binding assay, and for interaction with the MAR-binding proteins cut-like protein x/CCAAT-displacement protein (Cux/CDP), B cell regulator of IgH transcription (Bright), and special AT-rich sequence-binding protein (SATB1) by EMSA. Cux/CDP and SATB1 are associated with repression, while Bright is an activator of Ig transcription. Binding sites were identified in the vicinity of all analyzed Ig V genes, and were also found flanking TCR Vbeta genes. We also show that the binding sites of the different factors do not always occur at MAR sequences. MAR sequences were also found within the Ig V loci at a much higher frequency than throughout the rest of the genome. Overall, the frequency and location of binding sites relative to the coding regions, and the strength of DNA-protein interaction showed much heterogeneity. Thus, variations in factor binding and MAR activity could potentially influence the extent of localized accessibility to V(D)J recombination and thus could play a role in unequal rearrangement of individual V genes. These sites could also contribute to effective transcription of Ig genes in mature and/or activated B cells, bringing both the promoter as well as the enhancer regions into close proximity at the nuclear matrix.

Monte Carlo simulations of the peptide recognition at the consensus binding site of the constant fragment of human immunoglobulin G: the energy landscape analysis of a hot spot at the intermolecular interface

Monte Carlo simulations of molecular recognition at the consensus binding site of the constant fragment (Fc) of human immunoglobulin G (Ig) protein have been performed to analyze structural and thermodynamic aspects of binding for the 13-residue cyclic peptide DCAWHLGELVWCT. The energy landscape analysis of a hot spot at the intermolecular interface using alanine scanning and equilibrium-simulated tempering dynamics with the simplified, knowledge-based energy function has enabled the role of the protein hot spot residues in providing the thermodynamic stability of the native structure to be determined. We have found that hydrophobic interactions between the peptide and the Met-252, Ile-253, His-433, and His-435 protein residues are critical to guarantee the thermodynamic stability of the crystallographic binding mode of the complex. Binding free energy calculations, using a molecular mechanics force field and a solvation energy model, combined with alanine scanning have been conducted to determine the energetic contribution of the protein hot spot residues in binding affinity. The conserved Asn-434, Ser-254, and Tyr-436 protein residues contribute significantly to the binding affinity of the peptide-protein complex, serving as an energetic hot spot at the intermolecular interface. The results suggest that evolutionary conserved hot spot protein residues at the intermolecular interface may be partitioned in fulfilling thermodynamic stability of the native binding mode and contributing to the binding affinity of the complex.

Folding and oxidation of the antibody domain C(H)3

The non-covalent homodimer formed by the C-terminal domains of the IgG1 heavy chains (C(H)3) is the simplest naturally occurring model system for studying immunoglobulin folding and assembly. In the native state, the intrachain disulfide bridge, which connects a three-stranded and a four-stranded beta-sheet is buried in the hydrophobic core of the protein. Here, we show that the disulfide bridge is not required for folding and association, since the reduced C(H)3 domain folds to a dimer with defined secondary and tertiary structure. However, the thermodynamic stability of the reduced C(H)3 dimer is much lower than that of the oxidized state. This allows the formation of disulfide bonds either concomitant with folding (starting from the reduced, denatured state) or after folding (starting from the reduced dimer). The analysis of the two processes revealed that, under all conditions investigated, one of the cysteine residues, Cys 86, reacts preferentially with oxidized glutathione to a mixed disulfide that subsequently interacts with the less-reactive second thiol group of the intra-molecular disulfide bond. For folded C(H)3, the second step in the oxidation process is slow. In contrast, starting from the unfolded and reduced protein, the oxidation reaction is faster. However, the overall folding reaction of C(H)3 during oxidative folding is a slow process. Especially, dimerization is slow, compared to the association starting from the denatured oxidized state. This deceleration may be due to misfolded conformations trapped by the disulfide bridge.

Distribution of somatic H1 subtypes is non-random on active vs. inactive chromatin II: distribution in human adult fibroblasts

For nearly twenty years researchers have observed changes in the histone H1 subtype content of tissues as an organism develops into an adult. To better understand the consequences of such changes, immunofractionation of chromatin using previously characterized antibodies specific for human H1 subtypes was employed in the analysis of a fibroblast cell strain derived from a 37-year-old individual. DNAs isolated from immunoprecipitates were probed for the existence of a variety of DNA sequences. The results presented lend further support to a previously-proposed model (Parseghian et al. [2000] Chromosome Res 8:405-424) in which transcription of a sequence is accompanied by the selective depletion of subtypes. The data also suggest that there is more total H1 on actively transcribed sequences in these cells as compared to fetal fibroblasts and that there is less difference in the subtype compositions of active genes vs. inactive sequences in this strain. Specifically, the consequences of these changes appear to correlate with the attenuation of the heat shock response in aging fibroblasts. In a broader context, these results could explain why there are reductions in transcription in cells from mature tissue that approach senescence.

Influence of the isotype of the light chain on the properties of IgG

It is widely appreciated that the isotype of the H chain of the Ab molecule influences its functional properties. We have now investigated the contribution of the isotype of the L chain to the structural and functional properties of the Ab molecule. In these studies, the L chain variable region of a murine anti-dansyl Ab was joined to either human kappa or lambda constant region domains and expressed with mouse-human chimeric H chains of the four human IgG isotypes. The resulting Abs were secreted as fully assembled molecules although, as has been previously observed, IgG4 with either kappa or lambda L chains was also secreted as HL half-molecules. However, the isotype of the L chain can influence the kinetics of intracellular assembly with IgG1lambda, IgG2lambda, and IgG4lambda assembling more slowly than their kappa counterparts. The isotype of the L chain also influenced the susceptibility of the interchain disulfide bonds to attack by reducing agents with variable effects, depending on the isotype of the H chains. For IgG2, but not for IgG1, -3, and -4, the isotype of the L chain influenced the rate of clearance in mice, with IgG2lambda having a shorter in vivo half-life than IgG2kappa. Only slight differences were also observed between lambda and kappa molecules in their kinetics of binding to and dissociation from the hapten dansyl. These studies demonstrate that the isotype of the L chain has only a slight impact on the structural and functional properties of variable region identical Abs.

Unassembled Ig heavy chains do not cycle from BiP in vivo but require light chains to trigger their release

Unassembled Ig heavy chains are retained in the ER via the binding of BiP to the C(H)1 domain, which remains unoxidized. Interestingly, this domain folds rapidly, albeit nonproductively, when heavy chains are released from BiP in vitro with ATP. The in vivo cycling of BiP from heavy chains was monitored using BiP ATPase mutants as kinetic traps. Our data suggest that BiP does not cycle from the C(H)1 domain of free heavy chains. However, heavy and light chain assembly occurs rapidly and requires the ATP-dependent release of BiP. We propose that BiP's ATPase cycle is stalled or nonproductive when it is bound to free heavy chains. The binding of light chains to the complex reactivates the cycle and releases BiP.

The alternatively folded state of the antibody C(H)3 domain

The C(H)3 domain of antibodies is characterized by two antiparallel beta-sheets forming a disulfide-linked sandwich-like structure. At acidic pH values and low ionic strength, C(H)3 becomes completely unfolded. The addition of salt transforms the acid-unfolded protein into an alternatively folded state exhibiting a characteristic secondary structure. The transition from native to alternatively folded C(H)3 is a fast reaction. Interestingly, this reaction involves the formation of a defined oligomer consisting of 12-14 subunits. Association is completely reversible and the native dimer is quantitatively reformed at neutral pH. This alternatively folded protein is remarkably stable against thermal and chemical denaturation and the unfolding transitions are highly cooperative. With a t(m) of 80 degrees C, the stability of the alternatively folded state is comparable to that of the native state of C(H)3. The defined oligomeric structure of C(H)3 at pH 2 seems to be a prerequisite for the cooperative unfolding transitions.

The structure of the IgE Cepsilon2 domain and its role in stabilizing the complex with its high-affinity receptor FcepsilonRIalpha

The stability of the complex between IgE and its high-affinity receptor, FcepsilonRI, on mast cells is a critical factor in the allergic response. The long half-life of the complex of IgE bound to this receptor in situ ( approximately 2 weeks, compared with only hours for the comparable IgG complex) contributes to the permanent sensitization of these cells and, hence, to the immediate response to allergens. Here we show that the second constant domain of IgE, Cepsilon2, which takes the place of the flexible hinge in IgG, contributes to this long half-life. When the Cepsilon2 domain is deleted from the IgE Fc fragment, leaving only the Cepsilon3 and Cepsilon4 domains (Cepsilon3-4 fragment), the rate of dissociation from the receptor is increased by greater than 1 order of magnitude. We report the structure of the Cepsilon2 domain by heteronuclear NMR spectroscopy and show by chemical shift perturbation that it interacts with FcepsilonRIalpha. By sedimentation equilibrium we show that the Cepsilon2 domain binds to the Cepsilon3-4 fragment of IgE. These interactions of Cepsilon2 with both FcepsilonRIalpha and Cepsilon3-4 provide a structural explanation for the exceptionally slow dissociation of the IgE-FcepsilonRIalpha complex.

Novel control motif cluster in the IgH delta-gamma 3 interval exhibits B cell-specific enhancer function in early development

The majority of the human Ig heavy chain (IgH) constant (C) region locus has been cloned and mapped. An exception is the region between C delta and C gamma 3, which is unstable and may be a recombination hot spot. We isolated a pBAC clone (pHuIgH3'delta-gamma 3) that established a 52-kb distance between C delta and C gamma 3. Sequence analysis identified a high number of repeat elements, explaining the instability of the region, and an unusually large accumulation of transcription factor-binding motifs, for both lymphocyte-specific and ubiquitous transcription activators (IKAROS, E47, Oct-1, USF, Myc/Max), and for factors that may repress transcription (Delta EF1, Gfi-1, E4BP4, C/EBP beta). Functional analysis in reporter gene assays revealed the importance of the C delta-C gamma 3 interval in lymphocyte differentiation and identified independent regions capable of either enhancement or silencing of reporter gene expression and interaction with the IgH intron enhancer E mu. In transgenic mice, carrying a construct that links the beta-globin reporter to the novel delta-gamma 3 intron enhancer (E delta-gamma 3), transgene transcription is exclusively found in bone marrow B cells from the early stage when IgH rearrangement is initiated up to the successful completion of H and L locus recombination, resulting in Ab expression. These findings suggest that the C delta-C gamma 3 interval exerts regulatory control on Ig gene activation and expression during early lymphoid development.

The integrity of the ball-and-socket joint between V and C domains is essential for complete activity of a humanized antibody

AF2 is a high affinity murine Ab possessing potent neutralizing activity against human IFN-gamma. In carrying out the modifications to humanize this Ab, we discovered that an initial version displayed affinity for IFN-gamma that was slightly less than that of AF2, but exhibited IFN-gamma-neutralizing activity that was severely diminished. Characterization via site-directed mutagenesis revealed that the majority of this loss in IFN-gamma-neutralizing activity was due to altering the V(H) framework residue at position 11. V(H) position 11 is distal to the binding surface of the Ab; however, it, along with residues 110 and 112, have been identified as forming the socket of a molecular ball-and-socket joint between the V and C domains of the Ig Fab, which influences the elbow angle between these domains. To determine whether disrupting the structure of this joint was the basis for reduced IFN-gamma-neutralizing capacity, we altered residue 148 of C(H1), which with residue 149 comprises the corresponding ball portion of the joint. Changing this single C(H1) domain residue diminished the ability of the Ab to neutralize IFN-gamma to a level similar to that observed with the V(H) alteration. Thus, an intact ball-and-socket joint between the V and C domains in AF2 is required for potent neutralization of IFN-gamma. These results suggest the importance of the elbow angle between Ig V and C domains in Ab activity, and support the hypothesis that this joint can be an important functional element of Ab structure.

Fab chains as an efficient heterodimerization scaffold for the production of recombinant bispecific and trispecific antibody derivatives

Due to their multispecificity and versatility, bispecific Abs (BsAbs) are promising therapeutic tools in tomorrow's medicine. Especially intermediate-sized BsAbs that combine body retention with tissue penetration are valuable for therapy but necessitate expression systems that favor heterodimerization of the binding sites for large-scale application. To identify heterodimerization domains to which single-chain variable fragments (scFv) can be fused, we compared the efficiency of heterodimerization of CL and CH1 constant domains with complete L and Fd chains in mammalian cells. We found that the isolated CL:CH1 domain interaction was inefficient for secretion of heterodimers. However, when the complete L and Fd chains were used, secretion of L:Fd heterodimers was highly successful. Because these Fab chains contribute a binding moiety, C-terminal fusion of a scFv molecule to the L and/or Fd chains generated BsAbs or trispecific Abs (TsAbs) of intermediate size (75-100 kDa). These disulfide-stabilized bispecific Fab-scFv ("bibody") and trispecific Fab-(scFv)(2) ("tribody") heterodimers represent up to 90% of all secreted Ab fragments in the mammalian expression system and possess fully functional binding moieties. Furthermore, both molecules recruit and activate T cells in a tumor cell-dependent way, whereby the trispecific derivative can exert this activity to two different tumor cells. Thus we propose the use of the disulfide-stabilized L:Fd heterodimer as an efficient platform for production of intermediate-sized BsAbs and TsAbs in mammalian expression systems.

Structure of the human IgE-Fc C epsilon 3-C epsilon 4 reveals conformational flexibility in the antibody effector domains

IgE antibodies mediate antiparasitic immune responses and the inflammatory reactions of allergy and asthma. We have solved the crystal structure of the human IgE-Fc Cepsilon3-Cepsilon4 domains to 2.3 A resolution. The structure reveals a large rearrangement of the N-terminal Cepsilon3 domains when compared to related IgG-Fc structures and to the IgE-Fc bound to its high-affinity receptor, FcepsilonRI. The IgE-Fc adopts a more compact, closed configuration that places the two Cepsilon3 domains in close proximity, decreases the size of the interdomain cavity, and obscures part of the FcepsilonRI binding site. IgE-Fc conformational flexibility may be required for interactions with two distinct IgE receptors, and the structure suggests strategies for the design of therapeutic compounds for the treatment of IgE-mediated diseases.

Molecular basis for nonanaphylactogenicity of a monoclonal anti-IgE antibody

IgE Abs mediate allergic responses by binding to specific high affinity receptors (FcepsilonRI) on mast cells and basophils. Therefore, the IgE/FcepsilonRI interaction is a target for clinical intervention in allergic disease. An anti-IgE mAb, termed BSW17, is nonanaphylactogenic, although recognizing IgE bound to FcepsilonRI, and interferes with binding of IgE to FcepsilonRI. Thus, BSW17 represents a candidate Ab for treatment of IgE-mediated disorders. By panning BSW17 against random peptide libraries displayed on phages, we defined mimotopes that mimic the conformational epitope recognized on human IgE. Two types of mimotopes, one within the Cepsilon3 and one within the Cepsilon4 domain, were identified, indicating that this mAb may recognize either a large conformational epitope or eventually two distinct epitopes on IgE. On the basis of alignments of the two mimotopes with the human IgE sequence, we postulate that binding of BSW17 to the Cepsilon3 region predominantly blocks binding of IgE to FcepsilonRI, leading to neutralization of IgE. Moreover, binding of BSW17 to the Cepsilon4 region may explain how BSW17 recognizes FcepsilonRI-bound IgE, and binding to this region may also interfere with degranulation of IgE sensitized cells (basophils and mast cells). As a practical application of these findings, mimotope peptides coupled to a carrier protein may be used for the development of a peptide-based anti-allergy vaccine by induction of anti-IgE Abs similar to the current approach of using humanized nonanaphylactogenic anti-IgE Abs as a passive vaccine.

Identification of peptides that bind to the constant region of a humanized IgG1 monoclonal antibody using phage display

The pFc' fragments of a humanized IgG1 monoclonal antibody were generated by digestion with immobilized pepsin. These pFc' fragments were separated from F(ab')2 fragments by affinity chromatography. The pFc' fragments corresponding to the constant region of the humanized IgG1 monoclonal antibody were used as targets for phage display using variable-length peptide libraries. Interacting phage-displayed peptides were selected by repetitious cycles of target screening and phage amplification. Peptide sequences, deduced by sequencing DNA from isolated phage, were aligned and analyzed for amino acid motifs against each other and protein A. These results indicated that an amino acid motif has been identified using phage display technology that is sufficient for pFc' binding. Furthermore, the peptides derived from this study may prove useful in the development of peptidomimetic alternatives to protein A for use in affinity chromatography.

Correct immunoglobulin alpha mRNA processing depends on specific sequence in the C alpha 3-alpha M intron

The maturation of IgM-expressing B cells to IgM-secreting plasma cells is associated with both an increase in mu mRNA and the ratio of secreted to membrane forms of mu mRNA which differ at the 3' termini. In contrast, both in vitro and in vivo the secreted form of alpha mRNA is predominant at all stages in the development of a secretory IgA response. Previous studies demonstrated that preferential usage of the alpha s poly(A) site does not result from transcription termination and is independent of either the poly(A) sites or the 3' splice site associated with the exon encoding the membrane exon of IgA (alpha M). The present study demonstrates that a 349-bp region located 774 bp 3' to the alpha s poly(A) site is required for the preferential usage of the alpha s terminus. This region, which is the first isotype-specific cis-acting regulatory sequence not immediately adjacent to a secretory poly(A) site to be identified, contains regulatory elements that increase the efficiency of polyadenylation/cleavage. A ubiquitous, approximately 58-kDa RNA-binding protein interacts specifically with this regulatory region. These studies support the premise that cis-acting elements unique to each CH gene can impinge upon a common mechanism regulating Ig mRNA processing.

An antibody-avidin fusion protein specific for the transferrin receptor serves as a delivery vehicle for effective brain targeting: initial applications in anti-HIV antisense drug delivery to the brain

In the present study a novel Ab-avidin fusion protein has been constructed to deliver biotinylated compounds across the blood brain barrier. This fusion molecule consists of an Ab specific for the transferrin receptor genetically fused to avidin. The Ab-avidin fusion protein (anti-TfR IgG3-CH3-Av) expressed in murine myeloma cells was correctly assembled and secreted and showed both Ab- and avidin-related activities. In animal models, it showed much longer serum half-life than the chemical conjugate between OX-26 and avidin. Most importantly, this fusion protein demonstrated superior [3H]biotin uptake into brain parenchyma in comparison with the chemical conjugate. We also delivered a biotinylated 18-mer antisense peptide-nucleic acid specific for the rev gene of HIV-1 to the brain. Brain uptake of the HIV antisense drug was increased at least 15-fold when it was bound to the anti-TfR IgG3-CH3-Av, suggesting its potential use in neurologic AIDS. This novel Ab fusion protein should have general utility as a universal vehicle to effectively deliver biotinylated compounds across the blood-brain barrier for diagnosis and/or therapy of a broad range of CNS disorders such as infectious diseases, brain tumors as well as Parkinson's and Huntington's diseases.

Post-translational modifications of immunoglobulin G: a mouse IgG variant that lacks the entire CH1 domain

In the present study, we characterized the post-translational modifications of a short-chain variant of mouse IgG2a that lacks the entire CH 1 domain. The short-chain IgG2a and its proteolytic fragments were subjected to electrospray ionization- and fast atom bombardment-mass spectrometric analyses. It has been demonstrated that approximately 14% of the heavy chain of the short-chain IgG2a is O-glycosylated with a disaccharide of Ga1-GalNAc- at Thr220A in the hinge region. while the Oglycosylation does not occur in its parent IgG2a molecule. Two additional modifications have been detected at the C-termini of both the heavy and light chains of the short-chain IgG2a. Biological significance of the post-translational modifications of the short-chain IgG2a variant is briefly discussed.

Evidence that C1q binds specifically to CH2-like immunoglobulin gamma motifs present in the autoantigen calreticulin and interferes with complement activation

Calreticulin (CRT) is located predominantly in the endoplasmic reticulum (ER) of cells, where it functions as a quality control controller of protein folding. However, CRT is also a prevalent autoantigen in patients with systemic lupus erythematosus (SLE), where its release from the cell may arise as a results of dysfunctional apoptosis and inefficient removal of ER vesicles, which are an abundant source of CRT and other autoantigens. Indicative of this is the presence of autoantibodies against CRT in the sera of 40-60% of all SLE patients. Once released into the circulation, CRT might bind directly to C1q and we have suggested that this association may result in a defect in C1q-mediated clearance of antigen-antibody complexes. It has been previously shown that CRT under physiological salt conditions binds to the globular head of C1q. It is known that the globular head region of C1q binds to the CH2 domain in the Fc portion of immunoglobulin gamma (IgG). The N-terminal half of CRT contains a number of short regions of 7-10 amino acids that show sequence similarity to the putative C1q binding region in the CH2 domain of IgG. By use of a series of 92 overlapping CRT synthetic peptides, a number of C1q binding sites on the CRT molecule have been identified, including several containing a CH2-like motif similar to the ExKxKx C1q binding motif found in the CH2 domain of IgG. A number of these peptides were shown to inhibit binding of C1q to IgG and reduce binding of native CRT to C1q. Moreover, several of the peptides were capable of inhibiting the classical pathway of complement activation. These studies have identified specific binding sites on the CRT molecule for C1q and lend support to the hypothesis that interaction of CRT with C1q may interfere with the ability of C1q to associate with immune complexes in autoimmune-related disorders.

Effect of upstream RNA processing on selection of mu S versus mu M poly(A) sites

All of the regulatory factors responsible for augmenting microseconds mRNA levels preceding the dramatic increase in secretory IgM production upon B cell activation has not been totally elucidated. Whereas previous experiments have centered on the region of the gene specifying the choice between splicing to mu M exons versus selection of the mu S poly(A) site, we have found that upstream sequences within the Cmu gene, specifically the Cmu 4 acceptor splice site together with intronic sequences between the Cmu 3++ and Cmu 4 exons, play an important role in dictating the precision or the extent of splicing to the mu M exons even under conditions in which functional polyadenylation factors should be in excess. Therefore, splicing of upstream exons can affect remotely located downstream exons. These findings suggest that regulation of differential mu S/mu M mRNA expression may involve general processing enzymes that recognize specific cis -regulatory sequences residing within the body of the mu gene and account for the unique ability of activated B cells to secrete copious amounts of IgM.

Construction of phosphorylatable monoclonal antibody CC49 with a casein kinase II recognition site

A phosphorylation site for casein kinase II was introduced into chimeric monoclonal antibody CC49 (MAb-chCC49) by site-specific mutation of the coding sequence. The phosphorylation site for the casein kinase II was positioned at the carboxyl terminus of the heavy chain constant region of the MAb-chCC49. The resultant modified MAb-chCC49CKII was expressed in NS0 cells and purified. The MAb-chCC49CKII protein was phosphorylated by casein kinase II with [gamma-32P]ATP to high radiospecific activity. The 32P-labeled MAb-chCC49CKII binds to cells expressing TAG-72 antigens. The introduction of the phosphorylation sites for casein kinase II into monoclonal antibodies (MAb) provides a new reagent for the diagnosis and treatment of cancers. This demonstrates that the casein kinase II recognition site can also be used to introduce phosphorylation sites into proteins.

Effect of C2-associated carbohydrate structure on Ig effector function: studies with chimeric mouse-human IgG1 antibodies in glycosylation mutants of Chinese hamster ovary cells

The complex biantennary oligosaccharide at Asn297 of IgG is essential for some effector functions. To investigate the effect of carbohydrate structure on Ab function, we have now expressed mouse-human chimeric IgG1 Abs in Chinese hamster ovary (CHO) cells with defined defects in carbohydrate biosynthesis. We had previously shown that IgG1 Abs produced in the cell line Lec 1, which attaches a high-mannose intermediate carbohydrate, were severely deficient in complement activation, showed a slightly reduced affinity for Fc gammaRI, and had a reduced in vivo half-life. We have extended these studies by producing the same dansyl-specific IgG1 in cell lines deficient in attachment of sialic acid (Lec 2) and galactose (Lec 8). IgG1-Lec 1, IgG1-Lec 2, and IgG1-Lec 8 all showed varying reactivity with a mAb specific for an epitope in the amino terminal region of C(H)2, suggesting that the conformations of these proteins were altered by the different carbohydrate structures. Functionally, IgG1-Lec 2 and IgG1-Lec 8 were comparable to wild type with respect to in vivo half-life, affinity for Fc gammaRI, and capacity for complement-mediated hemolysis. While IgG1-Lec 2 was essentially identical to wild type in its capacity to interact with individual components of the classical complement activation pathway, IgG1-Lec 8 demonstrated equivalent maximal binding at lower concentrations and was preferentially bound by mannose-binding protein. Although IgG1-Lec 1 was deficient in activation of the classical pathway, it had a superior capacity to activate the alternative pathway. These studies demonstrate that Abs bearing C(H)2-linked carbohydrate of differing structures have different functional properties.

Mapping IgG epitopes bound by rheumatoid factors from immunized controls identifies disease-specific rheumatoid factors produced by patients with rheumatoid arthritis

We have mapped the specificity of 28 monoclonal IgM rheumatoid factors (RFs) produced by heterohybridomas derived from five healthy blood donors immunized with mismatched human red blood cells (HID). The HID-RFs did not differ in their binding specificity for IgG epitopes from RFs that we previously analyzed from patients with Waldenström's macroglobulinemia. However, IgM RFs produced by HID differed in their specificity for IgG compared with RFs expressed by patients with rheumatoid arthritis (RA-RFs). Only 1 of 28 HID-RFs bound all IgG subclasses (pan binding pattern) compared with 7 of 19 RA-RFs (p = 0.006). Three HID-RFs bound IgG3 compared with 9 RA-RFs (p = 0.007). Fine specificity differences were also identified between HID- and RA-RFs. Therefore, some RA-RFs show novel specificities for IgG not found among RFs from HID or individuals with Waldenström's macroglobulinemia who do not have joint disease. These Abs with unique specificities may represent disease-specific autoantibodies in patients with RA. Nine of the HID-RFs from the same individual were clonally related, and several contained somatic mutations. Even when the clonally related HID-RFs were considered as one RF for comparison, the reactivity of the HID-RFs differed significantly from RA-RFs in their inability to recognize all IgG subclasses (p = 0.044) and recognize IgG3 (p = 0.041). Interestingly, among the clonally related RFs, considerable differences in the specificity for IgG were also observed, with the RF containing the most somatic mutations in VH and VL showing the most distinctive specificity changes. Therefore, these studies also demonstrate a correlation between somatic mutation and binding specificity.

The C(H)1 domain of IgG is not essential for C3 covalent binding: importance of the other constant domains as targets for C3

The covalent binding of C3 to antigen-antibody complexes [immune complexes (IC)] plays a pivotal role in the elimination of antigens. C3 prevents the formation of large IC lattices promoting their solubilization. Subsequently, bound C3 fragments determine the efficacy of antigen presentation, and the generation of antibody responses and immunological memory. C3 binding to IgG-IC generates IgG-C3b-C3b complexes which are detected by SDS-PAGE as two major bands: C3alpha65-heavy chain and C3alpha65-C3alpha43 covalent complexes. Using human heat-aggregated IgG1 as a model of IC, a C3b binding site was localized only in the Cgamma1 domain. However, with true IC of ovalbumin and rabbit IgG anti-ovalbumin, C3b binds to both the Fab and Fc regions of IgG. To study the binding of C3b to the different domains of IgG and particularly to evaluate the involvement of the Cgamma1 domain, we have constructed recombinant single-chain antibodies without Cgamma1, which have the structure: V(H)-linker-V(L)-hinge-Cgamma2-Cgamma3 (scAb). The variable domains were from a mouse mAb anti-HSA and the constant region (hinge-C(H)2-C(H)3) from human IgG1 or rabbit IgG. C3 binds very efficiently to IC formed with human (h-scAb) or rabbit (r-scAb) recombinant antibodies (scAb-HSA) and generates also two bands on SDS-PAGE (C3alpha65-scAb and C3alpha65-C3alpha43), which are the counterparts of those of the complete antibody. In addition, IC formed with scAb activate the alternative pathway to a similar extent as IC of the entire IgG. These data indicate that the Cgamma1 domain is a dispensable region for C3b binding and that the remaining constant domains are as efficient as Cgamma1 in C3b binding. Overall these results support the view that C3 does not specifically recognize a unique site in the Cgamma1 domain. Rather it seems to be able to attach along the antibody molecule. Probably this implies an advantage for effective processing of C3b-IC and elimination of antigens in vivo.

The T/B cell interaction involved in induction of the mouse IgG2ab suppression is restricted by major histocompatibility complex class I, but not class II molecules

To determine the major histocompatibility complex (MHC) restriction of the T/ B cell interaction involved in a negative regulation of Ig production, we used mouse model of T cell-induced IgG2ab suppression in vivo. Normal or specifically triggered T splenocytes from mice of the Igha haplotype, when neonatally transferred into histocompatible Igha/b heterozygotes, are able to induce a specific and total suppression of the IgG2ab allotype. Nevertheless, only transfer of IgG2ab-primed Igha T splenocytes induces this suppression in Ighb/b homozygous congenic mice in which the whole IgG2a isotype production is inhibited. This suppression is chronically maintained by CD8+ T cells, but can be experimentally reversed. We have established that the suppression induction required a CD4+CD8+ T cell cooperation and operated via the recognition by the involved TCR of C gamma 2ab-derived peptides presented by the target B cells in an MHC haplotype-restricted manner. Here, by using Ighb mice genetically deficient for MHC class I (beta 2-microglobulin%, or beta 2m%) or class II (I-A beta%) molecules, we demonstrate functionally that the suppression induction implicates an MHC class I-, but not class II-restricted interaction. Indeed, the anti-IgG2ab T cells transferred into Ighb H-2b I-A beta% mice carry out the suppression process normally, while in Ighb H-2b beta 2m% recipients, their suppression induction capacity is significantly inhibited. Moreover, the C gamma 2ab 103-118 peptide, identified as the sole C gamma 2ab-derived peptide able to amplify the anti-IgG2ab T cell reactivity in Igha H-2b mice, is also able to stabilize the H-2Db, but not the H-2Kb class I molecules at the surface of RMA-S (TAP2-, H-2b) cells. These results indicate that, despite the CD4+/CD8+ T cell cooperation during the induction phase of suppression only MHC class I molecule expression is required at the surface of IgG2ab+ B cells for suppression establishment.

Elimination of N-linked glycosylation sites from the human IgA1 constant region: effects on structure and function

IgA1 Abs possess conserved N-linked glycosylation sites in the second C region and secreted tailpiece domains. To understand the role of these carbohydrates in the structure and function of human IgA1, site-directed mutants that produce human IgA1 lacking either one or both of the N-linked carbohydrate sites have been produced. When the mutant heavy chains are expressed in myeloma lines producing the relevant kappa-light chain, efficient secretion of the monomer and dimer forms of IgA1 is seen. In addition, higher polymer forms of the IgA molecules lacking the third domain carbohydrate, either singly or in the double mutant, are present. Functional analysis of the IgA1 proteins has shown significant differences between the various mutants and wild-type IgA. The carbohydrate mutants show a reduced affinity for their target Ag, dansyl. All of the IgA1 molecules retained the ability to bind to the polymeric Ig receptor. C3 binding was observed for all of the IgA molecules, with the IgA mutants lacking the third domain carbohydrate showing a reduced ability to bind C3; however, IgA did not effectively activate the alternative pathway, as determined by factor B cleavage and terminal complex binding. These studies demonstrate that N-linked glycosylation in the constant domain of human IgA1 plays an important role in the biologic properties of IgA1.

Subtle differences in antibody responses and hypermutation of lambda light chains in mice with a disrupted chi constant region

Analysis of lambda light chain use in normal mice is made difficult by the dominant chi light chain repertoire. We produced mice rendered deficient in chi light chain expression by gene targeting and focused on questions concerned with the generation of lambda light chain diversity. Whilst these mice compensate the chi deficiency with increased lambda liters, and their Ig level is therefore not significantly reduced, they show major differences in immunization titers, germinal center (GC) development and somatic hypermutation. After immunization, using antigens that elicit a restricted IgL response in normal mice, we obtained in the chi-/- mice elevated primary antibody titers but a subsequent lack in titer increase after repeated antigen challenge. Analysis of the Peyer's patches (PP) revealed a dramatically reduced cell content with rather small but highly active GC. Flow cytometric analysis showed different cell populations in the PP with enriched peanut agglutinin (PNA)hi/CD45R(B220)+ B cells, implying that the apparent compensation for the lack of lambda light chain expression involves the GC microenvironment in cell selection, the initiation of hypermutation and high affinity expansion. The three V lambda genes, V1, V2 and Vx, are mutated in the GC B cells, but show no junctional diversity. In contrast, a reduced rate of V lambda hypermutation is found in the hybridoma antibodies, which appears to reflect a selection bias rather than structural constraints. However, mechanisms of somatic mutation and specificity selection can operate with equal efficiency on the few V lambda genes.

A monovalent C mu 4-specific ligand enhances the activation of human B cells by membrane IgM cross-linking ligands

The ligand-receptor binding requirements for achieving full B cell activation through the membrane immunoglobulin (mIg) signaling pathway are relatively demanding, and mIg-antigen engagements which fall below these critical thresholds cause, at most, only the partial activation of B cells. In an effort to resolve new means of enhancing the efficacy of mIgM-mediated signal transduction, as well as to further understand the process by which mIgM-mediated signals are initiated, we have explored the mechanism for a previously reported synergy between certain mixtures of murine anti-IgM mAbs in eliciting human B cell DNA synthesis. We here report that striking synergy occurs when any of several relatively high affinity mAbs specific for diverse domains of mIgM are combined in culture with the relatively low affinity C mu 4-specific ligand, mAb IG6. Although B cell activation was dependent upon the bivalency, and hence mIgM cross-linking potential, of the high affinity ligand, low affinity mAb IG6 could enhance the activation process when present as a monovalent Fab' fragment. This did not appear due to F(ab')2 contamination or Fab' aggregation, since IG6 Fab' preparations were notably compromised in several other functions requiring ligand bivalency. Pulsing studies revealed that the C mu 4-specific ligand exhibits its functional effects only when stimulatory mIgM receptor cross-links are being formed by bivalent ligands, and that IG6 Fab' enhancement is most notable during the later interval of the prolonged mIgM signaling process that leads to S phase entry. A unique region of the membrane-proximal IgM domain may be important for Fab'-mediated enhancement, since Fab' fragments that bind with higher affinities to distinct sites on C mu 4 were not as effective at mediating this phenomenon. Several possibilities for the adjuvant effects of this C mu 4-specific Fab' on B cell responses triggered by mIgM crosslinking ligands are discussed, including the possibility that IG6 Fab' influences the potential for mIgM dimer formation or interactions of mIgM with other signal-transducing molecules.

Antibody constant region: potential to bind metal and nucleic acid

Environmental challenges appear to elicit similar patterns of cellular responses such as positive autoregulation and autoamplification whether one considers the generation of antibodies with identical antigen specificity or the accumulation of host-protective transcription factors. Therefore, I analyzed the structure of immunoglobulins (Ig) for motifs commonly found in transcription factors. Specifically, the well-known abundance and periodic location of cysteine residues in immunoglobulin chains prompted me to check antibody constant regions for the presence of putative metal-binding domains and zinc finger-like sequences. The constant regions of Ig light and heavy chains were found to harbor one or several copies, respectively, of a short cysteine- and histidine-containing sequence. Moreover, all four IgG subclasses were detected to comprise zinc finger-like motifs in their heavy chain constant and hinge domains. Yet another finding is the occurrence of several sequences of the form serine-proline-X-X and/or threonine-proline-X-X in the hinge sections of IgA and IgG3. These results suggest that antibody constant regions, as a fragment and/or embedded in a full-length immunoglobulin chain, may complex metal, thus acquiring conformations conducive to dimerization and nucleic acid binding. As such, my study provides a putative structural basis for the known requirement of divalent metal cations, particularly of zinc ions, for a normal immune response, and warrants further investigations, both theoretical and experimental, into the potential of antibody constant regions for metal binding and gene regulation. Moreover, future testing of the proposed zinc finger peptides from Ig constant domains should yield information relevant to zinc finger design with potentially wide applications in research and clinical medicine.(ABSTRACT TRUNCATED AT 250 WORDS)

Cre-loxP-mediated gene replacement: a mouse strain producing humanized antibodies

BACKGROUND

The bacteriophage-derived Cre-loxP recombination system operates efficiently in mammalian cells. This system is particularly useful in gene-targeting experiments in the mouse, and has already been used to generate 'clean' deletions of target genes in the germ line, as well as to inactivate target genes in a conditional manner (based on regulated expression of the Cre recombinase). In principle, Cre-loxP-mediated recombination should also allow gene replacement, and thus the introduction of virtually any kind of mutation into the genome.

RESULTS

We used the Cre-loxP system, in mouse embryonic stem cells, to replace the mouse gene C gamma 1, which encodes the constant region of the heavy chain of IgG1 antibodies, with its human counterpart. The mutation was transmitted through the mouse germ line, and the resulting mutant mice were crossed with mice expressing kappa light chains with a human, instead of a mouse, constant region. Mice homozygous for both mutations produce humanized, kappa-chain-bearing IgG1 antibodies at the same level and efficiency as wild-type mice produce murine IgG1 antibodies. These animals should enable the ex vivo production of humanized, chimeric monoclonal antibodies specific for any antigen to which the mouse can respond.

CONCLUSIONS

Cre-loxP-mediated gene replacement is a simple and efficient general method of targeted mutagenesis in the mouse.

Human high-affinity Fc IgG receptor (Fc gamma RI)-mediated phagocytosis and pinocytosis in COS cells

The high-affinity receptor (Fc gamma RI) for the constant (Fc) portion of immunoglobulin G (IgG) is one of three Fc IgG receptor classes (Fc gamma Rs) found on mononuclear phagocytes. The functional specialization of each of the Fc gamma R classes is not well understood. Previous studies utilizing anti-Fc gamma R monoclonal antibodies (mAbs) as opsonins suggest that Fc gamma RI, like the other Fc gamma Rs expressed by macrophages, is able to mediate phagocytosis. The ability of Fc gamma RI to mediate pinocytosis, however, had not been certain, since it binds, but does not mediate, internalization of monomeric IgG in the monocytoid U937 cells. We studied Fc gamma RI-mediated internalization by introducing it into the Fc gamma R-negative fibroblastic COS cells. We found, using electron microscopy and fluorescence microscopy, that COS cells expressing Fc gamma RI are able to phagocytose IgG-coated zymosan particles and sheep red blood cells (SRBC), as well as pinocytose cross-linked IgG. There was no intracellular accumulation of monomeric IgG. Chimeric receptors which retain the extracellular domains of Fc gamma RI but lack the entire wild-type transmembrane and intracellular regions of the receptor mediated both phagocytosis and pinocytosis with equal or increased efficiency when compared to the wild-type receptor. Control COS cells transfected with CD2 rosetted, but did not phagocytose, SRBC. Attachment of phagocytic targets to COS cells is therefore not sufficient for phagocytosis. Taken together, this suggests that the extracellular domain of Fc gamma RI is sufficient for it to mediate phagocytosis and pinocytosis in this system.

A single Fc binding domain--alkaline phosphatase gene fusion expresses a protein with both IgG binding ability and alkaline phosphatase enzymatic activity

A recombinant gene fusion was created and cloned using a previously constructed gene encoding a monodomain IgG Fc binding protein and the gene coding for bacterial alkaline phosphatase. The construct was able to express and secrete a fusion protein that exhibited both IgG binding and alkaline phosphatase enzymatic activities. Greater than 60% of the protein demonstrating both biological activities was detected from periplasmic space preparations. Nanogram concentrations of the Fc binding--alkaline phosphatase fusion protein allowed primary IgG antibody detection without the use of conjugated secondary antibodies. Removal of the domain coding for alkaline phosphatase resulted in decreased resistance of the protein to proteolytic degradation and the loss of IgG Fc binding ability. Using affinity-purified fusion protein, the specificity of binding to IgG, IgM and IgA was examined; binding was strong to IgG and barely detectable against IgM or IgA. Affinity for binding of the fusion protein to IgG (Kd = 6.7 x 10(-8) M) was determined to be equal to or greater than previously reported for protein A.

Gene synthesis and functional expression of a protein exhibiting monodomain IgG Fc binding

A gene encoding a bacterial IgG Fc binding domain was designed and synthesized. The synthetic DNA fragment was cloned 3' to an inducible trpE promoter such that expression of the gene in Escherichia coli produced abundant Fc binding protein fused to the first seven amino acids of the trpE protein. The recombinant protein contained a single Fc binding domain and demonstrated efficient binding to human IgG in Western blot analysis. This protein degraded rapidly following cell lysis in the absence of protease inhibitors, but could be effectively protected by the addition of protease inhibitor. After purification of the protein by IgG affinity chromatography, IgG Fc binding ability was retained for at least 24 h at either 23 or 37 degrees C and on heating for 15 min at temperatures up to 65 degrees C. No immunoprecipitation was observed in interactions between the monodomain Fc binding protein and IgG molecules. Unlike staphylococcal protein A, no detectable binding of the monodomain IgG Fc binding protein was observed to either IgM or IgA. Truncated proteins, expressed from a series of 3' deletions of the synthetic gene, were used to estimate the minimum portion of a monodomain Fc binding protein that retained Fc binding ability.

Production and tumour-binding characterization of a chimeric anti-CEA Fab expressed in Escherichia coli

A recombinant chimeric Fab (rcFab), with Fv derived from the monoclonal A5B7 antibody to carcinoembryonic antigen (CEA), and with human CHI and C kappa was cloned into pUC 19 and expressed in Escherichia coli. rcFab (10 to 12 mg per litre) was produced in bacterial culture fluid, and functional purified rcFab was isolated by affinity chromatography (using antibody to human C kappa) and size-exclusion gel filtration. The rcFab did not show reduced affinity for CEA, and reacted with human colorectal tumours showing a typical anti-CEA pattern by immunocytochemistry; it was also stable after iodination. Biodistribution studies in nude mice bearing human tumour xenografts showed no toxicity and good tumour localization. Therapeutic ratios at early time points were better than those obtained with whole murine antibody. The results demonstrate that bacterially produced anti-CEA Fab is of use for tumour targeting.