Rat immunoglobulins

Development and pharmacological validation of novel methods of B cell activation in rat whole blood

INTRODUCTION

Whole blood functional assays are pharmacologically relevant in the drug discovery process to evaluate potency in a relevant biological matrix, to support establishment of PK/PD relationships and to aid in human dose predictions. However development of B cell activation assays by BCR ligation in rat whole blood has not been previously described. The aim of the present study was to develop novel methods of B cell activation in rat whole blood.

METHODS

B cell activation in rat whole blood was evaluated by measuring CD86 up-regulation via flow cytometry. Rat B cells in whole blood were stimulated with dextran-coupled anti-IgD or a combination of anti-IgD and TLR9 agonist. BTK, SYK, and PI3Kδ inhibitors were added to rat whole blood prior to activation with dextran-coupled anti-IgD or anti-IgD and TLR9 agonist combination for pharmacological validation of the assay.

RESULTS

Both methods of stimulation in rat whole blood evoked robust B cell activation in a uni-modal fashion. Highly selective inhibitors of BTK, SYK, and PI3Kδ dose-dependently attenuated B cell activity evoked by both dextran-coupled anti-IgD and combined anti-IgD and TLR9 agonist. Compound potencies and rank order determined by the two assays were comparable.

DISCUSSION

Two novel methods were developed to stimulate B cells in rat whole blood, that have the potential to be used to support drug discovery efforts in the therapeutic targeting of B cells. Furthermore, we pharmacologically validated these whole blood assays using highly selective inhibitors of BTK, SYK, and PI3Kδ, signaling kinases which are downstream of the B cell receptor.

Quantitative analysis of immunoglobulin G subclasses in the rat

Quantitative ELISAs have been developed for the four immunoglobulin G (IgG) subclasses (IgG1, IgG2a, IgG2b and IgG2c) of the rat. These assays were shown to have excellent sensitivity, reproducibility and adequate specificity for studies of natural IgG and antigen-elicited IgG responses. The sensitivity (working range for each isotype assay) was: IgG1, 1.6 to 200 ng/ml; IgG2a, 8 to 200 ng/ml; IgG2b, 1.6 to 200 ng/ml; and IgG2c, 1.6 to 1000 ng/ml. The isotype sum and the direct assay of total plasma IgG agreed closely. The utility of these assays was demonstrated in a study of the development of total IgG and of the specific IgG response following immunization with tetanus toxoid (TT). For total IgG, the predominant isotype was IgG2b (64% of total IgG) followed by IgG2a (29%), IgG1 (4%) and IgG2c (3%). In contrast, the anti-TT response was mainly of the IgG1 subclass (57% of total anti-TT). These quantitative assays should prove useful for investigating the response to experimental vaccines and the influence of cytokines on class switching in vivo.

Experimental arthritis in rats induced by intra-articular injection of IgE aggregates: evidence for arthritogenic role of complexed IgE

An experimental arthritis model in the rat was used to study the arthritogenic potential of complexed IgE. IgE aggregates were produced in vitro by cross linking monoclonal rat IgE by dimethyl suberimidate and were injected into the knee joints. Animals which had not been injected and animals injected with phosphate buffered saline served as controls. The concentration of histamine in tissues, diffusion into the joint of bovine serum albumin labelled with iodine-125 injected intravenously, and the histology of the joints were studied. There was a significant decrease in the concentration of histamine in synovial tissue 8 and 24 hours after the injection of the IgE aggregates. A decreased number of stainable mast cells were found 8, 24, and 48 hours after exposure. A moderate hyperplasia of the synovial lining layer was also noted. These results provide further evidence for the arthritogenic potential of complexed IgE, especially in the initiation of arthritis through activation of mast cells.

Isolation and characterization of a VH domain fragment from monoclonal rat IgG of IgG1 and IgG2a subclasses

Digestion with trypsin of monoclonal rat IgG of IgG1 and IgG2a subclasses produced two fragments, isolated only in dissociating media. The larger fragment (mol. wt. 120,000 Da) was comprised of the two light chains covalently bound to shortened gamma chains. Amino acid sequence of the shortened gamma chain indicated that the site of cleavage is located at the beginning of the C gamma 1 domain at a position homologous to residue 139 of mouse gamma heavy chain of IgG1 MOPC 21. The smaller fragment (mol. wt. 13,000 Da) was found to consist of the entire variable domain of the heavy chain and probably a short stretch of the C gamma 1 domain. The unique susceptibility of rat monoclonal IgG1 and IgG2a is likely to be the result of the presence of a lysine residue in a loop of C gamma 1 domain, which therefore is accessible to trypsin. Tryptic cleavage of rat monoclonal antibodies of IgG1 and IgG2a subclasses can be considered as a simple method to produce a fragment related to the VH domain.

Isolation of rat IgM to IgG hybridoma isotype switch variants and analysis of the efficiency of rat Ig in complement activation

Sequential sublining was used in combination with enzyme-linked immunosorbent assays to isolate mu----gamma isotype switch variants of the rat IgM secreting mouse-rat B cell hybridoma line BA1.8. Switch variants to all four subclasses of IgG were obtained. The variant antibodies retained the antigen specificity of the parental IgM for the O18 (lipopolysaccharide) antigen of Escherichia coli. In sodium dodecyl sulfate-polyacrylamide gels the apparent molecular mass of the gamma heavy chains decreased in the order gamma 2b greater than gamma 1 greater than gamma 2c greater than gamma 2a. IgM, IgG1, IgG2a, IgG2b and IgG2c of the BA1.8 variant family and IgG2b, IgE and IgA of the previously described BA1.2 family were used for a comparative analysis of the capacity of rat Ig to activate complement. Efficient lysis of sheep erythrocytes coated with the O18 antigen was observed with IgM and all IgG subclasses, but no lysis was triggered by IgE or IgA. One hundred to 1000 IgG molecules were required to mediate the same hemolytic activity as one IgM molecule. The four IgG subclasses were equally efficient at mediating lysis by rat or human complement, while IgG2a was less efficient with guinea pig complement than the other three IgG subclasses. Antibody-triggered binding of C3 to pathogenic O18:K1 E. coli bacteria was measured using serum containing 125I-labeled C3. K1-encapsulated strains did not fix C3 efficiently in the absence of specific antibodies while acapsular mutants fixed C3 via the alternative pathway. IgM and all IgG subclasses triggered C3 binding to the K1 encapsulated bacteria. The capacity of IgM to mediate C3 fixation was not greater than that observed with IgG.

Studies of the IgE binding sites to rat mast cell receptor with proteolytic fragments and with a monoclonal antibody directed against epsilon heavy chain: evidence that the combining sites are located in the C epsilon 3 domain

The binding sites of rat IgE to mast cell receptor were investigated by the use of proteolytic fragments and a monoclonal antibody to epsilon chain (MARE-1). Three main fragments were characterized by short-time papain digestion of IgE: F(ab')2-E, a fragment related to the C, 4 domain, and an asymmetric fragment corresponding probably to an IgE molecule with one proteolyzed C, 3 domain. Neither F(ab')2-E nor C, 4 could interfere with the binding of IgE to rat mast cells. These two fragments did not show significant polymerization upon heating at 56 degrees C, while large amounts of polymers were produced from whole IgE, MARE-1 monoclonal antibody was found to react neither with F(ab')2 nor with C, 4, thereby suggesting its interaction with the C, 3 domain. MARE-1 was found to inhibit partially (about 55%) the binding of IgE to its receptor. Taken together the results indicate that the binding sites of IgE to rat mast cell receptor are located within the C, 3 domain. In addition, isolation of the C, 4 domain will be useful to evaluate its participation in the affinity of IgE to receptors of other cells such as lymphocytes or macrophages.

A monoclonal antibody to a cell wall component of Candida albicans

A heterologous fusion between mouse myeloma cells and rat lymphocytes resulted in the isolation of a rat immunoglobulin M monoclonal antibody with both agglutinating and precipitating activity. Indirect immunofluorescence and direct agglutination tests showed that the corresponding antigen was present in the cell wall of the three Candida species considered to be the most pathogenic, C. albicans, C. tropicalis, and C. glabrata, and also in the cell wall of C. guilliermondii. The antigen appeared to be predominantly polysaccharide in nature. Precipitation by counterimmunoelectrophoresis suggested that the epitope is shared by at least two separate molecules with different electrophoretic mobilities. Presence of this epitope varied from strain to strain within a given species and may be related to the morphological stage in the cell cycle. Antigen was shown to be present in the cytoplasm, in the periplasmic space, and at the cell surface of C. albicans. Indirect immunofluorescence also suggested that antigen is excreted from the cell.

Rat monoclonal antibodies. II. A rapid and efficient method of purification from ascitic fluid or serum

A technique for purifying rat monoclonal antibodies from ascitic fluid or serum is described which is based on 2 facts. First, approximately 95% of rat immunoglobulin light chains are of the kappa type. Second, an allotypy in the rat species is located on the constant part of the kappa light chain. By use of a mouse monoclonal antibody with specific binding affinity for the Ig kappa-1a allotype on the kappa light chains of the LOU inbred rat strain, it is possible with immunoaffinity chromatography to isolate LOU Ig kappa-1a-bearing immunoglobulins from the serum proteins, including the immunoglobulins, of rats of Ig kappa-1b allotype. LOU histocompatible hybridomas synthesizing the Ig kappa-1a allotype can be transplanted into rats congenic with the LOU inbred strain carrying the Ig kappa-1b allotype, since LOU rats with the Ig kappa-1a kappa light chain allotype and congenic LOU Ig kappa-1b rats with the Ig kappa-1b kappa light chain allotype are fully histocompatible. The serum or ascitic fluid of the recipients is applied to an immunoabsorbent column to which mouse monoclonal antibody against the Ig kappa-1a allotype is coupled. The serum proteins, including the host immunoglobulins pass through the column. An appropriate buffer is used to elute the monoclonal antibodies in a second step. The same technique may be employed for other monoclonal antibodies. A reciprocal system using mouse monoclonal antibodies against Ig kappa-1b rat allotype can be used, a plasmacytoma or hybridoma synthesizing Ig kappa-1b kappa light chain being transplanted into an Ig kappa-1a kappa light chain synthesizing rat. The method is rapid, efficient and inexpensive. Its limitation is with respect to lambda-type monoclonal antibodies, which are relatively rare.

Differential reduction of the inter-chain disulfide bonds of rat immunoglobulin E: relation to biological activity

Monoclonal rat IgE was reduced over a range of dithiothreitol (DTT) concns. The number of disulfide bonds reduced and their location in the IgE molecule were studied. One millimolar DTT was found to split the two inter-heavy-chain disulfide bonds of the C epsilon 2 domain while increasing DTT concn to 10 mM split the two inter-heavy-light-chain disulfide bridges. Therefore, the sensitivities to reduction of disulfide bonds in rat IgE were found to be the opposite of those in human IgE. In addition, the results indicated the absence, in rat IgE, of the intra-epsilon-chain labile disulfide bond of the C epsilon 1 domain, which is reduced by 2 mM DTT in human IgE. Circular dichroism studies showed significant modifications, mainly of tertiary structure, for rat IgE reduced with 10 mM DTT, but not for IgE reduced with 1 mM DTT. The ability to block passive sensitization with reaginic antibody was not modified when IgE was reduced with 1 mM DTT (which split the two inter-heavy-chain disulfide bonds), but was lost when inter-heavy-light-chain bridges were reduced with 10 mM DTT. In addition, a non-covalent epsilon-chain dimer was found to have the same blocking activity as native IgE (or IgE reduced with 1 mM DTT). Therefore, the results suggest that reduction of most or all the inter-chain disulfide bonds, in rat as in human IgE, induces changes in quaternary structure, more especially in the relationship between the Fab and Fc parts of the molecule, leading to steric blockade, by Fab, of the binding sites for mast cells present on Fc.

Characterization by Scatchard plots of monoclonal antibody enzyme conjugates directed against alpha-1-foetoprotein

A radioimmunoassay was used to investigate the affinity of 2 monoclonal antibodies against human alpha-1-foetoprotein before and after conjugation with horseradish peroxidase. The equilibrium constant of the high-affinity antibody was reduced 10-fold whereas it remained unaffected in the low-affinity antibody. With the Scatchard diagram, quantification of unlabelled antibodies in the unpurified conjugate mixture is also possible if antibody affinity is changed by the coupling procedure.

Rat monoclonal antibodies. I. Rapid purification from in vitro culture supernatants

A technique for purifying rat monoclonal antibodies quickly and efficiently from in vitro culture supernatants is described. It is based on the fact that more than 95% of rat immunoglobulins carry kappa light chains. A mouse monoclonal antibody with suitable binding affinity for rat kappa light chains is immobilized on solid support and used to purify rat immunoglobulins. Milligrams of rat monoclonal antibodies may be rapidly concentrated from culture supernatants with high recovery. Rat monoclonal antibodies expressing lambda light chains (about 5% of the total) may be purified in a similar way with an appropriate anti-rat lambda chain monoclonal antibody.

Optimal conditions for the preparation of Fab and F(ab')2 fragments from monoclonal IgG of different rat IgG subclasses

The optimal conditions for the preparation of Fab and F(ab')2 fragments from monoclonal rat IgG of different subclasses are described. Digestion of IgG for 2-4 h at 37 degrees C with 1% (w/w) papaïn at pH 7.0 in the presence of 0.01 M cysteine leads to almost complete cleavage into Fab and Fc fragments. Fab fragments are isolated by sequential chromatography on Ultrogel AcA 44 and on DEAE-cellulose columns. In the case of IgG2c subclass, Fab fragment may be directly isolated by chromatography on Protein A-Sepharose. Production of F(ab')2 fragments from rat IgG1 and IgG2a is obtained with best yield by treatment at acid pH (pH 2.8) before incubation with 1% (w/w) pepsin at pH 4.5 for 4 h at 37 degrees C. For monoclonal IgG2b the best procedure is incubation with S. aureus V8 protease at pH 7.8 (4 h at 37 degrees C with an E/S ratio of 1/30 (w/w]. The best yield of F(ab')2 from monoclonal IgG2c is obtained by incubation for 4 h at 37 degrees C with 1% (w/w) pepsin. F(ab')2 fragments (or the F(ab)2-like fragment released by digestion of IgG2b with S. aureus V8 protease) are isolated by gel filtration on Ultrogel AcA 44.

Proteolysis of rat IgG subclasses by Staphylococcus aureus V8 proteinase

Monoclonal IgG belonging to the four rat IgG subclasses (IgG1, IgG2a, IgG2b, IgG2c) and some IgG subclasses from normal rat serum were subjected to enzymatic degradation with Staphylococcus aureus V8 proteinase. The results show that only one subclass, IgG2b, is significantly cleaved by the enzyme, with the release of two main products identified as F(ab)2 and Fc-like fragments. This unique susceptibility of the IgG2b subclass represents therefore an easy means of identification and also offers a simple procedure for a preparation of F(ab)2 fragments from monoclonal IgG2b antibodies.

Subclass restriction of the antiidiotypic antibody response in the rat

Antiidiotypic antibodies were induced in LOU/M rats by immunization with two myeloma proteins of LOU origin: IR-162 (IgE) and IR-418 (IgG2a). Antibodies to IR-162 were easily obtained after a limited number of immunizations with protein in soluble form; polymerization with glutaraldehyde did not enhance immunogenicity. Antibodies to IR-418 appeared only after a large number of immunizations with protein in polymerized form or with protein copolymerized with rabbit IgG. All of the antibodies, either to IR-162 or to IR-418, were found to be idiotype-specific. In every case for which significant levels of antiidiotypic antibodies were produced, most or all of the antibodies belonged to rat IgG1 subclass. Since, in mice, antiidiotypic antibodies are restricted to the IgG1 subclass, our results indicate a functional analogy between rat and mouse IgG1. Our studies also suggest that the rat IgG1 subclass may be predominantly expressed in T-cell-dependent antibody responses, such as production of antiidiotypic antibodies.

Idiotype connectance in the immune system. I. Expression of a cross-reactive idiotype on induced anti-p-azophenylarsonate antibodies and on endogenous antibodies not specific for arsonate

A new cross-reactive idiotope family (CRIAD8) is described that contains subpopulations of antibodies binding to different epitopes. One subpopulation occurs naturally in normal sera from strain A mice, is found mainly on IgG2 and IgG3 subclasses, does not bind p-azobenzenearsonate (ABA)+, does not express CRI5Ci, and can be selectively stimulated by low doses of antiidiotype antibody (AD8). The second subpopulation is not found in normal serum, binds ABA, is found on all IgG subclasses, expresses CRI5Ci, and is selectively stimulated by ABA-conjugated proteins. Since CRIAD8 was found on both subpopulations of antibody, and since each subpopulation could be selectively expanded, it was possible to study the effect that expansion of the ABA- CRIAD8+ set had on subsequent responses elicited by ABA-keyhole limpet hemocyanin (KLH) in the ABA+ CRIAD8+ set. In these experiments, prior immunization with AD8 restricted the subsequent response of the ABA+ CRIAD8+ set to ABA-KLH. Furthermore, only those doses of AD8 that stimulated the ABA-CRIAD8+ set reduced the responsiveness of the ABA+ CRIAD8+ set to ABA-KLH, suggesting that the two phenomena are causally related. These findings argue that CRIAD8 correlates well with a regulatory idiotope and that immune responses by lymphocyte clones that have different antigen-binding specificities can affect one another as a result of their sharing such an idiotope. These results strongly favor a network organization of the immune system.

Comparative study of the conformational features of rat immunoglobulin G subclasses by circular dichroism

Four subclasses of rat immunoglobulin G and some of their fragments were studied by circular dichroism. These studies were carried out in different conditions in order to determine whether conformational differences exist between each subclass. The results showed that in their native state the subclasses cannot be distinguished by characteristic differences. Moreover, differences between monoclonal immunoglobulins G of the same subclass were found, particularly in the 200-250 nm region, and their relationship to aromatic side chain contributions (especially those of tyrosine) has been investigated. However, the conformations of the immunoglobulin G were found to be differently affected by denaturing treatments and a relation to subclass has been demonstrated.