Correlation of antibody multireactivity with variable region primary structure among murine anti-erythrocyte autoantibodies

Plasma Cell Differentiation Pathways in Systemic Lupus Erythematosus

Plasma cells (PCs) are responsible for the production of protective antibodies against infectious agents but they also produce pathogenic antibodies in autoimmune diseases, such as systemic lupus erythematosus (SLE). Traditionally, high affinity IgG autoantibodies are thought to arise through germinal center (GC) responses. However, class switching and somatic hypermutation can occur in extrafollicular (EF) locations, and this pathway has also been implicated in SLE. The pathway from which PCs originate may determine several characteristics, such as PC lifespan and sensitivity to therapeutics. Although both GC and EF responses have been implicated in SLE, we hypothesize that one of these pathways dominates in each individual patient and genetic risk factors may drive this predominance. While it will be important to distinguish polymorphisms that contribute to a GC-driven or EF B cell response to develop targeted treatments, the challenge will be not only to identify the differentiation pathway but the molecular mechanisms involved. In B cells, this task is complicated by the cross-talk between the B cell receptor, toll-like receptors (TLR), and cytokine signaling molecules, which contribute to both GC and EF responses. While risk variants that affect the function of dendritic cells and T follicular helper cells are likely to primarily influence GC responses, it will be important to discover whether some risk variants in the interferon and TLR pathways preferentially influence EF responses. Identifying the pathways of autoreactive PC differentiation in SLE may help us to understand patient heterogeneity and thereby guide precision therapy.

Emerging Functions of Natural IgM and Its Fc Receptor FCMR in Immune Homeostasis

Most natural IgM antibodies are encoded by germline Ig sequences and are produced in large quantities by both mice and humans in the absence of intentional immunization. Natural IgM are reactive with many conserved epitopes, including those shared by microorganisms and autoantigens. As a result, these antibodies play important roles in clearing intruding pathogens, as well as apoptotic/necrotic cells and otherwise damaged tissues. While natural IgM binds to target structures with low affinity due to a lack of significant selection by somatic hypermutation, its pentameric structure with 10 antigen-binding sites enables these antibodies to bind multivalent target antigens with high avidity. Opsonization of antigen complexed with IgM is mediated by cell surface Fc receptors. While the existence of Fc alpha/mu receptor has been known for some time, only recently has the Fc receptor specific for IgM (FCMR) been identified. In this review, we focus on our current understandings of how natural IgM and FCMR regulate the immune system and maintain homeostasis under physiological and pathological conditions.

High throughput monoclonal antibody generation by immunizing multiple antigens

Recognizing proteins via the production of highly specific monoclonal antibodies (mAbs) is crucial to identifying proteins for proteomic research. However, traditional mAb generation is time-consuming with low efficiency. In this study, we assessed the high throughput method of producing mAbs by immunizing mice with multiple antigens in order to obtain hybridomas against these multiple antigens in one cell fusion. We selected eight proteins that play important roles in human physiological or pathological processes. These proteins were mixed and simultaneously administered to one mouse. We observed the immunizing period for 10 d, and determined the effect of liquid medium and semi-solid medium in hybridoma generation. As a result, all eight immunogens induced antibodies in the immunized mouse in one cell fusion, we obtained hybridomas specific to all eight proteins by enzyme-linked immuno sorbent assay (ELISA) screening, hybridomas against five out of eight showed specific positive in Western-blotting assays. This indicates that we generated mAbs specific to eight proteins in one cell fusion, greatly increasing the efficiency of mAb generation. Furthermore, we observed that hybridomas selected from the liquid medium and semi-solid medium showed different reactivity to antigens. Our study established high-throughput and time-saving methods for production of mAbs. These results provide alternative approaches for increasing the efficacy of mAb generation.

Structural plasticity and the evolution of antibody affinity and specificity

The germline precursor to the ferrochelatase antibody 7G12 was found to bind the polyether jeffamine in addition to its cognate hapten N-methylmesoporphyrin. A comparison of the X-ray crystal structures of the ligand-free germline Fab and its complex with either hapten or jeffamine reveals that the germline antibody undergoes significant conformational changes upon the binding of these two structurally distinct ligands, which lead to increased antibody-ligand complementarity. The five somatic mutations introduced during affinity maturation lead to enhanced binding affinity for hapten and a loss in affinity for jeffamine. Moreover, a comparison of the crystal structures of the germline and affinity-matured antibodies reveals that somatic mutations not only fix the optimal binding site conformation for the hapten, but also introduce interactions that interfere with the binding of non-hapten molecules. The structural plasticity of this germline antibody and the structural effects of the somatic mutations that result in enhanced affinity and specificity for hapten likely represent general mechanisms used by the immune response, and perhaps primitive proteins, to evolve high affinity, selective receptors for so many distinct chemical structures.

Antibodies against Trypanosoma cruzi alkaline antigens are elicited in sera from acute but not chronic human chagasic patients

The aim of this work was to study the antibody response of acute and chronic chagasic patients against a Trypanosoma cruzi alkaline fraction (FI) in comparison with the reactivity against a T. cruzi acidic antigen, the main cystein proteinase of the parasite named cruzipain, and "natural" antigens. FI-specific antibodies were detected only during the acute phase of the infection and IgM was the main isotype produced, whereas cruzipain-specific antibodies were detected during all phases of the infection. By means of immunoblot and sequencing analysis we identified a 47-kDa FI proteic band recognized by IgM from acute chagasic patients as the T. cruzi glutamate dehydrogenase (GluDH). Furthermore, the antibody response against isolated GluDH showed similar characteristics as the one against FI. We also observed a strict association between the reactivity of IgM against FI and GluDH and IgM natural antibodies. However, reactivity against these alkaline antigens was not modified after absorption of natural antibodies in sera from acute chagasic patients, indicating that these parasite antigens are not recognized by the polyspecific natural antibodies. The most important goal of this report is that for the first time the T. cruzi antigen isoelectric point has been associated with its ability to trigger immunological memory, raising a novel antigen property that should be considered in the selection of antigens used in Chagas' disease diagnostic test and in the design of a vaccine against T. cruzi infection.

Production of IgM Hexamers by Normal and Autoimmune B Cells: Implications for the Physiologic Role of Hexameric IgM

Secreted IgM is predominantly found as pentameric molecules, but IgM can also be secreted as hexamers by B cell lines. Murine hexamers activate the complement cascade more efficiently than pentamers, but the physiologic significance of hexameric IgM remains unknown. Here, we report that IgM hexamers and pentamers are cleared from the circulation with similar kinetics, suggesting that the predominance of pentameric IgM in vivo reflects the regulation of polymer assembly and secretion in responding B cells. Normal IgM-secreting B cells, particularly those from the peritoneal cavity, are capable of secreting abundant hexameric IgM in vitro. The disparity between the ability of B cells to secrete IgM hexamers in vitro and the paucity of this polymer in vivo suggest that IgM hexamers might be deleterious. In support of this, we demonstrate that the autoantibodies from a number of patients with cold agglutinin (CA) disease include both IgM hexamers and pentamers. The CA IgM hexamers lyse human erythrocytes in the presence of human complement more efficiently than CA IgM pentamers, suggesting a potential role for hexameric IgM in the pathogenesis of this autoimmune syndrome.

Human monoclonal antibodies encoded by the V4-34 gene segment show cold agglutinin activity and variable multireactivity which correlates with the predicted charge of the heavy-chain variable region

We have characterized the reactivities of a panel of V4-34-encoded human IgM monoclonal antibodies (mAb) which bind the erythrocyte Rh D antigen, derived from an immunized individual. These were compared with the specificities of V4-34-encoded autoantibodies with I/i reactivity produced from patients with cold agglutinin disease (CAD), and other V4-34-encoded autoantibodies. The antibodies were evaluated for cold agglutinin activity using haemagglutination tests, immunofluorescence microscopy for reactivity with tissue components, and in solid phase radiobinding assays with purified antigens. We found that (i) cold agglutinin activity was a property of all the V4-34-encoded mAb (ii) the cold agglutinins from CAD patients were generally monospecific for I/i whereas most of the anti-D and the other V4-34-encoded mAb displayed multireactive properties, frequently binding to strongly acidic antigens (iii) computation of the net charge of the heavy-chain V regions showed that the multireactive mAb were generally more positively charged than the monospecific cold agglutinins, which could contribute to their multireactive phenotype. The involvement of charge interactions was further indicated by the effects of pH and ionic strength on the immunofluorescence staining patterns.

Inhibition of murine B1 lymphocytes by interleukin-12

B1 cells are a subset of B lymphocytes found in many species and are implicated in the development of autoimmunity. B1 cells have previously been shown to be suppressed by the T helper (Th)1 cytokine interferon (IFN)-gamma, and to be stimulated by the Th2 cytokines interleukin (IL)-2, IL-4, IL-5 and IL-10. To examine further the interactions of B1 cells and Th1 cells, we have now tested the effects of the Th1 cell-inducing cytokine IL-12 on murine B1 cells. BALB/c mice were immunized with phosphorylcholine conjugated to keyhole limpet hemocyanin (PC-KLH) and simultaneously treated with 1 microgram recombinant murine IL-12 for 3 consecutive days. In addition to altering the isotype and idiotype distribution of anti-PC antibodies, IL-12 treatment was found to cause a loss of peritoneal, but not splenic B lymphocytes in immunized mice. B cell depletion required exposure to IL-12 plus antigenic stimulation. Levels of peritoneal B lymphocytes were fully restored by day 45, but the majority of these cells belonged to the B2 subset. Additionally, proliferation of B1 cells in vitro induced by IL-5 was substantially inhibited by IL-12. IL-12 itself had no effect on viable cell recovery of peritoneal cells (PeC) cultured in vitro, but viable cell recovery was significantly decreased in PeC cultured with IL-5 plus IL-12. These results show that IL-12 causes the loss of murine peritoneal B1 cells and suggest that treatment with this cytokine may be useful for disease conditions that involve B1 cell dysfunction.

Chagas' disease: polyspecificity of antibodies against Trypanosoma cruzi acidic antigens

The aim of this work was to study the reactivity of chagasic patient sera against a panel of natural antigens and its relationship with the immune response against T. cruzi acidic antigens. The presence of IgG and IgM antibodies reactive with myosin, myoglobin, actin and thyroglobulin was investigated in sera with positive serology for Chagas' disease classified into groups (G) I, n = 7, with normal electrocardiogram (ECG) and no signs or symptoms of the disease; GII, n = 7, with ECG abnormalities but without cardiomegaly and GIII, n = 7, with cardiomegaly and congestive heart failure. Healthy individual sera were analyzed in parallel as controls. In the three groups of chagasic patients, a high proportion of sera exhibited an enhancement of IgG response anti actin ranging from 71 to 100%. IgM against this antigen was found positive in GI, 21%; GII and GIII, 57%. The antibodies binding to myosin and myoglobin were mainly of IgM type. When myosin was assayed, the frequency of reactive sera was gradually diminished as heart involvement increased: GI 57%, GII 28% and GIII 14%. Only IgG antibodies against thyroglobulin were detected in the three groups of chagasic patients ranging from 43 to 86%. IgG natural antibodies showed to be polyreactive, since a diminished reactivity against each one of the natural antigens assayed and against T. cruzi acidic antigens (FIV) was observed in the sera absorbed with any of the selected antigens irrespective of the absorbing ones. Moreover, the antibodies against FIV parasite's antigens purified by immunoabsorption showed a similar reactivity with FIV, myosin and actin, and a slight lower reactivity with thyroglobulin. These results indicate that in chagasic patients, the specific humoral response against FIV is associated with an increase of the natural autoantibodies along with their polyspecificity.

Reactivity and structure of a mouse anti-F(ab')2 IgM. Comparison of its variable region sequences with those of a structurally close polyreactive natural IgM

IE12 is a monoclonal IgM with strong anti-F(ab')2 activity that inhibits the binding of normal mouse IgG to self antigens. In this study, we found that this IgM was also reactive with several monoclonal antibodies (mAbs), myeloma proteins and B lymphocytes from normal BALB/c mouse. The nucleotide sequences of the variable region of the heavy and light chains of IE12 were determined, and compared to those of another mAb already described in the literature. This mAb uses the same light chain and also the same VH, D and JH segments, but unlike IE12, is polyreactive. The comparison of the amino acid composition of these two mAbs and of the computer predictions for their structure and hydrophilicity indicated that the most striking difference between them was located in the third complementarity determining region (CDR3) of the heavy chain. Indeed, they used the same D segment but translated in two different reading frames, leading to different amino acid compositions. The CDR3 of IE12 contains aliphatic amino acids, while that of the polyreactive IgM does not. In addition, IE12 has two prolines, one at each at each extremity of its D segment, that could confer a certain rigidity to this region. Finally, the CDR3 of IE12 is predicted to be hydrophobic, while the one of the polyreactive IgM is predicted to be hydrophilic and more flexible, suggesting that the hydrophilicity and the flexibility of this region might be critical for polyreactivity.

Restricted kappa chain expression in early ontogeny: biased utilization of V kappa exons and preferential V kappa-J kappa recombinations

To determine the extent of kappa chain diversity in the preimmune repertoire early in development, kappa cDNA libraries were analyzed from 15-d old fetal omentum, 18-d-old fetal liver, and 3-wk old bone marrow. An anchored polymerase chain reaction approach was used to avoid bias for particular V kappa families. From the sequence analysis of 27 bone marrow clones, 10 different families and 20 unique V kappa genes were identified. In contrast, the V kappa expression in the fetus is highly restricted and clearly differs from the broader distribution see in 3-wk-old bone marrow. Although several V kappa families were represented in the fetal library including V kappa 9, V kappa 10, V kappa 4,5, V kappa 8, and V kappa 1, one or two members of individual families were observed repeatedly. The fetal liver and omentum libraries were found to be largely overlapping. Given the V kappa families/exons identified in the fetal sequences, the mechanism of kappa rearrangements in the early repertoire appears to occur predominantly by inversion. Importantly, the fetal repertoire was further restricted by dominant V kappa-J kappa combinations such as V kappa 4,5-J kappa 5, V kappa 9-J kappa 4, and V kappa 10-J kappa 1. Since in some cases independent rearrangements could be established, the results indicate a bias for particular V kappa-J kappa joins. The results also suggest that clonal expansion/selection in the fetal repertoire takes place after light chain rearrangement as opposed to at the pre-B cell level in the bone marrow. The restriction observed in kappa light chain expression together with known restrictions in gene usage and junctional diversity at the heavy chain level indicate a remarkably conserved fetal repertoire.