Identification of rare immunoglobulin switch variants using the ELISA spot assay

CD81 is a novel immunotherapeutic target for B cell lymphoma

The anti-CD81 mAb (5A6) eliminates lymphoma tumor cells from patient follicular biopsy specimens while sparing the imbedded normal B and T lymphocytes. It has equivalent therapeutic effects as rituximab against a xenografted human B cell lymphoma.

The tetraspanin CD81 was initially discovered by screening mAbs elicited against a human B cell lymphoma for their direct antiproliferative effects. We now show that 5A6, one of the mAbs that target CD81, has therapeutic potential. This antibody inhibits the growth of B cell lymphoma in a xenograft model as effectively as rituximab, which is a standard treatment for B cell lymphoma. Importantly, unlike rituximab, which depletes normal as well as malignant B cells, 5A6 selectively kills human lymphoma cells from fresh biopsy specimens while sparing the normal lymphoid cells in the tumor microenvironment. The 5A6 antibody showed a good safety profile when administered to a mouse transgenic for human CD81. Taken together, these data provide the rationale for the development of the 5A6 mAb and its humanized derivatives as a novel treatment against B cell lymphoma.

Antibody Competition Reveals Surface Location of HPV L2 Minor Capsid Protein Residues 17-36

The currently available nonavalent human papillomavirus (HPV) vaccine exploits the highly antigenic L1 major capsid protein to promote high-titer neutralizing antibodies, but is limited to the HPV types included in the vaccine since the responses are highly type-specific. The limited cross-protection offered by the L1 virus-like particle (VLP) vaccine warrants further investigation into cross-protective L2 epitopes. The L2 proteins are yet to be fully characterized as to their precise placement in the virion. Adding to the difficulties in localizing L2, studies have suggested that L2 epitopes are not well exposed on the surface of the mature capsid prior to cellular engagement. Using a series of competition assays between previously mapped anti-L1 monoclonal antibodies (mAbs) (H16.V5, H16.U4 and H16.7E) and novel anti-L2 mAbs, we probed the capsid surface for the location of an L2 epitope (aa17-36). The previously characterized L1 epitopes together with our competition data is consistent with a proposed L2 epitope within the canyons of pentavalent capsomers.

CD81 as a tumor target

CD81 participates in a variety of important cellular processes such as membrane organization, protein trafficking, cellular fusion and cell-cell interactions. In the immune system, CD81 regulates immune synapse, receptor clustering and signaling; it also mediates adaptive and innate immune suppression. CD81 is a gateway in hepatocytes for pathogens such as hepatitis C virus and Plasmodium; it also confers susceptibility to Listeria infection. These diverse biological roles are due to the tendency of CD81 to associate with other tetraspanins and with cell-specific partner proteins, which provide the cells with a signaling platform. CD81 has also been shown to regulate cell migration and invasion, and has therefore been implicated in cancer progression. Indeed, we have recently shown that CD81 contributes to tumor growth and metastasis. CD81 is expressed in most types of cancer, including breast, lung, prostate, melanoma, brain cancer and lymphoma, and the overexpression or down-regulation of this molecule has been correlated with either good or bad prognosis. Here, we discuss the role of CD81 in cancer and its potential therapeutic use as a tumor target.

Isotype switching increases efficacy of antibody protection against staphylococcal enterotoxin B-induced lethal shock and Staphylococcus aureus sepsis in mice

Staphylococcal enterotoxin B (SEB) is a potent toxin that is produced by Staphylococcus aureus strains and is classified as a category B select agent. We have previously shown that monoclonal antibody (MAb) 20B1, a murine anti-SEB IgG1, successfully treats SEB-induced lethal shock (SEBILS) and bacteremia that is caused by SEB-producing S. aureus. In this study, we have generated two isotype switch variants of the original IgG1 MAb 20B1, an IgG2a and IgG2b, both bearing the same variable region sequence, and compared their neutralizing and protective activity in in vitro and in vivo assays, respectively. All 3 isotypes demonstrated comparable affinity to SEB and comparable 50% inhibitory concentrations (IC50s) in T cell proliferation assays. In vivo, however, the IgG2a isotype variant of 20B1 exhibited significantly greater protection than IgG1 or IgG2b in murine SEB intoxication and S. aureus sepsis models. Protection was associated with downmodulation of inflammatory host response. Our data demonstrate that changing the isotype of already protective MAbs, without affecting their antigen specificity or sensitivity, can result in an enhancement of their protective ability. Isotype selection, therefore, should be carefully considered in the development of toxin-neutralizing MAbs and the design of antibody therapeutics.

IMPORTANCE

The purpose of this study was to enhance the protective efficacy of an existing, protective monoclonal antibody against staphylococcal enterotoxin B. Using two in vivo mouse models, our study demonstrates that the protective efficacy of a monoclonal antibody may be improved by inducing an isotype switch at the Fc region of an antibody, without altering the antigen specificity or sensitivity of the antibody. The development of therapeutic MAbs with higher efficacy may allow for the achievement of equal therapeutic benefit with a lower dosage. In turn, the use of lower doses may reduce the cost of these therapies, while reducing the potential for adverse side effects.

Variable region identical immunoglobulins differing in isotype express different paratopes

The finding that the antibody (Ab) constant (C) region can influence fine specificity suggests that isotype switching contributes to the generation of Ab diversity and idiotype restriction. Despite the centrality of this observation for diverse immunological effects such as vaccine responses, isotype-restricted antibody responses, and the origin of primary and secondary responses, the molecular mechanism(s) responsible for this phenomenon are not understood. In this study, we have taken a novel approach to the problem by probing the paratope with (15)N label peptide mimetics followed by NMR spectroscopy and fluorescence emission spectroscopy. Specifically, we have explored the hypothesis that the C region imposes conformational constraints on the variable (V) region to affect paratope structure in a V region identical IgG(1), IgG(2a), IgG(2b), and IgG(3) mAbs. The results reveal isotype-related differences in fluorescence emission spectroscopy and temperature-related differences in binding and cleavage of a peptide mimetic. We conclude that the C region can modify the V region structure to alter the Ab paratope, thus providing an explanation for how isotype can affect Ab specificity.

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.

A requirement for FcγR in antibody-mediated bacterial toxin neutralization

Constant regions of antibodies influence toxin neutralization in a manner dependent on FcγR.

One important function of humoral immunity is toxin neutralization. The current view posits that neutralization results from antibody-mediated interference with the binding of toxins to their targets, a phenomenon viewed as dependent only on antibody specificity. To investigate the role of antibody constant region function in toxin neutralization, we generated IgG2a and IgG2b variants of the Bacillus anthracis protective antigen–binding IgG1 monoclonal antibody (mAb) 19D9. These antibodies express identical variable regions and display the same specificity. The efficacy of antibody-mediated neutralization was IgG2a > IgG2b > IgG1, and neutralization activity required competent Fcγ receptor (FcγR). The IgG2a mAb prevented lethal toxin cell killing and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase cleavage more efficiently than the IgG1 mAb. Passive immunization with IgG1 and IgG2a mAb protected wild-type mice, but not FcγR-deficient mice, against B. anthracis infection. These results establish that constant region isotype influences toxin neutralization efficacy of certain antibodies through a mechanism that requires engagement of FcγR. These findings highlight a new parameter for evaluating vaccine responses and the possibility of harnessing optimal FcγR interactions in the design of passive immunization strategies.

Circular Dichroism reveals evidence of coupling between immunoglobulin constant and variable region secondary structure

Antibodies (Ab) are bifunctional molecules with two domains, a constant region (C) that confers effector properties and a variable (V) region responsible of antigen (Ag) binding. Historically the C and V regions were considered to be functionally independent, with Ag specificity being solely determined by the V region. However, recent studies suggest that the C region can affect Ab fine specificity. This has led to the proposal that the C(H) domain influences the structure of the V region, thus affecting Ab affinity and fine specificity. An inference from this proposal is that V region identical monoclonal Abs (mAbs) differing in C region (eg isotype) would manifest different secondary structures arising from isotype-induced variation in the V-C regions after Ag binding. We hypothesized that such effects could translate into differences in Circular Dichroism (CD) upon Ag-Ab complexes formation. Consequently we studied the interaction of a set of V region identical IgG(1), IgG(2a), IgG(2b) and IgG(3) mAbs with glucuronoxylomannan (GXM). The native CD spectra of the pairs IgG(1)/IgG(2a) and IgG(3)/IgG(2b) were strikingly similar, implying similar secondary structure content. GXM binding by IgG(1), IgG(2a), IgG(2b) and IgG(3) produced different CD changes, with the pairs IgG(1)/IgG(2a) and IgG(3)/IgG(2b) again manifesting qualitatively similar trends in secondary structure changes. The magnitude of the changes differed among the isotypes with IgG(2a)>IgG(3)>IgG(2b)>IgG(1). These differences in CD changes were interpreted to reflect differences in V-C secondary structures.

Molecular characterization of hybridoma subclones spontaneously switching at high frequencies in vitro

The hybridoma technology allows the production of large quantities of specific antibodies of a single isotype. Since different isotypes have special effector functions and are distributed distinctively throughout the body, it is often useful to have a library of switch variants from the original monoclonal antibody. We have shown previously that forced expression of activation induced cytidine deaminase (AID) in hybridomas increased their very low frequency of class switch recombination (CSR) in vitro only approximately 7-13 fold. Since we had previously identified rare hybridoma subclones that spontaneously switched at more than 100 times higher frequencies, we have now examined those higher switching variants to search for ways to further increase the frequency of isotype switching in vitro. AID was not responsible for the approximately 100 fold increase in CSR, so we used whole-genome gene expression profiling to provide a platform for studying candidate molecular pathways underlying spontaneous CSR in hybridomas.

Isothermal titration calorimetry reveals differential binding thermodynamics of variable region-identical antibodies differing in constant region for a univalent ligand

The classical view of immunoglobulin molecules posits two functional domains defined by the variable (V) and constant (C) regions, which are responsible for antigen binding and antibody effector functions, respectively. These two domains are thought to function independently. However, several lines of evidence strongly suggest that C region domains can affect the specificity and affinity of an antibody for its antigen (Ag), independent of avidity-type effects. In this study, we used isothermal titration calorimetry to investigate the thermodynamic properties of the interactions of four V region-identical monoclonal antibodies with a univalent peptide antigen. Comparison of the binding of IgG1, IgG2a, IgG2b, and IgG3 with a 12-mer peptide mimetic of Cryptococcus neoformans polysaccharide revealed a stoichiometry of 1.9-2.0 with significant differences in thermodynamic binding parameters. Binding of this peptide to the antibodies was dominated by favorable entropy. The interaction of these antibodies with biotinylated peptides manifested greater enthalpy than for native peptides indicating that biotin labeling affected the types of Ag-Ab complexes formed. Our results provide unambiguous thermodynamic evidence for the notion that the C region can affect the interaction of the V region with an Ag.

The immunoglobulin heavy chain constant region affects kinetic and thermodynamic parameters of antibody variable region interactions with antigen

A central dogma in immunology is that antibody specificity is a function of the variable (V) region. However serological analysis of IgG(1), IgG(2a), and IgG(2b) switch variants of murine monoclonal antibody (mAb) 3E5 IgG(3) with identical V domains revealed apparent specificity differences for Cryptococcus neoformans glucuronoxylomannan (GXM). Kinetic and thermodynamic binding properties of mAbs 3E5 to a 12-mer peptide mimetic of GXM revealed differences in the affinity of these mAbs for a monovalent ligand, a result that implied that the constant (C) region affects the secondary structure of the antigen binding site, thus accounting for variations in specificity. Structural models of mAbs 3E5 suggested that isotype-related differences in binding resulted from amino acid sequence polymorphisms in the C region. This study implies that isotype switching is another mechanism for generating diversity in antigen binding and that isotype restriction of certain antibody responses may reflect structural constraints imposed by C region on V region binding. Furthermore, isotype affected the polyreactivity of V region identical antibodies, implying a role for C region in determining self-reactivity.

Forced expression of AID facilitates the isolation of class switch variants from hybridoma cells

Monoclonal antibodies are used in the treatment and diagnosis of diseases and to study the protective and adverse functions of antibodies in vitro and in vivo. Since the isotype determines the effector function, half-life in the serum and distribution throughout the body, it would be useful to have a battery of antibodies with the same binding site associated with different isotypes. However, since hybridomas switch isotypes at very low frequencies in tissue culture, it has been difficult and very labor intensive to isolate panels of class switch variants. We show here that stable transfection of activation-induced cytidine deaminase (AID) in hybridomas increased their frequency of switching to a level that greatly facilitated the isolation of subclones expressing monoclonal antibodies of different isotypes. Although forced expression of AID also increased the frequency of somatic hypermutation in the immunoglobulin variable regions that encode the antigen binding site, antigen recognition was retained in the isotype switched antibodies.

Variable-region-identical antibodies differing in isotype demonstrate differences in fine specificity and idiotype

A central tenet of the current understanding of the relationship between Ab structure and function is that the variable region domain is solely responsible for Ag specificity. However, this view was recently challenged by the observation that families of mouse-human chimeric Abs with identical V regions demonstrate differences in fine specificity and by reports of changes in Ab Id structure with isotype switching. Here we revisited this question by evaluating the reactivity of two families of murine IgG switch variants that differed in V region usage for Cryptococcus neoformans glucuronoxylomannan, glucuronoxylomannan peptide mimetics, and anti-Id mAbs. The results reveal isotype-related differences in fine specificities and Id for two mAb isotype switched families, thus establishing the validity of this observation with sets of homologous Abs. The results suggest that the C region affects V region protein conformation, leading to differences in fine specificity and Id. The finding that isotype can affect fine specificity has major implications for current concepts of the generation of secondary responses, idiotypic network regulation, and isotype function. Given that isotype class switching and Ig gene somatic hypermutation share molecular mechanisms, these observations unify these processes in the sense that both can alter specificity and affinity.

IgA autoimmune disorders: development of a passive transfer mouse model

IgA is present in the skin in several dermatoses, including dermatitis herpetiformis, linear IgA bullous dermatosis, and Henoch-Schoenlein purpura. The neutrophilic infiltration in the area of the IgA deposition suggests that IgA is responsible for the associated inflammatory events. The mechanism for this process is unproven, but is likely to involve IgA-mediated neutrophil chemotaxis with inhibition of chemotaxis by dapsone. Elucidation of the mechanism of IgA-mediated inflammation will require an animal model. We have established a model for linear IgA bullous dermatosis as a prototype disease to be studied. IgA mouse monoclonal antibodies against a linear IgA bullous dermatosis antigen have been passively transferred to SCID mice with human skin grafts. This has produced neutrophil infiltration and basement membrane vesiculation in 4 of 12 mice tested. We conclude that an animal model for the pathogenesis of IgA dermatoses with IgA deposition and inflammation can be produced by passive transfer of mouse IgA antibodies against a linear IgA antigen.

Isolation of Rare Isotype Switch Variants in Hybridoma Cell Lines Using an Agarose Gel Microdrop-Based Protein Secretion Assay

Using gel microdrop (GMD) encapsulation technology and fluorescence-activated cell storing (FACS), we have developed a rapid, sensitive, and reliable method for discriminating and recovering rare isotypic switch variants in hybridoma cell lines. Using the GMD-based IgSwitch assay, a novel approach for isolating subpopulations of IgG-secreting hybridoma cells present at a frequency of approximately 1-10 in 10(6), we successfully isolated spontaneous and in vitro-induced isotypic switch variants in less than half the time required for conventional sublining. The effectiveness and specificity of the assay are demonstrated.

The use of in vitro immunisation, as an adjunct to monoclonal antibody production, may result in the production of hybridomas secreting polyreactive antibodies

The possibility that immortalisation of in vitro immunised splenocytes may result in hybridomas secreting polyreactive antibodies was investigated. A panel of nine murine hybridomas, secreting IgM(kappa) anti-goat immunoglobulin G (anti-GIgG), was produced by immortalising splenocytes that had been immunised in vitro with GIgG. The ability of the corresponding monoclonal antibodies (Mabs) to bind multiple antigens was investigated using two techniques. First, the affinity constants characterising the interactions of each of the nine Mabs with each of a panel of six antigens were determined. Second, the specific anti-GIgG activities of each hybridoma supernatant and its corresponding affinity-purified IgM fraction were determined and compared. In total, these experiments indicated that eight of the nine hybridomas were polyreactive.

Both Th1 and Th2 cytokines affect the ability of monoclonal antibodies to protect mice against Cryptococcus neoformans

Variable-region-identical mouse immunoglobulin G1 (IgG1), IgG2b, and IgG2a monoclonal antibodies to the capsular polysaccharide of Cryptococcus neoformans prolong the lives of mice infected with this fungus, while IgG3 is either not protective or enhances infection. CD4+ T cells are required for IgG1-mediated protection, and CD8+ T cells are required for IgG3-mediated enhancement. Gamma interferon is required for both effects. These findings revealed that T cells and cytokines play a role in the modulation of cryptococcal infection by antibodies and suggested that it was important to more fully define the cytokine requirements of each of the antibody isotypes. We therefore investigated the efficacy of passively administered variable-region-identical IgG1, IgG2a, IgG2b, and IgG3 monoclonal antibodies against intravenous infection with C. neoformans in mice genetically deficient in interleukin-12 (IL-12), IL-6, IL-4, or IL-10, as well as in the parental C57BL/6J strain. The relative inherent susceptibilities of these mouse strains to C. neoformans were as follows: IL-12(-/-) > IL-6(-/-) > C57BL/6J approximately IL-4(-/-) >> IL-10(-/-). This is consistent with the notion that a Th1 response is necessary for natural immunity against cryptococcal infection. However, none of the IgG isotypes prolonged survival in IL-12(-/-), IL-6(-/-), or IL-4(-/-) mice, and all isotypes significantly enhanced infection in IL-10(-/-) mice. These results indicate that passive antibody-mediated protection against C. neoformans requires both Th1- and Th2-associated cytokines and reveal the complexity of the mechanisms through which antibodies modulate infection with this organism.

Expression of a Mycobacterium tuberculosis arabinomannan antigen in vitro and in vivo

The outermost layer of Mycobacterium tuberculosis contains two major polysaccharides, arabinomannan (AM) and glucan (GC). We studied the in vitro and in vivo expression of an M. tuberculosis AM antigen using monoclonal antibody (MAb) 9d8 (2a), an isotype-switched variant of the immunoglobulin G3 (IgG3) MAb 9d8. MAb 9d8 had been previously shown to bind M. tuberculosis AM and the M. tuberculosis surface. Our in vitro experiments showed that MAb 9d8(2a) bound strongly to whole-cell M. tuberculosis Erdman but not to the CDC 1551 strain grown in medium for an extended period. However, AM antigen was detected in the culture supernatant of both strains, and its concentration increased in a time-dependent manner. The detection of AM antigen from both strains was decreased in the presence of Tween 80. In mice infected with M. tuberculosis Erdman, AM antigen accumulated in organ homogenates concomitant to an increase in bacterial organ burden and an increase in IgG and IgM titer to AM. These results (i) indicate that the surface expression of AM during in vitro growth changes with culture age, is strain dependent, and is affected by the presence of Tween 80 in the culture media; (ii) show that AM is produced by bacteria growth in vivo; and (iii) demonstrate that the amount of in vivo-detected AM can be dependent on the number of bacteria in the infected organ.

Ig Heavy Chain Expression and Class Switching In Vitro from an Allele Lacking the 3′ Enhancers DNase I-Hypersensitive hs3A and hs1,2

The murine Ig heavy chain (IgH) 3′ regulatory region contains four enhancers: hs3A, hs1,2, hs3B, and hs4. Various studies have suggested a role for these enhancers in regulating IgH expression and class switching. Here we assess the role of hs3A and hs1,2 in these processes by exploiting a naturally occurring deletion of these enhancers from the expressed, C57BL/6 allele of the F1 pre-B cell line, 70Z/3. Equivalent μ expression in 70Z/3 and 18-81 (which has an intact 3′ region) indicated that hs3A and hs1,2 were not essential for μ expression at the pre-B cell stage. To further examine the role of hs3A and hs1,2 in IgH function at the plasma cell stage, we fused 70Z/3 with the plasmacytoma NSO. Electromobility shift assay analysis of the 70Z/3-NSO hybrids revealed a transcription factor complement conducive to the activation of the 3′ enhancers. Despite the lack of enhancers, hs3A and hs1,2, the level of μ RNA and protein in the 70Z/3-NSO fusion hybrids was substantially elevated relative to its pre-B parent and comparable with that observed in a number of μ-producing spleen cell hybridomas. Additionally, ELISAspot assays showed that the 70Z/3-NSO hybrid underwent spontaneous class switching in culture to IgG1 at a frequency comparable with that of most hybridomas. These results indicate that hs3A and hs1,2 are not essential for high levels of IgH expression or for spontaneous class switching in a plasma cell line.

Comparison of an anti-HSV-2 monoclonal IgG and its IgA switch variant for topical immunoprotection of the mouse vagina

An IgG2a monoclonal antibody (Mab) directed against glycoprotein D of herpes simplex virus 2 (HSV-2) was compared with an IgA heavy chain Mab switch variant to investigate the effect of isotype for topical immunoprotection of the murine vagina. The IgA Mab, a mixture of monomeric and polymeric IgA, was indistinguishable from its IgG parent in an in vitro HSV-2 neutralization assay. When these class switched Mabs were delivered to the mouse vagina, we also found no significant difference between the IgG and IgA for preventing vaginal transmission of HSV-2 infection. The implications of these results for active and passive immunization strategies against vaginal transmission of genital herpes infections are discussed.

Isotype switching increases efficacy of antibody protection against Cryptococcus neoformans infection in mice

The isotype and epitope specificities of antibodies both contribute to the efficacy of antibodies that mediate immunity to Cryptococcus neoformans, but the relationship between these properties is only partially understood. In this study, we analyzed the efficacy of protection of two sets of immunoglobulin G (IgG) isotype switch variants from two IgG3 monoclonal antibodies (MAbs) which are either not protective or disease enhancing, depending on the mouse model used. The two IgG3 MAbs 3E5 and 4H3 have different epitope specificities. Protection experiments were done with A/JCr mice infected intravenously with C. neoformans and administered with 3E5 IgG3 and its IgG1, IgG2a, and IgG2b switch variants. These experiments revealed that IgG1, IgG2b, and IgG2a were each more effective than IgG3. For 4H3 IgG3 and its IgG1 and IgG2b switch variants, the relative efficacy was IgG2b > IgG1 >> IgG3. The combination of 3E5 IgG3 and 4H3 IgG3 was more deleterious than either IgG3 alone. All IgG isotypes were opsonic for mouse bronchoalveolar cells, with the relative efficacy being IgG2b > IgG2a > IgG1 > IgG3. These results (i) confirm that a nonprotective IgG3 MAb can be converted to a protective MAb by isotype switching, (ii) indicate that the efficacy of protection of an IgG1 MAb can be increased by isotype switching to another subclass, (iii) show that protective and nonprotective IgG MAbs are opsonic, and (iv) provide additional evidence for the concept that the efficacy of the antibody response to C. neoformans is dependent on the type of MAb elicited.

Sequence dependent hypermutation of the immunoglobulin heavy chain in cultured B cells

The variable (V) regions of immunoglobulin heavy and light chains undergo high rates of somatic mutation during the immune response. Although point mutations accumulate throughout the V regions and their immediate flanking sequences, analysis of large numbers of mutations that have arisen in vivo reveal that the triplet AGC appears to be most susceptible to mutation. We have stably transfected B cell lines with gamma2a heavy chain constructs containing TAG nonsense codons in their V regions that are part of either a putative (T)AGC hot spot or a (T)AGA non-hot spot motif. Using an ELISA spot assay to detect revertants and fluctuation analysis to determine rates of mutation, the rate of reversion of the TAG nonsense codon has been determined for different motifs in different parts of the V region. In the NSO plasma cell line, the (T)AGC hot spot motif mutates at rates of approximately 6 x 10(-4)/bp per generation and approximately 3 x 10(-5)/bp per generation at residues 38 and 94 in the V region. At each of these locations, the (T)AGC hot spot motif is 20-30 times more likely to undergo mutation than the (T)AGA non-hot spot motif. Moreover, the AGA non-hot spot motif mutates at as high a rate as the hot spot motif when it is located adjacent to hot spot motifs, suggesting that more extended sequences influence susceptibility to mutation.

T cells cooperate with passive antibody to modify Cryptococcus neoformans infection in mice

Cryptococcus neoformans is an encapsulated fungus that is a major cause of meningitis in patients with AIDS. In immunocompetent mice, administration of IgG1 mAb protects against cryptococcal infection, whereas administration of IgG3 is not protective and can accelerate the infection. In beige mice with impaired natural killer cell function, the effects of IgG1 and IgG3 are similar to those observed in immunocompetent mice, suggesting that natural killer cells are not crucial for antibody-mediated modulation of cryptococcal infection. In mice lacking CD4+ T cells, IgG1 is not protective and IgG3 accelerates infection, indicating that CD4+ T cells are required for antibody-mediated protection. In mice lacking CD8+ T cells, both IgG1 and IgG3 antibodies prolong survival, indicating that acceleration of the disease process by IgG3 involves CD8+ T cells. Both IgG1-mediated protection and IgG3-mediated acceleration of infection require interferon gamma. These results reveal a functional dependence of passively administered antibody on cellular immunity in cryptococcal infection in mice and have implications for antibody-based therapies in humans in the setting of CD4+ lymphopenia.

Monoclonal antibodies to surface antigens of Mycobacterium tuberculosis and their use in a modified enzyme-linked immunosorbent spot assay for detection of mycobacteria

Three monoclonal antibodies (MAbs) were generated from splenocytes of a BALB/c mouse immunized with heat-killed Mycobacterium tuberculosis. All three MAbs bound to surface epitopes of M. tuberculosis as shown by whole-cell enzyme-linked immunosorbent assay (ELISA), indirect immunofluorescence, and immunoelectron microscopy. One immunoglobulin M (IgM) MAb bound to lipoarabinomannan, the second IgM MAb bound to mycolyl-arabinogalactan-peptidoglycan complex, and the third MAb, an IgG3, bound to a surface epitope of an uncertain nature. The MAbs demonstrated different cross-reactivity patterns with other mycobacteria. Two of the MAbs were used to develop a modified ELISA spot assay for the detection of mycobacteria.

Molecular comparison of cultured hybridoma cells that switch isotypes at high and low rates

We have previously reported the isolation of variants from the 36.65 and PC1.4.1 hybridoma cell lines that spontaneously switch from gamma 1 to gamma 2a and gamma 2b at high and low rates. In order to further characterize the phenotype of these variants, we have now investigated the production of germline transcripts and methylation which are two of the molecular correlates of isotype switching. While some of the correlations that exist in normal cells were present in some of the clonal variants, others were not. However, the higher switching variants of both cells lines had higher recombinational activity as measured with a shuttle vector. The distinct phenotypic characteristics of each cell line provide an opportunity to dissect the roles of individual molecular events in the process of isotype switching.

Immunoglobulin variable region hypermutation in hybrids derived from a pre-B- and a myeloma cell line

Somatic mutation of the variable (V) regions of immunoglobulin genes occurs in vivo at rates that have been estimated to be between 10(-3) and 10(-4) per bp per generation. To study this process in vitro, the 18.81 pre-B-cell line and hybrids derived by fusing 18.81 to the NSO myeloma fusion partner were transfected with a mu heavy-chain construct containing a nonsense mutation in the V region (Vn) or the constant region (Cn). Mutation was quantitated by reversion analysis using the ELISA spot assay to detect single cells secreting IgM. Fluctuation analysis revealed that V-region mutations spontaneously occurred in 18.81 cells at an average rate of 5.8 x 10(-6) per bp per cell generation and in selected 18.81-NSO hybrids at greatly increased rates of 1.6 x 10(-3) to 5.8 x 10(-4) per bp per generation. The Vn construct also reverted frequently in transgenic mice, indicating that it contained sufficient information to mutate at high rates both in vivo and in vitro. Sequence analysis of reverted genes revealed that reversion was due to point mutations. Since the rates and nature of the mutations that are occurring in these transfected genes are similar to those reported in vivo, it should be possible to use this system to identify the cis-acting sequences and trans-acting factors that are responsible for V-region somatic hypermutation.

A well-differentiated B-cell line is permissive for somatic mutation of a transfected immunoglobulin heavy-chain gene

pSV2neo plasmids containing an IgM heavy-chain gene with nonsense mutations in either the variable (V) or the constant (C) region were transfected into four differentiated mouse plasma cell lines: S107 and the NSO fusion partner (myeloma cell lines) and 2C3 and 36.65 (hybridoma cell lines). The frequencies of reversion of the nonsense mutations in multiple independent transfectants were determined with the spot ELISA and rates of reversion were calculated by fluctuation analysis. Mutations in both V and C regions were confirmed by sequence analyses. In the S107 cell line, spontaneous point mutations occurred in the V region at a rate of approximately 5 x 10(-5)/bp per cell generation, > 400-fold higher than the rate of V-region mutation in the NSO cell line and considerably higher than the rates in 2C3 and 36.65 hybridoma cell lines. These studies suggest that S107 is a relatively permissive cell line in which V-region mutations can occur constitutively, even though it represents a late stage of B-cell differentiation. Further, the results show that the construct used contains sufficient information in its flanking and coding sequences to allow a relatively high rate of V-region mutation, at least in the S107 cell line.

Use of mutagens to increase rate of immunoglobulin isotype switching of hybridoma cells

Isotype switching of hybridoma clones may be essential when the class of the antibody produced does not suit the task for which it was generated. In those instances immunoglobulin (Ig) switch variants can be isolated in vitro but the success of isolating these rare variants primarily depends on the frequency of switching of each individual hybridoma. Variations in the frequency are noted not only between hybridomas secreting different classes but also between fresh clones isolated from the same hybridoma. Immunoglobulin switch variants may be identified and isolated using the sib selection and the ELISA spot assay; however, when the frequency of switching is low, this may be extremely difficult and sometimes impossible. In the present article we demonstrate that ICR191 may increase the frequency of switching and that these Ig switch antibodies maintain the same antigen specificity and normal-sized heavy chain.

Antibodies to Cryptococcus neoformans glucuronoxylomannan enhance antifungal activity of murine macrophages

Monoclonal antibodies (MAbs) to the capsular polysaccharide of the pathogenic fungus Cryptococcus neoformans can prolong survival and decrease organ fungal burden in experimental murine cryptococcosis. To investigate the mechanism of antibody-mediated protection, the interaction of C. neoformans and murine macrophage-like J774.16 cells was studied in the presence and absence of MAbs differing in isotype. Immunoglobulin G2a (IgG2a) and IgG2b isotype switch variants were isolated from an IgM hybridoma to complete the IgG subclass set. IgM, IgG1, IgG2a, IgG2b, IgG3, and IgA MAbs were studied for their ability to promote phagocytosis and reduce the number of CFU in C. neoformans and J774.16 cell cocultures. The MAbs in this set had similar if not identical fine specificities and were derived from a single B cell. All isotypes promoted phagocytosis; however, the IgG subclasses were more effective opsonins than IgM or IgA. All isotypes enhanced J774.16 anti-C. neoformans activity in vitro, as measured by a reduction in the number of CFU. The IgG1 MAbs were consistently more active in promoting opsonization and reducing the number of CFU. Addition of IgG1 MAb to C. neoformans and J774.16 cocultures resulted in rapid reduction in the number of CFU, which is consistent with fungal killing. Electron microscopy revealed that MAb-opsonized C. neoformans cells were internalized and appeared damaged. Administration of IgM, IgG1, IgG2a, and IgG2b isotype switch variant MAbs revealed that the IgG2a and IgG2b subclasses were the most and least effective isotypes, respectively, in prolonging survival in an intraperitoneal murine infection model. The results indicate that murine antibody subclasses differ in their ability to enhance macrophage anti-C. neoformans activity and suggest that antibody enhancement of macrophage function is a mechanism by which antibodies modify infection in vivo.

Simultaneous expression of kappa and lambda light chains in a murine IgG3 anti-Cryptococcus neoformans hybridoma cell line

Allelic exclusion normally results in the expression of only one light and one heavy chain gene. However, some hybridomas have been reported to express two different heavy chain genes. Here we report that the IgG3 hybridoma 4H3.C8B expresses both kappa and lambda light chains. 4H3.C8B was originally recovered by screening for antibody binding to Cryptococcus neoformans polysaccharide antigen and characterized as gamma 3 lambda. The gamma 3 lambda but not the gamma 3 kappa binds to polysaccharide antigen. Some of the antibody molecules were heterodimers composed of both lambda and kappa. IgG1 and IgG2b isotype switch variants were identified and isolated by the technique of sib selection, using the ELISA spot assay. Like the parent IgG3 line, the switch variants continued to express both kappa and lambda light chains but only the lambda-containing antibodies bound to the antigen. Our experience suggests that hybridomas recovered by assays that are antigen dependent should also be tested for the expression of other isotypes and light chains in a non-antigen-binding immunoassay.

Mouse-human immunoglobulin G1 chimeric antibodies with activities against Cryptococcus neoformans

Passive antibody administration is a potentially useful approach for the therapy of human Cryptococcus neoformans infections. To evaluate the efficacy of the human immunoglobulin G1 (IgG1) constant region against C. neoformans and to construct murine antibody derivatives with reduced immunogenicities and longer half-lives in humans, two mouse-human IgG1 chimeric antibodies were generated from the protective murine monoclonal antibodies 2D10 (IgM) and 18B7 (IgG1). The 2D10 mouse-human IgG1 chimeric antibody (ch2D10) had significantly lower binding affinity than its parent murine antibody (m2D10), presumably because of a loss of avidity contribution on switching from IgM to IgG. The 18B7 mouse-human IgG1 chimeric antibody (ch18B7) had higher affinity for cryptococcal polysaccharide antigen than its parent murine antibody (m18B7). ch18B7 and ch2D10 promoted phagocytosis of C. neoformans by primary human microglial cells and the murine J774.16 macrophage-like cell line. ch18B7 and m18B7 enhanced fungistatic or fungicidal activity of J774.16 cells and prolonged the survival of lethally infected mice. We conclude that the human IgG1 constant chain can be effective in mediating antifungal activity against C. neoformans. ch18B7 or similar antibodies are potential candidates for passive antibody therapy of human cryptococcosis.

Clonal variants of hybridoma cells that switch isotype at a high frequency

As B cells differentiate under the influence of antigen and T cells, they frequently switch from the expression of IgM antibody to the expression of other isotypes. This is accomplished by rearranging the expressed variable region gene to downstream constant region genes and deleting the intervening sequences. Some B-cell lines that represent early stages in development switch constitutively in culture at frequencies that approach those of lipopolysaccharide- or lymphokine-stimulated normal B cells. Hybridoma cells represent a later stage of development and rarely switch in culture. In contrast to early B-cell lines, hybridomas produce large amounts of immunoglobulin, and single cells can be assayed easily for the expression of new isotypes. We have used the ELISA spot assay and fluctuation analysis to determine the rate of switching of two hybridoma cell lines. By identifying subclones that switched more frequently, we have progressively enriched for cells that switch spontaneously at higher rates. These cells, like normal cells, switch by rearrangement and deletion, and the frequency of switched cells in some of the clones is comparable to that which has been observed in less differentiated B-cell lines and in normal B cells.

Monoclonal antibodies to Cryptococcus neoformans capsular polysaccharide modify the course of intravenous infection in mice

Immunoglobulin G1 (IgG1) monoclonal antibodies (MAbs) to the capsular glucuronoxylomannan (GXM) were studied for their ability to modify the course of intravenous Cryptococcus neoformans infection in mice. A/J mice were given intraperitoneal injection of 1.0 mg of either a GXM-binding IgG1 MAb (2H1 or 2D10 gamma 1) or the irrelevant isotype-matched control MAb 36-65 prior to intravenous infection. Parameters used to study antibody efficacy were lung and brain tissue fungal burden, lung and brain weights, serum GXM levels, and histopathological examination of lung, brain, heart, kidney, and spleen tissues. Mice given GXM-binding MAb had significantly reduced lung tissue fungal burden as measured by CFU. In contrast to the reduction in lung tissue burden, the reduction in brain tissue burden was small and did not achieve statistical significance. Serum GXM levels were reduced in mice receiving GXM-binding MAb. Histopathological examination revealed reduced numbers of granulomas and C. neoformans organisms in the lungs, brains, and kidneys of MAb 2H1-treated mice relative to control mice. The lungs and brains of mice receiving GXM-binding MAb weighed significantly less than those of control animals, consistent with the reduced inflammation noted histologically. Subendocardial inflammation and kidney cortical infarctions were present in control infected mice but not in MAb 2H1-treated mice. Immunocytochemical staining for polysaccharide antigen revealed a marked reduction in the amount of tissue polysaccharide in mice treated with MAb 2H1 relative to control mice. The results support an useful role for passive antibody administration in C. neoformans infections.

A simple technique for the rapid enrichment of class and subclass hybridoma switch variants. A 1000-fold enrichment in half the time, for half the cost

Switching parental hybrids in vitro to downstream switch variant clones producing more desirable monoclonal antibodies (MoAbs) requires either labor intensive and time consuming subcloning techniques, or fluorescence activated sorting of the desired clones. We tested the hypothesis that enrichment of downstream switch variant clones might be achieved by selective lysis of upstream hybridoma cells followed by expansion of the enriched downstream clone. Using a parental hybridoma with surface and secretory IgM, we attempted to enrich downstream switch variant clones producing class (IgG) and subclass (IgG1 or IgG2a) MoAbs. Enrichment of downstream IgG, IgG1 and IgG2a MoAb-producing switch variants was achieved by single or repeated antibody-dependent, complement-mediated lysis of the upstream IgM-bearing parental hybridoma cells followed by limited subcloning. Two exposures of parental hybridoma cells to lysis followed by plating at 100 cells/well enriched the frequency of switch variants up to 1235-fold, enabling the development of IgG1 or IgG2a-producing subclones exhibiting high yield antibody production. Using this protocol, production time and costs were reduced by > 50% when compared to the standard technique. This novel technique for the rapid isolation and expansion of switch variant clones should be ideal for most laboratories, particularly those without access to cell sorting capabilities.

The use of chemiluminescence and the ELISA spot assay to identify and enumerate rare immunoglobulin switch variants

The use of chemiluminescence and the ELISA spot assay for identifying rare immunoglobulin switch variants is described. The technique utilizes nitrocellulose membranes and allows rapid screening of a large number of cells. The number of spots can be recorded either manually or automatically by using a commercially available colony counting program. This modification of the ELISA spot assay makes it less labor intensive and time consuming and can be adapted for the search for rare cells secreting small amounts of Ig or other macromolecules.

Quantitation of IgE and carcinoembryonic antigen (CEA) by optical beam deflection (OBD) measurement of dot-immunobinding assay patterns visualized by an ELISA technique

Dot-immunobinding assays of IgE and CEA were performed by a conventional dot-ELISA technique with diaminobenzidine staining, and the quantitative results were compared by densitometry and a new, spectroscopic, optical beam deflection (OBD) method using the same membrane. It was possible with the OBD method to detect quantities of these substances at least ten times smaller than with densitometry. Better intra-assay reproducibility for IgE and CEA measurements was obtained by the OBD method. The measurable ranges of the OBD method was broader than that of densitometry, because dark bands caused OBD in proportion to their color densities. When the dot-immunobinding assay with OBD measurement for CEA was also compared with a microtube ELISA using biotin-avidin conjugates, the sensitivities and reproducibilities of the two methods were found to be similar, with a correlation coefficient of 0.991.

Rapid quality evaluation of hybridomas using ELISPOT and cell-ELISA techniques

We have used the ELISPOT assay in combination with ELISA procedures for rapid evaluation of properties of three different murine hybridoma cell lines, 104-I, -B and -G, secreting IgG1 kappa monoclonal antibodies against a 104-mer synthetic peptide from the C-terminal part of HIV-1 p24. By conventional ELISA we obtained data suggesting that the three monoclonal antibodies had different affinities. By cell-ELISA we found that the IgG1 kappa secretion rate varied between cells (4,000 to 14,000 antibody molecules/cell/min), and ELISPOT showed that only 4-5% of 104-I cells gave antigen-specific spots, indicating a cell population with diverse properties. We recommend that the ELISPOT and cell-ELISA techniques should be used routinely to supplement conventional ELISA procedures for rapid evaluation of hybridoma properties.