Somatic cell genetic analysis of myelomas and hybridomas

The use of Bcl-2 over-expression to stabilize hybridomas specific to the HERG potassium channel

We encountered a high degree of clonal hybridoma loss in the course of generating antibodies specific for the hERG potassium channel. A protein that is crucial for controlling heart rhythm, is abundant in parts of the brain and is abnormally expressed in some tumors. Intracellular domains of the protein were used for immunogens and generated adequate antibody responses in mice. Subsequent hybridomas created using Ag8 myeloma fusion partner yielded clones that secreted specific antibody but none could be successfully maintained in culture. A variety of mechanisms, including polyploidy inherent to hybridoma development or production of cytotoxic antibodies, may be responsible for eventual loss of cell viability by mechanisms that may include apoptosis. When spleen cells were fused to the NSO myeloma cell line that stably over-expresses the anti-apoptotic protein Bcl-2, hybridoma clones were generated that remained viable in culture with high level of hERG-specific antibody production. When the parental NSO cell line not over-expressing Bcl-2 was used, no stable hybridomas were produced. Antibodies secreted by NSO-Bcl-2 hybridomas were specific for hERG and performed well in immunoblot, immunoprecipitation and immunofluorescence assays. This work demonstrates a feasible option when faced with antigens that seem to be associated with clonal instability in the process of generating monoclonal antibodies.

Mechanisms of HIV-tat-induced phosphorylation of N-methyl-D-aspartate receptor subunit 2A in human primary neurons: implications for neuroAIDS pathogenesis

HIV infection of the central nervous system results in neurological dysfunction in a large number of individuals. NeuroAIDS is characterized by neuronal injury and loss, yet there is no evidence of HIV-infected neurons. Neuronal damage and dropout must therefore be due to indirect effects of HIV infection of other central nervous system cells through elaboration of inflammatory factors and neurotoxic viral proteins, including the viral transactivator, tat. We previously demonstrated that HIV-tat-induced apoptosis in human primary neurons is dependent on N-methyl-D-aspartate receptor (NMDAR) activity. NMDAR activity is regulated by various mechanisms including NMDAR phosphorylation, which may lead to neuronal dysfunction and apoptosis in pathological conditions. We now demonstrate that tat treatment of human neurons results in tyrosine (Y) phosphorylation of the NMDAR subunit 2A (NR2A) in a src kinase-dependent manner. In vitro kinase assays and in vivo data indicated that NR2A Y1184, Y1325, and Y1425 are phosphorylated. Tat treatment of neuronal cultures enhanced phosphorylation of NR2A Y1325, indicating that this site is tat sensitive. Human brain tissue sections from HIV-infected individuals with encephalitis showed an increased phosphorylation of NR2A Y1325 in neurons as compared with uninfected and HIV-infected individuals without encephalitis. These findings suggest new avenues of treatment for HIV-associated cognitive impairment.

Adenovirus-mediated delivery of an anti-V antigen monoclonal antibody protects mice against a lethal Yersinia pestis challenge

Pneumonic plague, caused by inhalation of Yersinia pestis, represents a major bioterrorism threat for which no vaccine is available. Based on the knowledge that genetic delivery of monoclonal antibodies (MAbs) with adenovirus (Ad) gene transfer vectors results in rapid, high-level antibody expression, we evaluated the hypothesis that Ad-mediated delivery of a neutralizing antibody directed against the Y. pestis V antigen would protect mice against a Y. pestis challenge. MAbs specific for the Y. pestis V antigen were generated, and the most effective in protecting mice against a lethal intranasal Y. pestis challenge was chosen for further study. The coding sequences for the heavy and light chains were isolated from the corresponding hybridoma and inserted into a replication-defective serotype 5 human Ad gene transfer vector (AdalphaV). Western analysis of AdalphaV-infected cell supernatants demonstrated completely assembled antibodies reactive with V antigen. Following AdalphaV administration to mice, high levels of anti-V antigen antibody titers were detectable as early as 1 day postadministration, peaked by day 3, and remained detectable through a 12-week time course. When animals that received AdalphaV were challenged with Y. pestis at day 4 post-AdalphaV administration, 80% of the animals were protected, while 0% of control animals survived (P < 0.01). Ad-mediated delivery of a V antigen-neutralizing antibody is an effective therapy against plague in experimental animals and could be developed as a rapidly acting antiplague therapeutic.

A second major native von Hippel-Lindau gene product, initiated from an internal translation start site, functions as a tumor suppressor

The von Hippel-Lindau (VHL) tumor suppressor gene is inactivated in both sporadic and inherited clear cell renal carcinoma associated with VHL disease. We have identified two distinct native products of the human VHL gene, with apparent molecular masses of 24 and 18 kDa. The 18-kDa VHL protein was more abundant in nearly all cell lines examined. Reintroduction of the 18-kDa VHL gene product into renal carcinoma cells lacking wild-type VHL protein led to down-regulation of vascular endothelial growth factor (VEGF) mRNA and glucose transporter GLUT1 protein and suppressed tumor formation in nude mice. The 18-kDa VHL protein also demonstrated binding to elongins B and C. In an in vitro assay, the second in-frame AUG codon present in VHL mRNA was shown to be necessary and sufficient for production of the 18-kDa VHL protein, consistent with an internal translation mechanism. These data provide evidence for a second major VHL gene product, which contains the functional domains of the VHL gene. Moreover, these results indicate that internal translation initiation is an important mechanism for production of the major VHL protein.

Distinct STAT structure promotes interaction of STAT2 with the p48 subunit of the interferon-alpha-stimulated transcription factor ISGF3

Cells express a variety of STAT (signal transducer and activator of transcription) transcription factors that are structurally homologous and yet function specifically in response to particular cytokines. The functions of the individual STATs are dependent on distinct protein-protein interactions. STAT1 and STAT2 are activated by tyrosine phosphorylation in response to type I interferons-alpha/beta (IFN-alpha/beta) and subsequently form a multimeric transcription factor designated the IFN-alpha-stimulated gene factor 3 (ISGF3). ISGF3 is a unique STAT complex because it also contains a non-STAT molecule, p48, which is a critical DNA-binding component. We provide evidence that STAT2 specifically interacts with p48 in vivo before and after IFN-alpha stimulation. The specificity of ISGF3 formation is therefore a result of the distinct nature of the STAT2 molecule. Coimmunoprecipitation assays demonstrate p48 association with STAT2 but not STAT1. Hybrid STAT2. STAT1 molecules were used to identify a region of STAT2 which specifically associates with p48. The region of STAT2 interaction spans an amino-terminal region of two predicted coiled coils. The studies demonstrate the in vivo existence of a STAT2.p48 complex and a distinct STAT2.STAT1 complex after IFN-alpha stimulation. Data suggest that distinct bipartite complexes STAT2.p48 and STAT2.STAT1 translocate to the nucleus and associate on the DNA target site as ISGF3.

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.

In vitro activation of a nonproductive immunoglobulin allele by a single base pair insertion

We have shown that one of the alleles in a hybridoma was nonproductive because the sequence in the N region of the heavy chain caused it to be out of frame and to terminate prematurely. This allele became productive, and the gamma 1 heavy chain that it encoded was secreted when an adenosine was inserted to produce an open reading frame. This event occurred at a very low frequency following mutagenesis of the cultured cells. This result suggests that similar sorts of events must occur in vivo when both productive and nonproductive alleles undergo frequent mutations and that new genes may be expressed in hybridomas as they are being subcloned.

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.

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.

Inhibition of matrix metalloproteinase 9 activation by a specific monoclonal antibody

Two members of the matrix metalloproteinase (MMP)1 family of enzymes are expressed at elevated levels in highly aggressive human tumor cells and have been implicated in the catalytic functions of extracellular proteolysis. The zymogen forms of these enzymes are designated proMMP-2 and proMMP-9, also known as 72kDa and 92kDa type IV collagenases/gelatinases, respectively. The MMP family of enzymes can be activated in vitro by a number of compounds including the organomercurial 4-aminophenylmercuric acetate (APMA). The natural or in vivo activators of MMP-2 and MMP-9 are at present unknown. A partially purified preparation of MMP-9 was used to immunize mice for the isolation of monoclonal antibodies (mAbs). Three IgG1 mAbs were identified by immunoreactivity with purified MMP-9 and are designated 6-6B, 7-11C, and 8-3H. These mAbs react specifically with MMP-9 by ELISA and Western blot. Additionally, these mAbs react with N-glycanase treated 92kDa protein. These mAbs were tested for their ability to inhibit enzyme activation in a radio-labeled gelatin assay. The 6-6B mAb inhibited the activation of MMP-9, but had no effect on MMP-2. These mAbs are highly specific to human MMP-9 and the 6-6B mAb will be extremely useful for examining the autolytic and catalytic activity of MMP-9 in normal and abnormal biological processes.

Identification of rare immunoglobulin switch variants using the ELISA spot assay

We describe here the use of the ELISA spot assay to identify, quantify, and isolate rare hybridoma subclones that have switched to expressing a new class or subclass of Ig. This technique is less labor intensive and time consuming than sib selection and standard ELISA and eliminates the many false positives that complicated those techniques. The use of the ELISA spot assay also allows screening large populations of cells and accurate quantitation of the rate of isotype switching.