Production of human monoclonal IgG antibodies reacting with cytomegalovirus (CMV)

A human monoclonal antibody (MoAb) reacting with cytomegalovirus (CMV) has been produced using somatic cell hybridization between Epstein-Barr virus (EBV) infected B lymphocytes and a human-mouse heteromyeloma cell line (SHM-D33). The hybrids were selected in HAT medium containing 5 × 10⁻⁷ ouabain. The median level of Ig production was 5 (0.1−20) μg/10⁶ cells/day. One selected hybridoma (IB-8E9H5) has been maintained in continuous culture for more than 30 months with a stable IgG2, λ production. Molecular hybridization using EBV-specific probes demonstrate that our hybrids have lost the IR-1 EBV sequence during fusion. Unexpectedly, these blotting experiments revealed the presence of multiple EBNA-1 sequences dispersed among the genomic DNA of the SHM-D33 cell line. Screening for anti-CMV specificity was performed by ELISA and confirmed by immunofluorescence staining. Thus far, three CMV reference strains and 14 local strains are stained by the MoAb as early as 3 h after CMV infection of human fibroblasts, apparently through the recognition of a nuclear viral antigen of 67 kDa. In conclusion, this technique permits (a) the removal of the EBV genome contained in the lymphoblastoid parental cell line and (b) the production of human anti-CMV MoAb with potential applications in the prevention of life threatening CMV infections.     

Identification of human cytomegalovirus strain with immediate-early (IE) antigen-specific monoclonal antibody is prevented by point mutation in IE gene.

In an AIDS patient with a disseminated human cytomegalovirus (HCMV) infection, presence of HCMV in blood was repeatedly excluded by the shell vial culture method with the HCMV immediate-early (IE) antigen-specific monoclonal antibody (MAb) 5D2 currently employed for rapid HCMV identification, whereas it was repeatedly confirmed by all other assays (conventional virus isolation from blood, antigenemia, and DNAemia). Sequence analysis of the HCMV strain revealed a point mutation in exon 2 of the IE gene, which led to a serine-to-proline substitution at position 20 of the corresponding protein. Cloning and expression of a region of the IE gene containing the mutation showed that this was responsible for the lack of reactivity of MAb 5D2. A pool of IE antigen-reactive MAbs instead of a single MAb must be used for rapid HCMV identification to detect all viral strains.     

Characterization of human monoclonal antibodies directed against the pp65-kD matrix antigen of human cytomegalovirus.

Human monoclonal antibodies specific for human cytomegalovirus (CMV) antigens have been established using peripheral blood lymphocytes from a seropositive donor. Immortalization of antigen-specific B cells was achieved by Epstein-Barr virus transformation followed by somatic cell fusion of antigen-specific lymphoblastoid cells. Four clones producing high-affinity antibodies (0.2-7 x 10(9) M-1) specific for the viral matrix protein pp65 have been further characterized with respect to epitope specificity of secreted antibodies. The studied antigen represents a major protein produced by in vitro-cultivated virus, and is important in the serodiagnosis of CMV infection. The human monoclonal antibodies recognized different epitopes, some of which proved to be overlapping. The fine specificity of these antibodies was evaluated using synthetic peptides covering the sequence of pp65. The antibody MO58 recognized a linear epitope (residues 283-288) whereas antibody MO53 recognized a discontinuous epitope involving residues 208-216 and 280-285. Despite the close proximity of these epitopes, the antibodies did not compete with each other for the same binding site on intact antigen.     

Identification of a monoclonal antibody from Peromyscus maniculatus (deer mouse) cytomegalovirus (PCMV) which binds to a protein with homology to the human CMV matrix protein HCMV pp71

In this study we identified and characterized a monoclonal antibody against the matrix protein of a cytomegalovirus isolated from the common deer mouse (Peromyscus maniculatus) (PCMV). The monoclonal antibody was isolated using previously described technology which could be applied to the production of monoclonal antibodies against zoonotic disease. The antibody was found to react with a protein homologous to the human cytomegalovirus (HCMV) matrix protein (pp71), the product of the UL82 open reading frame (ORF). mAbs were generated from heterologous fusion of spleen cells from PCMV-positive mice and Balb/C P3X63-Ag8.653 myeloma cells. Using this approach, four monoclonal antibodies: B8C4, C12E8, G6A2 and P4E5 were generated. Antibody G6A2 reacted strongly with PCMV-infected cells as well as purified virions on ELISA and immunofluorescence. Western blot analysis, using sucrose gradient-purified virions, demonstrated that this mAb reacted specifically to a single protein with an apparent molecular weight of 71 kDa. The protein band was excised from the gel, purified and subjected to trypsin digestion followed by mass spectrometry. The protein sequences obtained were found to have identity to HCMV UL82 gene product. Sequence analysis indicated that it is the putative HCMV pp71 protein homolog of PCMV. G6A2 mAb did not cross-react with either human or murine recombinant pp71 proteins expressed in mammalian cells.     

Significance of cytomegalovirus (CMV)-pp65 antigenemia in the diagnosis of CMV disease in human immunodeficiency virus-infected patients

To establish the diagnostic value of the cytomegalovirus (CMV)-pp65 antigenemia in CMV disease occurring in human immunodeficiency (HIV)-infected patients, CMV-pp65 antigen in polymorphonuclear leukocytes (PMNLs) was assayed in 373 samples from 138 randomly included patients followed up for symptomatic HIV-1 infection and the correlation between CMV-pp65 antigenemia and diagnosis of CMV disease was investigated. Thirty-seven CMV disease episodes were observed in 30 patients and 89.2% of these episodes were associated with a positive CMV-pp65 antigenemia. In contrast, 94% of the patients negative for CMV-pp65 antigenemia remained free of CMV disease. Patients with CMV disease had significantly higher levels of CMV-pp65 antigenemia than CMV disease-free patients (695 positive cells/2 × 10⁵ PMNLs vs. 28 positive cells/2 × 10⁵ PMNLs). The positive and negative predictive values of the test were 45% and 94%, respectively, but were 93% and 80%, respectively, when a CMV-pp65 antigenemia level of >100 positive cells/2 × 10⁵ PMNLs was taken into consideration. These results indicate that the CMV-pp65 antigenemia assay is useful for the diagnosis and monitoring of CMV disease in HIV-infected patients. © 1996 Wiley-Liss, Inc.     

Generation of recombinant human monoclonal antibodies to rotavirus from single antigen-specific B cells selected with fluorescent virus-like particles

Technical difficulties have severely limited the yield of methods for the generation of human antiviral monoclonal antibodies (Mabs) in the past. We describe here a novel method for the efficient development of human Mabs against viruses. Rotavirus (RV) is a major cause of gastroenteritis in infants and adults worldwide. We generated fluorescent virus-like particles (VLPs) to identify and physically sort single RV-specific B cells from healthy adult blood donors, or RV-infected infants or adults. We expanded the sorted single B cells in culture, tested for RV-specific antibody secretion, and cloned and sequenced the antibody heavy and light chain variable region (VH and VL) genes. The percentage of wells that produced antibodies after sorting and expanding RV-specific adult B cell clones was high at 23%. The overall efficiency of RV-specific antibody gene recovery after the isolation, confirmation, and cloning of RV-specific VH segments was 1.3% of sorted cells in adults. RV-specific variable gene segments also were obtained from acutely infected infants, although infant B cells did not proliferate and differentiate in culture as well as adult B cells. We expressed recombinant Fabs incorporating the VH and VL genes from RV-specific B cell clones using a new modified bacterial Fab expression vector that we describe. Finally, we demonstrated binding of purified Fabs to RV proteins by immunofluorescence and ELISA. This method for the generation of recombinant human Mabs to RV from single antigen-specific B cell clones selected with fluorescent VLPs could be used to generate human Mabs to many other viruses whose proteins can self-assemble into VLPs.     

High-level sensitivity of quantitative pp65 cytomegalovirus (CMV) antigenemia assay for diagnosis of CMV disease in AIDS patients and follow-up.

Cytomegalovirus (CMV) antigenemia was evaluated in 174 patients positive for human immunodeficiency virus. Antigenemia could be detected in 96.7% of patients with CMV disease, 76.9% of patients suffering from a relapse of the disease, and 11.4% of asymptomatic patients with CD4 levels of < 100 cells per microliter. No antigenemia was detected in patients with CD4 levels of 250 to 500 cells per microliter. Specificity and the positive predictive value for CMV disease were increased only if more than 5 positive cells per slide were considered. However, CMV disease may also occur in patients with low-grade antigenemia.     

Salivary antibodies to cytomegalovirus (CMV) glycoprotein B accurately predict CMV infections among preschool children.

Among preschool children, we found an association between cytomegalovirus (CMV) infection and salivary immunoglobulin G antibodies to CMV glycoprotein B. All of the 20 infected children had immunoglobulin G to CMV glycoprotein B in their saliva, whereas 38 of 38 uninfected children lacked these antibodies. Testing saliva provides a sensitive and specific alternative to obtaining serum.     

The diversity of antigen-specific monoclonal antibodies from transgenic mice bearing human immunoglobulin gene miniloci

An approach to the preparation of antigen-specific human monoclonal antibodies focuses on mice transgenic for human immunoglobulin gene miniloci; the V gene segments in these miniloci undergo productive rearrangement to yield mouse B cells expressing human immunoglobulin (Ig) chains. The general usefulness of this strategy hinges on whether it is feasible to obtain specific, high-affinity antibodies following immunization of such animals with a variety of antigens. To test this, we have investigated the antigen-specific responses in mice which carry human IgH miniloci (constaining just one or two V_H segments) instead of a functional mouse IgH locus. Although serum responses were relatively weak, monoclonal antibodies were readily obtained to all immunogens tested (a hapten, foreign proteins and human lymphoma cells). The affinities of two of the hapten-specific (anti-2-phenyl-oxazol-5-one) antibodies were 60 and 160 nM, values intermediate between what is typically obtained in the primary and secondary response of normal mice. Sequence analysis of the rearranged V genes revealed that junctional events made a major contribution to diversity with a considerable amount of apparently non-templated sequence at the V-D and D-J borders. Somatic hypermutation was also evident within the expressed V gene segments of many of the antigen-specific hybridomas. These findings augur well for the general usefulness of the transgenic approach for the isolation of high-affinity human antibodies to a wide range of antigens and suggests that the miniloci need not be particularly large.     

A multisite trial comparing two cytomegalovirus (CMV) pp65 antigenemia test kits, biotest CMV brite and Bartels/Argene CMV antigenemia

A total of 513 blood specimens, predominantly from organ transplant recipients, human immunodeficiency virus-positive patients, and bone marrow transplant recipients, were tested for cytomegalovirus (CMV) by culture and pp65 antigenemia across four test sites. Peripheral blood leukocytes were examined by using both the Biotest CMV Brite and the Bartels/Argene CMV Antigenemia kits. A total of 109 specimens were positive for CMV, 106 (97%) were positive by antigenemia, and 34 (31%) were positive by culture. According to the manufacturers' instructions, 150,000 cells were applied per slide for the Biotest kit and 200,000 cells per slide for the Bartels kit. A total of 93 specimens (88%) were positive by the Biotest kit, and 86 (81%) were positive by the Bartels kit. In specimens found to be positive by only one kit, the positive cell counts were low (median, 1; range, 1 to 7). When the data from all four sites were combined and analyzed, there was no statistical difference between the performance of the two kits; the Biotest and Bartels kits were found to be equivalent in sensitivity, specificity, and positive and negative predictive values for the detection of CMV pp65 antigenemia.     

Comparison of different immunostaining techniques and monoclonal antibodies to the lower matrix phosphoprotein (pp65) for optimal quantitation of human cytomegalovirus antigenemia.

The main parameters of immunostaining techniques, i.e., the type of fixative, immunocytochemical reaction, and quality of monoclonal antibodies (MAbs), for quantitation of human cytomegalovirus (HCMV) antigenemia in peripheral blood polymorphonuclear leukocytes (currently performed by the indirect immunofluorescence or immunoperoxidase reaction by using MAbs to HCMV pp65) were investigated in order to optimize procedural steps and reagents. Significantly better results (in terms of the number of positive cells) were obtained on multiple cytospin preparations from heart transplant recipients with HCMV viremia when we used (i) formalin instead of methanol-acetone fixation and (ii) the indirect immunofluorescence reaction instead of the immunoperoxidase reaction, the avidin-biotin complex method, or the alkaline phosphatase antialkaline phosphatase procedure. In addition, comparison of the staining capabilities of three MAbs to pp65, which were developed in the laboratory and which were reactive to different epitopes of the protein, with a commercially available MAb (Clonab CMV) for determination of HCMV antigenemia showed that, while individual MAbs did not provide better results, the pool of MAbs detected a significantly higher number of positive peripheral blood polymorphonuclear leukocytes than Clonab CMV did. In addition, the sensitivity of the pool in detecting patients with low levels of viremia (less than 5/2 x 10(5) cells inoculated) as antigenemia positive was 100%, whereas the sensitivity of Clonab CMV was 47%. No differences in the specificities between the two MAb preparations were observed.     

Epitope analysis of human cytomegalovirus glycoprotein complexes using murine monoclonal antibodies

A panel of 10 monoclonal antibodies reactive with human cytomegalovirus (HCMV) glycoproteins was generated. These antibodies immunoprecipitated disulfide-linked complexes which contained glycoproteins with molecular weights of 130,000, 93,000, and 52,000. These complexes were designated gC-I. Epitope analysis of gC-I was done with a simultaneous two antibody binding assay. A network of epitopes was revealed which clustered in three major domains designated I, II, and III. Antibodies within individual domains I and II showed strong mutual inhibition of each other's binding. However, there were multiple antibody interactions between domains I and II. For example, the binding of most antibodies in domain I was augmented to some extent by antibodies from domain II. However, the binding of only one antibody from domain II was augmented by all antibodies from domain I. The augmentation in binding between two antibodies was dependent on the native structure of gC-I and was sensitive to conformational changes due to nonionic detergent extraction of gC-I and/or disruption of disulfide bonds. A synergistic effect was also observed between antibodies in domains I and II in a virus neutralization assay. A neutralizing antibody had a much greater neutralizing activity in the presence of a nonneutralizing antibody, which also enhanced the binding of the neutralizing antibody in the simultaneous two antibody binding assay. Also, two antibodies which were nonneutralizing individually were neutralizing when used in combination. Such antibodies also augmented each other's binding in the simultaneous two antibody binding assay. Finally, domain III consisted of a nonneutralizing antibody that inhibited the binding of all antibodies in domains I and II. This antibody also inhibited the neutralizing activity of a neutralizing antibody in a virus neutralizing assay.     

Quantitative analysis of cytomegalovirus (CMV) viremia using the pp65 antigenemia assay and the COBAS AMPLICOR CMV MONITOR PCR test after blood and marrow allogeneic transplantation

The performance of a commercially available qualitative PCR test for plasma (AMPLICOR CMV Test; Roche Diagnostics) and a quantitative PCR test for plasma and leukocytes (COBAS AMPLICOR CMV MONITOR Test; Roche Diagnostics) was evaluated with samples from 50 blood or marrow allogeneic transplant recipients who received short courses of sequential ganciclovir therapy (2 weeks intravenously followed by 2 weeks orally) based on a positive cytomegalovirus (CMV) pp65 antigenemia (AG) assay. The number of persons with a positive CMV test was significantly higher for leukocyte-based assays (AG, 67.5%; PCR, 62.5%) compared to both quantitative and qualitative PCR tests of plasma (42.5 and 35%, respectively). One person developed CMV disease during the study despite a negative AG assay; in this particular case, all PCR assays were found to be positive 10 days before his death. There was a trend for earlier positivity after transplantation and more rapid negativity after initiation of ganciclovir for the tests performed on leukocytes. The mean number of CMV copies as assessed by PCR was significantly higher in leukocytes than in plasma (P = 0.02). Overall, excellent agreement (kappa coefficient, >0.75) was found only between the two PCR assays (qualitative and quantitative) based on plasma. These results suggest that either the pp65 AG assay or the COBAS AMPLICOR CMV MONITOR Test using leukocytes could be used to safely monitor CMV viremia in related allogeneic blood or marrow transplant recipients. Such a strategy will result in preemptive treatment for about two-thirds of the persons with a relatively low rate (<33%) of secondary viremic episodes following short courses of ganciclovir therapy.     

Epitope mapping of human herpesvirus-7 gp65 using monoclonal antibodies

Summary.  Human herpesvirus (HHV)-7 encodes a unique 65-kDa heparin- binding glycoprotein, designated gp65. This molecule is thought to play a role in virus attachment and entry. To obtain reagents to map the structure and function of HHV-7 gp65, we produced monoclonal antibodies to this molecule. Ten monoclonal antibodies reacting with gp65 on ELISA were subdivided in four groups on the basis of their isotype and differential reactivity with (i) native versus denatured forms of gp65, and (ii) mature (virion-associated) versus immature (cell-associated) forms of the molecule. We were able to map the binding epitopes for eight of these ten antibodies, and these were found to cluster to one site on gp65 (amino acids 239–278); within this region, the antibodies reacted with at least three distinct domains (244–251, 255–262, 263–278). The reasons for the apparent immunodominance of this region are uncertain. Taken together, this panel of antibodies constitutes an extensive and well-characterized set of HHV-7 specific antibodies that may have utility for future analyses of the structure/function of gp65, and for studies on the virus life cycle. Accepted April 9, 2001 Received January 19, 2001     

Development of two potential diagnostic monoclonal antibodies against human cytomegalovirus glycoprotein B

Human cytomegalovirus glycoprotein B (gB) represents a target for diagnosis and treatment in view of the role it plays in virus entry and spread. Nevertheless, to our knowledge, rare detection of a gB antigen has been reported in transplant patients and limited information is available about diagnostic gB monoclonal antibodies (mAbs). Our aim was to develop gB mAbs with diagnostic potential. Hydrophilic gB peptides (ST: amino acids 27-40, SH: amino acids 81-94) of favorable immunogenicity were synthesized and used to immunize BALB/c mice. Two mAbs, named ZJU-FH6 and ZJU-FE6, were generated by the hybridoma technique and limited serial dilution and then characterized by indirect ELISA, Western blotting, immunoprecipitation, and immunohistochemical staining. The mAbs displayed high titers of specific binding affinities for the ST and SH synthetic peptides at an mAb dilution of 1:60,000 and 1:240,000, respectively. Western blotting and immunoprecipitation indicated that these mAbs recognized both denatured and native gB of the Towne and AD169 strains. The mAbs, when used as the primary antibody, showed positive staining in cells infected with both Towne and AD169 strains. The mAbs were then tested on patients submitted to allogeneic hematopoietic stem cell transplantation. The gB antigen positivity rates of the patients tested using ZJU-FH6 and ZJU-FE6 were 62.0 and 63.0%, respectively. The gB antigen showed a significant correlation with the level of pp65 antigen in peripheral blood leukocytes. In conclusion, two potential diagnostic gB mAbs were developed and were shown to be capable of recognizing gB in peripheral blood leukocytes in a reliable manner.     

Characterization of antigen-specific repertoire diversity following in vitro restimulation by a recombinant adenovirus expressing human cytomegalovirus pp65

Human cytomegalovirus (HCMV) and adenovirus cause significant morbidity and mortality in immunocompromised hosts undergoing allogeneic stem cell transplantation. We have previously established a procedure for the generation of polyclonal CTL with specificity against adenovirus and HCMV using a recombinant adenovirus encoding the HCMV pp65 protein (RAdpp65). However, specific CTL expanded after in vitro culture steps were subjected to several in vitro restimulations and, depending on the protocol adopted, this could lead to a selection bias, compromising the clinical benefit. To determine which part of the memory repertoire is selected after in vitro restimulation, we have followed the specificity and clonal composition of pp65-peptide-specific CD8(+) T cells in HLA-A*201 individuals before and after repeated in vitro restimulation of cells with RAdpp65, combining HLA tetrameric complexes and immunoscope analysis. Tetramer staining showed that, after in vitro restimulation, up to 60% of CD8(+) T cells were virus-specific. Immunoscope analysis showed that the predominant TCRBV diversity of pp65-specific clones was conserved, demonstrating that the memory repertoire was preserved all along the procedure. Altogether, these results suggest that the use of RAdpp65 to induce CMV- and adenovirus-specific CTL maybe appropriate for immunotherapy.     

Human antibody reactivity against the lower matrix protein (pp65) produced by cytomegalovirus.

The lower matrix protein (pp65) is a major product of many laboratory strains of cytomegalovirus (CMV). It is thus an integral part of many CMV serological assays based on native antigen. Recombinant fragments of pp65 have previously been investigated for their usefulness in more-defined assays. The latter antigens have, however, failed to develop a positive response with serum samples derived from a substantial number of infected individuals. Here we show that the human humoral immune response to CMV pp65 is highly diverse and recognizes at least seven distinct but in some cases partly overlapping epitopes. Most of these epitopes could not be mimicked by any of the investigated recombinant or synthetic antigens. Furthermore, when we investigated the ability of human CMV-seropositive serum samples to block the reactivity of pp65-specific antibodies recognizing five different epitopes within pp65, it was evident that several sera did not contain significant levels of antibodies against any of these or overlapping structures. It was thus concluded that the antibody response against CMV pp65 is weak in some CMV-infected individuals, making this antigen unsuitable for use alone in serological screening systems for CMV infection.     

Expression of tegument protein pp65 of human cytomegalovirus (CMV) and its application to the analysis of viral-specific cellular immunity in CMV-infected individuals

Summary.  Investigations into human cytomegalovirus (CMV)-specific cellular immunity are important to better understand and manage CMV infections. CMV phosphoprotein pp65 is thought to be a major antigen for CMV-specific cellular immunity. We newly synthesized protein pp65 with a baculovirus expression system and purified it via metal affinity chromatography in a soluble form. The recombinant protein pp65 was antigenic in an enzyme immuno-linked assay for pp65-specific IgG in sera from 196 children. Traditional lymphoproliferation assays have shown that pp65 protein promotes specific lymphoproliferation in CMV-seropositive donors. Using an intracellular cytokine detection system, we showed that this recombinant protein stimulated CD4-positive T cells to express interferon-γ. The results of these assays using protein pp65 were comparable with the use of CMV whole antigen. pp65- and CMV-specific cellular immunity, and CMV DNA load were also compared in four recipients of unrelated cord blood transplantation. The delay in re-constitution in CMV-specific cellular immunity was associated with reactivation of CMV. These results indicated that the recombinant protein pp65 can be used to study specific immunity in CMV infections. Received March 1, 2002; accepted May 28, 2002