Epstein-Barr virus nuclear antigen 1 linear epitopes that are reactive with immunoglobulin A (IgA) or IgG in sera from nasopharyngeal carcinoma patients or from healthy donors

The Epstein-Barr Virus Nuclear Antigen 1 Variant Associated with Nasopharyngeal Carcinoma Defines the Sequence Criteria for Serologic Risk Prediction

PURPOSE

Antibodies to select Epstein-Barr virus proteins can diagnose early-stage nasopharyngeal carcinoma (NPC). We have previously shown that IgA against Epstein-Barr virus nuclear antigen 1 (EBNA1) can predict incident NPC in high- and intermediate-risk cohorts 4 years before diagnosis. Here, we tested EBNA1 variants, with mutants, to define the sequence requirements for an NPC risk assay.

EXPERIMENTAL DESIGN

Mammalian-expressed constructs were developed to represent EBNA1 variants 487V and 487A, which can differ by ≥15 amino acids in the N- and C-termini. Denatured lysates were evaluated by a refined IgA and IgG immunoblot assay in a case-control study using prediagnostic NPC sera from two independent cohorts in Singapore and Shanghai, the People's Republic of China.

RESULTS

At 95% sensitivity, 487V yielded a 94.9% specificity compared with 86.1% for 487A. EBNA1 deleted for the conserved glycine-alanine repeats (GAr) reduced false positives by 22.8%. NPC sera reacted more strongly to the C-terminus than healthy controls, but the C-terminal construct (a.a. 390-641) showed lower specificity (84.8%) than the EBNA1 GAr-deleted construct (92.4%) at 95% sensitivity.

CONCLUSIONS

Although EBNA1 IgA was present in healthy sera, most epitopes localized to the immunodominant GAr. We conclude that a refined EBNA1 antigen deleted for the GAr, but with residues consistently detected in Southeast Asian NPC tumors, is optimized for risk prediction with an extended sojourn time of 7.5 years. Furthermore, distinct EBNA1 serologic profiles enhanced the utility of the EBNA1 IgA assay for risk stratification. This illustrates the importance of serologically relevant EBNA1 sequences for NPC risk prediction and early detection.

Assessing immune factors in maternal milk and paired infant plasma antibody binding to human rhinoviruses

Introduction

Before they can produce their own antibodies, newborns are protected from infections by transplacental transfer of maternal IgG antibodies and after birth through breast milk IgA antibodies. Rhinovirus (RV) infections are extremely common in early childhood, and while RV infections often result in only mild upper respiratory illnesses, they can also cause severe lower respiratory illnesses such as bronchiolitis and pneumonia.

Methods

We used high-density peptide arrays to profile infant and maternal antibody reactivity to capsid and full proteome sequences of three human RVs - A16, B52, and C11.

Results

Numerous plasma IgG and breast milk IgA RV epitopes were identified that localized to regions of the RV capsid surface and interior, and also to several non-structural proteins. While most epitopes were bound by both IgG and IgA, there were several instances where isotype-specific and RV-specific binding were observed. We also profiled 62 unique RV-C protein loop sequences characteristic of this species' capsid VP1 protein.

Discussion

Many of the RV-C loop sequences were highly bound by IgG from one-year-old infants, indicating recent or ongoing active infections, or alternatively, a level of cross-reactivity among homologous RV-C sites.

Assessing Immune Factors in Maternal Milk and Paired Infant Plasma Antibody Binding to Human Rhinoviruses

Before they can produce their own antibodies, newborns are protected from infections by transplacental transfer of maternal IgG antibodies and after birth through breast milk IgA antibodies. Rhinovirus (RV) infections are extremely common in early childhood, and while RV infections often result in only mild upper respiratory illnesses, they can also cause severe lower respiratory illnesses such as bronchiolitis and pneumonia. We used high-density peptide arrays to profile infant and maternal antibody reactivity to capsid and full proteome sequences of three human RVs - A16, B52, and C11. Numerous plasma IgG and breast milk IgA RV epitopes were identified that localized to regions of the RV capsid surface and interior, and also to several non-structural proteins. While most epitopes were bound by both IgG and IgA, there were several instances where isotype-specific and RV-specific binding were observed. We also profiled 62 unique RV-C dominant protein loop sequences characteristic of this species' capsid VP1 protein. Many of these RV-C sites were highly bound by IgG from one-year-old infants, indicating recent or ongoing active infections, or alternatively, a level of cross-reactivity among homologous RV-C sites.

Tumor microenvironment contributes to Epstein-Barr virus anti-nuclear antigen-1 antibody production in nasopharyngeal carcinoma

Nuclear antigen-1 (NA1) protein of Epstein-Barr virus (EBV) is expressed in EBV-infected cells in the microenvironment of cancer. Since immune cells infiltrate abundantly in nasopharyngeal carcinoma (NPC) tumor tissues, we hypothesized that the local tumor microenvironment may perform an important role in the production of antibodies directed at NA1. Furthermore, we hypothesized that anti-NA1 antibody originating in the local microenvironment could be secreted into the saliva of patients with NPC. In the present study, 20 healthy controls and 39 patients with NPC treated with intensity-modulated radiation therapy were recruited for the study. Saliva and serum samples were collected from the NPC patients, and nasopharyngeal tissue samples from the patients with NPC. The titers of anti-NA1 antibody [immunoglobulin A (IgA)] were determined by ELISA. Expression of NA1, human leukocyte antigen-antigen D related (HLA-DR), cluster of differentiation (CD)80, CD86, CD3, CD4, CD19 and IgA was detected by immunohistochemical staining on paraffin-embedded nasopharyngeal tissue sections. Anti-NA1 antibodies were detected in the serum and saliva samples of the patients with NPC. In infiltrating cells, expression of HLA-DR, CD80, CD86, CD3, CD4, CD19 and IgA was detected, indicating that dendritic cells, T lymphocytes and B lymphocytes were all present in the local tumor tissues. Furthermore, expression of EBNA1 protein was detected on the membrane of the NPC tumor cells. Therefore, the NPC tumor microenvironment has the potential to initiate a humoral response to EBNA1 by producing IgA antibodies.

High-Density Peptide Microarray Analysis of IgG Autoantibody Reactivities in Serum and Cerebrospinal Fluid of Multiple Sclerosis Patients

Intrathecal immunoglobulin G (IgG) synthesis and oligoclonal IgG bands in cerebrospinal fluid (CSF) are hallmarks of multiple sclerosis (MS), but the antigen specificities remain enigmatic. Our study is the first investigating the autoantibody repertoire in paired serum and CSF samples from patients with relapsing-remitting MS (RRMS), primary progressive MS (PPMS), and other neurological diseases by the use of high-density peptide microarrays. Protein sequences of 45 presumed MS autoantigens (e.g.MOG, MBP, and MAG) were represented on the microarrays by overlapping 15mer peptides. IgG reactivities were screened against a total of 3991 peptides, including also selected viral epitopes. The measured antibody reactivities were highly individual but correlated for matched serum and CSF samples. We found 54 peptides to be recognized significantly more often by serum or CSF antibodies from MS patients compared with controls (pvalues <0.05). The results for RRMS and PPMS clearly overlapped. However, PPMS patients presented a broader peptide-antibody signature. The highest signals were detected for a peptide mapping to a region of the Epstein-Barr virus protein EBNA1 (amino acids 392-411), which is homologous to the N-terminal part of human crystallin alpha-B. Our data confirmed several known MS-associated antigens and epitopes, and they delivered additional potential linear epitopes, which await further validation. The peripheral and intrathecal humoral immune response in MS is polyspecific and includes antibodies that are also found in serum of patients with other diseases. Further studies are required to assess the pathogenic relevance of autoreactive and anti-EBNA1 antibodies as well as their combinatorial value as biomarkers for MS.

EBV-Associated Cancer and Autoimmunity: Searching for Therapies

Epstein-Barr virus (EBV) infects B-, T-, and NK cells and has been associated not only with a wide range of lymphoid malignancies but also with autoimmune diseases such as lupus erythematosus, rheumatoid arthritis and, in particular, multiple sclerosis. Hence, effective immunotherapeutic approaches to eradicate EBV infection might overthrow cancer and autoimmunity incidence. However, currently no effective anti-EBV immunotherapy is available. Here we use the concept that protein immunogenicity is allocated in rare peptide sequences and search the Epstein-Barr nuclear antigen 1 (EBNA1) sequence for peptides unique to the viral protein and absent in the human host. We report on a set of unique EBV EBNA1 peptides that might be used in designing peptide-based therapies able to specifically hitting the virus or neutralizing pathogenic autoantibodies.

Combining HLA-DR risk alleles and anti-Epstein-Barr virus antibody profiles to stratify multiple sclerosis risk

Background:

Risk factors for multiple sclerosis (MS) include human leukocyte antigen (HLA)-DR and Epstein-Barr virus (EBV)-specific antibody responses, including an epitope within EBV nuclear antigen 1 (EBNA-1) that is of recent interest.

Objective:

The objective of this paper is to assess case-control associations between MS risk and anti-EBV antibody levels as well as HLA-DR profiles, gender and age in a population-based cohort.

Methods:

Serological responses to EBV were measured in 426 MS patients and 186 healthy controls. HLA-DR typing was performed using sequence-based methods.

Results:

MS patients had significantly higher levels of antibodies against epitope-specific and polyspecific EBNA-1 and viral capsid antigen (VCA), compared with controls (all p < 10−15). In regression analyses, anti-EBNA-1 and anti-VCA antibody levels, protective HLA-DR*04/07/09 alleles and gender (all p < 0.003) contributed independently to a model that classified cases and controls with an odds ratio > 20 (sensitivity 92%, specificity 64%). Notably, the strong influence of high-risk HLA-DR alleles was abrogated after inclusion of EBV serology results.

Conclusions:

The ability to discriminate MS cases and controls can be substantially enhanced by including anti-EBV serology as well as HLA-DR risk profiles. These findings support the relevance of EBV-specific immunity in MS pathogenesis, and implicate both HLA-dependent and HLA-independent immune responses against EBNA-1 as prominent disease risk factors.

Generation and characterization of a novel recombinant antibody against LMP1-TES1 of Epstein-Barr virus isolated by phage display

Latent Membrane Protein 1 (LMP1) is a primary target for controlling tumorigenesis in Epstein-Barr virus related malignancies; in this study, we aimed to develop a specific antibody against the TES1 domain of the oncogenic LMP1. We screened a full human naïve Fab phage library against TES1 peptide, which consisted of C terminal-activating regions proximal 44 amino acids. After three rounds of panning, enrichment and testing by phage ELISA and further analyzed by DNA sequencing, we selected a phage clone with the highest affinity to LMP1-TES1 and designated it as htesFab. The positive clone was expressed in Escherichia coli and the purified htesFab was characterized for its binding specificity and affinity to LMP1. ELISA, immunofluorescence and FACS analysis confirmed that htesFab could recognize LMP1 TES1 both in vitro and in LMP1 expressing HNE2-LMP1 cells. Furthermore, MTT assay showed that htesFab inhibited the proliferation of HNE2-LMP1 cells in a dose-dependent manner. In summary, this study reported the isolation and characterization of human Fab, which specifically targets the C terminal region/TES1 of LMP1, and has potential to be developed as novel tool for the diagnosis and therapy of Epstein-Barr virus related carcinoma.

Challenges in the management of nasopharyngeal carcinoma: a review

Nasopharyngeal carcinoma (NPC) is a non-lymphomatous, squamous cell carcinoma that occurs in the epithelial lining of the nasopharynx, an area that shows varying degrees of differentiation. Although relatively rare worldwide, NPC has substantial incidence and mortality in populations in Southeast Asia and in people with Southern Chinese ancestry. In Malaysia, NPC is a leading cancer type. In the clinic, NPC presents on a very wide spectrum. Therefore, a high degree of suspicion on the part of the clinician and an increased awareness by the patient is essential for the recognition of an early lesion. Early detection of the cancer is important as it affects the patient's prognosis and the mode of treatment. Managing patients with NPC is very challenging as patients usually present late when the cancer is already in an advanced stage. Here, we review the challenges in the management of NPC.

Evaluation of antibodies against different Epstein-Barr virus nuclear antigen 1 peptides in diagnosis of nasopharyngeal carcinoma

Epstein-Barr virus nuclear antigen 1 (EBNA1) is a protein expressed consistently in nasopharyngeal carcinoma (NPC). Although antibody levels against three different EBNA1 peptides were all high in NPC patients, the correlation between any two biomarkers was low. Therefore, the selection of distinct EBNA1 peptides could render different results in serological detection for individuals with NPC.

Native early antigen of Epstein-Barr virus, a promising antigen for diagnosis of nasopharyngeal carcinoma

The Epstein-Barr virus (EBV) early antigen (EA) complex consists of multiple proteins with relevance for diagnosis of acute, chronic and malignant EBV related diseases, including nasopharyngeal carcinoma (NPC). In a recent study, it was found that the molecular diversity of EBV-specific IgG and IgA antibody responses in NPC patients and demonstrated that these reflect independent B-cell triggering leading to distinct EBV antigen-recognition profiles. The fine-specificity of NPC-related IgG and IgA responses was explored further against defined recombinant and synthetic EBV-EA antigens using immunofluorescence, immunoblot and ELISA techniques and determined their diagnostic value in a large panel of sera from NPC (n = 154), non-NPC tumor patients (n = 133), acute mononucleosis patients (n = 70) and healthy EBV carriers (n = 259). Individual recombinant EBV-EA markers yielded sensitivity/specificity values not exceeding 86%, whereas selected EA-specific peptide epitopes were rather poorly recognized by IgG and IgA antibodies in NPC sera. Surprisingly, we found that a "low salt" native EA-protein extract reproducibly prepared from purified nuclei of EA-induced HH514 cells, and containing characteristic EA(D)-polypeptides, such as p47-54 (BMRF1), p138 (BALF2), p55-DNAse (BGLF5), and p65-TK (BXLF1), but without viral capsid (VCA) or nuclear antigen (EBNA) reactivity, gave highest sensitivity (90.4%) and specificity (95.5%) values for NPC diagnosis in both IgG and IgA ELISA. The data support further the notion that EBV-EA reactive IgG and IgA antibodies in NPC patients are directed against distinct conformational and-in part-linear epitopes on EBV-specific proteins, barely recognized in other EBV-related syndromes. The use of a defined native EBV EA-specific antigen opens the way to further improve serological diagnosis of NPC.

Serum antibody response to lytic and latent Epstein-Barr virus antigens in undifferentiated nasopharyngeal carcinoma patients from an area of nonendemicity

Epstein-Barr virus (EBV)-associated undifferentiated carcinoma of the nasopharyngeal type (UCNT) is highly prevalent in southeast China, where immunoglobulin A (IgA) antibodies to viral capsid antigen and early antigen (EA) represent important markers, routinely used to assist in diagnosing this malignancy. Our study aimed at determining the EBV serological profiles of 78 UCNT patients from Italy, an area of nonendemicity for this tumor, using different assays specific for both lytic and latent EBV antigens. Serum IgA against both EA and EBNA1 and IgG and IgA to the latent membrane protein 1 (LMP1), to EA, and to the EBV transactivator ZEBRA protein were assessed. These serological responses were then evaluated according to the clinicopathologic parameters at diagnosis. The sensitivities of the IgG assays were 37.7% for LMP1, 73.6% for EA, and 61.0% for ZEBRA. EA/EBNA1 IgA reactivity was 84.4%, and a high association (odds ratio [OR], 2.6; 95% confidence interval [CI], 1.7 to 4.0) with UCNT was observed. When EBV serological reactivities were analyzed according to the tumor, node, and metastasis staging system (TNM), a statistically significant association was found between N stage and IgG antibody rates for EA (OR, 3.6; 95% CI, 1.2 to 10.9) and ZEBRA (OR, 2.6; 95% CI, 1.2 to 5.5) and between M stage and IgG antibody rates for ZEBRA (OR, 7.1; 95% CI, 3.2 to 16.0) and LMP1 (OR, 14.0; 95% CI, 1.8 to 110.9). Our results show that no single serological marker allows the detection of all UCNT cases. EA/EBNA1 IgA represents a reliable marker for diagnosis, with a high predictive value also in areas where UCNT is not endemic, such as Italy. The analysis of serological results according to TNM classification is consistent with a progressive impairment of humoral immune response to EBV as the disease advances and may be used to improve the accuracy of diagnosis.

Comparison of three different serological techniques for primary diagnosis and monitoring of nasopharyngeal carcinoma in two age groups from Tunisia

Nasopharyngeal carcinoma (NPC) in Tunisia is characterized by its bimodal age distribution involving juvenile patients of 10-24 years and adult patients of 40-60 years. Three serological techniques were compared for primary diagnosis (N = 117) and post-treatment monitoring (N = 21) of NPC patients separated in two age groups. Immunofluorescence assay (IFA) was used as the "gold standard" for detection of IgG and IgA antibodies reactive with Epstein-Barr virus (EBV) early (EA) and viral capsid (VCA) antigens. Results were compared with ELISA measuring IgG and IgA antibody reactivity to defined EBNA1, EA, and VCA antigens. Immunoblot was used to reveal the molecular diversity underlying the anti-EBV IgG and IgA antibody responses. The results indicate that young NPC patients have significantly more restricted anti-EBV IgG and IgA antibody responses with aberrant IgG VCA/EA levels in 78% compared to 91.7% in elder patients. IgA VCA/EA was detected in 50% of young patients versus 89.4% for the elder group (P < 0.001). Immunoblot revealed a reduced overall diversity of EBV antigen recognition for both IgG and IgA in young patients. A good concordance was observed between ELISA and IFA for primary NPC diagnosis with 81-91% overall agreement. Even better agreement (95-100%) was found for antibody changes during follow-up monitoring, showing declining reactivity in patients in remission and increasing reactivity in patients with persistent disease or relapse. ELISA for IgA anti-VCA-p18 and immunoblot proved most sensitive for predicting tumor relapse. VCA-p18 IgA ELISA seems suitable for routine diagnosis and early detection of NPC complication.

Immune responses to Epstein-Barr virus lytic proteins in patients with nasopharyngeal carcinoma

Immune responses to three Epstein-Barr virus (EBV) lytic proteins, DNase, thymidine kinase (TK), and BMRF-1 gene products (50/52 kDa diffused early antigen, EA-D complex) were determined in EBV-infected control individuals and patients with nasopharyngeal carcinoma (NPC). Immunofluorescence assays (IFA) were used to detect their humoral immune responses using recombinant EBV lytic proteins expressed in a baculovirus system as antigens. Cell proliferation assays were performed to evaluate their cellular immune responses by monitoring 3H-thymidine incorporation. Seventy patients with NPC and 32 non-cancer controls were analyzed. The results of IFA showed antibody titers to all three EBV lytic proteins to be higher in the patients with NPC especially for the IgA class. Positivity rates of the three IgA antibodies also were higher in the patients with NPC population. Furthermore, the profiles of the IgA antibodies correlated with those to total early antigens (EA) expressed in the early phase and viral capsid antigen (VCA) expressed in the late phase, of EBV replication. The most interesting finding was that antibody titers to the three EBV lytic proteins were associated significantly with metastases of cervical lymph nodes in patients with NPC. As for cellular immunity to the EA-D complex and DNase, weak responses were observed in the cell proliferation assays. Peripheral blood cells from most individuals could not be stimulated to proliferate, except for a few patients with NPC whose antibody titers against the EA-D complex and DNase also were very high.

Molecular approach to find target(s) for oligoclonal bands in multiple sclerosis

OBJECTIVES

Oligoclonal bands are a characteristic finding in the CSF of patients with multiple sclerosis, yet their target antigen(s) remain unknown. The objective was to determine whether a filamentous phage peptide library could be employed to allow the oligoclonal bands to select their own target epitopes.

METHODS

CSF IgG antibody from 14 patients with multiple sclerosis and 14 controls was used to select individual phage clones from a bacteriophage library containing approximately 4 x 10(7) different hexamers expressed on its surface pIII protein. The amino acid sequence selected was deduced by sequencing the DNA of the genetically engineered insert.

RESULTS

In general, after three rounds of selection, CSF from both patients with multiple sclerosis and controls selected one to two consistent peptide motifs. Five out of 14 patients with multiple sclerosis, and one control, selected the amino acid sequence motif, RRPFF. Given 20 possible amino acids per position, the likelihood of five patients selecting the same linear five amino acid sequence is at most 1.6 x 10(-3), corrected for the number of clones sequenced. A GenBank computer search showed that this sequence is found in the Epstein-Barr Virus nuclear antigen (EBNA-1), and a heat shock protein alphaB crystallin. Human serum antibodies to a synthetic peptide containing RRPFF were virtually exclusively found in patients with prior infection by Epstein-Barr virus. Other studies have suggested a relation between Epstein-Barr virus infection and multiple sclerosis, including nearly 100% Epstein-Barr virus seropositivity among patients with multiple sclerosis and increased concentrations of antibody to EBNA in CSF of patients with multiple sclerosis. By antigen specific immunoblotting, antibodies to the RRPFF motif in the CSF were shown to correspond to a subset of oligoclonal bands in the CSF from the same patient.

CONCLUSION

This study shows that phage epitope display libraries may be used to select amino acid motifs which are potentially relevant to the pathogenesis of multiple sclerosis.

Antibody responses to defined epitopes in the Epstein-Barr virus BZLF1-encoded transactivator protein among human immunodeficiency virus-infected patients

The Epstein-Barr virus BZLF1-encoded replication activator (ZEBRA) is a key mediator of reactivation from latency to the viral productive cycle. In the present study, the serum antibody responses against three defined ZEBRA epitopes (designated ZEBRA-1, -19, and -22) were determined for 50 human immunodeficiency virus (HIV)-seropositive patients and 100 matched healthy control subjects. The anti-ZEBRA responses were more commonly found among HIV-seropositive patients than among healthy controls for all the three ZEBRA epitopes tested (P < 0.0003, P < 0.003, and P < 0.001, respectively). Comparison of ZEBRA antibody levels with the degree of immunodeficiency (CD4 cell counts), CDC grouping, and HIV p24 antigen positivity showed little association, suggesting that induction of ZEBRA antibodies is an early event after HIV infection.

Compact Epstein-Barr virus diagnosis based on a defined antigen mix and specific IgA

An ELISA was evaluated to measure EBV-specific IgM, IgG and IgA using a test plate coated with a controlled mix of EA, VCA and EBNA. Using the IgM version, the multi-reactive surface reduced false-negative findings in cases of infectious mononucleosis (sensitivity 98%), but was confronted with polyclonal IgM responses (specificity 85% to 95% depending on the population). For the IgG version, a national research standard was involved, and in combination with the IgA estimation, a new diagnostic strategy became possible for chronic EBV infections and reactivations. Patients screened by enhanced IgG (> 650 U/ml) were confirmed by an elevated IgA signal (> 0.6 A), with an efficacy of 95%. A raised virus-specific IgA response was not observed in cases of infectious mononucleosis, but was seen for healthy HIV-positive persons. Superior to the traditional antibody puzzle for EBV diagnosis, it will now become possible, using this compact diagnostic concept, to reduce laboratory expenses without loss of reliable results.

Linear epitopes of the replication-activator protein of Epstein-Barr virus recognised by specific serum IgG in nasopharyngeal carcinoma

The linear antigenic epitopes of the Epstein-Barr virus replication activator protein (ZEBRA), recognised by specific serum IgG in nasopharyngeal carcinoma (NPC), were determined. This was achieved by synthesizing the entire amino acid sequence of ZEBRA as a set of 29, 22-residue peptides with an overlap of 14 amino acids. The ZEBRA peptides were tested in enzyme-linked immunosorbent assay (ELISA) for IgG binding in sera from 37 selected NPC patients who had IgG antibodies to the native ZEBRA protein. The most immunogenic epitope was peptide 1 at the amino-terminal end with 36 of the sera reactive against it. Further analysis of peptide 1, using the multipin peptide-scanning technique, defined a 10-amino-acid sequence FTPDPYQVPF, which was strongly bound by IgG. Two other regions of ZEBRA were also identified as immunodominant IgG epitopes, namely peptide 11 (amino acids 82-103) and peptide 19/20 (amino acids 146-175) with 8-13 of the NPC sera reactive against the peptides. The number of peptides reactive with individual NPC serum varies from 1 to 6 or more and there is some correlation between a greater number of peptide (at least 4) bound and a higher (at least 1:40) titre of serum IgA to viral capsid antigen. The immunodominant ZEBRA peptide 1 could be utilised in IgG ELISA for the detection of NPC.

Screening for nasopharyngeal carcinoma with an ELISA using the Epstein-Barr virus nuclear antigen, EBNA 1: a complementary test to the IgA/VCA immunofluorescence assay

An ELISA using the Epstein-Barr virus nuclear antigen 1 (EBNA 1) was found to detect selectively specific IgA in sera from patients with nasopharyngeal carcinoma (NPC). The antigen, p107, was a 20-amino acid synthetic peptide, representing a major epitope of EBNA 1.267/294 (90.8%) of NPC patients had IgA antibodies to p107 but in normal individuals, only 41/577 (7.1%) had IgA/p107. In sera from patients with other cancers, 11/77 (14.3%) had IgA/p107 reactivity. 124 IgA/VCA positive and 86 IgA/VCA negative NPC sera were also tested for IgA/p107 binding in ELISA. The majority of IgA/VCA positive sera (117) also contained IgA/p107 antibodies. Of interest was the detection of 74/86 IgA/p107 reactive sera in the IgA/VCA negative group. The results suggest that the IgA/p107 ELISA could become a useful, complementary screening assay to the IgA/VCA immunofluorescence test for detection of NPC.

Functional importance of amino acid residues making up peptide antigenic determinants

The functional importance of each amino acid residue making up the antigenic determinants of three different peptide-mAb interactions was determined using complete series of substitution analogs of the three immunizing synthetic peptides. Fingerprint substitution profiles for the three different antigenic determinants were obtained separately by direct and competitive ELISA. Competitive ELISA was found to offer the advantage of being able to measure the concn of each peptide substitution analog necessary to inhibit antibody binding to the original peptide. In this manner, the relative functional contribution to antibody binding of each amino acid residue making up the antigenic determinant was determined and termed the relative positional importance factor (RPIF). Each antigenic determinant was found to contain one very highly specific residue (i.e., highest RPIF) that was, on average, the least replaceable with any of the natural L-amino acids (the average decrease in recognition ranged 250- to 28,000-fold). At the other extreme, two or three positions in each antigenic determinant were found to be only weakly involved in recognition. These positions were considered redundant since the average decrease in recognition of the substitution analogs for these positions was found to be 20-fold or less. The remaining antigenic determinant residues exhibited the fine specificity common to antigen-antibody interactions in that only relatively conservative substitutions for these residues were recognized by their respective antibodies. It is of interest that the positional arrangement of specific and nonspecific residues were different for each of the three antigenic determinants examined.