Human IgM monoclonal antibody to ganglioside GM2 and complement suppress virus propagation in ex vivo cultures of lymphocytes from HIV-1 infected patients.

HIV-1 infection induces aberrant ganglioside GM2 expression on infected cell lines, and human IgM anti-GM2 monoclonal antibody (L55 Ab) together with normal fresh human serum (FHS) as a source of complement causes complement mediated cytolysis of HIV-1 infected cells as well as HIV-1 particles. We report here that high expression of GM2 was also detected on HIV-1 infected lymphocytes from HIV-1 seropositive patients. L55 Ab effectively suppressed the generation of HIV in the presence of FHS in primarily cultured lymphocytes from HIV-1 infected patients in ex vivo experiments, and the suppression was enhanced additively by AZT. These data suggest that L55 Ab may increase the therapeutic effect of chemotherapy.

Cytotoxic T lymphocytes that recognize decameric peptide sequences of retinoblastoma binding protein 1 (RBP-1) associated with human breast cancer.

Retinoblastoma binding protein 1 (RBP-1) is a 143-kDa nuclear phosphoprotein that promotes cell growth by inhibiting the product of retinoblastoma tumour suppressor gene (pRB). We recently found that RBP-1 contains KASIFLK, a heptameric peptide (250-256) recognized by human antibodies and overexpressed by breast cancer cells. In the present study, we demonstrate that human T-cells stimulated with RBP-1 decameric peptides containing KASIFLK can kill human breast cancer cells. These decamers, GLQKASIFLK (247-256) and KASIFLKTRV (250-259), have anchor motifs for both HLA-A2 and HLA-A3. Peripheral blood lymphocytes from 41 normal donors were stimulated by these peptides in culture media containing 15 IU ml(-1) interleukin-2, 25 IU ml(-1) interleukin-7 and 500 IU ml(-1) granulocyte-macrophage colony-stimulating factor. Cytotoxic activity of the T-cells was assessed against autologous B lymphoblastoid cells pulsed with each peptide. Stimulation by GLQKASIFLK generated specific cytotoxic T lymphocyte (CTL) lines from HLA-A2, A3 donors, HLA-A2 donors and HLA-A3 donors. Stimulation with KASIFLKTRV generated specific CTL lines from HLA-A2 donors. No HLA-A2-, A3 CTL line showed specific cytotoxicity against these target cells. These CTL lines were also cytotoxic against HLA-A2 and HLA-A3 breast cancer cells but not against normal fibroblastoid cell lines, normal epidermal cell lines, or a melanoma cell line. RBP-1 peptide antigens may be of clinical significance as a potential peptide vaccine against human breast cancer.

Recognition of an epitope of a breast cancer antigen by human antibody.

A novel tumor-associated peptide epitope KASIFLK expressed preferentially by breast cancer cells was identified using an IgG antibody from a breast cancer patient. A cDNA library from a MCF-7 breast cancer cell line was screened to isolate three cDNA clones that were immunoreactive with this antibody. KASIFLK was located in clones 27 and 40, both of which were identical to the cDNA and protein sequence of retinoblastoma binding protein 1 (RBP1, 250-256). An affinity-purified IgG antibody against the peptide epitope was completely absorbed by cytoplasmic extracts of MCF-7 cells. Immunohistochemical staining using this antibody revealed the antigen in MCF-7 cells and in 12 of 15 primary breast cancer tissues and 3 of 34 other cancer tissues, but in none of 6 normal breast tissues. Anti-KASIFLK antibody titers were significantly higher in sera of 55 breast cancer patients than in sera from 30 normal healthy donors (P>0.001). These results suggest that KASIFLK or its cross-reactive epitope is a breast cancer antigen and is immunogenic in humans.

IgM anti-ganglioside antibodies induced by melanoma cell vaccine correlate with survival of melanoma patients.

Melanoma cells express ganglioside antigens GM3, GD3, GM2, and GD2 on their surface. This study examined whether immunization with a melanoma cell vaccine induced anti-ganglioside antibody responses in melanoma patients and whether these responses were correlated with survival. Sixty-six patients who had received melanoma cell vaccine immunotherapy after surgical removal of regional metastatic melanoma were identified. Cryopreserved serum samples from these patients were used in an enzyme-linked immunsorbent assay to determine the IgM antibody levels to GM2, GD2, GM3, and GD3 prior to melanoma cell vaccine treatment and 4 wk after the first melanoma cell vaccine immunization. All antibody levels significantly increased by week 4 (p < 0.001 for all four antibodies) and all increases were significantly associated with survival (anti-GD2, p < 0.001; anti-GM2, p = 0.001; anti-GD3, p < 0.001; anti-GM3, p < 0.001). Anti-tumor activity of these antibodies was proved using five representative antibody-positive sera in a complement-dependent cytotoxicity assay with cultured melanoma cell lines. These studies suggest that GM2, GD2, GM2, and GD3 expressed by melanoma cells can induce specific IgM antibodies and that high levels of these antibodies might have a beneficial impact on survival.

Complement-mediated anti-HIV-1 effect induced by human IgM monoclonal antibody against ganglioside GM2.

HIV-infected cells aberrantly express a high level of antigenic glycosidic structures such as GM2 and Gg4. Some normal sera containing natural IgM Abs to GM2 and/or Gg4 cause C-mediated cytolysis of HIV-infected cells. In the present study we demonstrated that a human IgM anti-GM2 mAb (L55 Ab) can induce cytolysis of HIV-infected cells. Increased GM2 expression by HIV-1 infection of a human T cell line (MOLT4), a human monocyte cell line (U937), and human lymphoblastoid cells was confirmed by immunofluorescence staining with L55 Ab. These infected cells were readily lysed by L55 Ab in the presence of fresh human serum as a C source that alone did not cause cytolysis. L55 Ab also had the ability to destroy HIV-1 particles via C-mediated lysis. By adding L55 Ab together with human C to mixed culture of HIV-infected cells and naive cells, HIV-1 replication was significantly suppressed, and this effect was synergistic when L55 Ab was combined with a reverse transcriptase inhibitor and a proteinase inhibitor. Therefore, a human IgM anti-GM2 mAb may be effective in treating HIV-infected patients, especially when used together with chemotherapeutic agents.

Anti-tyrosinase-related protein-2 immune response in vitiligo patients and melanoma patients receiving active-specific immunotherapy.

Several melanosome glycoproteins have been shown to be antigenic in humans. Correlation of antigen-specific immune responses in patients with the autoimmune disease vitiligo, therapy-induced hypopigmentation, and cutaneous melanoma has not been well studied. We examined antibody responses to a melanocyte autoantigen, tyrosinase-related protein-2 (TRP-2), as it is highly expressed in cutaneous melanoma and melanocytes. TRP-2 recombinant protein was synthesized for western blot and affinity anti-TRP-2 enzyme-linked immunosorbent assay. We demonstrated that patients with malignant melanoma, vitiligo, and active-specific immunotherapy-induced depigmentation had significant anti-TRP-2 IgG titers. The highest level of anti-TRP-2 IgG response was found in vitiligo patients. Induction and enhancement of anti-TRP-2 IgG responses were observed in melanoma patients treated with a polyvalent melanoma cell vaccine containing TRP-2. Active-specific immunotherapy could induce and/or augment the TRP-2 IgG antibody titers. Melanoma patients who developed hypopigmentation and had improved survival after polyvalent melanoma cell vaccine had significantly augmented anti-TRP-2 antibody responses compared with patients with poor prognosis. This study demonstrates that TRP-2 autoantigen is immunogenic in humans. TRP-2 antibody responses provide a linkage between autoimmune responses by vitiligo patients and melanoma patients responding to immunotherapy who have induced hypopigmentation.

Augmentation of IgM antibody to gp43 tumor-associated antigen peptide by melanoma cell vaccine.

We previously reported that gp43 tumor-associated antigen peptide (DLTMKYQIF; designated 810 antigen) on human melanoma cells is recognized by IgM human monoclonal antibody L92 and by cytotoxic T lymphocytes (CTL). In this study, we retrospectively tested sera of 44 patients with regional metastatic melanoma (22 who recurred within 1 year and 22 who survived longer than 5 years) to determine if antibody responses to 810 antigen could be induced by immunization with an allogeneic melanoma cell vaccine that contained 810 peptide. IgM and IgG antibodies were assessed by enzyme-linked immunosorbent assay using a synthetic 810 nonamer peptide. A significant augmentation of IgM antibody was demonstrated 4 weeks after initiation of vaccine therapy, and the IgM level was significantly higher in patients who survived more than 5 years. The antigen epitope recognized by antibodies was located within TMKYQI. Of this epitope sequence, K appears to play a central role in antigenicity. The 810 antigen recognized by antibody and CTL may have clinical relevance as a potential source of melanoma vaccine.

Cellular immuno-PCR. Detection of a carbohydrate tumor marker.

Carbohydrate tumor-antigens are important tumor markers for diagnosis and functional characteristics of human cancer cells. Detection of these carbohydrate tumor antigens on metastatic cancer cells in blood is a difficult task. We developed a highly sensitive method to detect a cell surface carbohydrate antigen using a hybrid technology referred to as cellular immuno-PCR. This technique uses the human monoclonal antibody (HumAb) L612, specific to a tumor-related antigen (ganglioside) GM3 that is expressed on the cell surface of human tumor cells and not normal cells. L612 coupled to a DNA oligonucleotide for exponential amplification by DNA polymerase chain reaction (PCR) can be used to enhance the detection signal. The DNA-HumAb conjugate was assessed for detection of a small number of human cancer cells after PCR amplification and Southern blot analysis. To assess the assay specificity human melanoma and other cancer cell lines, as well as healthy donor and melanoma patients, bloods were assessed. Cellular immuno-PCR requires < 1 ng/ml DNA-HumAb complex and was shown to have a detection level of < 10 cells in titration studies in which melanoma cells were diluted in 2 million healthy donor peripheral blood lymphocytes. The assay was shown to be very sensitive and could detect low levels of GM3 antigen expression by tumor cells. This novel approach for detecting a carbohydrate tumor antigen on tumor cells in blood provides a potential useful clinicopathological assay.

Assessment of messenger RNA of beta 1-->4-N-acetylgalactosaminyl-transferase as a molecular marker for metastatic melanoma.

Gangliosides GM2 [GalNAc beta 1-4(NeuAc alpha 2-3)Gal beta 1-4Glc beta 1-1Cer] and GD2 [GalNAc beta 1-4(NeuAc alpha 2-8NeuAc alpha 2-3)Gal beta 1-4Glc beta 1-1Cer] are cell surface tumor-associated antigens and have been demonstrated to be important markers of human malignant melanoma progression. Expression of these glycolipid antigens on melanoma tissues can be assessed by immunohistochemistry or biochemical analysis. These methodologies, however, are not logistically practical or sensitive for testing metastatic melanoma cells in blood or in tissue biopsies. In the present study, we hypothesized that the enzyme involved in GM2 and GD2 synthesis, beta 1-->4-N-acetylgalactosaminyltransferase (beta 1-->4GalNac-T), can be a useful marker for detection of occult metastatic melanoma. A reverse transcription PCR and Southern blot assay to detect beta 1-->4GalNac-T mRNA expression was developed. Beta 1-->4GalNac-T mRNA was detected in all 13 melanoma cell lines tested. Metastatic melanoma of lymph nodes and different organ sites expressed beta 1-->4GalNac-T mRNA at various levels. Detection sensitivity of the reverse transcription PCR assay was 1 ng of total RNA extracted from tumor specimens and approximately 5 melanoma cells in 20 million normal donor peripheral blood lymphocytes. In assessment of blood from 126 melanoma patients, beta 1-->4GalNac-T mRNA was more frequently found in advanced-stage melanomas and in patients showing more aggressive tumor progression. Normal donor blood samples (n = 37) were all negative for beta 1-->4GalNac-T mRNA expression. These results suggest that beta 1-->4GalNac-T mRNA is a promising molecular marker for detecting melanoma cells, characterizing antigen expression, and monitoring tumor progression.

707-AP peptide recognized by human antibody induces human leukocyte antigen A2-restricted cytotoxic T lymphocyte killing of melanoma.

We recently identified a tumor-associated antigen that was recognized by human monoclonal antibody L94. The antibody-reactive 707-AP sequence RVAALARDAP, cloned from a melanoma cDNA library, was also found to be recognized by peripheral blood lymphocytes (PBLs) from melanoma patients. In this study, 707-AP was used to stimulate melanoma patients' PBLs for the establishment of peptide-specific CTL cell lines. CTL cell lines derived from 258 melanoma patients of different human leukocyte antigen (HLA)-A and HLA-B allele expressions were assessed by a 51Cr cytotoxicity assay against the peptide-pulsed autologous B lymphoblastoid cells and T2 HLA-A2 antigen-presenting cells and autologous and allogeneic melanoma cell lines. The analysis of 707-AP CTL activity demonstrated that only HLA-A2 patients' PBLs could be stimulated with 707-AP. 707-AP CTLs were able to specifically lyse HLA-A2 autologous and allogeneic melanoma cell lines. This verified the endogenous processing and presentation of 707-AP by melanoma cells. 707-AP CTL cytotoxicity against peptide-pulsed autologous HLA-A2 B lymphoblastoid cells and T2 HLA-A2 cells was also demonstrated. The killing activity of HLA-A2 707-AP CTL cell lines (CD8+ CD3+) was inhibited by anti-HLA class and anti-HLA-A2 monoclonal antibodies. The amino acid substitution or deletion analysis of the 707-AP sequence in CTL stimulation and recognition confirmed that position 2, amino acid V and position 9, amino acid A were essential. Both positions are known as supermotif anchors for HLA-A2 peptide sequences. Our studies demonstrated that 707-AP is a potent stimulator of CTLs that can induce peptide-specific HLA-A2 melanoma cell killing. The recognition of 707-AP by both antibody and CTLs suggests its potential significance as a peptide immunotherapeutic.

Recognition of gp43 tumor-associated antigen peptide by both HLA-A2 restricted CTL lines and antibodies from melanoma patients.

Previously, we detected a 43-kDa tumor-associated antigen (TAA) using the human monoclonal antibody L92, which recognizes the tetramer peptide KYQI. In the present study, cell lines of cytotoxic T lymphocytes (CTL) specific to the gp43 peptide (DLTMKYQIF) were established from peripheral blood lymphocytes (PBL) of melanoma patients. Patients' PBL (n = 326) of different HLA Class I types were assessed for gp43 CTL activity. CTL specific to gp43 peptide were generated only from HLA-A2 melanoma patients and not normal donors. gp43 CTL recognized gp43 peptide-pulsed autologous BLC and T2 HLA-A2 target cell lines. Furthermore, CTL lines were shown to kill both HLA-A2 autologous and HLA-A2 allogeneic melanoma cell lines, indicating that gp43 peptide can be processed endogenously and presented by melanoma cells as a common TAA. The gp43 CTL lines did not kill normal cells. Specific amino acids of the peptide were shown to be important determinants in stimulation and recognition of CTL. gp43 peptide, recognized by both antibodies and T cells of melanoma patients, is a novel TAA peptide that may play an important role in anti-tumor immunity in human.

Current status of human melanoma vaccines: can they control malignant melanoma?

The characterisation of human melanomas as immunogenic and the observation of spontaneous regression have led to the development of active specific immunotherapy in the form of vaccines for treatment of malignant melanoma. These vaccines have recently been the subject of considerable interest, particularly since the introduction of melanoma antigen cloning, the identification of specific peptide sequences recognised by the immune system, and better understanding of antigen presentation. Today, melanoma vaccines are a significant therapeutic agent in treatment of malignant melanoma. There are many melanoma vaccines that are in clinical trials which have produced very encouraging clinical responses. This review discusses the different forms of melanoma vaccines used in the 1990s and their current status in clinical trials.

Development of a human monoclonal antibody to ganglioside G(M2) with potential for cancer treatment.

A human B-lymphoblastoid cell clone, L55-81, that produces human monoclonal antibody (MAb) to ganglioside G(M2) was established from peripheral blood B lymphocytes of a melanoma patient. L55-81 secretes IgMkappa light chain antibody in a serum-free medium. G(M2) specificity of the antibody was tested by immune adherence assay, TLC immunostaining, and ELISA. Anti-G(M2) antibody was shown to have the ability to kill the G(M2)-rich human melanoma cell line M14 in the presence of human or rabbit complement. A purified L55-81 MAb (>99.5% purity in protein concentration) was biotinylated and tested for its reactivity to various histological-type biopsied tumor and normal tissues in an avidin-biotin detection system. L55-81 MAb (20 microg/ml) reacted with several types of tumor tissues such as melanoma (7 of 10), colon carcinoma (4 of 5), ovary carcinoma (4 of 5), breast carcinoma (1 of 5), kidney carcinoma (1 of 5), and prostate carcinoma (1 of 5). None of the normal tissues derived from 24 different organs and adjacent normal tissues surrounding the cancerous tissues were stained. Production of the antibody in a serum-free medium, the cytotoxic potential with human complement, the inability to react to normal tissues, and the ability to target antigen-specific target cells make L55-81 a potential therapeutic agent for the treatment of cancers expressing ganglioside G(M2).

Cytotoxic T cell recognition of a human melanoma derived peptide with a carboxyl-terminal alanine-proline sequence.

Recently, we defined the antigenic epitope recognized by the human monoclonal antibody L94 to be a protein with a C-terminal sequence of alanine-proline (AP). An antigenic peptide no. 707 (RVAALARDAP), which was identified by the use of cDNA libraries of an antigen positive melanoma cell line M14, was evaluated for cellular immune responses in melanoma patients. PBMC from 16 of 19 melanoma patients were shown to lyse autologous B lymphoblastoid cell lines (BCL) pulsed with synthetic peptide no. 707 (hereafter no. 707). This specific cytotoxicity to the peptide significantly increased in 84% of melanoma patients after in vivo immunization with a melanoma cell vaccine (MCV). In contrast, peptide specific cytotoxicity was observed in only one of 19 normal volunteer donors. In vitro restimulation of MCV treated patients' PBMC with no. 707 augmented cytotoxicity against autologous no. 707-pulsed BCL. This cytotoxicity was specific to the C-terminal sequence AP, since the removal of C-terminal AP completely abolished the specific lysis. no. 707 restimulation of PBMC enhanced cytotoxicity against autologous melanomas. Autologous melanoma and peptide-pulsed BCL targets were lysed by CD8+CTL in a HLA class I-restricted manner. The strong cytotoxicity was obtained from patients of HLA A24. CTL lysis of autologous no. 707-pulsed BCL was partially blocked by unlabeled autologous melanomas in a cold target inhibition test. This suggested that the epitope identical or cross-reactive to no. 707 may be presented on the melanoma cell surface by HLA class I antigens. Our findings suggest that peptide no. 707 presented on human melanoma cells is recognized by CTL and that C-terminal AP plays a critical role in both antibody and T cell recognition.

Polysialogangliosides expressed by amelanotic melanoma: a possible explanation for the poor response to anti-monosialoganglioside antibody 202 in a patient with melanoma.

A 52-year-old Japanese woman developed numerous amelanotic metastatic melanomas on the skin and in various organs three years after a surgical operation for primary melanoma on the right axilla. The patient was treated with monosialoganglioside specific monoclonal antibody 202; however, no apparent clinical effects were observed. Ganglioside analysis of a metastatic tumor demonstrated that it expressed GM3, GM2, GD3, GD2, and polysialogangliosides. Since polysialogangliosides rarely appear in melanomas, their expression may explain the patient's poor response to MAb 202. The relationship between ganglioside composition and the effect of anti-ganglioside monoclonal antibody is discussed.

Peptides with carboxyl-terminal sequence of alanine-proline: detection by a human monoclonal antibody.

A human B lymphoblastoid cell line JWCI-L94 secretes an IgM human monoclonal antibody (HuMAb) that reacts with human melanoma cell lines, M14 and M12. To identify the antigenic epitope of this antibody, we screened lambda gt11 expression libraries constructed from M14 and M12. A total of 12 immunoreactive clones were isolated, and their DNA sequences were determined. The only sequence shared by all these clones was alanine-proline (A-P) at the carboxyl (C) terminal. HuMAb L94 reacted not only with C-terminal A-P-containing fusion proteins, but also with the synthetic dipeptide A-P. None of the peptides containing A-P internally or amino terminally reacted to HuMAb L94. Proline or alanine alone had no ability to bind to HuMAb L94. When alanine was replaced by glycine (G-P) or proline (P-P), the binding activity of these peptides was similar to that of A-P. On the other hand, when alanine was replaced by serine, valine, leucine, glutamine, lysine, methionine, phenylalanine, or hydroxyl proline, the resulting peptide completely lost the antigenic activity of HuMAb L94. These results demonstrate that HuMAb L94 recognizes C-terminal A-P, G-P, or P-P, and that a human antibody can recognize peptides as small as a two-amino acid residue.

A decapeptide (Gln-Asp-Leu-Thr-Met-Lys-Tyr-Gln-Ile-Phe) from human melanoma is recognized by CTL in melanoma patients.

The decapeptide 810 (QDLTMKYQIF) contains the antigenic epitope (KYQI) recognized by human mAb L92. This sequence is found in a 43-kDa protein associated with human melanoma M14. We examined the expression of 810 in human cells and its involvement in the cellular immune responses of melanoma patients. Nineteen stage III melanoma patients and 19 normal donors were studied for their responses to 810. All patients were immunized in vivo with an allogeneic melanoma cell vaccine. PBMC cytotoxicity was tested on autologous EBV-transformed B lymphoblastoid cell lines (BCL) pulsed with 810 and autologous melanomas. Proliferative responses of PBMC to 810 were evaluated by using [3H]Tdr incorporation assays. Western blotting revealed that the 43-kDa protein was not specific to melanoma but was common to various cells. However, the percentage of cytotoxicity of PBMC against autologous 810-pulsed BCL was significantly greater in melanoma patients than in normal controls (p < 0.005). Cytotoxicity was increased after melanoma cell vaccine immunization in 15 patients (78%). Proliferative responses to 810 were observed only in melanoma patients and were enhanced in 12 patients (63%) after vaccination. Restimulation of PBMC from vaccinated patients with 810 increased cytotoxicity against both autologous 810-pulsed BCL and melanomas. These targets were lysed by CD8+ T lymphocytes in an HLA class I-restricted manner. HLA-A2 and -A11 seemed to serve as the 810-presenting molecule. Our findings indicate that 810 may function as an epitope for CTL on human melanoma and can be used as a vaccine.

Attachment of monophosphoryl lipid A (MPL) to cells and liposomes augments antibody response to membrane-bound gangliosides.

Antibodies to gangliosides are found in low levels in normal individuals, and attempts to augment their production have had limited success. Murine studies suggest that the antibody response to membrane-bound cryptic antigens, such as phospholipids and gangliosides, can be induced and augmented by attaching lipid A to membranes. Therefore, we assessed the ability of monophosphoryl lipid A, a non-toxic derivative of lipid A, to augment antibody response against membrane-associated gangliosides. Anti-ganglioside antibodies were IgM after the first and second immunizations; in contrast, anti-phospholipid antibodies were IgM after the first immunization and IgG after the second immunization. Mice (BALB/c) immunized with MPL-attached human cells as well as mice (C57BL/6J) immunized with MPL-attached syngeneic tumor cells (B16 melanoma) produced a significant IgM response. Mice (C57BL/6J) immunized with MPL-attached liposomes containing GM3 developed significantly higher IgM responses than those immunized with purified gangliosides, MPL or MPL-free B16 cells. However, the antibody response after immunization with MPL-GM3-liposomes is similar to that after immunization with MPL-attached tumor cells, even though the MPL-liposomes contained a 27-fold higher level of gangliosides than the tumor cells. Our results emphasize that co-expression of MPL with membrane-bound gangliosides is necessary to augment the anti-ganglioside antibody response. These findings may shed light on the elevated titers of anti-ganglioside IgM antibodies found in patients with motor neuron diseases, various neuropathies and classical ALS, and are relevant to clearance of circulating immunosuppressive gangliosides in cancer patients.

Human monoclonal antibody identified an immunoreactive tetrapeptide sequence (Lys-Tyr-Gln-Ile) in M(r) 43,000 protein of human melanoma.

The human monoclonal antibody (HuMAb) L92 reacts to an M(r) 43,000 protein associated with human melanoma. To identify the gene encoding its antigenic epitope, a complementary DNA expression library constructed from the human melanoma cell line UCLASO M14 was screened with HuMAb L92. DNA sequence analysis of the isolated clone revealed that the immunoreactive peptide was composed of 10 amino acids (QDLT-MKYQIF). The peptide was expressed in Escherichia coli with beta-galactosidase as a fused protein. There is no homology between the cloned sequence and other reported DNA sequences. Western blot analysis showed that the fused protein had specific binding to HuMAb L92. An antigen-encoding peptide with 10 amino acids was synthesized and tested for its immunoreactivity in vitro. HuMAb L92 reacted specifically to the 10-amino acid peptide in both an antibody-binding inhibition to the M(r) 43,000 protein and a solid-phase enzyme-linked immunosorbent assay. Using several truncated fusion proteins, we found the minimum number of amino acids required for the antibody binding to be 4 (KYQI). These results suggest that the identified peptide sequence encodes the antigenic epitope of the M(r) 43,000 protein.