抗ARL-1单克隆抗体的制备及初步应用

目的: 制备抗醛糖还原酶相似蛋白(aldosereductaselikeprotein, ARL- 1)的单克隆抗体 (mAb)并进行初步应用。方法: 用纯化的ARL- GST蛋白免疫BALB/c小鼠, 采用杂交瘤技术制备mAb。间接ELISA法、Westernblot及免疫组织化学染色法, 对mAb进行鉴定及初步应用。结果: 获得 1株可稳定分泌抗ARL -1蛋白mAb的杂交瘤细胞系。mAb的Ig亚类为IgG2b, 轻链属于κ型。腹水mAb的效价为: 1∶4. 096×105。Westernblot的结果表明, ARL 1蛋白在被检测的肝癌组织中呈高表达, 而在相应的癌旁组织中不表达。免疫组织化学染色的结果表明, ARL -1蛋白在肝癌组织中呈高表达且主要定位在胞质内。结论: 成功地制备 1株抗ARL -1蛋白的mAb。初步应用的结果表明, ARL- 1蛋白在肝癌组织中呈高表达, 为进一步研究ARL -1蛋白的功能, 以及其与肝癌的确切关系奠定了基础。     

抗SARS冠状病毒M蛋白片段单克隆抗体的制备与初步鉴定

目的:制备抗SARS冠状病毒(SARS-CoV)M蛋白N端1~43氨基酸(aa)单克隆抗体(mAb),并对其特性进行初步鉴定。方法:用纯化的SARS-M/GST融合蛋白抗原免疫BALB/c小鼠,采用杂交瘤技术制备抗M蛋白片段的mAb。用间接ELISA法筛选能分泌抗M蛋白片段mAb的杂交瘤细胞株。Westernblot和间接ELISA鉴定所获mAb的特异性,并用小鼠mAb亚类测定试剂盒检测所获的mAbIg亚类。为分析mAb识别位点,进一步将M蛋白片段截短为部分重叠的2段表达,以Westernblot初步定位mAb识别位点。结果:获得1株可分泌特异性抗SARS-CoVM蛋白片段的mAb杂交瘤细胞株(3H9),Ig亚类鉴定为IgG2a,轻链为κ型。Westernblot显示其mAb可特异识别SARS-CoVM蛋白N端1~43aa,间接ELISA证实mAb可与包被于聚苯乙烯微孔板上的SARS病毒全蛋白抗原发生特异性反应,其识别位点位于M蛋白N端16~28aa。结论:成功获得1株抗SARS-CoVM蛋白N端1~43aamAb杂交瘤细胞,并初步定位其识别位点。     

S100A9蛋白单克隆抗体的制备及初步鉴定

目的制备S100A9重组蛋白及其单克隆抗体(mAb),并对抗体进行特异性鉴定。方法以成人肝cDNA表达文库为模板,构建重组表达质粒pGEX-4T-1-S100A9和pET-32a-S100A9。融合蛋白在大肠杆菌中表达,纯化后以His-S100A9作为免疫原制备鼠mAb。采用ELISA和Western blot法鉴定筛选抗体。挑取杂交瘤细胞株制备腹水并纯化,采用Western blot法和间接免疫荧光染色鉴定mAb的特异性。结果 GST-S100A9和His-S100A9融合蛋白均成功构建表达。共筛选到抗S100A9杂交瘤细胞18株,其中15株在Western blot检测中与重组蛋白呈强阳性反应,3株呈弱阳性反应。2株可识别肝癌组织间隙液中的S100A9天然蛋白。结论成功地制备出多株抗S100A9的mAb。     

降钙素原抗原的表达及其抗体的制备与初步鉴定

目的:构建降钙素原(PCT)原核表达载体, 制备其多克隆抗体(pAb)和单克隆抗体(mAb), 并进行特性鉴定.方法:以甲状腺癌细胞cDNA文库为模板, 构建重组表达质粒pGEX- 4T-1-PCT和PET-32a-PCT, 在大肠杆菌中分别表达GST-PCT和His-PCT融合蛋白, 用His-PCT免疫家兔和BALB/c小鼠制备兔pAb和鼠mAb.通过ELISA法测定抗人PCT抗血清效价;用Western blot、 间接免疫荧光鉴定pAb的特异性.使用间接ELISA法筛选分泌特异性抗PCT的杂交瘤细胞株, 采用Western blot和间接免疫荧光鉴定mAb的特异性. 结果:构建了重组表达质粒pGEX- 4T-1-PCT和PET-32a-PCT, 并在大肠杆菌中表达、纯化.经ELISA法测得抗人PCT抗血清效价是1∶ 256 000.制备的抗PCT兔多克隆抗体可特异地识别重组和天然的PCT蛋白.获得8株可稳定分泌抗PCT的杂交瘤细胞株, 可识别重组人PCT蛋白, 其中有4株可特异性结合TT细胞质中的天然蛋白. 结论: 成功地制备出兔抗人PCT抗血清和8株抗PCT的mAb, 为进一步研究PCT在严重的细菌感染和脓毒症中的病理及生理作用奠定了基础.     

抗3种标签蛋白单克隆抗体的制备及特性鉴定

目的：制备并鉴定抗GST、His x6及FLAG的单克隆抗体（mAb）。方法：分别以含GST、His x6及FLAG标签的融合蛋白为抗原免疫BALB／c小鼠，通过细胞融合制备抗相应标签蛋白的mAb。用Western blot检测mAb对变性融合蛋白的反应性。用流式细胞术（FCM）检测抗FLAG mAb对细胞膜表面融合蛋白的反应性。结果：其获得12株可稳定分泌抗3种标签蛋白mAb的杂交瘤细胞（其中5株可分泌抗GST mAb，5株可分泌抗His x6 mAb，2株可分泌抗FLAG mAb）。11株mAb均可用于相应融合蛋白的Western blot检测。1株抗FLAG mAb检测细胞膜表面融合蛋白的阳性率，与商品化的mAb M2相接近。用1株抗His x6 mAb制备亲和层析柱，并用于纯化IL-8-His融合蛋白，也获得较满意的结果。结论：成功地制备了抗GST、抗His x6及抗FLAG的mAb，为含标签的融合蛋白的研究和应用提供了重要的工具。     

His标签单克隆抗体的制备、鉴定及初步应用

目的:制备和鉴定抗His标签单克隆抗体(mAb),初步建立检测和纯化带His标签融合蛋白的方法.方法:用碳化二亚胺法合成His-tag完全抗原,免疫BALB/c小鼠,采用杂交瘤技术进行细胞融合,通过有限稀释法和间接ELISA法克隆和筛选阳性杂交瘤细胞株.常规制备腹水,用辛酸-硫酸铵法纯化,并对纯化的mAb进行特异性鉴定.结果:His-tag完全抗原偶联成功,经细胞融合、筛选及克隆化,成功获得1株分泌His标签mAb的杂交瘤细胞株.制备腹水测得腹水效价高于 1:106,且此株mAb与其他融合蛋白标签无交叉反应,并在含His标签融合蛋白的鉴定实验中取得了满意效果.结论:成功制备了1株His标签mAb,为带His标签融合蛋白的研究应用提供了重要的工具.     

抗人生长转化因子15单克隆抗体制备及鉴定

目的:制备抗人生长转化因子15(GDF15)单克隆抗体(mAb)并鉴定其特性。方法:构建GDF15原核表达载体pGEX-4T-2-gdf15,诱导表达并纯化GST-GDF15融合蛋白。以该融合蛋白免疫BALB/c小鼠,取小鼠的脾细胞与同系小鼠Sp2/0骨髓瘤细胞常规融合,依次进行阳性杂交瘤细胞株的筛选及亚克隆,最终获得稳定分泌抗GDF15 mAb杂交瘤细胞株。以间接ELISA法检测抗体效价;Western blot鉴定抗体特异性;免疫共沉淀法初步检测在肝癌患者血清中GDF15表达水平。结果:获得1株稳定分泌抗人GDF15 mAb杂交瘤细胞株,命名为9G3;抗体免疫球蛋白亚类为IgG1,轻链为κ型;免疫共沉淀及质谱分析证实抗GDF15 mAb9G3能与肝癌患者血清中GDF15蛋白特异性结合并且GDF15水平在肝癌患者血清中较正常人显著升高。结论:成功制备了抗人GDF15mAb,为后期肝癌早期诊断试剂盒的开发奠定了良好的基础。     

鼠抗人CD20单克隆抗体的制备与鉴定

目的:制备CD20融合蛋白及制备CD20的单克隆抗体(mAb),并对其进行特性鉴定.方法:以人单核细胞cDNA文库为模板,构建重组表达质粒pGEX- 4T-1-CD20 (即CD20-GST) 和pET-32a-CD20(即CD20-His).以融合蛋白CD20-GST为免疫源免疫BALB/c雌性小鼠制备(mAb),用间接ELISA法测定抗体的效价,Western blot、免疫荧光鉴定其特异性及免疫组化染色法对mAb相应的抗原进行组织定位.结果:CD20-GST 和CD20-His蛋白在大肠杆菌中可高效表达,经纯化后可获得高纯度的CD20蛋白,经细胞融合得到1株稳定分泌CD20抗体的杂交瘤细胞株,Western blot和免疫组化显示抗CD20 mAb与CD20蛋白具有良好的反应性.结论:成功制备高纯度CD20蛋白并以此为抗原制备了1株能够稳定分泌抗人CD20的鼠mAb的杂交瘤细胞株BD11F4,为进一步研究CD20对非霍奇金淋巴瘤的诊断治疗提供基础.     

抗血型M、N及抗血型糖蛋白A/B单克隆抗体的制备及其特性鉴定

目的:制备抗血型M、N及抗血型糖蛋白A/B(GPA/GPB)的单克隆抗体(mAb),并进行特性鉴定。方法:用人“O”型血红细胞作为免疫源,免疫BALB/c小鼠。采用淋巴细胞杂交瘤技术制备mAb,用谱红细胞筛选阳性克隆;采用直接、间接血凝试验检测杂交瘤细胞培养上清及腹水中mAb的效价。分别用快速定性试纸和酶处理红细胞检测mAb的Ig亚类及抗原表位。用Western blot鉴定抗GPA/GPB mAb的特异性。结果:获得4株分泌抗M、1株抗N及3株抗GPA/GPB mAb的杂交瘤细胞株。杂交瘤细胞培养上清mAb的效价介于1×2-4~1×2-8之间,腹水mAb的效价在1×2-7~1×2-12之间。除1株mAb 1C1C9C4为IgM外,其他7株mAb均为IgG。通过杂交瘤细胞培养上清与谱红细胞的反应格局,结合mAb抗原表位的检测,确定4株和1株mAb可分别特异性结合于GPA的M、N抗原表位;另3株mAb 6D7C9、7C9H4和7C9G11与“O”型血红细胞膜的Western blot结果显示,均可结合GPA、GPB蛋白。结论:成功地建立了4株分泌抗M、1株分泌抗N及3株分泌抗GPA/GPB mAb的杂交瘤细胞株,可用于MNSs血型系统的研究及鉴定,并为制备双功能抗体用于病毒、肿瘤疾病的诊断和治疗打下了坚实的基础。     

抗果蝇ECP蛋白单克隆抗体的制备及特性鉴定

目的:制备抗果蝇ECP蛋白的单克隆抗体(mAb)并进行特性鉴定。方法:用大肠杆菌表达并纯化的GST-ECP融合蛋白,免疫6~8wk的雌性BALB/c小鼠,取其脾细胞与Sp2/0骨髓瘤细胞融合,经间接ELISA筛选和克隆化培养制备杂交瘤细胞系。用间接ELISA及Western blot等方法对mAb的Ig亚类(型)、腹水效价及特异性进行鉴定。结果:获得1株杂交瘤细胞株,命名为8G9。Ig亚类(型)鉴定表明,此株mAb为IgG1(κ型)。腹水mAb的效价为1∶1×105。Western blot分析表明,该株mAb可与果蝇的幼虫、蛹及成虫组织中表达的ECP特异性结合,可特异性的识别ECP抗原的231~300aa区段。结论:获得1株能稳定分泌抗ECP mAb的细胞株,为进一步研究ECP的功能奠定了基础。     

以乙肝病毒核心蛋白为载体制备肠道病毒71 型非结构蛋白3D 单克隆抗体

目的：预测肠道病毒71型（EV71）非结构蛋白3D的表位,以HBc蛋白为载体展示多肽,制备并鉴定抗EV71-3D的特异性单克隆抗体（mAb）。方法：应用生物信息学方法分析预测出EV71-3D蛋白亲水性和免疫原性指数较高的多肽片段,并运用HBc颗粒型蛋白载体展示肽段,构建多肽融合蛋白,免疫BALB/c雌鼠,通过杂交瘤技术和亲和层析技术制备和纯化抗EV71-3D蛋白的特异性mAb,用间接ELISA、ELISPOT、IFA和IHC对mAb的性质进行初步鉴定。结果：构建表达分别嵌合3D蛋白34~43位氨基酸残基、 61~76位氨基酸残基、151~164位氨基酸残基的HBc重组蛋白,免疫并经过多轮克隆化筛选,获得抗EV71-3D单克隆抗体3E1,其亚类为IgG2a;免疫荧光试验、ELISPOT法和免疫组织化学染色结果显示其可与EV71特异性结合。结论：成功制备可特异性识别EV71的单克隆抗体3E1,为病毒的检测及进一步研究3D蛋白的功能奠定了基础,同时还验证了生物信息学技术与HBc颗粒型载体展示多肽技术相结合可快速高效地制备单克隆抗体。     

Mapping of B-cell epitopes of hemagglutinin protein of rinderpest virus

Monoclonal antibodies (mAbs) against secreted hemagglutinin (H) protein of rinderpest virus (RPV) expressed by a recombinant baculovirus were generated to characterize the antigenic sites on H protein and regions of functional significance. Three of the mAbs displayed hemagglutination inhibition activity and these mAbs were unable to neutralize virus infectivity. Western immunoblot analysis of overlapping deletion mutants indicated that three mAbs recognize antigenic regions at the extreme carboxy terminus (between amino acids 569 and 609) and the fourth mAb between amino acids 512 and 568. Using synthetic peptides, aa 569-577 and 575-583 were identified as the epitopes for E2G4 and D2F4, respectively. The epitopic domains of A12A9 and E2B6 mAbs were mapped to regions encompassing aa 527-554 and 588-609. Two epitopes spanning the extreme carboxy terminal region of aa 573 to 587 and 588 to 609 were shown to be immunodominant employing a competitive ELISA with polyclonal sera form vaccinated cattle. The D2F4 mAb which recognizes a unique epitope on RPV-H is not present on the closely related peste des petits ruminant virus HN protein and this mAb could serve as a tool in the seromonitoring program after rinderpest vaccination.     

The latex allergen hev B 5 is an antigen with repetitive murine B-cell epitopes

BACKGROUND: Hev b 5 is a potent latex allergen. In this study, we characterize the linear B-cell epitopes for three monoclonal antibodies (mAbs) to Hev b 5. METHODS: The mAbs included 2 IgG1 (6A10, 3G3) and 1 IgG2b (6F6) isotypes. We used SPOTscan analysis with overlapping octapeptides to identify the binding regions for the antibodies and then methionine substitution analysis to further define the critical amino acids (aa) in each epitope. Site-directed mutagenesis was used to selectively eliminate the IgG binding for each epitope and single and multiple mutations were expressed as recombinant GST fusion proteins. Antibody recognition of the mutant proteins was determined by inhibition ELISA. RESULTS: All three mAbs recognized the same aa sequence by SPOTs analysis with slight variations, and this epitope was repeated 3 times in the Hev b 5 sequence; APETEK (63-68), PAEGEK (120-125), and PAEEEK (126-131). Sequential methionine substitution by SPOTsalogue identified K68, E122, and K131 as critical aa in each epitope to change by site-directed mutagenesis. Inhibition ELISA with the mutant proteins indicated that epitope 126-131 was the dominant epitope, but mutation of epitope 120-125 was also required to eliminate mAb reactivity to Hev b 5. The antibodies did not appear to recognize the epitope 63-68 in the recombinant fusion protein. CONCLUSIONS: We identified an immunodominant B-cell epitope in Hev b 5 that is repeated 3 times within the sequence, making Hev b 5 multivalent. Well-characterized monoclonals recognizing repeated epitopes would be a good choice for immunodetection of Hev b 5 in glove extracts where individual epitopes could get altered by the manufacturing process.     

抗MDV-1 VP22羧基端单克隆抗体的制备与免疫学特性鉴定

目的：制备抗血清Ⅰ型马立克氏病病毒（MDV-1）VP22的单克隆抗体（mAb），并鉴定其免疫学特性。方法：在原核系统中表达VP22羧基端区域（94～243aa），获得融合表达产物GST-VP22C。将该表达产物切胶免疫小鼠，利用杂交瘤技术制备抗MDV-1VP22C的mAb，并通过ELISA、间接免疫荧光（IFA）、Western blot鉴定其特性。结果：获得了2株可稳定分泌抗MDV-1VP22C的mAb的杂交瘤细胞，命名为3F7、4FA。IFA鉴定表明，两株mAb均能与感染MDV-1的成纤维细胞反应，其中，3F7mAb染色呈现MDV空斑，而4FAmAb呈现整个单层的细胞核荧光。ELISA和Westernblot分析表明，3F7能与杆状病毒表达的VP22反应，4FA不具备该特性。对3F7mAb进一步鉴定，确定了该mAb针对的具体位置在94～193aa处，是蛋白转导域的预测位置。结论：成功地制备了抗MDV-1VP22C的mAb，为深入研究VP22蛋白的转导功能提供了有用的试剂。     

Human and murine B-cells recognize the HBeAg/beta (or HBe2) epitope as a linear determinant.

The complete amino acid (aa) sequence of the hepatitis B virus (HBV) core protein (HBcAg), deduced from the genome of the HBV ayw subtype, was synthesized as decapeptides with five overlapping aas. The peptides were tested for reactivity with monoclonal antibodies (mAbs) to the beta (or HBe2) epitope of hepatitis B e antigen (HBe/b mAbs; 57/8, 78/3, 141/158 and 141/207). Cross-competition between the mAbs with a mAb to the HBe/alpha epitope (or HBe1) and an anti-HBc mAb showed that all the HBe/b mAbs specifically inhibited human anti-HBe/b binding. Screening the HBc/e peptides showed that all anti-HBe/b mAbs recognized a peptide covering the residues 126-135. Three of the mAbs, 78/3, 141/152 and 141/207, had a less restricted reactivity than the other two, suggesting the recognition of the HBe/b as a discontinuous determinant. Fine mapping of the region aa 126-135 was performed by synthesizing decapeptides with nine overlapping aas, covering residues aa 121-140. All mAbs, except 78/3, reacted with the linear sequence TPPAYR, at residues 128-133. An additional set of peptides was synthesized, where the six aas within the epitope 128-133 were substituted in turn by the other 19 possible aas. By this approach, the essential aas for mAb 57/8 were found to be the sequence of PPA at residues 129-131, and for mAb 141/158 the sequence PP-Y, at residues 129, 130 and 132, respectively. Human recognition of the linear HBe/b epitope was investigated by using a peptide covering residues 121-140 (p 33). Thirty-one sera from chronic carriers of HBsAg, of which seven were positive for HBeAg and the remaining 24 for anti-HBe, were investigated. Of the sera with HBeAg, two had low levels of anti/-HBe/b in the p 33 assay. Out of the sera with anti-HBe, eight were positive in the p 33 EIA. Thus, murine monoclonals and human sera may recognize the HBe/b epitope as a linear determinant residing around aa 130.     

Conserved amino acids W423 and N424 in receptor-binding domain of SARS-CoV are potential targets for therapeutic monoclonal antibody

The receptor-binding domain (RBD) on spike protein of severe acute respiratory syndrome-associated coronavirus (SARS-CoV) is the main region interacting with the viral receptor-ACE2 and is a useful target for induction of neutralizing antibodies against SARS-CoV infection. Here we generated two monoclonal antibodies (mAbs), targeting RBD, with marked virus neutralizing activity. The mAbs recognize a new conformational epitope which consists of several discontinuous peptides (aa. 343-367, 373-390 and 411-428) and is spatially located neighboring the receptor-binding motif (RPM) region of the RBD. Importantly, W423 and N424 residues are essential for mAb recognition and are highly conserved among 107 different strains of SARS, indicating that the residues are the most critical in the epitope which is a novel potential target for therapeutic mAbs. A human-mouse chimeric antibody, based upon the original murine mAb, was also constructed and shown to possess good neutralizing activity and high affinity.     

Monoclonal antibodies recognizing EVETPIRN epitope of influenza A virus M2 protein could protect mice from lethal influenza A virus challenge

Based on the fact that the 24 amino acid extracellular domain of M2 protein (M2e) is nearly invariant in all influenza A strains, several different M2e vaccine constructs and vaccination modalities have been developed by others and us. Although most of these vaccines could induce efficient and broad-spectrum immunity inhibiting influenza A virus infection in mice model, information of the refined protective epitope on M2e was scarce. In this paper, two M2e specific monoclonal antibodies (mAbs) conferring protective immunity in vivo were reported, which in passive administration could protect 75% mice from five LD(50) (50% lethal dose) challenge of influenza virus A/PR/8/34. In addition, higher M2e specific antibody titer (over 1:1600) could be detected after 12h of intraperitoneal passive administration in mice sera. Peptide mapping assay indicated that both mAbs strongly interacted with N-terminus and middle part peptides of M2e (NM2, aa2-12; MM2, aa8-18), but not with the C-terminus peptide (CM2, aa13-24). More importantly, M2e specific mAbs could recognize EVETPIRN (aa6-13) peptide, which were the overlapping region of NM2 and MM2 peptide and the neighboring amino acid residues. In contrast, M2e domain that was deleted EVETPIR sequence could not be recognized by either mAb in immunoblotting assay. All these results indicated that the epitope EVETPIRN (aa6-13) on M2e could be responsible for the induction of the protective immunity.     

Epitope mapping and biological function analysis of antibodies produced by immunization of mice with an inactivated Chinese isolate of severe acute respiratory syndrome-associated coronavirus (SARS-CoV)

Inactivated severe acute respiratory syndrome-associated coronavirus (SARS-CoV) has been tested as a candidate vaccine against the re-emergence of SARS. In order to understand the efficacy and safety of this approach, it is important to know the antibody specificities generated with inactivated SARS-CoV. In the current study, a panel of twelve monoclonal antibodies (mAbs) was established by immunizing Balb/c mice with the inactivated BJ01 strain of SARS-CoV isolated from the lung tissue of a SARS-infected Chinese patient. These mAbs could recognize SARS-CoV-infected cells by immunofluorescence analysis (IFA). Seven of them were mapped to the specific segments of recombinant spike (S) protein: six on S1 subunit (aa 12-798) and one on S2 subunit (aa 797-1192). High neutralizing titers against SARS-CoV were detected with two mAbs (1A5 and 2C5) targeting at a subdomain of S protein (aa 310-535), consistent with the previous report that this segment of S protein contains the major neutralizing domain. Some of these S-specific mAbs were able to recognize cleaved products of S protein in SARS-CoV-infected Vero E6 cells. None of the remaining five mAbs could recognize either of the recombinant S, N, M, or E antigens by ELISA. This study demonstrated that the inactivated SARS-CoV was able to preserve the immunogenicity of S protein including its major neutralizing domain. The relative ease with which these mAbs were generated against SARS-CoV virions further supports that subunit vaccination with S constructs may also be able to protect animals and perhaps humans. It is somewhat unexpected that no N-specific mAbs were identified albeit anti-N IgG was easily identified in SARS-CoV-infected patients. The availability of this panel of mAbs also provided potentially useful agents with applications in therapy, diagnosis, and basic research of SARS-CoV.     

Characterisation of a linear binding site for a monoclonal antibody to hepatitis B core antigen.

The complete amino acid (aa) sequence of the hepatitis B virus (HBV) core protein (HBcAg), ayw subtype, was synthesized as decapeptides with five overlapping aas. The peptides were tested for reactivity with monoclonal antibodies (mAbs) to HBcAg (35/312, 37/275, and 7/275). All the mAbs specifically inhibited human anti-HBc by cross competition in assays for anti-HBc and anti-HBe. The mAb 35/312 recognised a peptide covering residues 76-85 of the HBcAg sequence. The other two mAbs did not react specifically with any linear peptide, suggesting discontinuous epitopes for these mAbs. The linear sequence EDPASR at residues 77-82 was found to constitute the epitope for mAb 35/312 when fine mapping the binding site. The most essential aas for mAb 35/312 were found to be the DP at residues 79-80, when peptides were synthesized where the aas at 77-83, were substituted by the other 19 aas. Since the mAb 35/312 inhibits the binding of human anti-HBc positive sera, which are known to recognise an SDS labile epitope, the sequence 77-82 might be a part of a larger discontinuous epitope. Alternatively the mAb 35/312 blocks the binding of human anti-HBc by steric hindrance.