Molecular Basis for the Interaction of the Hepatitis B Virus Core Antigen with the Surface Immunoglobulin Receptor on Naive B Cells

The nucleocapsid of the hepatitis B virus (HBV) is composed of 180 to 240 copies of the HBV core (HBc) protein. HBc antigen (HBcAg) capsids are extremely immunogenic and can activate naive B cells by cross-linking their surface receptors. The molecular basis for the interaction between HBcAg and naive B cells is not known. The functionality of this activation was evidenced in that low concentrations of HBcAg, but not the nonparticulate homologue HBV envelope antigen (HBeAg), could prime naive B cells to produce anti-HBc in vitro with splenocytes from HBcAg- and HBeAg-specific T-cell receptor transgenic mice. The frequency of these HBcAg-binding B cells was estimated by both hybridoma techniques and flow cytometry (B7-2 induction and direct HBcAg binding) to be approximately 4 to 8% of the B cells in a naive spleen. Cloning and sequence analysis of the immunoglobulin heavy- and light-chain variable (VH and VL) domains of seven primary HBcAg-binding hybridomas revealed that six (86%) were related to the murine and human VH1 germ line gene families and one was related to the murine VH3 family. By using synthetic peptides spanning three VH1 sequences, one VH3 sequence, and one VLκV sequence, a linear motif in the framework region 1 (FR1)complementarity-determining region 1 (CDR1) junction of the VH1 sequence was identified that bound HBcAg. Interestingly, the HBcAg-binding motif was present in the VL domain of the HBcAg-binding VH3-encoded antibody. Finally, two monoclonal antibodies containing linear HBcAg-binding motifs blocked HBcAg presentation by purified naive B cells to purified HBcAg-primed CD4(+) T cells. Thus, the ability of HBcAg to bind and activate a high frequency of naive B cells seems to be mediated through a linear motif present in the FR1-CDR1 junction of the heavy or light chain of the B-cell surface receptor.

乙型肝炎病毒（hepatitis B virus, HBV）核衣壳由180至240拷贝的HBV核心（HBc）蛋白组装而成。乙型肝炎病毒核心抗原（HBcAg）衣壳具备极强的免疫原性，可通过交联初始B细胞的表面受体激活此类细胞。目前，HBcAg与初始B细胞之间相互作用的分子基础尚未阐明。
该激活过程的功能性已得到实验验证：低浓度的HBcAg（而非其非颗粒型同源物——乙型肝炎病毒包膜抗原（HBeAg））能够致敏初始B细胞，使其在与HBcAg及HBeAg特异性T细胞受体转基因小鼠的脾细胞共培养时，可在体外产生抗HBc抗体。
研究人员通过杂交瘤技术与流式细胞术（含B7-2诱导实验及直接HBcAg结合实验）估算，结合HBcAg的B细胞频率约占初始脾脏B细胞群体的4%~8%。
对7株原发性HBcAg结合型杂交瘤的免疫球蛋白重链可变区（variable heavy chain, VH）和轻链可变区（variable light chain, VL）进行克隆与序列分析后发现，其中6株（占比86%）归属于鼠源及人源VH1种系基因家族，剩余1株则属于鼠源VH3家族。
借助覆盖3条VH1序列、1条VH3序列及1条VLκV序列的合成肽，研究人员在VH1序列的骨架区1（framework region 1, FR1）与互补决定区1（complementarity-determining region 1, CDR1）的接头区域中，鉴定出了一个可结合HBcAg的线性基序。值得注意的是，该HBcAg结合基序同样存在于编码HBcAg结合抗体的VH3序列的VL结构域中。
最终，2株携带线性HBcAg结合基序的单克隆抗体能够阻断纯化的初始B细胞将HBcAg呈递给纯化的HBcAg致敏CD4阳性T细胞。综上，HBcAg结合并激活高频率初始B细胞的能力，似乎是通过B细胞表面受体重链或轻链的FR1-CDR1接头区域存在的线性基序所介导的。