BioVector® pMECS 噬菌体展示质粒载体 (Phagemid Vector)

BioVector® pMECS Phagemid Vector for Nanobody / VHH Phage Display

第一部分：中文说明

一、 载体基本信息与用途

• 载体名称：BioVector® pMECS

• 载体类型：噬菌体显性/隐性展示质粒载体（Phagemid Vector）。

• 核心用途：专门用于驼科动物（骆驼、大羊驼、羊驼）外周血淋巴细胞的重链单域抗体（VHH / Nanobody，即纳米抗体）的基因克隆与噬菌体展示文库（Phage Display Library）构建。

• 载体大小：约 4510 bp（4.5 kb）。

• 抗性标记：氨苄青霉素抗性（Ampicillin，$Amp^R$）。

• 常用宿主菌：E. coliTG1、ER2738 等琥珀抑制型（Amber Suppressor）大肠杆菌株，用于文库的扩增与噬菌体拯救。

二、 关键结构域与元件配置

• 启动子（Promoter）：带有阻遏位点的 $P_{lac}$（乳糖启动子/操纵子系统），可通过 IPTG 进行诱导转录，在无诱导剂时保持较低的背景表达。

• 信号肽（Signal Peptide）：含有 BioVector® PelB 信号肽，引导表达的纳米抗体定向分泌至大肠杆菌的周质空间（Periplasmic Space），有利于抗体的高效可溶性折叠。

• 多克隆位点（MCS）与克隆策略：

  • 核心克隆位点为 PstI 和 NotI。

  • 在 PstI 与 NotI 之间包含一个唯一的 XbaI 位点。在酶切制备文库载体时，常利用 XbaI 进行双酶切质控，或用于将未完全双酶切的空载体切断，以最大程度降低文库构建的“空载背景”。

• 融合标签与融合蛋白：

  • 目标 VHH 序列插入后，其 C 端将与基因III蛋白（g3p / pIII 噬菌体外壳蛋白）以及下游的 HA 标签（表达标签）和 His 标签（纯化标签）融合表达。

  • 琥珀终止密码子（Amber Stop Codon, TAG）：位于抗体标签与 g3p 基因之间。

    • 在琥珀抑制株（如 TG1）中：TAG 被读码为氨基酸，表达出 VHH-HA-His-g3p 融合蛋白，组装在辅助噬菌体表面进行高通量筛选（Panning）。

    • 在非琥珀抑制株（如 WK6, BL21）中：TAG 被识别为终止符，直接分泌表达不带 g3p 的可溶性单体纳米抗体（VHH-HA-His），方便直接进行 ELISA 验证或纯化。

三、 纳米抗体文库构建基本流程

1. RNA 提取与 cDNA 合成：提取免疫后驼科动物外周血淋巴细胞的总 RNA，反转录合成 cDNA。

2. 巢式 PCR 扩增 VHH：通过第一轮 PCR 扩增重链抗体片段（约 700 bp，包含 VHH、Hinge 及部分 CH2 结构域），第二轮 PCR 引入 PstI 和 NotI 限制性酶切位点，获得专一性的 VHH 片段（约 400 bp）。

3. 限制性内切酶消化：使用 PstI 和 NotI 分别对 VHH 片段和 BioVector® pMECS 空载体进行双酶切。

4. 连接与电转化：将酶切产物进行 T4 连接，高效率电转化至 E. coli TG1 感受态细胞中，通过平板稀释计数计算文库的库容（通常需达到 $10^7\text{--}10^9$ CFU 级别）。

5. 噬菌体拯救与淘洗：加入辅助噬菌体进行超感染（Superinfection），回收携带 VHH 的噬菌体颗粒，针对靶抗原进行 3–4 轮固相或液相富集淘洗。

四、 核心科研应用

1. 天然/免疫纳米抗体文库筛选：作为全球实验室最经典的 VHH 噬菌体展示骨架之一，用于高效分离针对肿瘤靶点、病毒表面蛋白、免疫 checkpoint 的高亲和力单域抗体。

2. 多价/多特异性抗体串联构建（Manifold Constructs）：利用 BioVector® pMECS-VHH 载体中的独特连接序列，可将不同的 VHH 基因以串联形式克隆，快速组装为双价（Bivalent）、双特异性（Bispecific）或双表位（Biparatopic）的纳米抗体衍生物，以提升亲和力或实现多靶点阻断。

3. 原位可溶性周质表达：得益于 Amber 密码子设计，筛选出的阳性克隆无需更换载体，可直接转换至非抑制型宿主菌中实现周质空间内高纯度单域抗体的中试级别分泌表达。

PART 2: ENGLISH SECTION

I. General Description and Applications

• Vector Name: BioVector® pMECS

• Vector Type: Phagemid Vector (Phage Display Vector).

• Primary Application: Specifically designed for cloning single-domain heavy-chain antibody fragments (VHH / Nanobody) sourced from immunized camelids and generating high-diversity phage display libraries.

• Vector Size: Approximately 4510 bp (~4.5 kb).

• Selection Marker: Ampicillin resistance ($Amp^R$).

• Common Host Strains: E. coliTG1 or ER2738 (amber-suppressing strains utilized for library amplification, propagation, and helper-phage rescue protocols).

II. Vector Anatomy and Component Configuration

• Promoter System: Driven by the tight, IPTG-inducible $P_{lac}$ (lac promoter/operator system), which minimizes background cytotoxicity before induction.

• Signal Peptide: Outfitted with a BioVector® PelB leader sequence to route translated nanobody products efficiently across the inner membrane into the E. coliperiplasmic space for optimal, soluble disulfide bond formation.

• Cloning Site Strategy:

  • Core target genes are inserted using the PstI and NotI restriction boundaries.

  • Features a unique XbaI restriction site nestled directly between the PstI and NotI markers. This design allows diagnostic digestion during background vector preparation, where XbaI linearizes any singly cut, non-recombinant empty vectors, drastically dropping false-positive background rates during library ligation.

• Tags & Fusion Partners:

  • Inserted VHH fragments are expressed as a fusion protein joined to an HA tag, a Hexa-Histidine (His) tag, and the bacteriophage gene III protein (g3p/pIII).

  • Amber Stop Codon (TAG): Positioned strategically between the purification tags and the g3p gene sequence.

    • In Amber-suppressor strains (e.g., TG1): The TAG codon is read through as an amino acid, yielding the full VHH-HA-His-g3p fusion partner anchored onto the rescued phage coat for biological panning cascades.

    • In Non-suppressor strains (e.g., WK6, BL21): The TAG codon acts as a hard stop, directly generating a soluble, free-floating monomeric nanobody (VHH-HA-His) in the periplasm, optimized for direct ELISA profiling or IMAC purification.

III. Basic Outline of VHH Library Construction

1. RNA Extraction & cDNA Synthesis: Isolate total RNA from peripheral blood lymphocytes of an immunized camelid and transcribe it into primary cDNA templates.

2. Nested PCR Amplification: A first-round PCR isolates heavy-chain only fragments (~700 bp encoding VHH, hinges, and partial CH2 domains). A second nested PCR appends the PstI and NotI recognition sites, isolating the VHH region (~400 bp).

3. Restriction Digestion: Digest both the clean VHH amplicons and the BioVector® pMECS phagemid backbone using PstI and NotI endonucleases.

4. Ligation & Electroporation: Ligate the cohesive ends via T4 DNA Ligase and electroporate into electrocompetent E. coli TG1 cells. Serial dilution plating is deployed to confirm library functional sizes (typically hitting scales of $10^7$ to $10^9$ CFU).

5. Rescue & Biopanning: Superinfect the log-phase library with helper phages, harvest the physical VHH-displaying virions, and execute 3–4 rounds of affinity panning against immobilized target antigens.

IV. Core Research Directions

1. Immune and Synthetic Single-Domain Discovery: Serves as a gold-standard phagemid architecture to screen and fish out high-affinity binders targeting tumor biomarkers, viral envelope spikes, and receptors involved in immune checkpoints.

2. Manifold Multivalent Engineering: Highly adaptable for stitching separate VHH building blocks in tandem. Researchers use BioVector® pMECS configurations to link identical or distinct single domains, formatting bivalent, bispecific, or biparatopic nanobody cocktails that leverage avidity or multi-epitope neutralization profiles.

3. Direct Soluble Periplasmic Interrogations: Because of the integrated amber switch, verified positive binders do not require sub-cloning steps into new expression vectors; switching to a non-suppressive bacterial host yields fast production of un-fused, soluble nanobodies right out of the periplasmic fraction.

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