BioVector? pMECS 噬菌體展示質粒載體 (Phagemid Vector)

BioVector? pMECS Phagemid Vector for Nanobody / VHH Phage Display

第一部分：中文說明

一、 載體基本信息與用途

• 載體名稱：BioVector? pMECS

• 載體類型：噬菌體顯性/隱性展示質粒載體（Phagemid Vector）。

• 核心用途：專門用于駝科動物（駱駝、大羊駝、羊駝）外周血淋巴細胞的重鏈單域抗體（VHH / Nanobody，即納米抗體）的基因克隆與噬菌體展示文庫（Phage Display Library）構建。

• 載體大小：約 4510 bp（4.5 kb）。

• 抗性標記：氨芐青霉素抗性（Ampicillin，$Amp^R$）。

• 常用宿主菌：E. coliTG1、ER2738 等琥珀抑制型（Amber Suppressor）大腸桿菌株，用于文庫的擴增與噬菌體拯救。

二、 關鍵結構域與元件配置

• 啟動子（Promoter）：帶有阻遏位點的 $P_{lac}$（乳糖啟動子/操縱子系統(tǒng)），可通過 IPTG 進行誘導轉錄，在無誘導劑時保持較低的背景表達。

• 信號肽（Signal Peptide）：含有 BioVector? PelB 信號肽，引導表達的納米抗體定向分泌至大腸桿菌的周質空間（Periplasmic Space），有利于抗體的高效可溶性折疊。

• 多克隆位點（MCS）與克隆策略：

  • 核心克隆位點為 PstI 和 NotI。

  • 在 PstI 與 NotI 之間包含一個唯一的 XbaI 位點。在酶切制備文庫載體時，常利用 XbaI 進行雙酶切質控，或用于將未完全雙酶切的空載體切斷，以最大程度降低文庫構建的“空載背景”。

• 融合標簽與融合蛋白：

  • 目標 VHH 序列插入后，其 C 端將與基因III蛋白（g3p / pIII 噬菌體外殼蛋白）以及下游的 HA 標簽（表達標簽）和 His 標簽（純化標簽）融合表達。

  • 琥珀終止密碼子（Amber Stop Codon, TAG）：位于抗體標簽與 g3p 基因之間。

    • 在琥珀抑制株（如 TG1）中：TAG 被讀碼為氨基酸，表達出 VHH-HA-His-g3p 融合蛋白，組裝在輔助噬菌體表面進行高通量篩選（Panning）。

    • 在非琥珀抑制株（如 WK6, BL21）中：TAG 被識別為終止符，直接分泌表達不帶 g3p 的可溶性單體納米抗體（VHH-HA-His），方便直接進行 ELISA 驗證或純化。

三、 納米抗體文庫構建基本流程

1. RNA 提取與 cDNA 合成：提取免疫后駝科動物外周血淋巴細胞的總 RNA，反轉錄合成 cDNA。

2. 巢式 PCR 擴增 VHH：通過第一輪 PCR 擴增重鏈抗體片段（約 700 bp，包含 VHH、Hinge 及部分 CH2 結構域），第二輪 PCR 引入 PstI 和 NotI 限制性酶切位點，獲得專一性的 VHH 片段（約 400 bp）。

3. 限制性內(nèi)切酶消化：使用 PstI 和 NotI 分別對 VHH 片段和 BioVector? pMECS 空載體進行雙酶切。

4. 連接與電轉化：將酶切產(chǎn)物進行 T4 連接，高效率電轉化至 E. coli TG1 感受態(tài)細胞中，通過平板稀釋計數(shù)計算文庫的庫容（通常需達到 $10^7\text{--}10^9$ CFU 級別）。

5. 噬菌體拯救與淘洗：加入輔助噬菌體進行超感染（Superinfection），回收攜帶 VHH 的噬菌體顆粒，針對靶抗原進行 3–4 輪固相或液相富集淘洗。

四、 核心科研應用

1. 天然/免疫納米抗體文庫篩選：作為全球實驗室最經(jīng)典的 VHH 噬菌體展示骨架之一，用于高效分離針對腫瘤靶點、病毒表面蛋白、免疫 checkpoint 的高親和力單域抗體。

2. 多價/多特異性抗體串聯(lián)構建（Manifold Constructs）：利用 BioVector? pMECS-VHH 載體中的獨特連接序列，可將不同的 VHH 基因以串聯(lián)形式克隆，快速組裝為雙價（Bivalent）、雙特異性（Bispecific）或雙表位（Biparatopic）的納米抗體衍生物，以提升親和力或實現(xiàn)多靶點阻斷。

3. 原位可溶性周質表達：得益于 Amber 密碼子設計，篩選出的陽性克隆無需更換載體，可直接轉換至非抑制型宿主菌中實現(xiàn)周質空間內(nèi)高純度單域抗體的中試級別分泌表達。

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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