BioVector® KM13 辅助噬菌体

BioVector® KM13 Helper Phage

第一部分 中文说明

一 产品基本信息与科研用途

• 产品名称：BioVector® KM13 辅助噬菌体

• 基因组分类：单链 DNA（ssDNA）丝状噬菌体（基于 M13 噬菌体改构）。

• 核心用途：专门用于噬菌体展示技术（Phage Display）中的高通量抗体/多肽文库筛选（Biopanning）。KM13 是一种典型的含有“蛋白酶切位点”的特殊辅助噬菌体，旨在通过化学促选手段，极其剧烈地提高针对高亲和力靶标的淘选效率和克隆回收率。

• 抗性选择标记：带有 卡那霉素抗性基因（Kanamycin，$Km^R$）。这一设计便于在共感染大肠杆菌宿主时，通过抗生素加压筛选出成功装配的辅助噬菌体。

• 常用宿主菌：大肠杆菌抗性/缺陷型菌株（如 TG1、XL1-Blue、ER2738、SS320 等带有 $F'$ 因子纤毛的菌株）。

二 关键结构域与酶切促选（Trypsin-cleavable）机制

• 传统 M13 辅助噬菌体（如 M13KO7、VCSM13）的瓶颈：在利用常规噬菌体表达载体（Phagemid）进行抗体文库包装时，由于 phagemid 只编码外源抗体-pIII 融合蛋白而缺乏其他结构基因，必须依靠野生型结构的辅助噬菌体来提供完整的衣壳蛋白和复制酶。然而，辅助噬菌体自身带有完整的、未融合的野生型 pIII 基因（负责结合大肠杆菌纤毛进而完成侵染）。

  • 这会导致最终包装排出的噬菌体颗粒中，绝大多数都装配了野生型的 pIII 蛋白（称为“空载颗粒”），而非展现外源抗体的融合蛋白。在淘选（Biopanning）过程中，这些不表达抗体但具备超级侵染力的空载噬菌体会造成极高的物理背景残留。

• KM13 的分子改构与胰蛋白酶切选机制（$\Delta pIII$ 蛋白酶原敏感性突变）：

  • 改构核心：BioVector® KM13 对其自身的 pIII 外膜衣壳蛋白编码区进行了精准工程化改造，在 pIII 蛋白的突触结构域（N1 与 N2 结构域之间）引入了一个高度特异性的胰蛋白酶（Trypsin）剪切位点。

  • 促选原理：

    1. 在包装出的混合噬菌体文库中，由 KM13 贡献的衣壳蛋白带有胰蛋白酶敏感位点；而由 Phagemid 载体（如 pCANTAB 5E、pHEN2）驱动表达的外源抗体-pIII 融合蛋白由于去除了该敏感位点，对胰蛋白酶表现为天然耐受。

    2. 在噬菌体与靶抗原结合、洗涤去除未结合的颗粒后，直接向孔内加入适量的 胰蛋白酶（Trypsin） 进行短时间温育。

    3. 此时，所有未展示异源抗体、纯粹由辅助噬菌体衣壳组装的“空载”颗粒其 pIII 蛋白会发生全面断裂，彻底丧失侵染大肠杆菌的能力。

    4. 相比之下，那些真正展示了高亲和力抗体的重组噬菌体颗粒，由于抗体-pIII 融合链不被剪切，依然能够完整无损地高效侵染宿主菌 TG1 进行扩增。该技术通过化学“淘汰法”，将文库淘选的背景假阳性率降低了数个数量级。

三 标准噬菌体文库包装与淘选步骤

1. 噬菌体超感染包装（Helper Phage Superinfection）：

   • 将含有重组抗体 Phagemid 文库的大肠杆菌 TG1 接种于 2x YT（含 2% 葡萄糖和氨苄青霉素）液体中，37 摄氏度振荡培养至对数生长中期（$OD_{600} = 0.4 \sim 0.5$）。

   • 按感染复数 MOI = 10 到 20 的比例加入定量的 BioVector® KM13 辅助噬菌体液，37 摄氏度静置保温 30 分钟以令其吸附。

   • 离心去除富含葡萄糖的旧培养基（葡萄糖会阻遏 lac 启动子），用不含葡萄糖、但含有氨苄青霉素和卡那霉素（$50 \ \mu\text{g/mL}$）的 2x YT 新鲜培养基重悬，30 摄氏度（或 25 摄氏度低速）过夜振荡培养包装。

2. 噬菌体沉淀与分级回收：

   • 次日离心收获过夜培养基上清，加入 1/5 体积的 PEG/NaCl 沉淀液（20% PEG 8000, 2.5 M NaCl），冰浴放置 1 小时。

   • 超速或常规高转速离心沉淀，用无菌 PBS 缓冲液重悬沉淀颗粒，获得滴度在 $10^{12} \sim 10^{13} \ \text{pfu/mL}$ 的 KM13 包装文库。

3. 胰蛋白酶促选洗脱法（Trypsin Elution / Selection）：

   • 依照标准固相包被法将靶抗原固定于 Immunotube 或 96 孔板中，加入重悬的噬菌体文库孵育 2 小时。

   • 使用 PBST 缓冲液进行常规 10 到 20 次的强力洗涤，以完全去除物理粘附的未结合病毒。

   • 洗涤结束后，向孔内加入最终浓度为 $1 \ \text{mg/mL}$ 的精制胰蛋白酶（Trypsin） 溶液，室温温育 10 到 15 分钟。胰蛋白酶不仅能通过降解 KM13 pIII 蛋白来瘫痪空载噬菌体，还能高效洗脱已经结合的高亲和力特异性克隆。

   • 迅速收集洗脱液，直接中和并投入新制备的对数期大肠杆菌 TG1 液体中，37 摄氏度侵染扩增，进入下一轮淘选循环。

四 核心科研应用方向

1. 超大规模全人源单克隆抗体文库（ScFv / Fab Library）的特异性淘选：BioVector® KM13 是全球抗体工程与高通量药物筛选核心实验室、研究机构首选的高效辅助噬菌体系统。它尤其适用于从 $10^{10}$ 以上库容的天然全人源单链抗体文库中，富集和筛选针对微量高毒性抗原、肿瘤特异性膜受体的高亲和力中和抗体克隆。

2. 靶向高难度膜蛋白（GPCRs）与微量复杂糖蛋白的噬菌体展示文库筛选：由于 7 次跨膜蛋白（GPCR）等靶点在体外极难维持天然构象且背景结合率极高，使用 KM13 进行文库包装可在淘选洗脱阶段运用“胰蛋白酶灭活法”，最大程度上消除因物理非特异性吸附导致的假阳性克隆膨胀。

3. 基于噬菌体表面展示的肿瘤靶向归巢肽（Homing Peptides）体内筛选（In vivo Panning）：在活体动物内注射多肽文库进行器官特异性归巢肽筛选时，回收的组织样本常含有大量非特异性残留颗粒。利用 KM13 的胰蛋白酶敏感特性，可在回收后进行体外酶解，从而精准捕获真正靶向内皮细胞的高效肽段载体。

PART 2 ENGLISH SECTION

I General Information and Applications

• Product Name: BioVector® KM13 Helper Phage

• Genomic Classification: Single-Stranded DNA (ssDNA) Filamentous Phage (engineered M13 phage derivative).

• Primary Application: Specialized for high-throughput biopanning and antibody/peptide selection within Phage Display technology platforms. KM13 represents a premier textbook "trypsin-cleavable" helper phage designed to drastically enhance selection stringency against low-abundance or high-affinity targets via strategic chemical inactivation.

• Selection Flags: Houses a functional Kanamycin resistance gene ($Knamycin, Km^R$) configured into its genome, allowing for streamlined selective pressure application when coordinating multi-plasmid replication workflows inside E. coli.

• Common Host Systems: E. coli strains carrying the $F'$ episome pili architecture (e.g., TG1, XL1-Blue, ER2738, SS320).

II Architecture and Trypsin-Cleavable Selection Mechanics

• Limitations of Legacy Systems (e.g., M13KO7, VCSM13): When utilizing phagemid vectors to express an antibody library, the phagemid itself provides only the heterologous antibody-pIII fusion block but lacks all other structural and replicative codes. It relies on a helper phage to complement the missing machinery. However, classic helper phages introduce their own fully un-fused, highly infectious wild-type pIII capsids into the packaging pool.

  • This creates a scenario where the overwhelming majority of egressed viral progeny carry wild-type pIII arrays (termed "empty or passenger particles") instead of the target antibody fusion. During biopanning cycles, these non-displaying but highly infectious contaminants generate major non-specific binding and physical background noise.

• KM13 Design and Trypsin-Cleavable Differentiation Core ($\Delta pIII$ Protease-Sensitive Engineering):

  • Molecular Modification: BioVector® KM13 features a highly specific, genetically engineered trypsin cleavage site integrated into its own pIII minor coat protein matrix, positioned precisely between the N1 and N2 structural domains.

  • Selection Workflow:

    1. In an amplified hybrid phage library pool, the coat proteins contributed by KM13 maintain high susceptibility to trypsin degradation; conversely, the heterologous antibody-pIII fusions driven by the phagemid vectors (e.g., pCANTAB 5E, pHEN2) lack this protease-sensitive target and are naturally resistant.

    2. Following library engagement against immobilized target antigens and consecutive vigorous washing routines, the assay matrix is briefly treated with a calibrated dose of Trypsin.

    3. This enzymatic step cleaves the pIII structures of empty, non-displaying phages, leaving them unable to infect E. coli.

    4. Meanwhile, the true antibody-displaying clones remain structurally sound, retaining full infectivity to transform log-phase TG1 hosts for downstream amplification. This biochemical elimination method dramatically drives down false-positive rates by multiple orders of magnitude.

III Library Packaging and Trypsin-Driven Biopanning Protocols

1. Helper Phage Superinfection Execution:

   • Cultivate an E. coli TG1 library carrying the recombinant phagemid constructs in 2x YT media (supplemented with 2% glucose and ampicillin) at 37 degrees Celsius with vigorous shaking until reaching mid-log phase ($OD_{600} = 0.4 \sim 0.5$).

   • Inoculate the cell suspension with BioVector® KM13 helper phage liquid at a standard Multiplicity of Infection of MOI = 10 to 20. Incubate undisturbed at 37 degrees Celsius for 30 minutes to facilitate pilus attachment and genome injection.

   • Pellet the biomass to fully eliminate residual glucose (purging catabolite repression blocks off the lac promoter). Resuspend the cell mass in fresh glucose-free 2x YT broth enriched with both Ampicillin and Kanamycin ($50 \ \mu\text{g/mL}$), and run an overnight expression layout at 30 degrees Celsius (or 25 degrees Celsius for optimized folding configurations).

2. Phage Isolation and Concentration via PEG Precipitation:

   • Clarify the overnight culture matrix via high-speed centrifugation, harvest the virus-rich supernatant, and mix with a 1/5 volume of standard PEG/NaCl solution (20% PEG 8000, 2.5 M NaCl). Chill on ice for 1 hour to precipitate the viral structures.

   • Centrifuge to collect the phage pellet, and resuspend thoroughly in sterile PBS buffer, adjusting baseline functional titers to approximately $10^{12} \sim 10^{13} \ \text{pfu/mL}$.

3. Trypsin Selection & Elution Routine:

   • Apply the concentrated KM13-packaged library to immunotubes or ELISA wells pre-coated with the specific target antigen, allowing binding for 2 hours.

   • Wash the matrices rigorously 10 to 20 times with PBST buffer to thoroughly clear non-specific, physically trapped background viral units.

   • To recover the specific binders, inject a finalized concentration of $1 \ \text{mg/mL}$ purified Trypsin solution into the wells, incubating at room temperature for 10 to 15 minutes. The trypsin enzymatically de-adheres the high-affinity binders from the matrix while systematically disabling the infectivity of any co-purified empty helper phage contamination.

   • Collect the eluate promptly, quench the reaction, and add the suspension directly into freshly prepared log-phase E. coli TG1 cells for immediate infective expansion and downstream enrichment rounds.

IV Strategic Research Applications

1. High-Stringency Biopanning of Ultra-Large Human Antibody Repertoires (ScFv / Fab Networks): BioVector® KM13 stands as an internationally recognized gold-standard helper phage system in advanced antibody engineering facilities. It is uniquely prioritized for extracting rare, high-affinity therapeutic candidates out of synthetic or naive human antibody libraries exceeding $10^{10}$ in functional diversity, especially when isolating blocks against toxic, transient, or conserved target antigens.

2. Phage Selection Against Complex Low-Abundance G-Protein Coupled Receptors (GPCRs): Because native multi-pass membrane architectures like GPCRs are notoriously difficult to stabilize and present massive hydrophobic background binding risks, deploying KM13 permits researchers to clean up non-specific passenger artifacts during the elution phase, keeping selection tracks focused purely on true conformation-specific binders.

3. In Vivo Panning of Tissue-Specific Homing Peptides: When injecting peptide libraries directly into animal models to discover organ-specific vascular homing peptides, harvested tissues often yield high background retention. Utilizing the trypsin-cleavable mechanics of KM13 allows for effective post-harvest enzymatic processing, ensuring only high-affinity target-integrated motifs are amplified.

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