#monoclonal antibodies

Immunological Applications

Immunotoxins are composed of:

Immunotoxins constructed to overcome lack of selectivity of conventional chemotherapy achieve tumor specific delivery of ultrapotent protein toxins that cannot enter mammalian cells unaided. Design consists of targeting moiety typically single-chain variable fragment scFv or disulfide stabilized Fab derived from murine or humanized antibody recognizing tumor antigen CD22 on hairy cell leukemia, CD25 on adult T cell leukemia, mesothelin on mesothelioma, linked via flexible glycine-serine peptide or reducible disulfide bond formed between engineered cysteines to effector toxin devoid of native receptor binding domain to prevent off target internalization. Toxin moiety from Pseudomonas aeruginosa exotoxin A truncated to 38 kDa PE38 containing ADP-ribosylation domain plus translocation domain or diphtheria toxin DT388 lacking receptor domain 1-389, or plant toxin ricin A chain 32 kDa. This mechanistic insight supports diagnostic and therapeutic applications while reinforcing core immunological and cell biology principles taught in advanced curricula. This mechanistic detail underpins practical applications in diagnostics, vaccine design, and biopharmaceutical manufacturing.

Ref: Pastan et al Nature Rev Cancer 2006 6:559 immunotoxins PE38; Kreitman Clin Cancer Res 2009 fusion toxin de-immunized.

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Immunological Applications

Monoclonal antibodies bind:

Monoclonal antibody originates from unique genetic event where one developing B cell successfully completed VDJ recombination on heavy chain locus chromosome 14 mediated by RAG1/2 recognition of recombination signal sequences, non-homologous end joining with TdT adding N nucleotides creating unique complementarity determining region three loop that dominates antigen contact, plus VJ recombination on light chain locus producing paired variable domains. All progeny via mitosis carry identical nucleotide sequence encoding paratope whose three-dimensional shape, electrostatic surface, and hydrophobic pockets complement single epitope topology defined by side chain arrangement and glycan. Binding affinity Kd remains uniform across preparation because combining site does not vary, allowing quantitative analysis via Scatchard plot and predictable off-rate. Even bivalent IgG displays same intrinsic affinity per Fab arm, avidity only increases with multivalent antigen. Such uniformity reduces nonspecific background in immunohistochemistry where inconsistent binding creates noise, enables precise epitope mapping using alanine scanning mutagenesis, and ensures reproducible pharmacokinetics with linear clearance. Uniform binding to single epitope with constant affinity distinguishes monoclonal from heterogeneous polyclonal mixture containing varying avidities and cross-reactivities that compromise quantitation.

Ref: Abbas Cellular Molecular Immunology 10th ed monoclonal single epitope uniform affinity; Roitt Essential Immunology specificity.

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Immunological Applications

Myeloma cells used in hybridoma technology are selected to be:

Establishing stringent selection requires myeloma background auxotrophic for purine salvage, achieved by exposure to 8-azaguanine, purine analogue structurally similar to guanine. Salvage competent cells convert 8-azaguanine via HGPRT to 8-azaguanosine monophosphate incorporated into mRNA and rRNA causing miscoding, translational errors, and lethal proteotoxicity triggering unfolded protein response. Spontaneous HPRT1 mutants survive because they cannot metabolize analogue, accumulating no toxic nucleotides. Surviving subclones harbor frameshift, nonsense, or missense mutations in coding exons 2-9, leading to premature stop codon and nonsense mediated mRNA decay, enzymatic activity undetectable via radiometric assay. In complete medium containing folate, deficiency tolerated because de novo purine synthesis via amidophosphoribosyltransferase using glutamine nitrogen supplies sufficient IMP via ten-step pathway. Once switched to HAT where aminopterin blocks that pathway, reliance shifts entirely to salvage, now impossible in mutants. ATP depletion, accumulation of PRPP, activation of AMPK, inhibition of mTORC1, and caspase 9 mediated apoptosis eliminate myeloma. Clones SP2/0-Ag14, P3X63-Ag8.653 routinely used worldwide ensure background below 1 per million, maximizing hybrid recovery efficiency.

Ref: Szybalski & Szybalska 1962 HGPRT negative mutants 8-azaguanine; ATCC SP2/0 Ag14 datasheet selection mechanism.

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Immunological Applications

Hybridoma cells survive in HAT medium because they:

Survival in HAT environment demands functional purine and pyrimidine salvage enzymes inherited from normal B parent. B lymphocytes express HPRT1 locus on X chromosome encoding 24 kDa hypoxanthine-guanine phosphoribosyltransferase that catalyzes phosphoribosyl transfer from PRPP to hypoxanthine yielding IMP plus pyrophosphate and to guanine yielding GMP, bypassing de novo steps requiring folate. They also express cytosolic thymidine kinase 1 cell-cycle regulated peaking in S phase and mitochondrial kinase 2 constitutive, phosphorylating thymidine supplied in medium to TMP using ATP. Hybridoma cells retain active alleles, enabling uptake of exogenous bases via equilibrative nucleoside transporters ENT1 and ENT2 and concentrative transporters CNT, generating nucleotides sufficient for DNA synthesis even when de novo synthesis ablated by aminopterin. Energy charge maintained allowing progression through G1/S checkpoint via cyclin E CDK2. Myeloma parents selected with 8-azaguanine lack HGPRT activity, cannot recycle hypoxanthine, undergo purine starvation, decrease in ATP and GTP pools activates p53 and causes mitochondrial depolarization. Unfused B cells possess salvage enzymes but inherently short lived due to withdrawal of BAFF and CD40L, undergoing apoptosis within seven days, leaving only hybrids as long-term proliferating population capable of indefinite expansion in selective medium.

Ref: Janeway Immunobiology salvage pathway HGPRT; Freshney Animal Cell Culture hybrids survive HAT via HGPRT TK from B cell.

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Immunological Applications

Aminopterin in HAT medium functions by:

Aminopterin classifies as antifolate antimetabolite structurally analogous to folic acid with pteridine ring substitution preventing enzymatic reduction. Dihydrofolate reductase normally catalyzes NADPH dependent reduction of dihydrofolate to tetrahydrofolate, central one-carbon carrier crucial for biosynthesis. Tetrahydrofolate derivatives required at two distinct steps: 10-formyl tetrahydrofolate donates formyl groups to glycinamide ribonucleotide transformylase and aminoimidazole carboxamide ribonucleotide transformylase in de novo purine pathway synthesizing inosine monophosphate precursor to adenine and guanine nucleotides, and 5,10-methylene tetrahydrofolate provides methyl group to thymidylate synthase converting deoxyuridine monophosphate to thymidine monophosphate essential for DNA replication. Aminopterin competitive inhibition depletes all tetrahydrofolate pools, halts purine and thymidine triphosphate production, cellular ATP drops, AMPK activated, p53 stabilization triggers apoptosis within hours. Rapidly proliferating lymphocytes depend heavily on de novo route because salvage alone insufficient during S phase. Provision of hypoxanthine and thymidine allows salvage-competent cells to circumvent block via HGPRT and TK, explaining selective toxicity exploited in hybridoma selection and historically in cancer chemotherapy similar to methotrexate.

Ref: Goodman & Gilman Pharmacol 13th ed DHFR antifolate; Lodish MBoC 9th ed Fig 7-32 aminopterin blocks de novo purine thymidylate.

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Immunological Applications

Immunotoxins are composed of:

Immunotoxins chimeric proteins integrating targeting domain derived monoclonal antibody and cytotoxic domain protein toxin to achieve picomolar potency against malignant cells expressing defined surface antigen. Antibody moiety typically Fab prime 50 kDa scFv 25 kDa composed heavy variable light variable domains connected glycine serine linker disulfide-stabilized Fv dsFv recognizing tumor antigens CD22 135 kDa hairy cell leukemia CD25 IL-2 receptor alpha cutaneous T-cell lymphoma mesothelin 40 kDa GPI-anchored mesothelioma pancreatic adenocarcinoma HER2 with affinity 1-10 nM. Toxin component truncated Pseudomonas exotoxin PE38 38 kDa lacking domain Ia binding LRP1 ubiquitous retaining domain II translocation domain III ADP-ribosylating diphtheria toxin DT388 receptor-binding domain deleted but translocation catalytic retained ricin A chain N-glycosidase depurinating 28S RNA ribosome-inactivating protein saporin Saponaria. Components genetically fused peptide linker GGS chemically conjugated thioether preserving binding enzymatic functions. Upon antigen-mediated endocytosis clathrin-coated pits binding trafficking early endosomes pH 6.0 furin cleavage trans-Golgi network generating heterodimer retrograde via KDEL receptor ER translocation Sec61 cytosol where one molecule sufficient killing cell protein synthesis inhibition triggering apoptosis.

Ref: Pastan Annu Rev Med 2007 Immunotoxin Construction Principles Review; FDA Lumoxiti Moxetumomab Design Label; Janeway Immunotoxin Antibody Toxin Fusion Mechanism.

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Immunological Applications

Rituximab targets which antigen?

Rituximab first chimeric mouse-human IgG1 kappa mAb targeting CD20 tetra-spanning membrane protein 33-37 kDa encoded MS4A1 11q12 four hydrophobic transmembrane domains intracellular N C termini extracellular small large loops 44 aa large loop epitope. CD20 forms homodimers tetramers lipid rafts regulating calcium flux store-operated entry Orai1 STIM1 and BCR signaling amplifying PLC gamma. Expression pre-B mature B lymphocytes but absent plasma cells HSC no shedding soluble form slow internalization excellent therapeutic target. Binding rituximab large loop crosslinks CD20 redistribution lipid rafts activating Src kinases Lyn Fyn phosphorylating ITAM leading caspase-3/9 dependent apoptosis mitochondrial cytochrome c release PARP cleavage. Fc effector potent complement-dependent cytotoxicity initiated C1q binding CH2 forming C4b2a C3 convertase lysing cells especially high CD20 density NHL plus ADCC via Fc gamma RIIIa V158F polymorphism high affinity correlating response enhanced NK activity ADCP macrophages. B-cell depletion blood 24h lasts 6-9 months while plasma cells lacking CD20 preserve preexisting IgG titers allowing regeneration CD20-negative pro-B progenitors without prolonged hypogammaglobulinemia but increasing infection risk nadir requiring monitoring.

Ref: FDA Rituxan Rituximab CD20 MOA Label; NEJM Rituximab Lymphoma 1997 Maloney; Janeway CD20 Depletion ADCC CDC Mechanism Chapter 5.

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Immunological Applications

Monoclonal antibodies bind:

Bevacizumab recombinant humanized IgG1 mAb 149 kDa neutralizes soluble VEGF-A isoforms 121 165 189 206 secreted tumor stromal inflammatory cells hypoxia HIF1alpha binding hypoxia response elements driving transcription. VEGF-A binds VEGFR1 FLT1 and R2 KDR tyrosine kinases endothelial cells dimerization autophosphorylation Tyr1175 recruiting PLC gamma generating IP3 DAG increasing calcium PKC stimulating proliferation permeability via VE-cadherin internalization gaps survival Akt phosphorylation caspase-9. Binding epitope Arg82 Lys84 affinity 0.5 nM overlapping receptor binding interface prevents ligand engagement suppresses downstream ERK activation inhibits new capillary sprouting pruning immature vessels lacking pericyte expressing Ang2 and normalizes remaining vasculature reducing interstitial fluid pressure 20 to 5 mmHg improving perfusion enhancing penetration co-administered cytotoxic drugs fluorouracil irinotecan. Approved metastatic colorectal first-line chemotherapy showing improvement PFS 10.6 vs 6.2 months non-small cell lung glioblastoma exemplifying anti-angiogenic strategy targeting microenvironment rather malignant cells directly requiring exclusion bleeding risk hemoptysis bowel perforation management hypertension side effect class effect.

Ref: Ferrara NEJM Bevacizumab VEGF Targeted Therapy 2004; FDA Avastin Mechanism Label VEGF Neutralization; Lodish Angiogenesis VEGF Signaling Pathway Chapter 23.

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Basic Applied Biology

Hybridoma technology produces:

Hybridoma technology developed by Georges Köhler and César Milstein in 1975 and awarded Nobel 1984 created paradigm shift by immortalizing antibody-producing B lymphocytes through somatic cell fusion. Mice immunized with antigen develop germinal center reaction where B cells undergo VDJ recombination, somatic hypermutation and class switching to generate high affinity plasma cells. Splenic lymphocytes harvested shortly after boost are fused with HGPRT-deficient myeloma partner such as SP2/0-Ag14 or NS0 using polyethylene glycol 1500 that perturbs lipid bilayers promoting membrane fusion forming heterokaryons containing nuclei from both parents. Resulting hybrid cells combine immortal growth driven by myeloma oncogenes like c-myc deregulation with functional immunoglobulin heavy and light chain transcription from B cell. Clonal expansion and selection yields everlasting lines secreting monospecific antibody of defined isotype typically IgG1 kappa recognizing single epitope, known as monoclonal antibody. Such reagents revolutionized diagnostics, research tools like western blotting, and therapeutics by提供 unlimited standardized antibody with consistent affinity replacing heterogeneous polyclonal sera plagued by batch variation.

Ref: Köhler & Milstein Nature 1975 Continuous cultures of fused cells; Abbas Cellular and Molecular Immunology 10th ed. monoclonal antibody generation.

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