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#CD20

3 public questions tagged with this topic.

Immunological Applications

Rituximab targets which antigen?

MS4A1 gene on chromosome 11q12.2 encodes CD20 differentiation antigen, 33 to 37 kDa tetra-spanning membrane phosphoprotein with both N and C termini intracellular, four transmembrane helices forming tetrameric oligomer functioning as store-operated calcium channel component modulating B cell receptor induced calcium flux via interaction with calcium release-activated calcium modulator ORAI1. Expression tightly regulated during ontogeny: absent on hematopoietic stem cells and early pro-B, appears at late pro-B stage after productive immunoglobulin heavy chain rearrangement, persists through mature naive, germinal center, memory B cells, but extinguished upon plasma cell differentiation when transcription factors BLIMP1 and XBP1 repress MS4A1 and induce secretory program. Lack on stem cells ensures regenerative capacity after depletion; absence on long-lived plasma cells preserves humoral memory maintaining protective antibody titers against prior vaccines. Rituximab binding extracellular loops between transmembrane domains 3-4 induces translocation to lipid rafts enriched in cholesterol and sphingolipid, enhancing Lyn mediated PLCγ2 activation, calcium mobilization, and Bax mediated mitochondrial apoptosis. Concurrent opsonization promotes phagocytosis by macrophages via FcγRI and complement. Reconstitution from pro-B precursors occurs within six to twelve months, infection risk transient compared to continuous pan-B ablation.

Ref: NCBI Gene MS4A1 CD20 tetra-span; Janeway Immunobiology CD20 calcium channel rituximab ADCC CDC.

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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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Basic Applied Biology

Rituximab targets antigen:

Rituximab represents first chimeric anti-cancer monoclonal antibody approved 1997 targeting CD20, a 33 to 37 kDa non-glycosylated tetra-span membrane phosphoprotein encoded by MS4A1 gene on chromosome 11q12 expressed on pre-B through mature B lymphocytes but absent on hematopoietic stem cells, pro-B cells and terminally differentiated plasma cells. Protein architecture comprises four transmembrane domains with short intracellular termini and two extracellular loops accessible for antibody binding. Physiologic function involves regulation of calcium flux through modulating B cell receptor signaling threshold, affecting activation and differentiation. Rituximab binding via Fab region to CD20 extracellular loop triggers multiple effector mechanisms: complement-dependent cytotoxicity via C1q recruitment and membrane attack complex formation, antibody-dependent cellular cytotoxicity mediated by Fc region engaging Fc-gamma-RIIIa on natural killer cells releasing perforin, direct apoptosis induction via cross-linking, lipid raft clustering and activation of caspase 3, and phagocytosis by macrophages. Specific lineage restriction allows B cell depletion in non-Hodgkin lymphoma, chronic lymphocytic leukemia, rheumatoid arthritis with subsequent regeneration from stem cells lacking CD20. Absence on other lineages minimizes off-target toxicity compared to CD4, CD8 or HER2 targets.

Ref: Maloney et al. Blood 1997 Rituximab ideal target; Reff et al. CD20 biology; PubMed review CD20 therapeutic target.

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