Antigen Receptor Systems: BCRs, TCRs, and V(D)J Recombination

Vocabulary practice flashcards defining core concepts, enzymes, DNA sequences, and mechanisms involved in B-cell and T-cell receptor gene rearrangement and diversity.

Complementarity-Determining Regions (CDRs)

Three hypervariable amino acid loops (designated CDR1, CDR2, and CDR3) located within the variable heavy ($V_H$) and variable light ($V_L$) domains of immunoglobulins that collectively form the antigen-binding surface.

Dreyer and Bennett Hypothesis

The 1965 theoretical model proposing that immunoglobulin variable ($V$) and constant ($C$) regions are encoded by distinct, separated genomic segments that must be recombined somatically to form an intact heavy or light chain gene.

Hozumi and Tonegawa Experiment

The Southern blot experiment demonstrating that variable and constant gene segments are separated in embryonic (germ-line) tissue but joined together via DNA rearrangement in mature antibody-producing B-cell tumor lines, earning Tonegawa the 1987 Nobel Prize.

Recombination Signal Sequences (RSSs)

Conserved DNA sequence motifs directly flanking $V$, $D$, and $J$ coding segments, consisting of a conserved heptamer and nonamer sequence separated by a non-conserved spacer of either $12\,\text{bp}$ or $23\,\text{bp}$.

12/23 Rule

The strict enzymatic requirement dictating that productive V(D)J recombination can only occur between an RSS bearing a $12\,\text{bp}$ spacer and an RSS bearing a $23\,\text{bp}$ spacer, preventing illegal rearrangements such as direct $V_H$-to-$J_H$ joining.

RAG1/2 Recombinase

A lymphoid-specific protein complex essential for antigen receptor gene rearrangement; RAG1 carries out sequence-specific DNA binding and catalytic cleavage, while RAG2 stabilizes the complex and directs epigenetic targeting via binding to $\text{H3K4me3}$.

Coding Joint

The permanent chromosomal junction formed by the ligation of two coding gene segments (such as $V$ to $J$, or $D$ to $J$), which is modified by exonuclease trimming and nucleotide additions to generate CDR3 diversity.

Signal Joint

The precise head-to-head joined segment formed by the direct ligation of two excised heptamer RSS ends, which is excised from the chromosome and typically lost as an extrachromosomal circular episome.

Artemis

A non-lymphoid endonuclease of the non-homologous end-joining (NHEJ) repair pathway that, upon phosphorylation by DNA-PKcs, opens hairpin-sealed coding ends, predominantly generating a $3'$ single-stranded overhang.

DNA-PKcs

A DNA-dependent protein kinase catalytic subunit of the NHEJ pathway that forms a complex with Ku70/80 to phosphorylate and activate Artemis, while recruiting DNA ligase IV machinery to broken ends.

Ku70/80

A heterodimeric DNA end-binding protein of the NHEJ pathway that is recruited to double-strand breaks to stabilize and align DNA ends for coding and signal joint repair, and to recruit DNA-PKcs.

Terminal Deoxynucleotidyl Transferase (TdT)

A lymphoid-specific enzyme that randomly adds non-templated (N) nucleotides in a template-independent manner to the free coding ends of immunoglobulin heavy-chain and all TCR-chain gene segments.

P-Nucleotide Addition

The generation of short palindromic nucleotide sequences at coding joints created when Artemis cleaves a DNA hairpin asymmetrically, resulting in a single-stranded overhang that is filled in by DNA repair polymerases.

N-Nucleotide Addition

The template-independent addition of up to $20$ non-templated nucleotides to cleaved coding joints by TdT (or DNA Pol $\mu$), creating significant junctional diversity in heavy chains and TCR chains.

Combinatorial Diversity

The diversity generated in naive lymphocytes by the random combination of distinct germline $V$, $D$, and $J$ segments, multiplied by the random pairing of separate heavy and light (or $\alpha$ and $\beta$) protein chains.

Junctional Diversity

The massive diversity introduced specifically at the joining regions of antigen receptors through imprecise hairpin cleavage, exonuclease trimming, P-nucleotide addition, and N-nucleotide insertion.

Allelic Exclusion

The developmental control mechanism that ensures each individual B or T cell expresses an antigen receptor of single specificity by downregulating rearrangement machinery once a productive allele is expressed.

Surrogate Light Chain (SLC)

An invariant, light-chain-like protein complex that pairs with a newly synthesized $\mu$ heavy chain on the surface of pre-B cells to form the pre-BCR, signaling successful heavy-chain recombination and initiating light-chain rearrangement.

Receptor Editing

A secondary V(D)J rearrangement event in immature B cells whereby an autoreactive light-chain gene is modified or inactivated by recombining an upstream $V$ segment with a downstream $J$ segment to eliminate self-reactivity.

Prkdc^scid Mutation

A mutant murine genotype causing defective DNA-PKcs function that impairs coding joint resolution, producing severe combined immunodeficiency accompanied by generalized radiosensitivity and leaky B/T-cell production with age.

Rag1/Rag2 Knockout Phenotype

A genetic condition in mice characterized by a complete failure of DNA cleavage at the RSS, causing an absolute developmental arrest of B cells at the pro-B stage and T cells at the DN3 stage with zero leakiness and normal radiosensitivity.

HMGB1/2 (High Mobility Group Box 1 and 2)

Non-lymphoid architectural proteins that stabilize the binding of the RAG1/2 recombinase to RSS elements and promote DNA bending at the $23\,\text{bp}$ spacer RSS.

Ataxia Telangiectasia Mutated (ATM) Protein

A DNA damage sensor kinase recruited by the MRN complex to double-strand breaks during V(D)J recombination to arrest the cell cycle; its loss causes lymphopenia and thymic lymphomas carrying TCR translocations.

Pre-T-Cell Receptor (pre-TCR)

A transient developmental receptor complex consisting of a functional TCR-$\beta$ chain paired with an invariant surrogate $\text{pT}\alpha$ chain and the CD3 signaling complex, required to signal T-cell survival and $\alpha$-chain rearrangement.