VDJ recombination

VDJ recombination is a somatic recombination process that generates the vast diversity of antigen receptors (immunoglobulin heavy/light chains in B cells, TCR alpha/beta or gamma/delta chains in T cells) needed for adaptive immunity. During lymphocyte development, RAG1 and RAG2 enzymes recognize recombination signal sequences (RSS) flanking V, D, and J gene segments and introduce double-strand DNA breaks, allowing random combination of one V, one D (for heavy chain/TCR beta), and one J segment. Terminal deoxynucleotidyl transferase (TdT) adds random nucleotides (N regions) at the junctions, further increasing diversity—this is called junctional diversity. Non-homologous end joining (NHEJ) repair machinery (including Artemis, DNA-PK, XRCC4, Ligase IV) then rejoins the DNA ends. This combinatorial and junctional diversity theoretically allows generation of >10^9 unique antigen receptors from a limited genomic sequence, forming the basis of the immune repertoire. Clinically, defects in RAG1/RAG2 cause a form of severe combined immunodeficiency (SCID) with absent T and B cells (T-B-NK+ SCID), while defects in Artemis or DNA-PK cause radiosensitive SCID (T-B-NK+ SCID with radiosensitivity). Omenn syndrome results from hypomorphic RAG mutations, causing partial VDJ recombination activity with autoreactive T cells, leading to erythroderma, lymphadenopathy, and eosinophilia. Understanding VDJ recombination is also important for understanding lymphoid malignancies, as aberrant recombination events can lead to oncogenic translocations (e.g., BCL2-IGH in follicular lymphoma, MYC-IGH in Burkitt lymphoma).