The recombination difference between mouse kappa and lambda segments is mediated by a pair-wise regulation mechanism

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• In mice, kappa light chains significantly outnumber lambda light chains in the immunoglobulin repertoire.
• Recombination signal sequences (RSS) are known contributors to this differential representation, but the underlying mechanism remains unclear.

Purpose of the Study:

• To investigate the mechanism by which kappa and lambda RSS influence differential V(D)J recombination and immunoglobulin light chain usage.

Main Methods:

• Utilized extra-chromosomal recombination substrates to monitor V(D)J recombination efficiency.
• Performed in vitro cleavage assays to assess RAG/DNA complex formation and cleavage frequencies.
• Employed electrophoretic mobility shift assays (EMSA) to analyze RAG-1/2 binding to kappa and lambda RSS with flanking sequences.

Main Results:

• Kappa RSS, with their natural flanks, supported 6-9 fold higher recombination levels compared to lambda RSS.
• Individual kappa and lambda RSS cleavage frequencies were comparable, but kappa RSS pairs showed significantly higher double cleavage.
• RAG-1/2 binding to individual kappa RSS was only modestly higher than to lambda RSS.

Conclusions:

• The preferential usage of kappa over lambda light chain segments is not due to differences in RAG-1/2 binding or single RSS cleavage.
• Enhanced pair-wise cleavage of kappa RSS by RAG-1/2 is the likely mechanism driving kappa overuse.
• Biased light chain segment usage is likely imposed at the level of synaptic RSS pairs during V(D)J recombination.