Uneven TCR chain pairing constraints govern epitope recognition.

Combinatorial pairing of independently recombined T cell receptor (TCR) α- and β-chains is central to diversifying the TCR repertoire. Although sequence motifs in one chain correlate with epitope recognition, the extent to which a single chain dictates specificity remains unclear. Here, we systematically tested TCR chain coupling constraints by enforcing the pairing of individual chains with hundreds of thousands of partners. Although most chains paired stably, the preservation of epitope specificity was rare and highly variable, with the frequency of compatible partners ranging from ~10 to <0.1%. This approach identified >70,000 epitope-specific TCRs across 10 epitopes. Our work illuminates the distinct contributions of TCR chains, highlights the limitations of single-chain data, and provides an experimental and analytical framework for refining TCR-peptide-major histocompatibility complex specificity inference.