A diverse T cell receptor (TCR) repertoire is essential for adaptive immune responses and is generated by somatic recombination of TCRβ and TCRα gene segments in the thymus. Previous estimates of the total TCR diversity have studied the circulating mature repertoire, identifying 1-3 x 106 unique TCRβ and 0.5 x 106 TCRα sequences. Here we provide the first estimate of the total TCR diversity generated in the human thymus, an organ which in principle can be sampled in its entirety. High-throughput sequencing of samples from four pediatric donors detected up to 10.3 x 106 unique TCRβ sequences and 3.7 x 106 TCRα sequences, the highest directly observed diversity so far for either chain. To obtain an estimate of the total diversity we then used three different estimators, preseq and DivE, which measure the saturation of rarefaction curves, and Chao2, which measures the size of the overlap between samples. Our results indicate that the thymic repertoire consists of 40-70 x 106 unique TCRβ sequences and 60-100 x 106 TCRα sequences. The thymic repertoire is thus extremely diverse, but the data also suggest that it is transient, with different clones produced at different times.
