TCR β CDR3 repertoire signatures in paired PBMC and BALF from children with mycoplasma pneumoniae pneumonia

Background Mycoplasma pneumoniae (MP) is a leading cause of community-acquired pneumonia in children, yet the characteristics of the T cell receptor (TCR) repertoire in the local pulmonary environment remain poorly defined. Methods Paired PBMC and bronchoalveolar lavage fluid (BALF) samples from four MP-infected children were subjected to TCR β CDR3 repertoire sequencing, followed by analysis of clonotype diversity, repertoire overlap, V/J gene usage, CDR3 length distribution, database annotation and k-mer profiling. The main findings were further validated in an independent cohort including 13 MP patients and 20 healthy children (HC). Results Although BALF contained fewer clonotypes than paired PBMC samples, it showed a trend toward a higher proportion of recurrent clonotypes and long (≥16 AA) CDR3 sequences. Only a small fraction of clonotypes were shared between paired PBMC and BALF samples, with minimal inter-individual overlap; however, V and J gene usage patterns were more similar within paired samples than between different individuals. A minority of shared clonotypes matched known antigen-specific sequences, mainly against viruses, and k-mer analysis revealed distinct motif patterns in BALF. In the expanded cohort, repertoire diversity was significantly reduced in both MP_BALF and MP_PBMC compared with HC_PBMC. Several TRBV and TRBJ genes showed differential usage between MP patients and healthy controls, and the proportion of long (≥16 AA) CDR3 sequences was significantly increased only in MP_PBMC. Recurrent clonotypes identified in BALF also showed a trend toward higher frequencies in MP_PBMC than in HC_PBMC. Conclusion Pediatric MP pneumonia is associated with distinct compartment-specific TCR β repertoire features, including altered repertoire diversity, biased TRBV/TRBJ gene usage, and compartment-dependent CDR3 characteristics, providing insight into systemic and local adaptive immune responses during MP infection.