Mature tertiary lymphoid structures support B cell-mediated antitumour immunity and are disrupted by neoadjuvant therapy in rectal cancer: a multicentre, retrospective study.

BACKGROUND: The tumour immune microenvironment (TIME), particularly the presence and maturation of tertiary lymphoid structures (TLS), shapes antitumour immunity and therapy response. However, the role of mature TLS (mTLS) in rectal cancer (RC) and their modulation by neoadjuvant therapy (neoTx) remain unclear. METHODS: In this multicentre, retrospective study, we analysed patients with RC from two cohorts. Multi-omics profiling in patients with locally advanced rectal cancer (LARC) receiving no treatment (NT) included bulk RNA-seq (n = 123), immunohistochemistry and multiplex immunofluorescence (n = 161), scRNA-seq (n = 10) with paired scBCR-seq (n = 10). An independent neoTx cohort was used to assess treatment-induced immune changes, including bulk RNA-seq (n = 19) and immunohistochemistry (n = 125). FINDINGS: mTLS tumours were characterised by plasma cells and CD8(+) T cells being located in close spatial proximity to each other. B cell-related signatures-including plasma cells, germinal centre B cells, and follicular B cells-as well as CD138, IgG, and IgA expression were elevated in mTLS tumours. scRNA-seq and scBCR-seq analyses further revealed that mTLS tumours harboured a greater abundance of plasma cells, broader clonal diversity, and a higher proportion of IgG(+) and IgA(+) plasma cells. High CD138 expression correlated with favourable survival. Post-neoTx tumours showed higher CD4(+), CD8(+), and CD45RO(+) T cell densities and lower mTLS presence. Notably, B cell gene signatures and CD20(+) cell density were enriched in responders to neoTx, despite no difference in TLS maturation. INTERPRETATION: mTLS are associated with enhanced B cell-mediated immune features and favourable prognosis. Post-neoTx is correlated with increased T cell infiltration but decreased TLS presence. The sustained B cell activation observed in non-responders raises the possibility that therapeutic strategies aimed at preserving or enhancing humoural immunity may benefit this patient subset. FUNDING: This study was supported by grants from the Natural Science Foundation of Beijing (7242034), New Technologies and Businesses of the PLAGH (5156ZE1X).