The formation and function of tertiary lymphoid structures

Tertiary lymphoid structures (TLSs) are immune cell aggregates that emerge in nonlymphoid tissues during various disease states, including chronic inflammation, autoimmunity, and cancer. TLSs are structurally and functionally analogous to secondary lymphoid organs, and exhibit a maturation continuum (progressing from initial aggregation to mature structures with germinal centers). TLS formation is synergistically regulated by local chemokine networks (e.g. CXCL13, CCL19, and CCL21), lymphotoxin signaling axes, stromal cells, metabolic reprogramming, and the microbiome. This review comprehensively elucidates the biological foundations of TLSs, including their cellular composition, spatial architecture, and developmental dynamics of maturation. We explore the crucial roles of TLSs as favorable prognostic factors and predictors of the immunotherapy response in various solid tumors, including melanoma, breast cancer, lung cancer, hepatocellular carcinoma, and colorectal cancer. Additionally, we analyze their "pathogenic" role in causing tissue damage and disease progression in autoimmune disorders such as rheumatoid arthritis and Sjögren's syndrome, as well as chronic inflammatory diseases such as COPD, IgA nephropathy, and atherosclerosis. In addition, we thoroughly examine TLS research methodologies, covering a wide range of approaches from conventional hematoxylin and eosin (H&E) and immunohistochemical staining to advanced multiplex fluorescence staining, imaging mass spectrometry, and spatial transcriptomic techniques. We summarize multiple gene expression signatures (e.g. the 12-chemokine signature and TLS score) for TLS identification and quantification. Finally, we highlight multiple strategies for artificially inducing TLS formation, including cytokine delivery, immunotherapy, engineered scaffolds, microbiome modulation, and organoid technologies, designed to enhance antitumor immunity or reverse immunopathology. This review provides a comprehensive framework for understanding the complex functions of TLSs in human disease and explores their clinical translation potential as biomarkers and therapeutic targets.