Dissecting Tumor-Associated Tertiary Lymphoid Structure Formation and Maintenance Using Mouse Models

The solid tumor microenvironment is a highly complex system shaped by dynamic interactions among immune and non-immune cell populations. The formation and organization of tumor-associated tertiary lymphoid structures (TA-TLS) have been observed in patients and linked to favorable prognosis and improved response to immunotherapy against a variety of cancers. Since then, preclinical mouse models have been extensively used to recapitulate TA-TLS formation, dissect the underlying mechanisms, and define the cellular interactions licensing TLS anti-tumor activity. Importantly, mouse models allow temporal dissection of TA-TLS development and progression, an aspect that is difficult to capture in humans where analyses are typically limited to endpoint surgical material. Although many steps leading to TLS formation mirror secondary lymphoid organogenesis, a lot is left unknown about what initiates TA-TLS formation and how these transient immune hubs can be manipulated therapeutically in cancer. Notably, while the persistence of TA-TLS in the tumor microenvironment provides a prolonged local anti-tumor immune response, it also enables the accumulation of regulatory cell components (regulatory T and B cells), thus making TLS the sites of both promotion and regulation of the ongoing endogenous immune response. The disruption of germinal centers and the progressive disaggregation of TLS following tumor regression and neoantigen clearance is also an important point where TLS differ from lymph nodes. Whether a form of local tissue memory exists following TA-TLS resolution, and whether this memory allows for faster and more efficient response upon tumor rechallenge, remain open questions. Different preclinical tumor models used to address these questions have their own strengths and limitations. These models are empowered by advances in high-dimensional imaging and spatial profiling technologies. The field of TA-TLS is now reaching a turning point, where emerging techniques promise to help shift from mostly descriptive studies toward a more mechanistic and functional understanding of TLS biology in cancer. Here, we review insights gained from mouse models into TA-TLS formation, persistence, and function, and highlight experimental and technological advances shaping future TLS research.