LTβR Agonism Promotes Antitumor Immune Responses via Modulation of the Tumor Microenvironment.

The presence of high endothelial venules (HEV) and tertiary lymphoid structures (TLS) in solid tumors is correlated with favorable prognosis and better responses to immune checkpoint blockade in many cancer types. Elucidation of the molecular mechanisms underlying intratumoral HEV and TLS formation and their contribution to antitumor responses may facilitate the development of improved treatment strategies. Lymphotoxin β receptor (LTβR) signaling is a critical regulator of lymph node organogenesis and can cooperate with antiangiogenic and immune checkpoint blockade treatment to augment tumor-associated HEV formation. In this study, we demonstrated that LTβR signaling modulates the tumor microenvironment via multiple mechanisms to promote antitumor T-cell responses. Systemic activation of the LTβR pathway via agonistic antibody treatment induced tumor-specific HEV formation, upregulated the expression of TLS-related chemokines, and enhanced dendritic cell (DC) and T-cell infiltration and activation in syngeneic tumor models. In vitro studies confirmed direct effects of LTβR agonism on DC activation and maturation and associated DC-mediated T-cell activation. Single-agent LTβR agonist treatment inhibited syngeneic tumor growth in a CD8+ T-cell-dependent and HEV-dependent manner, and the LTβR agonist enhanced antitumor effects of anti-PD-1 and CAR T-cell therapies. An in vivo tumor screen for TLS-inducing cytokines revealed that the combination of LTβR agonism and lymphotoxin ⍺ expression promoted robust intratumoral TLS induction and enhanced tumor responses to anti-CTLA4 treatment. Collectively, this study highlights crucial functions of LTβR signaling in modulating the tumor microenvironment and could inform future HEV/TLS-based strategies for cancer treatments. Significance: LTβR mediates tumor-specific  high endothelial venule formation and immunomodulation of the tumor microenvironment that promotes antitumor immune responses, supporting LTβR agonism as an approach to enhance the antitumor efficacy of immunotherapies.