Profiling of the immune microenvironment in FL

Tumor cells orchestrate their microenvironment. Here, we provide biochemical, structural, functional and clinical evidence that Cathepsin S (CTSS) alterations induce a tumor-promoting immune microenvironment in follicular lymphoma (FL). We found CTSS mutations at Y132 in 6% of FL (19/305). Another 13% (37/286) had CTSS amplification, which was associated with higher CTSS expression. CTSS Y132 mutations lead to accelerated autocatalytic conversion from pro-CTSS to active CTSS and increase substrate cleavage, including CD74 which regulates MHC-II-restricted antigen presentation. Lymphoma cells with hyperactive CTSS more efficiently activated antigen-specific CD4+ T-cells in vitro. Tumors with hyperactive CTSS showed increased CD4+ T-cell infiltration and proinflammatory cytokine perturbation in a mouse model and in human FLs. In mice, this CTSS-induced immune microenvironment promoted tumor growth. Clinically, patients with CTSS-hyperactive FL had better treatment outcomes with standard immunochemotherapies, indicating that these immunosuppressive regimens target both the lymphoma cells and the tumor-promoting immune microenvironment. Digital multiplexed gene expression profiling of formalin-fixed and paraffin-embedded biopsy specimens of the validation cohort was performed as previously described (Hellmuth et al., Blood 2018) 104 follicular lymphoma patient samples were analysed with a 12 gene nCounter® Elements™ XT Reagents gene kit from NanoString.