Tumor associated inflammation predicts response to immune checkpoint blockade in human melanoma. Established mechanisms that underlie therapy response and resistance center on anti-tumor T cell responses.Here we show that tumor-associated B cells are vital to tumor associated inflammation.Autologous B cells were directly induced by melanoma conditioned medium, expressed proand anti-inflammatory factors, and differentiated towards a plasmablast-like phenotype in vitro .We could identify this phenotype as a distinct cluster of B cells in an independent public single-cell RNA-seq dataset from melanoma tumors. There, plasmablast-like tumor-associated B cells showed expression of CD8+T cell-recruiting chemokines such as CCL3, CCL4, CCL5 and CCL28. Depletion of tumor associated B cells in metastatic melanoma patients by anti-CD20 immunotherapy decreased overall inflammation and CD8+T cell numbers in the human melanoma TME. Conversely, the frequency of plasmablast-like B cells in pretherapy melanoma samples predicted response and survival to immune checkpoint blockade in two independent cohorts. Tumor-associated B cells therefore orchestrate and sustain tumor inflammation, recruit CD8+ T effector cells and may represent a predictor for response and survival to immune checkpoint blockade in human melanoma.2 Cancers such as melanoma, lung, and kidney cancer often present with an inflamed but immunosuppressive tumor microenvironment (TME). Immune checkpoint blocking (ICB) antibodies have significantly improved cancer therapy by overcoming inhibition of T cell effector functions. Yet, a considerable number of patients does not benefit from ICB therapy 1 . It is therefore key to understand the mechanisms that regulate inflammation within the TME to develop novel therapies and improve patient survival. B cells promote both acute immune-associated inflammation for protection against foreign pathogens as well as chronic inflammation in autoimmune diseases and persistent infection. Mouse cancer models show that tumor-associated B cells (TAB) promote tumor inflammation 2,3 but may also inhibit anti-tumor T cell-dependent therapy responses 4-7 . The immuno-inhibitory function of TAB in these models resembles that of regulatory B cells (Breg), which are an established source of inhibitory cytokines such as IL-10 and TGF-b (reviewed in 8 ). In human cancer, Breg were described by either phenotyping, direct detection of immunoinhibitory cytokines or surface molecules, and/or immunosuppressive function 4,[9][10][11][12][13] .Often Breg frequencies increase with tumor progression and are enriched in tumors compared to peripheral blood or adjacent normal tissue. Increased IL-10 + B cell numbers can also be accompanied by increased numbers of CD4 + CD25 +/high CD127 low/and Foxp3 + Tregs in tumor tissues 10,12,14,15 which were independently associated with tumor progression or reduced patient survival.In human melanoma, up to 33% of the immune cells can be TAB 16,17 and phenotypic analysis has revealed CD20+ TAB (reviewed in 18 ) and CD1...