5, 200ra116. immune status in the LTB state. Thus, Brigatinib (AP26113) elevated IFN- secretion in the LTB state contributes to the development of an immune-intrinsic mechanism of resistance to combination checkpoint blockade, highlighting the importance of achieving the optimal magnitude of immune stimulation for successful combination immunotherapy strategies. Graphical Abstract In Brief Although immune checkpoint blockades are being combined to enhance anti-tumor efficacy, Pai et al. find that this approach can lead to therapy resistance in the low tumor burden setting. Potent immunotherapy in this setting overdrives tumor-reactive T cells, leading to their death. Optimal immunotherapy could therefore be disease-context dependent. INTRODUCTION In recent years, immune checkpoint inhibitors have been rapidly approved for the management of advanced malignancies, including melanoma, non-small-cell lung malignancy (NSCLC), renal cell carcinoma (RCC), urothelial carcinoma, and head and neck malignancy (Callahan et al., 2016). However, only a small subset (10%C30%) of patients respond to single-agent immune checkpoint therapy (Robert et al., 2015), and a myriad of combination strategies are currently being actively investigated in clinical trials with the goal of enhancing anti-tumor immunity and clinical efficacy. Co-targeting of cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) and programmed death-1 (PD-1) immune checkpoint pathways is usually one strategy that demonstrates significantly improved clinical outcomes in advanced melanoma (Larkin et al., 2015). Despite these improvements, a significant proportion of patients still do not accomplish objective responses to checkpoint inhibitors. Recent clinical observations suggest that treatments with checkpoint inhibitors do not usually lead to better outcomes in patients. In multiple large randomized trials (Bellmunt et al., 2017; Borghaei et al., 2015; Kwon et al., 2014), patients receiving immune checkpoint inhibitors experienced worse survival outcomes than did control arms during the initial months of treatment, at a time before immune-related toxicities Brigatinib (AP26113) fully manifest. Indeed, some malignancy patients (9%) exhibit accelerated tumor growth upon treatment with immune checkpoint inhibitors, a phenomenon termed tumor hyper-progression (Champiat et al., 2017). Therefore, an improved understanding of the mechanisms underlying differential responses to checkpoint inhibition is needed to inform the future development of combinatorial therapeutic strategies. Several studies have contributed to the understanding of mechanisms underlying differential responses and mechanisms of resistance to immune checkpoint strategies (Sharma et al., 2017). These include adaptive resistance mediated by interferon-dependent expression of inhibitory ligands on malignancy cells (Benci et al., 2016; Tumeh et al., 2014), exclusion of CD8+ T cell infiltration by transforming growth factor- (TGF-) signaling within the tumor microenvironment (Mariathasan et al., 2018), and the acquisition of resistance by loss-of-function mutations in Janus kinases 1 and 2 (JAK1/2) or truncating mutations in histocompatibility leukocyte antigen (HLA) class I molecules (Zaretsky et al., 2016). Recently, tumor burden has emerged as a key factor determining clinical responses of immune checkpoint blockade (Huang et al., Brigatinib (AP26113) 2017). Early administration of PD-1 blockade in a relatively low disease burden can reinvigorate the dysfunctional T cells, whereas continuous exposure of CSF2RA tumor antigens can ultimately develop into fixed T cell exhaustion status, resulting in poor response to anti-PD-1 (Schietinger et al., 2016). Similarly, favorable clinical outcomes in patients with low disease burdens treated with PD-1 blockade are associated with a higher ratio of reinvigorated CD8+ T cells to tumor burden (Huang et al., 2017). However, different immune-checkpoint blockade treatments contribute to unique immune landscapes (Wei et al., 2017), and whether the combining of checkpoint blockades favors the clinical response in low disease burden is usually questionable. Sub-group analyses of treatment responses of two recent clinical trials significantly favor high, as opposed to low, disease burdens in patients who received anti-CTLA-4 plus anti-PD-1 (42%C29% in RCC and 51%C13% in NSCLC; Hellmann et al., 2018; Motzer et al., 2018), further indicating the complexity of disease burden and therapeutic responses to combination checkpoint blockade. Here, we sought to further understand the effects of different.