Gene Expression profile for the characterization of PD-L1 blockade in melanoma model

Only a subset of patients responds to immune checkpoint blockade in melanoma. A preclinical model recapitulating the clinical activity of ICB would provide a valuable platform for mechanistic studies. We used melanoma tumors arising from an Hgftg;Cdk4R24C/R24C genetically engineered mouse (GEM) model to evaluate the efficacy of an anti-mouse PD-L1 antibody similar to the anti-human PD-L1 antibodies durvalumab and atezolizumab. Consistent with clinical observations for ICB in melanoma, anti-PD-L1 treatment elicited complete and durable response in a subset of melanoma-bearing mice. We also observed tumor growth delay or regression followed by recurrence. For early treatment assessment, we analyzed gene expression profiles, T cell infiltration, and T cell receptor (TCR) signatures in regressing tumors compared to tumors exhibiting no response to anti-PD-L1 treatment. We found that CD8+ T cell tumor infiltration corresponded to response to treatment, and that anti-PD-L1 gene signature response indicated an increase in antigen processing and presentation, cytokine-cytokine receptor interaction, and natural killer cell-mediated cytotoxicity. TCR sequence data suggest that an anti-PD-L1-mediated melanoma regression response requires not only an expansion of the TCR repertoire that is unique to individual mice, but also tumor access to the appropriate TCRs. Thus, this melanoma model recapitulated the variable response to ICB observed in patients and exhibited biomarkers that differentiate between early response and resistance to treatment, providing a valuable platform for prediction of successful immunotherapy. Mice were treated with anti-mouse PD-L1 mIgG1 D265A (clone 80; AstraZeneca Cat# AB740080, Batch# SP15-221) in PBS at 10mg/kg or 20mg/kg intraperitoneally twice weekly for a maximum of 6 doses over 3 weeks in the initial efficacy study, or over a longer period to assess tumor response as indicated in the text. For each study the vehicle control group received PBS (10ml/kg) and an additional antibody control group received mouse IgG1 D265A (AstraZeneca clone: Cat# NIP228, Batch# SP16-017) in PBS at 10 or 20mg/kg. Tumor growth was monitored by caliper measurement. Tumors were harvested from treated mice at several different endpoints (delayed growth, stabilization, regression) or when they reached a maximum tumor volume of 2000 mm3. At necropsy, tumors were divided into sections for fixation or flash-freezing. Blood samples were taken from each mouse prior to treatment and at the time of tumor harvest. To evaluate response in biomarker studies (Study 2 or 3), tumors from mice treated with anti-PD-L1 were harvested after 2-12 doses based on regression in tumor volume after two consecutive measurements (Responders). Anti-PD-L1-treated tumors that exhibited growth progression were harvested at matching timepoints (Non-responders). Tumors that regressed to below measurable volumes after initial treatment but regrew after 4-10 doses of anti-PD-L1 were also harvested (Relapse). PBS- and control IgG-treated tumors were also harvested at multiple time points for comparison. Additionally, any mice that did not experience initial tumor growth, but developed tumors after anti-PD-L1 treatment were designated as Growth Delay. Any tumor that decreased in volume by 5% or more was designated as a Responder. Within the Responder group, tumors with no volume change between 2 consecutive measurements were designated as growth stabilization.