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Cancer immunotherapy often depends on recognition of peptide epitopes by cytotoxic T lymphocytes (CTLs). The tumor microenvironment (TME) is enriched for peroxynitrite (PNT), a potent oxidant produced by infiltrating myeloid cells and some tumor cells. We demonstrate that PNT alters the profile of MHC class I bound peptides presented on tumor cells. Only CTLs specific for PNT-resistant peptides have a strong antitumor effect in vivo, whereas CTLs specific for PNT-sensitive peptides are not effective. Therapeutic targeting of PNT in mice reduces resistance of tumor cells to CTLs. Melanoma patients with low PNT activity in their tumors demonstrate a better clinical response to immunotherapy than patients with high PNT activity. Our data suggest that intratumoral PNT activity should be considered for the design of neoantigen-based therapy and also may be an important immunotherapeutic target.

Background Gut microbial diversity is associated with improved response to immune checkpoint inhibitors (ICI). Based on the known detrimental impact that antibiotics have on microbiome diversity, we hypothesized that antibiotic receipt prior to ICI would be associated with decreased survival. Methods Patients with stage III and IV melanoma treated with ICI between 2008 and 2019 were selected from an institutional database. A window of antibiotic receipt within 3 months prior to the first infusion of ICI was prespecified. The primary outcome was overall survival (OS), and secondary outcomes were melanoma-specific mortality and immune-mediated colitis requiring intravenous steroids. All statistical tests were two-sided. Results There were 568 patients in our database of which 114 received antibiotics prior to ICI. Of the patients, 35.9% had stage III disease. On multivariable Cox proportional hazards analysis of patients with stage IV disease, the antibiotic-exposed group had statistically significantly worse OS (hazard ratio [HR] = 1.81, 95% confidence interval [CI] = 1.27 to 2.57; P Conclusion Patients with stage III and IV melanoma exposed to antibiotics prior to ICI had statistically significantly worse OS than unexposed patients. Antibiotic exposure was associated with greater incidence of moderate to severe immune-mediated colitis. Given the large number of antibiotics prescribed annually, physicians should be judicious with their use in cancer populations likely to receive ICI.

BackgroundGamma-delta (γδ) T lymphocytes are primed to potently respond to pathogens and transformed cells by recognizing a broad range of antigens. However, adoptive immunotherapy with γδT cells has exhibited mixed treatment responses. Better understanding of γδT cell biology and stratifying healthy donors for allogeneic adoptive therapy is clinically needed to fully realize the therapeutic potential of γδT cells.MethodsWe examine 98 blood samples from healthy donors and measure their expansion capacity after zoledronate stimulation, and test the migration and cytotoxic effector function of expanded γδT cells in 2D culture, 3D tumor spheroid and patient-derived melanoma organoid assays.ResultsWe find that γδT cell expansion capacity is independent of expansion methods, gender, age and HLA type. Basal γδT cell levels in Peripheral blood mononuclear cell (PBMC) correlate well with their expansion, migration and cytotoxic effector capacity in vitro. Circulating γδT cells with lower expression of PD-1, CTLA-4, Eomes, T-bet and CD69, or higher IFN-γ production expand better. γδT cells with central memory and effector memory phenotypes are significantly more abundant in good expanders. A cut-off level of 0.82% γδT cells in PBMC stratifies good versus poor γδT cell expansion with a sensitivity of 97.78%, specificity of 90.48% and area under the curve of 0.968 in a healthy individual. Donors with higher Vδ2 Index Score in PBMC have greater anti-tumor functions including migratory function and cytotoxicity.ConclusionsOur results demonstrate that the interindividual γδT cell functions correlate with their circulating levels in healthy donors. Examination of circulating γδT cell level may be used to select healthy donors to participate in γδT-based immunotherapies.

10054 Background: Checkpoint blockade improves survival in patients with melanoma, with durable complete responses (CR) after stopping therapy. Based on data from KEYNOTE-001, immunotherapy is often continued for 24 months in patients with confirmed CR. Outcomes with treatment of less than 24 months hav not been adequately evaluated and reported. If equally efficacious, shorter courses would potentially reduce health care costs and toxicity. Methods: 45 patients with locally advanced stage III and IV melanoma who received immunotherapy (pembrolizumab, nivolumab or ipilimumab/nivolumab) as 1st line or subsequent therapy, achieved a CR, and stopped therapy were identified under an IRB approved protocol at Penn. Disease Free Survival (DFS) was defined as time from declaration of CR until recurrence or date of analysis (1/15/20). Landmark DFS from time of CR was analyzed based on duration of therapy (less than or greater than 7 months, based on early trial requirements to treat patients with confirmed CR for at least 6 months). Rationale for stopping (toxicity or CR) was also analyzed. Results: Of 45 patients with CR, 27 (60%) were treated less then 7 months (median 4.8, range 1 day to 6.7 months) and 18 (40%) were treated for greater than 7 months (median 12.4, range 7.5 to 24.2 months). Patients who were treated for less than 7 months had a median DFS from time of CR of 30.4 months (95% CI 23.7 to 37.2, range 2.9 to 65.7 months). Patients treated for greater than 7 months had a median DFS of 28.0 months (95% CI 18.9 to 37, range 8.5 to 73.7 months). Patients who stopped due to toxicity (N = 17, 40%) had a median treatment duration of 3.7 months. Their median DFS from time of CR was 30.4 months (95% CI 20.7 to 40.1, range of 2.9 to 65.7 months). Patients who stopped due to CR (N = 28, 60%) had a median treatment duration of 8.5 months. Their median DFS was 27.6 months (95% CI 21.2 to 34 range 7.2 to 73.7 months). Two of 27 (7.4%) patients treated for less then 7 months and 3 out of 18 (16%) patients treated greater than 7 months recurred after stopping. One out of 17 (5.8%) recurred after stopping for toxicity vs. 4/28 (14.3%) who stopped after CR. Conclusions: Patients who stop therapy at less than 7 months have CRs that are equally durable as those treated longer than 7 months, without reduction in landmark DFS. Patients who stopped therapy due to toxicity and then achieved a CR had no difference in DFS compared to patients treated until CR. There was no significant difference in recurrence after achieving a complete response in patients treated for a longer vs shorter treatment course.

56 Background: Recent studies suggest that changes in the gut microbiome modulate response to cancer treatment, including immune checkpoint inhibitors (ICI). Broad-spectrum antibiotics (Abx) are known to cause significant dysbiosis. We hypothesize that recent Abx exposure worsens outcomes in patients (pts) with stage III/IV melanoma (MM) receiving ICI. Methods: We identified MM pts treated with ICI from our institutional database. All received their first ICI between 2004-2019. Antibiotic exposure was defined as receipt of Abx within 3 months prior to the first infusion of ICI. The primary outcome was overall survival (OS) and the secondary outcome was immune-mediated colitis requiring intravenous (IV) steroids. Stage III and IV pts were analyzed separately for the primary analysis. Results: Of 568 pts in our database, 20% received Abx within the 3 months prior to ICI. 36% of pts had stage III disease and 26% of pts were treated with either adjuvant or neoadjuvant ICI. 1.6% of pts died of causes other than MM. The Abx+ and Abx- groups were balanced in terms of stage, race, age, sex, BRAF status, LDH, prior systemic therapies, and class of ICI received. Only 4 pts were hospitalized due to the infection prompting the Abx, and no pts died due to the infection. In the Stage IV group, Abx+ pts had worse OS on MV analysis (HR 1.6, 95% CI 1.1-2.2). Stage III Abx+ also had worse OS (HR 2.8, 95% CI 1.3-5.9). In a sensitivity analysis excluding pts who received IV Abx or were admitted due to infection, survival was still worse for Abx+ pts (HR 1.7, 95% CI 1.2-2.4). In a Fine-Grey competing risk MV model, Abx+ had a higher rate of immune-mediated colitis requiring IV steroids (HR 2.1, 95% CI 1.02-4.5). Conclusions: In MM pts treated with ICI, receipt of Abx within 3 months prior to ICI initiation was associated with decreased OS and increased colitis. Future research should include prospective studies to better define the risk/benefit profile of antibiotics in close proximity to ICI. [Table: see text]

Summary Metastatic melanoma is an aggressive disease, despite recent improvements in therapy. Eradicating all melanoma cells even in drug-sensitive tumors is unsuccessful in patients because a subset of cells can transition to a slow-cycling state, rendering them resistant to most targeted therapy. It is still unclear what pathways define these subpopulations and promote this resistant phenotype. In the current study, we show that Wnt5A, a non-canonical Wnt ligand that drives a metastatic, therapy-resistant phenotype, stabilizes the half-life of p53 and uses p53 to initiate a slow-cycling state following stress (DNA damage, targeted therapy, and aging). Inhibiting p53 blocks the slow-cycling phenotype and sensitizes melanoma cells to BRAF/MEK inhibition. In vivo, this can be accomplished with a single dose of p53 inhibitor at the commencement of BRAF/MEK inhibitor therapy. These data suggest that taking the paradoxical approach of inhibiting rather than activating wild-type p53 may sensitize previously resistant metastatic melanoma cells to therapy.

Antibodies targeting CTLA-4 induce durable responses in some patients with melanoma and are being tested in a variety of human cancers. However, these therapies are ineffective for a majority of patients across tumor types. Further understanding the immune alterations induced by these therapies may enable the development of novel strategies to enhance tumor control and biomarkers to identify patients most likely to respond. In several murine models, including colon26, MC38, CT26, and B16 tumors cotreated with GVAX, anti–CTLA-4 efficacy depends on interactions between the Fc region of CTLA-4 antibodies and Fc receptors (FcR). Anti–CTLA-4 binding to FcRs has been linked to depletion of intratumoral T regulatory cells (Treg). In agreement with previous studies, we found that Tregs infiltrating CT26, B16-F1, and autochthonous BrafV600EPten−/− melanoma tumors had higher expression of surface CTLA-4 (sCTLA-4) than other T-cell subsets, and anti–CTLA-4 treatment led to FcR-dependent depletion of Tregs infiltrating CT26 tumors. This Treg depletion coincided with activation and degranulation of intratumoral natural killer cells. Similarly, in non–small cell lung cancer (NSCLC) and melanoma patient-derived tumor tissue, Tregs had higher sCTLA-4 expression than other intratumoral T-cell subsets, and Tregs infiltrating NSCLC expressed more sCTLA-4 than circulating Tregs. Patients with cutaneous melanoma who benefited from ipilimumab, a mAb targeting CTLA-4, had higher intratumoral CD56 expression, compared with patients who received little to no benefit from this therapy. Furthermore, using the murine CT26 model we found that combination therapy with anti–CTLA-4 plus IL15/IL15Rα complexes enhanced tumor control compared with either monotherapy.