Key Inclusion Criteria:

          1. Stage IIIB, IIIC or IV melanoma

          2. Previously untreated and eligible for pembrolizumab treatment (prior treatment with
             BRAF and MEK inhibitors permitted) 3) Adequate blood, liver and kidney function 4)
             Consent to undergo tumor biopsies during the study

        Exclusion Criteria:

          1. Uveal or ocular malignant melanoma

          2. History of hematologic or primary solid tumor malignancy with the exception of
             patients in remission for at least 5 years, as judged by the investigator are allowed

          3. Prior systemic treatment for unresectable or metastatic melanoma. Exception: Prior
             treatment with BRAF and MEK inhibitors permitted. A washout period of at least
             3-half-lives (median terminal half-life) prior to the first dose of trial treatment
             must have elapsed.

          4. Prior therapy with an anti-CTLA4, anti-PD-1, anti-PD-L1, anti-PD-L2 agent or oncolytic
             virus.

          5. Known hypersensitivity to GM-CSF

          6. Women who are breastfeeding, pregnant or expect to be pregnant during the study
             through 6 months after the last dose

          7. Men who plan to become a father during the study through 4 months after the last dose
             of study medication

          8. Known history of, or any evidence of active, non-infectious pneumonitis

          9. History of cardiac disease