Study visit requirements are outlined in the Study Calendar ( Section 9 ). 4.10 Duration of Follow Up Patients will be followed for 30 days after th eir last dose of study drug or until death, whichever occurs first. SAEs that occur w ithin 90 days of the last infusion of MK- 3475 or before initiation of a new antineopl astic treatment shoul d also be followed and recorded. Patients removed from study for unacceptable adverse event(s) will be followed until resolution or stabil ization of the adverse event. Patients who discontinue from treatment s hould be contacted every three months to monitor overall survival. In formation of other cancer th erapies after discontinuation from the study treatment will be collected. At the end of the study, subjects will be re quired to enroll in a long-term follow-up protocol, an FDA requirement for genetically modified products, in which they will be followed for at least 5 years (or until death) after receiving the last dose of any investigational agent for disease progres sion, survival, and potential long term toxicity of gene therapy. The subjects will be asked to enroll in the study entitled, “Long-Term Follow-Up of Patients who Recei ved the Allogeneic Colon Cancer Cell Vaccine Administered with a GM-CSF Producing Bystander Cell Line.” 4.11 Criteria for Removal from Study Treatment Phase 2 Study of GVAX (with CY) and pembrolizum ab in MMR-p advanced colorectal cancer J16154/Version 5 / January 17, 2018 32Patients will be removed from study treatme nt when any of the criteria listed in Section 4.8 applies. The reason for study removal and the date the patient was removed must be documented in the Case Report Form. Patients with disease progression by radiogr aphic imaging or laboratory parameters during the 12-week evaluation but without rapi d clinical deterioration or significant change in performance status that requires additional immediate therapy may continue to receive treatment on study. Subjects that meet the above criteria and continue on study therapy must discontinue treatment upon documentation of disease progression on the second scan six weeks later, with the date of progression being backdated to the first RECIST 1.1 progression time point. NOTE: For patients who progress and remain on study, at the time of the next restaging, stable disease with clinical stability as determined by their treating physician/team can remain on study. Specifica lly, these patients do not have to have regression to <20% growth from baseline. If they do not have 20% increase from the last restaging scan, th ey may remain on study. 5. DOSING DELAYS/DOSE MODIFICATIONS 5.1 Dose Modifications Dose reduction or dose increase of CY, GVAX, and pembrolizumab will not be permitted in individual patients. 5.2 Dosing Delays 5.2.1 All scheduled cycles with in a course are to be given approximately 3 weeks apart. If necessary, a cycle may be de layed for up to 1 week. In this case, subsequent cycles should continue so that a subject can still receive all cycles given that the cycles are a minimum of 3 weeks apart and they have not experienced an AE necess itating discontinuation. If de layed more than 1 week, the Principal Investigator must be c ontacted for further instructions on continued treatment. Additional delays or modifications to the treatment schedule must be approved by the Prin cipal Investigator or IND Sponsor. If a delay occurs between Day 1 and 2 in a cycle:  Pembrolizumab-related infusion reactions must resolve to baseline prior to administration of GVAX.  Resume Day 2 treatment and assessme nt schedule without repeating Day 1 study treatments if the delay is within 72 hours.  If the delay is longer than 72 hours, repeat Day 1 and Day 2 (if applicable) study treatments/assessments with a minimum of 1 week from the previous Day 1 treatment. This includes steroid treatment requiring at least a 14 day washout prior to resu ming study-related treatments. Pembrolizumab will be withheld for drug-related Grade 4 hematologic toxicities, non-hematological toxicity ≥ Grade 3 including laboratory Phase 2 Study of GVAX (with CY) and pembrolizum ab in MMR-p advanced colorectal cancer J16154/Version 5 / January 17, 2018 33abnormalities, and severe or life-threatening AEs as per Table 5 . Table 5: Pembrolizumab Dose Del ay and Discontinuation Criteria General instructions: 1. Corticosteroid taper should be initiated upon AE improving to Gr ade 1 or less and continue to taper over at least 4 weeks. 2. For situations where pembrolizumab has been withheld, pemb rolizumab can be resumed after AE has been reduced to Grade 1 or 0 and corticosteroid has been tapered. Pembrolizumab should be permanently discontinued if AE does not resolve within 12 weeks of last dose or corticosteroids cannot be reduced to ≤10 mg prednisone or equivalent per day within 12 weeks. 3. For severe and life-threatening irAEs, IV corticosteroid should be initiated first followed by oral steroid. Other immunosuppressive treatment should be initiated if irAEs cannot be controlled by corticosteroids. Immune-related AEs Toxicity grade or conditions (CTCAEv4.0) Action taken to pembrolizum ab irAE management with corticosteroid and/or other therapies Monitor and follow-up Pneumonitis Grade 2 Withhold  Administer corticosteroids (initial dose of 1-2 mg/kg prednisone or equivalent) followed by taper  Monitor participants for signs and symptoms of pneumonitis  Evaluate participants with suspected pneumonitis with radiographic imaging and initiate corticosteroid treatment  Add prophylactic antibiotics for opportunistic infections Grade 3 or 4, or recurrent Grade 2 Permanently discontinue Diarrhea / Colitis Grade 2 or 3 Withhold  Administer corticosteroids (initial dose of 1-2 mg/kg prednisone or equivalent) followed by taper  Monitor participants for signs and symptoms of enterocolitis (ie, diarrhea, abdominal pain, blood or mucus in stool with or without fever) and of bowel perforation (ie, peritoneal signs and ileus).  Participants with ≥ Grade 2 diarrhea suspecting colitis should consider GI consultation and performing endoscopy to rule out colitis.