The primary objective is to assess the effect of vaccination with neopeptide-loaded dendritic cells on disease-free survival (DFS) compared to placebo in LS subjects who are known to be carrier of a germline MMR-gene mutation with no signs of disease.

A phase III, multicentre, randomised, double-blind, placebo-controlled trial evaluating disease-free survival after vaccination with with neopeptide-loaded dendritic cells or placebo in LS subjects aged 35-75 who are confirmed to carry a germline MMR-gene mutation in MLH1, MSH2 or MSH6 without clinical signs of disease. Participants will be treated for 6 months + 4 weeks, with an additional maximum follow-up of 51.5 months (total study duration 58.5 months).

Inclusion Criteria:

• a confirmed gPV in MLH1 or MSH2 and without a prior history of MMR-D cancer.
• a confirmed gPV in MSH6, whether or not they have a history of MMR-D cancer. MSH6 subjects with a history of MMR-D cancer have to be cancer-free for more than 1 year.
• previous surgical treatment may include any resection up to and including hemicolectomy; subjects who have undergone (sub)total colectomy are excluded due to the substantially reduced risk of cancer occurrence.
• aged between 35 and 75 for subjects with gPV in MLH1 and MSH2; and aged between 40 and 75 for subjects with gPV in MSH6.
• Lynch syndrome subjects without clinical signs of disease.
• Lynch syndrome subjects without prior treatment for LS-associated cancer, except for MSH6 subjects who are >1 year disease-free and whose prior surgical treatment did not include subtotal colectomy.
• Routine surveillance colonoscopy must be performed within 16 weeks prior to start of study, to exclude (pre)malignancy.
• HLA-A02.01 genotype
• Adequate hematologic, renal, and liver function as defined by laboratory values: WBC >3.0^109/l, lymphocytes >0.8^109/l, platelets >100^109/l, haemoglobin >7,0 mmol/l (9.0 g/dl), estimated glomerular filtration rate > 45 ml/min/1.73m2, AST/ALT <3 x ULN, serum crea¬tinine <150 µmol/l, serum bilirubin <1.5 x ULN (exception: Gilbert's syndrome is permitted).
• WHO performance status of 0 or 1
• No concomitant use of immunosuppressive drugs orally or intravenously. Topical and intranasal steroids are permitted.
• No uncontrolled infectious disease, i.e., negative testing for HIV, Hepatitis B Virus (HBV), Hepatitis C Virus (HCV) and syphilis (Treponema Pallidum Hemagglutination Assay (TPHA)).
• No autoimmune disease such as, but not limited to, inflammatory bowel disease, multiple sclerosis, and lupus. Subjects with type 1 diabetes mellitus, hypothyroidism after autoimmune thyroiditis and skin disorders are not excluded.
• No serious (bleeding and clotting) condition that may interfere with safe leukapheresis.
• No pregnant or lactating women. hCG tests will be performed regularly during the trial to confirm absence of pregnancy.
• No Women Of Child-Bearing Potential (WOCBP) or male partners of WOCBP who are unwilling or unable to use an acceptable method to avoid pregnancy for up to 8 weeks after the last administration of the treatment. WOCBP include any female who has experienced menarche and who has not undergone successful surgical sterilization (hysterectomy, bilateral tubal ligation or bilateral oophorectomy) or is not postmenopausal [defined as amenorrhea > 12 consecutive months].
• Subjects must have absence of any psychological, familial, sociological, or geographical condition potentially hampering compliance with the study protocol and follow-up schedule; those conditions must be discussed with the subject before registration in the trial.
• Expected adequacy of follow-up.
• Written informed consent.

Exclusion Criteria:

• Individuals with a history of malignancy in the past. Allowed malignancies are adequately treated basal cell carcinoma and squamous cell carcinoma of the skin, carcinoma in situ (e.g., DCIS/LCIS/cervical CIS), papillary thyroid carcinoma, and low-risk prostate carcinoma; all locally resected with negative surgical margins and not treated with systemic therapy.
• Organ allografts.
• Known allergy to shellfish.
• Inability to understand and communicate in Dutch.