• Have a diagnosis of MZL, of extranodal (EMZL) or splenic (SMZL based on the Matutes score and CD20 + CD11c + CD180 + CD43 + CD200 expression and validated by a centralized review) or nodals (NMZL) subtypes. In case of large dissemination, disseminated MZL will be included as DMZL and included in NMZL subtype.

• Have been treated with at least one prior systemic treatment and not more than three prior lines. Previous line must include at least one systemic line with a drug targeting CD20 (monoclonal antibody at least 2 cycles) with or without chemotherapy (R-CHOP, R-Bendamustine, R-CVP, R-Chlorambucil at least 2 cycles) or targeted treatment such as Ibrutinib (at least 1 month). Patients should not have received Lenalidomide before. Prior local therapy (including surgery, radiotherapy antibiotics for H. pylori-positive gastric lymphoma, and antiviral for hepatitis C virus) is not considered as one line of treatment

• Signed Informed Consent Form

• Age ≥ 18 years at the time of signing the informed consent form

• Ability to comply with the study protocol and procedures and required hospitalizations, in the investigator's judgement

• Eastern Cooperative Oncology Group (ECOG) performance score (PS) of ≤ 2

• Have a symptomatic disease requiring a systemic treatment

• Not eligible for a local treatment including radiotherapy or surgery

• Stage I disease of EMZL, SMZL or NMZL may be eligible only if not candidate to local therapy (surgery or radiotherapy).

⁃ Measurable disease in at least two perpendicular dimensions on an imaging scan is defined as: lymph node or nodal mass bi-dimensional measurement with ≥ 15 mm in longest transverse diameter or the short diameter must measure ≥ 10 mm regardless of the longest transverse diameter.

⁃ Spleen is considered as a measurable disease if vertical axis is higher than 13 cm.

⁃ Adequate hematopoietic function at screening as follows unless cytopenia is clearly due to marrow involvement of MZL or hypersplenism or autoimmune thrombocytopenia:

‣ 1. Platelet count ≥ 75 G/L; in cases of thrombocytopenia clearly due to marrow involvement of MZL or hypersplenism or auto-immune thrombocytopenia, platelet count should be ≥ 30 G/L Washout platelet transfusion is 7 days between transfusion and D1 of starting treatment 11.2. Absolute Neutrophil Count (ANC ) ≥ 1 G/L unless neutropenia is clearly due to marrow involvement of MZL or hypersplenism. Granulocyte Colony-Stimulating Factor (G-CSF) is not allowed within 7 days before starting treatment 11.3. Total hemoglobin ≥ 8 g/dL unless anemia is clearly due to marrow involvement of MZL or hypersplenism or autoimmune hemolytic anemia. Washout erythrocyte transfusion is 7 days between transfusion and D1 of starting treatment

⁃ Serum total bilirubin ≤ 1.5 x the upper limit of normal (ULN) (or ≤3 x ULN for patients with Gilbert syndrome),

⁃ Aspartate Transaminase (AST) or ALanine Transaminase (ALT) ≤ 2.5 x ULN, unless directly attributable to the patient's MZL

⁃ Measured or estimated creatinine clearance ≥ 40 mL/min by institutional standard method

⁃ Patients who are hepatitis B surface antigen (HBsAg) negative and hepatitis B core antibody (HBcAb) positive, must be negative for hepatitis B virus (HBV) polymerase chain reaction (PCR) to be eligible for study participation. Patients who are hepatitis B surface antigen (HBsAg) negative, hepatitis B surface antibody (anti-HBsAb) positive and hepatitis B core antibody (HBcAb) negative are eligible,

⁃ 16\. Contraception: 16.1. For women of childbearing potential (WOCBP) (refer to section 14.6.1 and 14.6.1.1): Serum test pregnancy at screening and Day 1 before first dose. And then monthly until end of treatment. Efficient contraceptive method is required during the treatment period (including periods of treatment interruption), for at least 28 days after the final dose of Lenalidomide (if applicable), 3 months after the final dose of Mosunetuzumab (if applicable), 6 months after the final dose of chemotherapies (if applicable) and 12 months after the final dose of Rituximab (if applicable).

‣ 2. For men: agreement to remain abstinent (refrain from heterosexual intercourse) or use a condom, and agreement to refrain from donating sperm, as defined below: With a female partner of childbearing potential or pregnant female partner, men must remain abstinent or use a condom during the treatment period (including periods of treatment interruption), and for at least 28 days after the final dose of Lenalidomide (if applicable), 3 months after the final dose of Mosunetuzumab and Tocilizumab (if applicable), 6 months after the final dose of chemotherapies (if applicable) and 12 months after the final dose of Rituximab) (if applicable).

⁃ Patient covered by any social security system (France)