Inclusion Criteria:

          -  Signed written informed consent.

          -  Male or female subjects aged ≥ 18 years.

          -  Histologically proven locally advanced or metastatic GIST. C-Kit (CD117) positive
             tumors detected by immunohistochemistry

          -  Known mutational status KIT or PDGFRA.

          -  Documented disease progression (as per RECIST 1.1) within 3 months before study entry

          -  No more than 3 previous lines of treatment, which must include imatinib and sunitinib
             .

          -  Performance status ≤ 2 at trial entry and an estimated life expectancy of at least 3
             months.

          -  Disease must be measurable with at least 1 unidimensional measurable lesion by RECIST
             1.1. Clinically and/or radiographically documented measurable disease within 21 days
             prior to registration

          -  Adequate hematological function defined by the following laboratory tests results,
             obtained within 14 days prior to initiation of study treatment

          -  - white blood cell (WBC) count ≥ 3 × 109/L with absolute neutrophil count (ANC) ≥ 1.5
             × 109/L,

          -  - lymphocyte count ≥ 0.5 × 109/L,

          -  - platelet count ≥ 100 × 109/L,

          -  - hemoglobin ≥ 9 g/dL (patients may be transfused).

          -  Adequate hepatic function defined by

          -  - total bilirubin level ≤ 1.5 × the upper limit of normal range (ULN),

          -  - an AST level ≤ 2.5 × ULN, and an ALT level ≤ 2.5 × ULN or, for subjects with
             documented metastatic disease to the liver, AST and ALT levels ≤ 5 × ULN.

          -  Serum creatinine ≤ 1.5 x ULN or creatinine clearance ≥ 50 ml/min on the basis of the
             Cockroft- Gault glomerular filtration rate estimation:

          -  (140-age) x (weight in kg) x (0,85 if female)/72x(serum creatinine in mg/dl)

          -  No clinically significant (i.e., active) cardiovascular disease: cerebral vascular
             accident/stroke (< 6 months prior to enrollment), myocardial infarction (< 6 months
             prior to enrollment), Uncontrolled hypertension (systolic blood pressure >150 mmHg
             and/or diastolic blood pressure >100 mmHg , unstable angina, congestive heart failure
             (≥2 New York Heart Association Classification medication) serious cardiac arrhythmia.
             All other significant diseases (e.g., inflammatory bowel disease), which, in the
             opinion of the investigator, might impair the subject's tolerance of trial treatment;

          -  Absence of any psychological, familial, sociological or geographical condition
             potentially hampering compliance with the study protocol and follow-up schedule; those
             conditions should be discussed with the patient before registration in the trial

          -  Patients not receiving anticoagulant medication who have an International Normalized
             Ratio (INR) >1.5 or an activated partial thromboplastin time (aPTT) >1.5 x ULN within
             7 days prior to first study treatment. Note: Patients receiving full dose oral or
             parenteral anticoagulants may be included in the study as long as anticoagulant dosing
             has been stable for at least 2 weeks prior to study entry and the appropriate
             coagulation monitoring tests are within local therapeutic limits;

          -  Effective contraception for both male and female subjects if the risk of conception
             exists. (Note: The effects of the study drugs on the developing human fetus are
             unknown. Thus, women of childbearing potential and men must agree to use effective
             contraception, defined as 2 barrier methods e.g. implants, injectables, combined oral
             contraceptives in combination with a barrier method, some intrauterine contraceptive
             devices, sexual abstinence, or a vasectomized partner;)

          -  Availability of representative formalin-fixed paraffin-embedded (FFPE) tumor specimens
             in paraffin blocks.

        Exclusion Criteria:

          -  Concurrent anticancer treatment within 14 days before the start of trial treatment
             (e.g., cytoreductive therapy, radiotherapy [with the exception of palliative bone
             directed radiotherapy], immune therapy, or cytokine therapy except for
             erythropoietin); major surgery within 28 days before the start of trial treatment
             (excluding prior diagnostic biopsy); use of hormonal agents within 7 days before the
             start of trial treatment; or use of any investigational drug within 28 days before the
             start of trial treatment.

          -  Patients with PDGFRA D842V mutations are not eligible for this study.

          -  Previous treatment with anti-PD-1 or anti-PD-L1 antibodies, previous therapy with
             axitinib.

          -  Persisting toxicity related to prior therapy Grade > 1 CTCAE v 4.0,

          -  Major surgical procedure within 28 days prior to the first study treatment, or
             anticipation of the need for major surgery during the course of the study treatment;
             or minor surgical procedures, within 24 hours prior to the first study treatment;

          -  Subjects with active, known or suspected autoimmune disease. Subjects with vitiligo,
             type I diabetes mellitus, residual hypothyroidism due to autoimmune condition only
             requiring hormone replacement, psoriasis not requiring systemic treatment, or
             conditions not expected to recur in the absence of an external trigger are permitted
             to enroll.

          -  Subjects with a condition requiring systemic treatment with either corticosteroids (>
             10 mg daily prednisone equivalents) or other immunosuppressive medications within 14
             days of study drug administration except for adrenal replacement steroid. The use of
             inhaled corticosteroids for chronic obstructive pulmonary disease is allowed.

          -  History of abdominal fistula, grade 4 bowel obstruction or gastrointestinal
             perforation, intra-abdominal abscess within 6 months of enrollment;

          -  History of idiopathic pulmonary fibrosis (including pneumonitis), drug-induced
             pneumonitis, organizing pneumonia (i.e., bronchiolitis obliterans, cryptogenic
             organizing pneumonia), or evidence of active pneumonitis on screening chest CT scan

          -  Significant acute or chronic infections including, among others: positive testing for
             human immunodeficiency virus (HIV) or known acquired immunodeficiency syndrome (AIDS);

          -  Active hepatitis B virus (HBV) infection (chronic or acute), defined as having a
             positive hepatitis B surface antigen (HBsAg) test at screening. Patients with past or
             resolved HBV infection, defined as having a negative HBsAg test and a positive total
             hepatitis B core antibody (HBcAb) test at screening, are eligible for the study.

          -  Active hepatitis C virus (HCV) infection, defined as having a positive HCV antibody
             test and positive HCV RNA test at screening

          -  Active tuberculosis

          -  Severe infection within 2 weeks prior initiation of study treatment, including, but
             not limited to, hospitalization for complications of infection, bacteriemia or severe
             pneumonia

          -  Brain or leptomeningeal metastases, history of intracranial hemorrhage

          -  Treatment with a live, attenuated vaccine within 4 weeks prior to initiation of study
             treatment, or anticipation of need for such a vaccine during the course of the study

          -  Known alcohol or drug abuse.

          -  Inability or unwillingness to swallow pills

          -  Pregnant or breastfeeding, or intending to become pregnant during the study. Woman of
             childbearing potential must have a negative serum pregnancy test result within 14 days
             prior to initiation of study treatment.

          -  Symptomatic brain metastases, history of intracranial hemorrhage

          -  Previous enrollment in this study