Inclusion Criteria:

          1. Aged 18 years or older at the time of consent;

          2. Able to give informed consent;

          3. Has a confirmed histological diagnosis of MPM with histological type epithelioid or
             biphasic (predominantly [>50%] epithelioid). Histological diagnosis of MPM will be
             confirmed centrally using specimens or slides from tumor specimens obtained at the
             time of initial presentation or a subsequent procedure. Central confirmation of
             diagnosis with immunohistochemistry will be performed, and independent central
             confirmation will be required for study entry;

          4. Measurable disease, per modified Response Evaluation Criteria in Solid Tumor [RECIST]
             for pleural mesothelioma;

          5. Has failed a minimum of 1 treatment regimen and a maximum of 2 treatment regimens,
             which may have been chemotherapeutic and/or immunotherapeutic treatment regimens for
             MPM which included at least 1 anti-folate and platinum combination regimen. Patients
             who have undergone primary surgical resection and/or radiation therapy to the
             pulmonary site are eligible to participate. For clarity, surgical resection and/or
             radiation therapy to the pulmonary site are not exclusionary and are not considered a
             line of therapy;

          6. Has a pleural space accessible for pleural catheter insertion. Patients with a
             previously inserted pleural catheter may be enrolled, and the pre-existing catheter
             can be used for vector administration as long as it is functional and has no evidence
             of local infection;

          7. Life expectancy >12 weeks in the judgement of the Investigator;

          8. Eastern Cooperative Oncology Group (ECOG) status of 1 or 0;

          9. Female and male patients:

               -  Female patients must be either postmenopausal (no menstrual period for a minimum
                  of 12 months) or surgically sterile upon entry into the study. Female patients of
                  childbearing potential must have a negative pregnancy test upon entry into this
                  study and agree to use a highly effective method of contraception from Screening
                  until 1 month following administration of gemcitabine;

                    -  Highly effective methods of contraception that result in a low failure rate
                       (i.e., <1% per year) when used consistently and correctly include combined
                       (estrogen and progestogen containing) hormonal contraception associated with
                       inhibition of ovulation (oral, intravaginal, or transdermal),
                       progestogen-only hormonal contraception associated with inhibition of
                       ovulation (oral, injectable, or implantable), intrauterine device,
                       intrauterine hormone-releasing system, bilateral tubal occlusion,
                       vasectomized partner, or sexual abstinence;

                    -  True abstinence, when in line with the preferred and usual lifestyle of the
                       patient, is considered a highly effective method only if defined as
                       refraining from heterosexual intercourse during the entire period of study
                       participation and for 1 month post-gemcitabine administration. The
                       reliability of sexual abstinence needs to be evaluated in relation to the
                       duration of the clinical study and the preferred and usual lifestyle of the
                       patient. Periodic abstinence (e.g., calendar, ovulation, symptothermal, and
                       post-ovulation method) and withdrawal are not acceptable methods of
                       contraception; and

               -  Male patients must be either surgically sterile or agree to use a double-barrier
                  contraception method from Screening until 1 month post-gemcitabine
                  administration; and

         10. Adequate laboratory values at Screening:

               -  Hemoglobin 9 g/dL;

               -  White blood cell count 3500/µL;

               -  Absolute neutrophil count 1500/µL;

               -  Platelet count 100,000/µL;

               -  International normalized ratio (INR) and activated partial thromboplastin time
                  (aPTT) below the upper limit of normal (ULN). It is expected that patients
                  receiving anticoagulation therapy will not have INR and aPTT results that fall
                  within normal limits. It is not intended to exclude these patients and,
                  therefore, medical discretion is permitted for patients who have clinically
                  acceptable results in regards to their current concomitant anticoagulant therapy;

               -  Aspartate aminotransferase (AST) 3 × ULN;

               -  Alanine aminotransferase (ALT) 3 × ULN;

               -  Total bilirubin 2 × ULN;

               -  Estimated glomerular filtration rate 50 mL/min/1.73 m2; and

               -  Serum albumin 2.5 g/dL.

        Exclusion Criteria:

          1. Is "treatment-naïve" (i.e., has not received at least 1 anti-folate and platinum
             combination regimen);

          2. Has previously received 3 or more lines of systemic chemotherapeutic or
             immunotherapeutic treatment;

          3. Has previously received treatment with gemcitabine;

          4. Has stage IV extrathoracic metastatic disease;

          5. Inadequate pulmonary function of clinical significance as per Investigator review;

          6. Clinically significant pericardial effusion (i.e., as judged by the Investigator
             and/or requiring drainage) detected by computed tomography (CT) scan at Screening;

          7. Prior therapy(ies), if applicable, must be completed according to the criteria below
             prior to vector administration:

               -  Cytotoxic chemotherapy, at least 21 days from last dose;

               -  Non-cytotoxic chemotherapy (e.g., small molecule inhibitor), at least 14 days
                  from last dose;

               -  Monoclonal antibody, at least 3 half-lives from last dose;

               -  Non-antibody immunotherapy (e.g., tumor vaccine), at least 42 days from last
                  dose;

               -  Radiotherapy, at least 14 days from last local site radiotherapy;

               -  Hematopoietic growth factor, at least 14 days from last dose; or

               -  Study drug, 30 days or 5 half-lives, whichever is longer, from last dose;

          8. Patient previously treated with IFNs (e.g., for chronic active hepatitis);

          9. Suspected/known hypersensitivity to IFN-α2b;

         10. Known hypersensitivity to celecoxib or sulfonamides;

         11. Impaired cardiac function or clinically significant cardiac disease including the
             following:

               -  New York Heart Association class III or IV congestive heart failure;

               -  Myocardial infarction within the last 12 months; and

               -  Patients known to have impaired left ventricular ejection fraction per
                  institutional standards and of clinical significance as per Investigator review;

         12. Women who are pregnant or breastfeeding;

         13. Uncontrolled intercurrent illness including, but not limited to, ongoing or active
             infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac
             arrhythmia, depression, or psychiatric illness/social situations within the last 12
             months;

         14. Patients with active, known, or suspected auto-immune disease or a syndrome that
             requires systemic or immunosuppressive agents (oral prednisolone or equivalent at a
             dose of <10 mg per day is permitted); NOTE: patients with vitiligo, residual
             hypothyroidism due to auto immune disease 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;

         15. History of asthma, urticaria, or other allergic-type reactions after taking aspirin or
             other NSAIDs;

         16. History of ulcer disease or gastrointestinal bleeding;

         17. Uncontrolled or poorly controlled hypertension requiring 3 or more anti-hypertensive
             drugs;

         18. Patients receiving lithium;

         19. Any significant disease which, in the opinion of the Investigator, would place the
             patient at increased risk of harm if he/she participated in the study;

         20. History of malignancy of other organ system within the past 5 years, except treated
             basal cell or squamous cell carcinoma of the skin, or early stage prostate cancer
             (stage T2a or smaller, prostate specific antigen <10 ng/mL, Gleason score <6); or

         21. Has a congenital or acquired immunodeficiency, including patients with known history
             of infection with human immunodeficiency virus.