Sequential Intranodal Immunotherapy (SIIT) Combined With Anti-PD1 (Pembrolizumab) in Follicular Lymphoma

Inclusion Criteria: 1. Be willing and able to provide written informed consent/assent for the trial. 2. Be \>= 18 years of age on day of signing informed consent. 3. Histologically confirmed incurable asymptomatic untreated or relapsed follicular lymphoma grade I-IIIA stage III-IV 4. Lymphoma nodes greater than 1.5 cm at sites suitable for radiation and ultrasound-guided injections 5. Have measurable disease outside irradiated sites. 6. Be willing to provide tissue from an excisional biopsy of a tumor lesion 7. Have a performance status of 0 on the ECOG Performance Scale. 8. Demonstrate adequate organ function as defined below. Absolute neutrophil count (ANC) \>=1,500 /mcL, Platelets \>=100,000 / mcL, Hemoglobin \>=9 g/dL, Serum creatinine \>=1.5 X upper limit of normal (ULN) Serum total bilirubin \>= 1.5 X ULN. AST (SGOT) and ALT (SGPT) \>= 2.5 X ULN OR \>= 5 X ULN for subjects with liver metastases, Albumin \>=2.5 mg/dL International Normalized Ratio (INR) or Prothrombin Time (PT) Activated Partial Thromboplastin Time (aPTT) \>=1.5 X ULN 9. Female subject of childbearing potential should have a negative urine or serum pregnancy within 72 hours prior to receiving the first dose of study medication. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required. 10. Female subjects of childbearing potential should be willing to use 2 methods of birth control or be surgically sterile, or abstain from heterosexual activity for the course of the study through 120 days after the last dose of study medication (Reference Section 5.7.2). Subjects of childbearing potential are those who have not been surgically sterilized or have not been free from menses for \> 1 year. 11. Male subjects should agree to use an adequate method of contraception starting with the first dose of study therapy through 120 days after the last dose of study therapy. Exclusion Criteria: 1. Has progressive lymphoma in need of standard therapy 2. Has transformation to more aggressive disease like diffuse large B cell lymphoma 3. Is currently participating and receiving study therapy or has participated in a study of an investigational agent and received study therapy or used an investigational device within 4 weeks of the first dose of treatment. 4. Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of trial treatment. 5. Has a known history of active TB (Bacillus Tuberculosis) 6. Hypersensitivity to rituximab, GM-CSF, pembrolizumab or any of its excipients. 7. Has had a prior anti-cancer monoclonal antibody (mAb) within 4 weeks prior to study Day 1 or who has not recovered (i.e., = Grade 1 or at baseline) from adverse events due to agents administered more than 4 weeks earlier. 8. Has had prior chemotherapy, targeted small molecule therapy, or radiation therapy within 2 weeks prior to study Day 1 or who has not recovered (i.e., = Grade 1 or at baseline) from adverse events due to a previously administered agent. * Note: Subjects with = Grade 2 neuropathy are an exception to this criterion and may qualify for the study. * Note: If subject received major surgery, they must have recovered adequately from the toxicity and/or complications from the intervention prior to starting therapy. 9. Has a known additional malignancy that is progressing or requires active treatment. Exceptions include basal cell carcinoma of the skin or squamous cell carcinoma of the skin that has undergone potentially curative therapy or in situ cervical cancer. 10. Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis. 11. Has active autoimmune disease that has required systemic treatment in the past 2 years (i.e. with use of disease modifying agents, corticosteroids or immunosuppressive drugs). Replacement therapy (eg., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency, etc.) is not considered a form of systemic treatment. 12. Has known history of, or any evidence of active, non-infectious pneumonitis. 13. Has an active infection requiring systemic therapy. 14. Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the trial, interfere with the subject's participation for the full duration of the trial, or is not in the best interest of the subject to participate, in the opinion of the treating investigator. 15. Has known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial. 16. Is pregnant or breastfeeding, or expecting to conceive or father children within the projected duration of the trial, starting with the pre-screening or screening visit through 120 days after the last dose of trial treatment. 17. Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent. 18. Has a known history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies). 19. Has known active Hepatitis B (e.g., HBsAg reactive) or Hepatitis C (e.g., HCV RNA \[qualitative\] is detected). 20. Has received a live vaccine within 30 days of planned start of study therapy.