IL-12 Genetically Engineered Myeloid Cells in Participants With Relapsed, Refractory Solid Tumors

Background: Myeloid cells are a type of immune cell found in most tumors. Interleukin 12 (IL-12) is a protein that helps the immune system kill tumor cells. Researchers want to know if myeloid cells that have been genetically engineered to produce IL-12 (IL-12 GEMys) can activate the immune system to attack cancer cells in solid tumors. Objective: To test IL-12 GEMys in people with cancer. Eligibility People aged 18 years and older with cancer that returned or failed to respond to treatment. Design: Participants will be screened. They will have a physical exam with blood tests. They will have tests of their heart and lung function. They will have imaging scans of their tumors. A sample of tumor tissue may be taken. Participants will have daily injections for few days to prepare them to undergo leukapheresis: Blood will be taken from the body through a needle inserted into a vein. The blood will pass through a machine that separates out stem cells. The remaining blood will be returned to the body through a different needle. The collected stem cells will be modified in a lab to create IL-12 GEMys. Participants will check in to the hospital. They will receive drugs for 5 days to prepare their body for the treatment. Then they will have their own IL-12 GEMys infused through a needle inserted into a vein. They will stay in the hospital until they are well enough to go home. This may be 7 to 14 days or longer. Some participants may receive a second treatment with IL-12 GEMys within 2 years after the first. Participants will have follow-up visits for about 5 years. These will include imaging scans and blood tests.

* INCLUSION CRITERIA: * Relapsed or refractory solid tumor malignancies for whom standard measures do not exist or are no longer effective. Must have histologic confirmation of original diagnosis or relapse. * Participants must have evaluable (measurable or not measurable) disease. * Part B only: Participants must: * be willing to undergo mandatory pre- and post-treatment tumor biopsies. Tumor tissue should either be taken from non-target lesions or from target lesions where sampling can be done without impacting lesion measurement and --have disease amenable to biopsy to allow to perform pre- and post-tumor biopsies. * Participants with treated brain metastases are eligible if follow-up brain imaging after central nervous system (CNS)-directed therapy and close monitoring for over 3 months shows no active progression. * At least one prior cancer treatment when upfront standard therapy exists. Note: There is no limit to the number or type of prior treatment regimens. * The indicated time must have elapsed since any systemic anti-cancer therapy prior to leukapheresis. * Age \>= 18 years. * Eastern Cooperative Oncology Group (ECOG) performance status \= 1000/mm\^3 * Platelet count \>= 100,000/mm\^3 * Hemoglobin (Hgb) \>= 8 g/dL (transfusion independent) * Prothrombin time (PT) \= 60 mL/minute/1.73m\^2 (calculated by the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula) * Aspartate Aminotransferase (AST) \ 6 weeks prior to leukapheresis. * Any participant who developed autoimmunity (\>= grade 3 per CTCAE v. 6.0) with checkpoint inhibitor use. Note: Exceptions will be allowed for vitiligo and hypothyroidism that has been stable on thyroid replacement medications for \> 6 weeks prior to leukapheresis. * Participants who have a major surgical procedure, other than for diagnosis, within 4 weeks prior to leukapheresis or anticipated to need a major surgical procedure during the study. * History of prior solid organ transplantation. * Participants that require urgent therapy due to tumor mass effects or spinal cord compression within 2 weeks prior to leukapheresis. * Any investigational therapy within 2 weeks prior to leukapheresis. * Pregnancy confirmed with Beta-human chorionic gonadotropin (Beta-HCG) serum or urine pregnancy test in WOCBP at screening. Note: In case of a suspected false-positive serum or urine test result, additional evaluation must be done to rule out pregnancy. * Uncontrolled intercurrent illness or medical condition(s) evaluated by medical history, physical exam, or situations that would unacceptably increase risk for the participant or impair the ability to evaluate the endpoints of the study.