• Histologically confirmed chronic lymphocytic leukemia (including small lymphocytic lymphoma)

‣ Relapsed after 2 or more lines of therapy, including a BTK inhibitor and a BCL2 inhibitor.

• No evidence of CNS lymphoma.

• Male or female ≥ 18 years of age.

• ECOG Performance status ≤ 2 \[See Appendix 1\].

• Presence of Presence of active disease for participants with CLL and SLL and presence of measurable disease for participants with SLL.

• A. CLL/SLL (note that SLL participants must have both measurable disease and active disease): Active disease as defined by the International Workshop on Chronic Lymphocytic Leukemia (iwCLL), with at least one of the following criteria21:

⁃ Evidence of progressive marrow failure as manifested by the development of, or worsening of, anemia and/or thrombocytopenia. Cutoff levels of Hb \<10 g/dL or platelet counts \<100 × 109/L are generally regarded as indication for treatment.

⁃ Massive (i.e., ≥6 cm below the left costal margin) or progressive or symptomatic splenomegaly.

⁃ Massive nodes (i.e., ≥10 cm in longest diameter) or progressive or symptomatic lymphadenopathy.

⁃ Symptomatic or functional extranodal involvement (e.g., skin, kidney, lung, spine, etc.).

⁃ Disease-related symptoms as defined by any of the following:

• Unintentional weight loss ≥10% within the previous 6 months.

∙ Significant fatigue (ie, ECOG performance scale 2 or worse; cannot work or unable to perform usual activities).

∙ Fevers ≥100.5°F or 38.0°C for 2 or more weeks without evidence of infection.

∙ Night sweats for ≥1 month without evidence of infection. B. Measurable disease for participants with SLL: At least one measurable lesion according to Lugano Revised Response Criteria for Malignant Lymphoma.

• \>2 weeks since prior radiation therapy or 5 half-lives for systemic therapy at the time of leukapheresis, whichever is shorter (Note: Obinutuzumab pre-treatment of CLL/SLL participants must start at the latest 14 days prior to apheresis).

• Total bilirubin ≤ 1.5 X upper institutional limit of normal (except in participants with Gilbert's syndrome, active hemolysis or disease involvement of the liver).

• AST (SGOT)/ALT ≤ 2.5 X institutional upper limit of normal.

• Calculated creatinine clearance ≥ 30ml/min.

⁃ Cardiac ejection fraction of ≥50%

⁃ Adequate pulmonary function as defined as pulse oximetry ≥ 92% on room air.

⁃ Participants (or legal guardians) must have the ability to understand and the willingness to sign a written informed consent document.

⁃ For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use a contraceptive method with a failure rate of \< 1% per year during the treatment period and for at least 90 days after the BAFF CAR-T cell infusion.

⁃ A woman is considered to be of childbearing potential if she is postmenarcheal, has not reached a postmenopausal state (\< 12 continuous months of amenorrhea with no identified cause other than menopause), and has not undergone surgical sterilization (removal of ovaries and/or uterus).

⁃ Examples of contraceptive methods with a failure rate of \< 1% per year include bilateral tubal ligation, male sterilization, hormonal contraceptives that inhibit ovulation, hormone-releasing intrauterine devices, and copper intrauterine devices.

⁃ The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the participant. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable methods of contraception.

⁃ For men: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures, and agreement to refrain from donating sperm, as defined below:

⁃ With female partners of childbearing potential, men must remain abstinent or use a condom plus an additional contraceptive method that together result in a failure rate of \< 1% per year during the treatment period and for at least 6 months after the BAFF CAR-T cell infusion. Men must refrain from donating sperm during this same period. With pregnant female partners, men must remain abstinent or use a condom during the treatment period and for at least 6 months after the BAFF CAR- T cell infusion to avoid potential embryonal or fetal exposure.

⁃ The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the participant. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable methods of contraception.