Trial of an investigational drug after the rejection of an allogeneic transplant.

You likely can't join if

• 1. Active systemic immunosuppressive therapy. 2. Patients who have previously received CAR-T Anti-BCMA treatment. Patients who have received another type of anti-BCMA therapy will not be excluded, although in this case the positivity for BCMA in myeloma cells must be confirmed at the time of inclusion. 3. Absolute lymphocyte count <0.2x109/L. 4. Previous malignancy, except if in complete remission >3 years, with the exception of cutaneous carcinoma (non-melanoma). 5. Active infection requiring treatment. 6. Active HIV, HBV, or HCV infection. 7. Uncontrolled medical illness. 8. Severe organic disease that meets any of the following criteria: EF <40%, DLCO < 40%, GFR < 50 ml/min, bilirubin > 3 NV (except Gilbert's syndrome). 9. Previous diagnosis of symptomatic AL amyloidosis or POEMS syndrome. 10. Pregnant or lactating women. 11. Women of childbearing potential who are unable or unwilling to use highly effective contraceptive methods 12. Men unable or unwilling to use highly effective contraceptive methods 13. Contraindication to receive lymphodepletive chemotherapy.