This is a prospective, single-arm, single-center clinical study designed to evaluate the safety of autologous hematopoietic stem cell infusion for the treatment of prolonged cytopenia following CAR-T cell therapy. Approximately 20 patients will be enrolled.

Inclusion Criteria:

1. Ability to understand and willingness to sign a written informed consent form (ICF).
2. Age ≥ 18 years.
3. Diagnosis of multiple myeloma according to the International Myeloma Working Group (IMWG) criteria.
4. Prior treatment with CAR-T cell therapy and currently in remission or, in the opinion of the investigator, with refractory/progressive disease not requiring salvage therapy.
5. Presence of grade 3-4 cytopenia (absolute neutrophil count < 1.0 × 10⁹/L, or platelet count < 50 × 10⁹/L, or hemoglobin < 80 g/L) either persisting or newly developed between day 30 and day 90 after CAR-T cell infusion; or deemed suitable for inclusion by the investigator based on clinical judgment.
6. Availability of ≥ 1 × 10⁶/kg autologous hematopoietic stem cells in reserve.

Exclusion Criteria:

1. Known intolerance, hypersensitivity, or contraindication to autologous hematopoietic stem cell infusion.
2. Active hepatitis B or hepatitis C virus infection.
3. Known HIV infection.
4. Life expectancy <6 months.
5. Woman who are pregnant or breastfeeding.
6. Evidence of uncontrolled dysfunction of heart, lung, brain, and other important organs.
7. Any other conditions that are not eligible for the trial in the judgement of the principal investigator.