Inclusion Criteria:

1. Adults 18 years of age and older.
2. Relapsed or refractory measurable multiple myeloma following prior treatment with ≥4 lines of anti-myeloma therapy slated for teclistamab monotherapy
3. Adequate bone marrow function including:

* Hemoglobin ≥ 8g/dL (unless ≥50% bone marrow involvement by MM),
* Absolute neutrophil count \>1000 / µL (unless bone marrow involvement by MM)
* Platelet count ≥30,000 / µL (unless bone marrow involvement by MM)
4. ECOG performance status 0 - 2
5. 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 3 months after the last dose of siltuximab.

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 subject. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable methods of contraception.
6. For men: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures, and agreement to refrain from donating sperm, as defined below:

1. 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. Men must refrain from donating sperm during this same period.
2. With pregnant female partners, men must remain abstinent or use a condom during the treatment period.

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 subject. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable methods of contraception.

Exclusion Criteria:

1. Waldenström's macroglobulinemia, POEMS syndrome (polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, and skin changes), or AL amyloidosis.
2. Known to be seropositive for human immunodeficiency virus or acquired immune deficiency syndrome.
3. Hepatitis B infection as defined according to the American Society of Clinical Oncology guidelines. In the event the infection status is unclear, quantitative levels are necessary to determine the infection status (Attachment 10). Hepatitis C (anti-hepatitis C virus \[HCV\] antibody positive or HCV-RNA quantitation positive) or known to have a history of hepatitis C. If positive, further testing of quantitative levels to rule out positivity is required.
4. Active central nervous system or meningeal involvement by MM.
5. Active bacterial, viral, fungal, mycobacterial, parasitic or other infection requiring systemic therapy within 2 weeks prior to first dose of study drug.
6. Active malignancy except for any of the following:

* Adequately treated cutaneous basal cell carcinoma, squamous cell carcinoma, or in situ cervical cancer
* Adequately treated Stage I cancer from which the subject is currently in remission and has been in remission for 2 years
* Low-risk prostate cancer with Gleason score \<7, prostate-specific antigen \<10 ng/mL, and a stage of cancer at most cT2a, cN0, and CM0
* Any other cancer from which the subject has been disease-free for ≥2 years
7. Participants with uncontrolled intercurrent illness including, but not limited to, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, pulmonary abnormalities or psychiatric illness/social situations that would limit compliance with study requirements.
8. Pregnant or breastfeeding women are excluded from this study because siltuximab therapy may be associated with the potential for teratogenic or abortifacient effects. Women of childbearing potential must have a negative serum pregnancy test. Because there is an unknown, but potential risk for adverse events in nursing infants secondary to treatment of the mother with siltuximab, breastfeeding should be discontinued during treatment and for 3 months after the last dose of siltuximab. These potential risks may also apply to other agents used in this study.
9. Participants with history of clinically relevant and active CNS pathology such as epilepsy, seizure disorders, paresis, aphasia, uncontrolled cerebrovascular disease, severe brain injuries, dementia and Parkinson's disease.
10. Participants with history of severe hypersensitivity reaction to siltuximab or any of the excipients