Inclusion Criteria:
1. Male and female HLH patients of any age.
2. Patients diagnosed with confirmed or suspected pHLH, based on; a molecular diagnosis or familial history consistent with pHLH or fulfilment of HLH-2004 diagnostic criteria, i.e., five out of eight of the criteria below:
* Fever
* Splenomegaly
* Cytopenias affecting 2 of 3 lineages in the peripheral blood (hemoglobin \<90 g/L; platelets \<100 x 109/L; neutrophils \<1 x 109/L)
* Hypertriglyceridemia (fasting triglycerides ≥3 mmol/L or ≥265 mg/dL) and/or hypofibrinogenemia (≤1.5 g/L)
* Hemophagocytosis in bone marrow, spleen, or lymph nodes, with no evidence of malignancy.
* Low or absent NK-cell activity
* Ferritin ≥500 μg/L
* Soluble CD25 (sCD25; i.e., soluble IL-2 receptor) ≥2400 U/mL
3. Presence of active HLH disease as assessed by the investigator.
4. Patients must fulfil one of the following criteria as assessed by the investigator:
* Having not responded to previous conventional treatment of HLH
* Having not achieved a satisfactory response to previous conventional treatment of HLH or worsened
* Having reactivated HLH
* Showing intolerance to previous conventional treatment of HLH At the time of enrollment, eligible patients might still be receiving treatment (induction or maintenance) or might have already discontinued it.
5. Expectation of survival beyond 1 week as judged by the investigator.
6. Patient has expectation of proceeding to HSCT
7. Informed consent signed by the patient (as required by local law), or by the patient's legally authorized representative(s) with the assent of patients who are legally capable of providing it, as applicable.
8. Willing to use highly effective methods of contraception from study drug initiation to 6 months after the last dose of study drug, if female and of childbearing potential.
Exclusion Criteria:
1. Diagnosis of secondary HLH consequent to a proven rheumatic, metabolic or neoplastic disease.
2. Active mycobacteria, Histoplasma capsulatum, Salmonella, or Leishmania infections.
3. Evidence of latent tuberculosis.
4. Presence of malignancy.
5. Existence of any severe co-morbidity or any other medical condition which, in the opinion of the investigator, makes the patient unsuitable for the treatment
6. History of hypersensitivity or allergy to any component of the study regimen (e.g., polysorbate).
7. Receipt of a Bacillus Calmette-Guérin (BCG) vaccine within 12 weeks prior to Screening.
8. Receipt of a live or attenuated live (other than BCG) vaccine within 4 weeks prior to Screening.
9. Pregnant or lactating female patients.
10. Enrollment in another concurrent clinical interventional study, or intake of an IMP, within three months prior to inclusion in this study
11. Any condition or circumstance that in the opinion of the Investigator may make the patient unlikely to complete the study or comply with study procedures or requirements.
