Allogeneic Hematopoietic Cell Transplantation for Refractory Systemic Juvenile Idiopathic Arthritis.

Inclusion Criteria:

* Patients \<35 years of age are eligible, and participants must meet all the following criteria to be eligible for this study:
* Diagnosis: Systemic Juvenile Idiopathic Arthritis (sJIA) diagnosed prior to age 16, fulfilling ILAR classification criteria for sJIA or the provisional PRINTO criteria for systemic arthritis. The diagnosis and disease course must be reviewed and confirmed by a pediatric rheumatologist.
* Refractory Disease: sJIA that is refractory to standard therapy, defined as meeting at least one of these conditions despite appropriate treatment:
* Persistent JIA activity: Ongoing active disease (e.g. uncontrolled arthritis or systemic features such as fever, rash, organ inflammation) or progressive joint damage or liver dysfunction that has not responded to biologic agents (IL-1/IL-6 inhibitors) and conventional therapies, or inability to wean from high-dose corticosteroids (steroid dependence).
* Recurrent MAS: Two or more episodes of macrophage activation syndrome within a 2-year period, indicating a failure to control the hyperinflammation with medical management. (MAS is defined by clinical and laboratory criteria such as fever, cytopenias, hyperferritinemia, hemophagocytosis, etc., per standard definitions.)
* sJIA-LD: Presence of lung disease related to sJIA, either symptomatic LD (e.g. chronic cough, tachypnea, hypoxemia) or progressive LD on imaging (worsening interstitial changes), meeting clinical/imaging features such as patchy ground-glass opacities, septal and pleural thickening, or clubbing of digits. sJIA-LD should be confirmed by a pulmonologist/radiologist (e.g. HRCT findings).

Exclusion Criteria:

* Uncontrolled Infection: Presence of a severe, uncontrolled infection at time of enrollment (e.g. sepsis, pneumonia not responding to therapy).
* Significant Organ Failure:
* Cardiac: EF \< 40%
* Renal: GFR \< 30 ml/hr
* Active Malignancy: Concurrent cancer diagnosis or history of malignant disease requiring active treatment.
* Pregnancy or Lactation: Female patients who are pregnant or breastfeeding. Female participants of childbearing potential must have a negative pregnancy test clinical result at screening (pregnancy testing is done as standard of care for all transplant patients).