A Dose Escalation and Expansion Study Evaluating the Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of RO7283420.

RO7283420 will be administered to participants by intravenous (IV) infusion Q3W at a starting dose of 0.15mg. Starting dose levels (double step-up regimen, Q3W) for SC injections was the same as the highest dose levels that have been cleared in the IV double step-up cohorts at that timepoint. Each participant will receive up to 6, 9, and 18 cycles of treatment with RO7283420, when treated with Q3W, Q2W, or QW dosing regimens, respectively.

RO7283420 at RP2D will be administered by IV infusion or SC injection as per dosing schedule determined in Part B.

Tocilizumab will be administered as an IV infusion 8 mg/kg (for participants with a weight of 30 kg and above) and 12 mg/kg (for participants with a weight of less than 30 kg). Tocilizumab will be given as rescue medication.

Dasatinib 100 mg film-coated tablets will be administered daily until symptom resolution (up to 100 mg twice daily \[BID\] for a maximum 3 days); orally. Dasatinib will be given as rescue medication.

20 mg IV of dexamethasone will be administered as pre-medication at least 60 minutes prior to the all RO7283420 infusions or injections during cycle 1.

500 or 1000 mg of paracetamol/acetaminophen will be administered orally or by IV as pre-medication at least 30 minutes prior to each RO7283420 infusion or injection.

25 mg or 50 mg of diphenhydramine will be administered orally or by IV as pre-medication at least 30 minutes prior to each RO7283420 infusion or injection.