A Study to Evaluate the Safety and Efficacy of Nivolumab Monotherapy and Nivolumab in Combination With Ipilimumab in Pediatric Participants With High Grade Primary Central Nervous System (CNS) Malignancies

The number of Safety Lead-In Participants who experienced an Adverse Event (AE) during the course of the study that lead to discontinuation of study therapy.

Time frame: From first dose to 30 days post-last dose (up to approximately 6 weeks)

Population: Safety Lead-in participants: In Module A, the first 6 DLT-evaluable participants in Cohort 1 and the first 10 DLT-evaluable participants in Cohorts 2-5; in Module B, the first 10 DLT-evaluable participants.

A dose-limiting toxicity (DLT) is defined as a drug-related AE occurring in the first 6 weeks of study treatment. A participant was considered evaluable for a DLT if study treatment was delayed \> 2 weeks or was discontinued due to a related Adverse Event (AE), or if planned study treatment (3 doses of nivolumab in Module A, 2 doses of nivolumab plus ipilimumab in Module B) was administered and safety evaluation after 6 weeks on study is available to the study steering committee (SSC).

Time frame: up to 6 weeks post-dosing

Population: Safety Lead-in participants: In Module A, the first 6 DLT-evaluable participants in Cohort 1 and the first 10 DLT-evaluable participants in Cohorts 2-5; in Module B, the first 10 DLT-evaluable participants.

The number of Safety Lead-In Participants who experienced a Serious Adverse Event (SAE) during the course of the study.

Time frame: up to 6 weeks post-dosing

Population: Safety Lead-in participants: In Module A, the first 6 DLT-evaluable participants in Cohort 1 and the first 10 DLT-evaluable participants in Cohorts 2-5; in Module B, the first 10 DLT-evaluable participants.

Overall survival (OS) is defined as the time between the date of diagnosis and the date of death in Cohort 1.

Time frame: up to approximately 42 months

Population: All treated participants

Progression-free survival (PFS) is defined as the time from first dose to the date of the first documented tumor progression or death due to any cause.

Time frame: up to approximately 42 months

Population: All treated participants

Progression-free survival (PFS) is defined as the time from first dose to the date of the first documented tumor progression or death due to any cause.

Time frame: up to approximately 42 months

Population: All treated participants

The number of treated participants who died during the course of the study.

Time frame: From first dose to the date of death (up to approximately 55 months)

Population: All treated participants

The number of treated participants who experienced an Adverse Event (AE) during the course of the study. An AE is defined as any new untoward medical occurrence or worsening of a preexisting medical condition in a clinical investigation participant administered study drug and that does not necessarily have a causal relationship with this treatment. An AE can therefore be any unfavorable and unintended sign (such as an abnormal laboratory finding), symptom, or disease temporally associated with the use of study drug, whether or not considered related to the study drug.

Time frame: From first dose to 30 days post-last dose (up to approximately an average of 3 months and a maximum of 51 months)

Population: All treated participants

The number of treated participants who experienced an Adverse Event leading to discontinuation during the course of the study. An AE is defined as any new untoward medical occurrence or worsening of a preexisting medical condition in a clinical investigation participant administered study drug and that does not necessarily have a causal relationship with this treatment. An AE can therefore be any unfavorable and unintended sign (such as an abnormal laboratory finding), symptom, or disease temporally associated with the use of study drug, whether or not considered related to the study drug.

Time frame: From first dose to 30 days post-last dose (up to approximately an average of 3 months and a maximum of 51 months)

Population: All treated participants

The number of treated participants who experienced a Drug-Related Adverse Event during the course of the study. An AE is defined as any new untoward medical occurrence or worsening of a preexisting medical condition in a clinical investigation participant administered study drug and that does not necessarily have a causal relationship with this treatment. An AE can therefore be any unfavorable and unintended sign (such as an abnormal laboratory finding), symptom, or disease temporally associated with the use of study drug, whether or not considered related to the study drug.

Time frame: From first dose to 30 days post-last dose (up to approximately an average of 3 months and a maximum of 51 months)

Population: All treated participants

The number of treated participants who experienced a Serious Adverse Event (SAE) during the course of the study. SAE is defined as any untoward medical occurrence that, at any dose: * Results in death * Is life-threatening * Requires inpatient hospitalization or causes prolongation of existing hospitalization * Results in persistent or significant disability/incapacity * Is a congenital anomaly/birth defect * Is an important medical event Note: The reporting timeframe of the SAEs for this Outcome Measure (first dose to 30 days post last dose) differs than that of the reporting timeframe of the SAEs reported under the AE section of the results form (first dose to 100 days post last dose) and thus, the data in each table of SAEs reflects the specific timeframe applied.

Time frame: From first dose to 30 days post-last dose (up to approximately an average of 3 months and a maximum of 51 months)

Population: All treated participants

The number of treated participants who experienced a laboratory abnormality of the liver during the course of the study. Aspartate aminotransferase (AST) Alanine aminotransferase (ALT) Upper Limit of Normal (ULN) Units per Liter (U/L) Results reported in International System of Units (SI)

Time frame: From first dose to 30 days post-last dose (up to approximately an average of 3 months and a maximum of 51 months)

Population: All treated participants

The number of treated participants who experienced a laboratory abnormality of the thyroid during the course of the study. Free T3 (FT3) Free T4 (FT4) Thyroid stimulating hormone (TSH) Lower Limit of Normal (LLN) Upper limit of normal (ULN) Milliunits per Liter (mlU/L) Results reported in International System of Units (SI)

Time frame: From first dose to 30 days post-last dose (up to approximately an average of 3 months and a maximum of 51 months)

Population: All treated participants with at least one on-treatment TSH measurement

Overall survival at 12 months (OS12) is defined as the percentage of participants who are alive at 12 months, measured as the survival rate at 12 months from Kaplan-Meier product limit cumulative probability.

Time frame: From first dose to up to 12 months after first dose

Population: All treated participants in Cohorts 1-4

Overall survival (OS) is defined as the time between date of first dose and the date of death for Cohorts 2-5.

Time frame: From first dose to the date of death (up to approximately 55 months)

Population: All treated participants in Cohorts 2-5

Progression-free survival at 6 months (PFS6) is defined as the percentage of participants who are progression free and alive at 6 months following first dose date, measured as the survival rate at 6 months from Kaplan-Meier product limit cumulative probability of progression free. Progression is defined as: * ≥ 25% increase in sum of the products of perpendicular diameters of enhancing lesions compared with the smallest tumor measurement * Significant increase in T2 or fast fluid-attenuated inversion recovery (FLAIR) non-enhancing lesions on stable or increasing doses of corticosteroids * Any new lesion * Clear clinical deterioration not attributable to other causes apart from the tumor * Failure to return for evaluation as a result of death or deteriorating condition * Clear progression of non-measurable disease

Time frame: From first dose to up to 6 months after first dose

Population: All treated participants in Cohorts 2-5

Progression-free survival (PFS) is defined as the time from first dose to the date of the first documented tumor progression or death due to any cause. Progression is defined as: * ≥ 25% increase in sum of the products of perpendicular diameters of enhancing lesions compared with the smallest tumor measurement * Significant increase in T2 or fast fluid-attenuated inversion recovery (FLAIR) non-enhancing lesions on stable or increasing doses of corticosteroids * Any new lesion * Clear clinical deterioration not attributable to other causes apart from the tumor * Failure to return for evaluation as a result of death or deteriorating condition * Clear progression of non-measurable disease

Time frame: From first dose to the date of the first documented tumor progression or death due to any cause (up to approximately 55 months)

Population: All treated participants in Cohort 1

Overall survival (OS) is defined as the time between the date of diagnosis and the date of death in Cohort 1. Note: This outcome measure represents an updated version of the primary endpoint to include additional data collection that has occurred after the primary completion date. (Assessments were made until 17-Jan-2022).

Time frame: From first dose to the date of death (up to approximately 55 months)

Population: All treated participants in Cohort 1

Progression-free survival (PFS) is defined as the time from first dose to the date of the first documented tumor progression or death due to any cause. Note: This outcome measure represents an updated version of the primary endpoint to include additional data collection that has occurred after the primary completion date. (Assessments were made until 17-Jan-2022). Progression is defined as: * ≥ 25% increase in sum of the products of perpendicular diameters of enhancing lesions compared with the smallest tumor measurement * Significant increase in T2 or fast fluid-attenuated inversion recovery (FLAIR) non-enhancing lesions on stable or increasing doses of corticosteroids * Any new lesion * Clear clinical deterioration not attributable to other causes apart from the tumor * Failure to return for evaluation as a result of death or deteriorating condition * Clear progression of non-measurable disease

Time frame: From first dose to the date of the first documented tumor progression or death due to any cause (up to approximately 55 months)

Population: All treated participants in Cohort 5

Progression-free survival (PFS) is defined as the time from first dose to the date of the first documented tumor progression or death due to any cause. Note: This outcome measure represents an updated version of the primary endpoint to include additional data collection that has occurred after the primary completion date. (Assessments were made until 17-Jan-2022). Progression is defined as: * ≥ 25% increase in sum of the products of perpendicular diameters of enhancing lesions compared with the smallest tumor measurement * Significant increase in T2 or fast fluid-attenuated inversion recovery (FLAIR) non-enhancing lesions on stable or increasing doses of corticosteroids * Any new lesion * Clear clinical deterioration not attributable to other causes apart from the tumor * Failure to return for evaluation as a result of death or deteriorating condition * Clear progression of non-measurable disease

Time frame: From first dose to the date of the first documented tumor progression or death due to any cause (up to approximately 55 months)

Population: All treated participants in Cohorts 2-4