Inclusion Criteria:
* Age: Between 1 and 18 years old;
* ECOG PS score: 0 to 1 point;
* Histopathologically confirmed locally advanced or metastatic lymphoepithelioma-like carcinoma in children or adolescents;
* Must have at least one measurable lesion defined by the RECIST or WHO criteria;
* Estimated survival time ≥ 6 months;
* Cardiac function:Left ventricular ejection fraction (LVEF) detected by echocardiography ≥ 50%; Electrocardiogram (EKG) shows no signs of myocardial ischemia;
* No history of arrhythmia requiring drug intervention before enrollment;
* No history of severe immune-related adverse events (CTCAE V4.03 Grade 3 or Grade 4);
* For patients known not to have bone marrow (BM) involvement:Absolute neutrophil count (ANC) ≥ 1.0×10⁹/L; Platelet count ≥ 100.0×10⁹/L;Hemoglobin ≥ 90 g/L;
* Liver and kidney functions should meet the following criteria: Total bilirubin (conjugated + unconjugated) ≤ 2.5× the upper limit of normal value (ULN) (corresponding to the age). Patients with confirmed Gilbert's syndrome can be considered for enrollment at the discretion of the investigator;Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 2.5× ULN; Estimated glomerular filtration rate ≥ 30 mL/min/1.73 m² or serum creatinine (Cr) ≤ 1.5× ULN;
* During participation in the study, be able to comply with outpatient treatment, laboratory monitoring and necessary clinical visits;
* The parents/guardians of child or adolescent subjects are capable of understanding, consenting to and signing the Informed Consent Form (ICF) for the study and the applicable child consent form before any protocol-related procedures are initiated. With the consent of the parents/guardians, the subject is capable of expressing consent (when applicable).
Exclusion Criteria:
* Have received anti-PD-1 or anti-PD-L1 monoclonal antibodies or targeted drugs of related pathways;
* Known to be allergic to PD-1 monoclonal antibody or any of its excipients; Known to have a history of allergic diseases or have a severe allergic constitution;
* Have other malignant tumor diseases other than the tumor being treated in this study, except for: Malignant tumors that have been cured and have not recurred within 3 years before study enrollment, completely resected basal cell and squamous cell skin cancers, and any type of in-situ cancer that has been completely resected;
* Active central nervous system metastases (whether treated or not), including symptomatic brain metastases, meningeal metastases, spinal cord compression, etc.; Except for: Asymptomatic brain metastases (no progression within at least 4 weeks after radiotherapy and/or no neurological symptoms or signs after surgical resection, and no need for dexamethasone or mannitol treatment).
* Uncontrollable pleural effusion, pericardial effusion or ascites that requires repeated drainage;
* The toxicity of previous treatment is still \> Grade 1 (CTCAE V4.03 criteria), except for alopecia and neurotoxicity;
* Have a history of mental disorders;
* Have a history of drug abuse or drug addiction as determined by inquiry;
* Have a history of idiopathic pulmonary fibrosis or idiopathic pneumonia;
* Have comorbidities that require treatment with immunosuppressive drugs, or have comorbidities that require systemic or local use of corticosteroids at immunosuppressive doses (prednisone \> 10 mg/day or equivalent dose of similar drugs).
* Have a history of autoimmune diseases, including but not limited to systemic lupus erythematosus, psoriasis, rheumatoid arthritis, inflammatory bowel disease, Hashimoto's thyroiditis, etc., except for: Type 1 diabetes mellitus, hypothyroidism that can be controlled only by hormone replacement therapy, skin diseases that do not require systemic treatment (such as vitiligo, psoriasis), celiac disease that has been controlled, or diseases that are not expected to recur without external stimulating factors;
* Have a history of previous or current active tuberculosis infection;
* Have active infections that require systemic treatment;
* Uncontrolled hypertension (systolic blood pressure ≥ 140 mmHg and/or diastolic blood pressure ≥ 90 mmHg), or pulmonary hypertension, or unstable angina pectoris; Have had a myocardial infarction or undergone coronary artery bypass grafting or stenting surgery within 6 months before drug administration; Have a history of chronic heart failure meeting New York Heart Association (NYHA) criteria Class 3-4; Have clinically significant valvular heart disease; Have severe arrhythmias that require treatment (excluding atrial fibrillation and paroxysmal supraventricular tachycardia), including a QTc interval of ≥ 450 ms for men and ≥ 470 ms for women (calculated by the Fridericia formula); Have had a cerebrovascular accident (CVA) or transient ischemic attack (TIA) within 6 months before drug administration, etc.;
* Have severe medical comorbidities, including but not limited to: Uncontrolled diabetes mellitus, active peptic ulcer, active bleeding, etc.;
* Positive for Anti-HIV, TP-Ab, HCV-Ab; Positive for HBV-Ag and the HBV DNA copy number \> the upper limit of the normal value of the testing unit;
* Abnormal thyroid function test results (FT3, FT4, T3, T4);
* Expected to receive live vaccines or attenuated vaccines within 4 weeks before drug administration, during the treatment period, or within 5 months after the last drug administration;
* Have participated in another clinical trial and received the investigational drug treatment within 30 days before drug administration;
* Patients who, according to the judgment of the investigator, are not suitable for participating in this trial for other reasons.
