- criteria linked to the disease severity : - patients hospitalized in icu or constant care unit. - patients with clinical symptoms requiring icu admission (respiratory rate>30/min, oxygen requirement> 4liters/min (using high concentration mask) to reach and maintain o2saturation>90%, qsofa≥ 2(quick score of sepsis-related organ failure assessment), or associated multi-visceral failure. - patients with high biological probability of macrophage activation syndrome (hemoglobin < 9.2 g/dl and a blood platelets < 110000/mm3 and ast > 30 u/l and ferritin > 600 mg/l). criteria linked to the treatment toxicity : - patients currently treated for cancer or with personal history of cancer within the last 3 years. - patients with chronic obstructive pulmonary disease (copd) (gold 3 and 4 stages). - chronic respiratory insufficiency treated with oxygen. - patients aged above 70 years old. - active smoking. - personal history of thoracic radiotherapy. - patients with known sensibility to nivolumab or one of its component. - patients upon immunosuppressive dosage of corticoids. - patients upon immunosuppressive therapy or immunosuppressed patients. - patients already presenting severe autoimmune disease, for whom additional immunologic activation response would potentially precipitate lethal prognosis general criteria: - minor patients - mentally unbalanced patients, under supervision or guardianship, - patient deprived of liberty, - patient who does not understand french/ is unable to give consent, - patient already included in a trial who may interfere with the study or in a period of exclusion following participation in a previous study. - pregnant (controlled by a pregnancy test) or lactating woman

- criteria linked to the disease severity : - patients hospitalized in icu or constant care unit. - patients with clinical symptoms requiring icu admission (respiratory rate>30/min, oxygen requirement> 4liters/min (using high concentration mask) to reach and maintain o2saturation>90%, qsofa≥ 2(quick score of sepsis-related organ failure assessment), or associated multi-visceral failure. - patients with high biological probability of macrophage activation syndrome (hemoglobin < 9.2 g/dl and a blood platelets < 110000/mm3 and ast > 30 u/l and ferritin > 600 mg/l). criteria linked to the treatment toxicity : - patients currently treated for cancer or with personal history of cancer within the last 3 years. - patients with chronic obstructive pulmonary disease (copd) (gold 3 and 4 stages). - chronic respiratory insufficiency treated with oxygen. - patients aged above 70 years old. - active smoking. - personal history of thoracic radiotherapy. - patients with known sensibility to nivolumab or one of its component. - patients upon immunosuppressive dosage of corticoids. - patients upon immunosuppressive therapy or immunosuppressed patients. - patients already presenting severe autoimmune disease, for whom additional immunologic activation response would potentially precipitate lethal prognosis general criteria: - minor patients - mentally unbalanced patients, under supervision or guardianship, - patient deprived of liberty, - patient who does not understand french/ is unable to give consent, - patient already included in a trial who may interfere with the study or in a period of exclusion following participation in a previous study. - pregnant (controlled by a pregnancy test) or lactating woman