• Hospital-acquired pneumonia (HAP) according to European guidelines (Torres et al. Eur Respir J 2017): Association of two criteria among (body temperature \> 38°C, leukocytosis\>12000 cells per mL, leucopenia \<4000 cells per mL and purulent pulmonary secretions), appearance of a new infiltrate or change in an existing infiltrate on chest radiography, and respiratory sample (Sputum, AET, BAL, mini-BAL or blind BAL) collected for bacteriological diagnosis (results can be pending at inclusion). The diagnosis of HAP can have been made outside of ICU. Diagnosis is done at least 48 hours after hospital admission.

• HAP severity defined as a PaO2/FiO2 ratio \< 300 under mechanical ventilation.

• Biological systemic inflammatory response defined as CPR≥ 150 mg/L (15 mg/dL)\*

• Receiving curative antimicrobial therapy for the current episode of HAP pneumonia for less than 48 hours.

• Informed consent from a legal representative, or emergency procedure (when possible, according to national regulation, see below). If it is not possible to obtain the patient consent prior the inclusion (comatose patients), patient consent for the study continuation will be obtained as soon as deemed possible.

• Person insured under a health insurance scheme.

• Female of childbearing age who agree and who are able to comply with effective contraception for the 28 first days of the study.