Patient description: The patient is a 23-year-old man who came to the hospital with high blood pressure (175/95 mmHg) and signs of septicemia. He developed respiratory failure requiring mechanical ventilation and renal failure requiring hemodialysis. His blood smear showed microangiopathic hemolytic anemia and thrombocytopenia. His blood tests revealed elevated lactate dehydrogenase and reduced human complement C3 levels with a normal coagulation profile. He was diagnosed with atypical hemolytic uremic syndrome. He was treated with plasma exchange and corticosteroids. He has been previously vaccinated with meningococcal group ACWY conjugate vaccine and meningococcal group B vaccine. The genetic survey revealed ADAMTS13 >10%.

Clinical trial eligibility criteria: eligible ages (years): 18.0-85.0,  Sepsis, Severe A written, signed informed consent, by the patient or the patient's legally authorized representative 2. Participants with an absolute lymphocyte count (ALC) ≤ 900 cells/mm3, at two time points at least twelve hours apart, following diagnosis of vasopressor dependent sepsis and, 1. the second time point should not be performed earlier than 48 hours after sepsis diagnosis, 2. study drug treatment initiation is required no later than 120 hours (up to 5 days) after the last qualifying ALC ≤ 900 cells/mm3 measure, and 3. the average value of the two qualifying ALC counts will serve as a baseline to express the percent increase at day 29, or at hospital discharge. 3. Patients in the ICU with onset of vasopressor dependent sepsis defined as hypotension requiring treatment with any vasopressor(s) for at least 6 hours to maintain a systolic pressure ≥ 90 mmHg or a mean arterial pressure ≥65 mmHg AND at least 1 of the 2 organ dysfunction below: 1. Acute respiratory failure defined as the need for invasive mechanical ventilation for at least 24 hours to support pulmonary function 2. Acute kidney injury defined as creatinine > 2.0 mg/dL (based on new abnormal result following onset of sepsis) OR urine output < 0.5 mL/kg/hr for > 4 hours despite adequate fluid resuscitation. In the presence of pre-existing impairment of renal function (defined as a serum creatinine concentration >2 times the upper limit of the normal reference range prior to the onset of sepsis), the patient must meet the other organ dysfunction criteria. 4. Anticipated hospital duration of up to approx. three weeks after initiating study drug treatment to allow 6 study drug administrations (Days 18 or 19 would be final dose) 5. This study permits the re-enrollment of a participant who may have been discontinued as a pre-treatment screen failure and/or prior to study drug treatment. 6. Age and reproductive status: 1. Women of childbearing potential (WOCBP) must have a negative serum or urine pregnancy test (minimum sensitivity 25 IU/L or equivalent units of HCG) within 24 hours prior to the start of study treatment 2. Women must not be breastfeeding 3. Women of childbearing potential (WOCBP) must agree to follow instructions for method(s) of contraception for the duration of treatment with CYT107 plus 5 half-lives of CYT107 (the terminal half-life of CYT107 is up to 2 days) plus 30 days (duration of ovulatory cycle) for a total of 2 months post-treatment completion. 4. Males who are sexually active with WOCBP must agree to follow instructions for method(s) of contraception for the duration of treatment with CYT107 plus 5 half-lives of CYT107 plus 90 days (duration of sperm turnover) for a total of 7 months post-treatment completion. In addition, male participants must be willing to refrain from sperm donation during this time. 5. Azoospermic males are exempt from contraceptive requirements. 6. WOCBP who are continuously not heterosexually active are also exempt from contraceptive requirements but still must undergo pregnancy testing  Cancer with current chemotherapy or radiotherapy (receipt of chemotherapy or radiotherapy for cancer within the last 6 weeks). All patients with current, or history of, hematologic malignancy (including, but not limited to, ALL, AML, CLL, CML, etc.) or lymphoma will be excluded, regardless of receipt of recent chemotherapy 2. Patients with minimal chance of survival and life expectancy less than 3-5 days as defined by an II score of ≥ 35 at time of consideration for study 3. Patients with history or current evidence of autoimmune disease including for example: myasthenia gravis, Guillain Barre syndrome, systemic lupus erythematosus, multiple sclerosis, scleroderma, ulcerative colitis, Crohn's disease, autoimmune hepatitis, Wegener's etc. 4. Patients who have received a solid organ transplant or bone marrow transplant. 5. Patients with active or a history of acute or chronic lymphocytic leukemia 6. AIDS-defining illness (category C) diagnosed within the last 12 months prior to study entry 7. Known history of chronic HBV infection and not on treatment with HBV nucleoside analogues prior to the current hospitalization or HBV DNA > 100 IU/mL 8. Known history of infection with HCV and currently undergoing treatment for HCV infections or has detectable HCV RNA 9. Known history of tuberculosis and currently undergoing treatment for tuberculosis 10. History of splenectomy 11. Any hematologic disease associated with hypersplenism, such as thalassemia, hereditary spherocytosis, Gaucher's Disease, and autoimmune hemolytic anemia 12. Participation in another investigational interventional study testing a drug or a medical device within the last 3 months prior to study entry 13. Patients receiving immunosuppressive drugs, e.g., TNF-alpha inhibitors, for any reason, or systemic corticosteroids other than hydrocortisone at a dose of 300 mg/day 14. Patients receiving concurrent immunotherapy or biologic agents; including growth factors, cytokines and interleukins other than the study medication : IL-2, Interferons α, β and γ, GM-CSF, G-CSF, HIV vaccines, immunosuppressive drugs, hydroxyurea, immunoglobulins, adoptive cell therapy 15. Prior exposure to IL 7 or other drugs specifically targeting T cells 16. Presence of an advanced directive to withhold or withdraw life-sustaining treatment, DNR order or no CPR order, or comfort measures only order 17. Patients for whom prognosis is poor and source control of septic event is considered unlikely per the clinical and research teams. 18. Patients under guardianship