Inclusion Criteria:  Pediatric patients with or without comorbidity Age ≥ 1 month to < 21 years  Confirmed MIS-C by signs and symptoms as detailed by the CDC Health Advisory (https://www.cdc.gov/mis-c/hcp/; May 14, 2020)  Persistent fever/chills (>38.0°C for ≥24 hours, or report of subjective fever lasting ≥24 hours); AND One or more laboratory parameters (evidence of inflammation); AND,  i) elevated C-reactive protein (CRP) ii) elevated erythrocyte sedimentation rate (ESR) iii) elevated ferritin iv) elevated lactic acid dehydrogenase (LDH) v) elevated d-dimer vi) elevated fibrinogen vii) elevated procalcitonin viii) elevated interleukin 6 (IL-6) ix) increased neutrophils x) reduced lymphocytes xi) low albumin c) Evidence of clinically severe illness requiring hospitalization, with multisystem (>2) organ involvement (cardiac, renal, respiratory, hematologic, gastrointestinal, dermatologic, or neurological)-MUST include GI symptoms, such as nausea, vomiting, diarrhea and/or abdominal pain; AND, d) No alternative plausible diagnoses; AND e) Positive for current or recent SARS-CoV-2 infection by RT-PCR, serology, or antigen test  Subject (or legal authorized representative) capable of understanding and signing an informed consent form and assent form, when appropriate.  Exclusion Criteria:  Female participants pregnant and/or lactating. Female participant has childbearing potential and is unwilling to use an acceptable method of birth control for the duration of the study. Participant has a significant co-morbid disease that by the Investigator's determination would make the participant unsuitable for enrollment, including unstable medical conditions. Participation in any other clinical investigation using an experimental drug within 30 days prior to screening or intends to participate in another clinical study while participating in AT1001 MIS-C 101 study. Have participated in a blood/plasma donation or blood loss greater than 400 mL within 90 days, or greater than 200 mL within 30 days prior to Screening. Known hypersensitivity to any of the formulation components of AT1001.