This R21 proposes to fill a major scientific gap by investigating a new therapeutic approach for multisystem inflammatory syndrome in children (MIS-C) using an innovative and clinically relevant feline model. We propose to test a novel multi-pronged therapeutic paradigm targeting viral replication, lymphoid tissue injury and hyper-inflammatory host response in cats with naturally occurring feline infectious peritonitis (FIP) to accelerate viral clearance and immune restoration. Our long-term objective is to develop new therapeutic approaches for the treatment of MIS-C. The overall objectives of this proposal are to test a novel multi-pronged chemo-biologic therapeutic strategy and determine its underlying mechanism of action in a clinically relevant animal model. The central hypothesis is that a combined GS-441524-multipotent stem/stromal cells (MSC) therapy synergistically restores injured lymphoid tissues, decreases systemic inflammation and enhances specific anti-coronavirus (CoV) immunity in cats with FIP. The rationale for this project is supported by our preliminary data indicating that GS-441524 is a potent anti-CoV agent, and that MSC have a novel role in viral infections by enhancing anti-viral immunity, dampening systemic inflammation and regenerating lymphoid tissue structure and function. The central hypothesis will be tested by pursuing two specific aims: 1) Determine the effect of GS-441524-MSC combination treatment on viral loads, lymphoid tissue injury and repair, and elucidate the molecular networks that govern its mechanism of action; and 2) Determine the effect of GS- 441524-MSC combination treatment on T cell activation/exhaustion, inflammation and lymphocyte depletion in peripheral blood compartment of cats with FIP. To test our hypothesis we will enroll client owned cats with FIP into a double blinded trial with two experimental groups: GS-441524 only, or a combined GS-441524 -MSC treatment. Blood, effusion and lymph node tissue samples will be serially collected throughout the study timeline. Parallel samples will be collected from healthy controls. We will further leverage our bio-banked tissues from cats with FIP that succumbed to the disease. Under the first aim we will determine the role of the combined treatment approach in inducing IL-7, IFN type-1 and antigen presentation pathways within lymphoid tissues to mitigate lymphoid depletion and mount an effective and balanced immune response to CoV infection. For the second aim, we will determine the role of the combined treatment approach in inhibiting lymphocyte apoptosis and T cell exhaustion in peripheral blood, and reducing systemic inflammation. The proposed research is innovative because it will determine how monotherapy is affecting immune recovery, and if MSC can enhance and accelerate viral clearance and lymphoid tissue regeneration in a clinically relevant animal model of MIS-C. The proposed research is significant because it is expected to provide a strong scie...