Coronavirus disease 19 (COVID-19) is a rapidly growing health threat in the United States with a fatality rate ten times greater than with seasonal influenza. The virus is highly transmissible and has rapidly infected large numbers of people in China, in several European countries, and now in the US. From these recent clinical experiences and emerging preclinical studies, it is apparent that much of the morbidity and mortality related to this infection is due to excessive and maladaptive host innate and adaptive immune responses to the virus. These unchecked responses produce inflammatory lung injury leading to severe acute lung injury in 15 to 20% of patients and death in approximately 3%. Patients at risk of severe COVID-19 infection are older and have underlying comorbid health conditions. Cancer is one such condition that increases infection risk related either to the immunosuppressive effects of the disease itself or the therapies it is treated with. However, a rapidly increasing number of patients with cancer are now being treated with checkpoint inhibitors (CPIs), which rather than being suppressive, actually stimulate the hosts immune response to cancer. Monoclonal antibodies (mAb) directed against the immune checkpoint molecules programmed death-1 (PD-1), its ligand programmed death-ligand 1 (PD-L1), or cytotoxic T-lymphocyte antigen-4 (CTLA-4) augment tumor-reactive cytotoxic T-cell function and have demonstrated great efficacy for several different cancers. However, the greatest adverse effect with the use of CPIs is the development of immune related adverse events, side effects associated with increased activity of the immune system. These adverse events can affect any organ system, including the lung. Checkpoint inhibitor pneumonitis can occur with a frequency of up to 10% and it can affect patients with any malignancy treated with CPIs. Given the large number of US cancer patients now receiving CPIs and who are at risk of developing COVID-19 infection, a critical question is whether patients on this treatment are going to experience increased inflammatory lung injury during pneumonia with the virus. Alternatively, it is possible that existing activation of host immunity related to CPI therapy might actually be protective during the onset of this pneumonia. To address these two questions, we are going to develop a mouse hepatitis virus (MHV) model of acute lung injury which can be conducted at Biosafety Level 2 in Building 28. MHV is a coronavirus that primarily infects liver and central nervous tissue. However, several key viral proteins such as the nucleocapsid protein necessary for replication, have close homology between MHV and human and severe acute respiratory syndrome coronavirus (SARS). Furthermore, intranasal (IN) and intratracheal (IT) delivery of several strains of the virus has been shown to produce inflammatory lung injury and pneumonitis that simulates key aspects of the pathology of human coronavirus lung injury. One particular strain of MHV, MHV-1, used in the A/J inbred strain of mice produces lung findings most compatible with human coronavirus respiratory infection. Using this MHV-1 model of acute injury, we will examine whether a two or 4 week regimen of treatment with a murine PD-L1 mAb that has known anti-neoplastic effects in mice, increases or decreases lung injury and lethality during the development of MHV induced pneumonia. The model will employ the MHV-1 strain of virus and the A/J mouse strain. Specific Aim 1: To develop a murine model of coronavirus-induced pneumonia. Current status: We have acquired the virus from a commercial entity. This virus along with the carrier control has been propagated and titered by an outside company. Experiments have been performed and we have completed the dose finding part of this study. A dose has been established that will yield the desired mortality rate of 50%. Specific Aim 2: To determine the effects of 4 or 8 doses of checkpoint inhibitor therapy on the phenotype of lung immune cells. Current status: We have completed this part of the study. Four or 8 doses of checkpoint inhibitors block detection of PD-L1 on immune cells in the lung to a similar extent. Additionally, circulating levels of antibody are also similar in the two dosing regimen groups. The 4 dose regimen has been chosen for this protocol. Specific Aim 3: To determine whether prior checkpoint inhibitor therapy has an effect on mortality in coronavirus-infected animals. Current status: These experiments have been completed. Prior checkpoint blockade has no effect on coronavirus mortality in this model. Specific Aim 4: To examine the effect of checkpoint blockade on immune cell phenotypes under the same experimental conditions as #3 above. Current status: These experiments have been completed. By evaluating lung tissue cells by flow cytometry, lung tissue by histology, and total lung tissue by probing for total protein, we have shown that: 1- Prior checkpoint inhibitor therapy leads to lung immune cell phenotype changes. Additionally, prior checkpoint inhibitor treatment leads to circulating protein changes suggesting an inflammatory and anti-apoptotic phenotype. 2- We additionally show an increase in inflammatory proteins that have also been shown to have a role in COVID-19. Manuscript is currently in preparation: Anti-PD-L1 therapy altered inflammation but not survival in a lethal coronavirus pneumonia model. Colleen S. Curran, Xizhong Cui, Yan Li, Mark Jaekle, Junfeng Sun, Cumhur Yusuf Demirkale, Rhea Dhamapurkar, Samuel Minkove, Victoria Hoffman, Peter Q. Eichacker, Parizad Torabi-Parizi