Markers and Mechanisms of PASC

SUMMARY Inflammatory responses to SARS-CoV-2 infection play a major role in COVID-19 pathogenesis, as the expression of various inflammatory mediators/regulators correlates with the disease severity and mortality rates. While the majority of COVID-19 patients fully recover from the infection and become asymptomatic, approximately one-third of COVID-19 survivors experience a wide spectrum of symptoms and/or complications beyond 4 weeks from the onset of the disease, now designated as “post-acute sequelae of COVID-19 (PASC)”; and this syndrome has become increasingly recognized and concerning. While the molecular basis of PASC is unknown, its clinical healthcare impacts are enormous, raising an urgent need to study long-term outcomes in our Veteran COVID-19 survivors. The objectives of this proposal are to identify and characterize biomarker(s) and molecular/cellular mechanisms for the PASC in COVID-19 survivors to better understand and manage this post-infection syndrome. Our recent investigations into the biochemical and cellular responses to SARS-CoV-2 infection revealed sustained inflammation and immunosuppression in COVID-19 survivors with PASC - also called COVID-19 long haulers (COV-LH). Specifically, we discovered significant changes in the expression of multiple inflammatory proteins in COVID-19 survivors, especially in COV-LH, suggesting persistent inflammation in these subjects. We also found significant increases in myeloid-derived suppressor cells (MDSCs), along with decreases in the numbers and functions of natural killer (NK) cells in these subjects. Importantly, the increase in MDSCs was associated with a decrease in NK cells in COV-LH compared with COVID-19 asymptomatic survivors (COV-AS), indicating sustained immunosuppression in these subjects. Based on our findings, we designed a research plan to address three critical questions: 1) Can unique signature molecules be identified as biomarkers for COV-LH? 2) Do these regulatory molecules promote MDSC expansion and suppressive functions in COV-LH? 3) Are these regulatory molecules and/or is MDSC-mediated immunosuppression responsible for the development of PASC in COV-LH? Based on our preliminary studies, we hypothesize that SARS-CoV-2 infection induces systemic inflammation and epigenetic and metabolic changes, leading to persistent inflammation and immunosuppression and the development of PASC in COVID-19 survivors. We propose three specific aims to test our hypothesis: Aim 1 will identify potential biomarkers and investigate their transcriptional dysregulations in COVID-19 survivors. Aim 2 will determine the molecular mechanisms that promote MDSC expansion and suppressive functions in COVID-19 survivors. Aim 3 will determine the role of regulatory molecules and MDSC-induced immunosuppression in the development of PASC in COVID-19 survivors. Because there are no known molecular or cellular biomarkers for the diagnosis and treatment of PASC, information gained from this research will not only fill major knowledge gaps in understanding this post-COVID- 19 syndrome, but will also facilitate identifying and managing PASC symptoms in our Veteran COVID-19 survivors.