Factor V is an immune inhibitor that is expressed at increased levels in leukocytes of patients with severe Covid-19

Wang, J.; Kotagiri, P.; Lyons, P.; Mescia, F.; Bergamaschi, L.; Turner, L.; Al-Lamki, R.; Morgan, M. D.; Calero-Nieto, F. J.; Bach, K.; Mende, N.; Wilson, N. K.; Watts, E. R.; Cambridge Institute of Therapeutic Immunology and Infectious Disease - NIHR Covid BioResource,; Chinnery, P.; Kingston, N.; Papadia, S.; Stirrups, K.; Walker, N.; Gupta, R. K.; Toshner, M.; Weekes, M.; Nathan, J. A.; Walmsley, S.; Ouwehand, W. H.; Kasanicki, M.; Gottgens, B.; Marioni, J. C.; Smith, S. G.; Pober, J. S.; Bradley, J. R.

Severe Covid-19 is associated with elevated plasma Factor V (FV) and increased risk of thromboembolism. We report that neutrophils, T regulatory cells (Tregs), and monocytes from patients with severe Covid-19 express FV, and expression correlates with T cell lymphopenia. In vitro full length FV, but not FV activated by thrombin cleavage, suppresses T cell proliferation. Increased and prolonged FV expression by cells of the innate and adaptive immune systems may contribute to lymphopenia in severe Covid-19. Activation by thrombin destroys the immunosuppressive properties of FV. Anticoagulation in Covid-19 patients may have the unintended consequence of suppressing the adaptive immune system.