Unfolded Von Willebrand Factor Binds Protein S and Reduces Anticoagulant Activity

Protein S (PS), the critical plasma cofactor for the anticoagulants tissue factor (TF) pathway inhibitor (TFPI) and activated protein C (APC), circulates in two functionally distinct pools: free (anticoagulant) or bound to complement component 4b-binding protein (C4BP) (anti-inflammatory). Acquired free PS deficiency is detected in several viral infections, but its cause is unclear. Here, we identified a shear-dependent interaction between PS and von Willebrand Factor (VWF) by mass spectrometry. Consistently, plasma PS and VWF comigrated in both native and agarose gel electrophoresis. The PS/VWF interaction was blocked by TFPI but not APC, suggesting an interaction with the C-terminal sex hormone binding globulin (SHBG) region of PS. Microfluidic systems, mimicking arterial laminar flow or disrupted turbulent flow, demonstrated that PS stably binds VWF as VWF unfolds under turbulent flow. PS/VWF complexes also localized to platelet thrombi under laminar arterial flow. In thrombin generation-based assays, shearing plasma decreased PS activity, an effect not seen in the absence of VWF. Finally, free PS deficiency in COVID-19 patients, measured using an antibody that binds near the C4BP binding site in SHBG, correlated with changes in VWF, but not C4BP, and with thrombin generation. Our data suggest that PS binds to a shear-exposed site on VWF, thus sequestering free PS and decreasing its anticoagulant activity, which would account for the increased thrombin generation potential. As many viral infections present with free PS deficiency, elevated circulating VWF, and increased vascular shear, we propose that the PS/VWF interaction reported here is a likely contributor to virus-associated thrombotic risk.

HSF1 is necessary for reactive oxygen species (ROS) homeostasis and proliferation in T cells at fever temperatures. (87.47)

HSF1 is a major heat and stress inducible transcription factor in eukaryotic cells responsible for the regulation of a variety of genes involved in maintenance of cellular homeostasis. We have previously shown that HSF1 is activated at physiologic fever temperatures (39oC) in T cells vs. much higher, non-physiologic temperatures (42oC) in most other cell types. T cells from HSF1−/− mice proliferate normally at 37oC, but are severely inhibited at 39oC. These T cells are blocked at the G1-S phase transition of the cell cycle. T cells normally increase ROS levels as a result of activation. Antioxidants which block ROS increases also inhibit proliferation. We found that fever temperatures also inhibited the normal activation induced increase of ROS in T cells within 2h. Wild type T cells can overcome this inhibition in ROS generation within 5h and go on to proliferate normally. In contrast, HSF1−/− T cells were unable to overcome this dysregulation and failed to proliferate. Thus, fever temperature causes a dysregulation in ROS homeostasis in T cells and HSF1 is critical in restoring this regulation.