Platelet Regulation of Monocyte Responses in Vascular Inflammation

Trained immunity is a biologic process in which prior immune stimuli durably ‘trains’ innate immune cell responses to future stimulation. Trained immunity is driven by changes in glycolysis and its metabolites, leading to histone modifications, DNA methylation, or changes in non-coding RNA (ncRNA) that immune programs subsequent monocyte/macrophage responses to either be greater or lessened. Trained immunity research has largely focused on how pathogen derived molecules immune train monocytes and macrophages (mono/macs), but there are also indications of cell based trained immunity. We recently made the novel discovery that resting platelet interactions with mono/macs via platelet CD47 increased glycolysis and durably programmed mono/macs through changes in histone methylation to limit TLR responses. This was the first demonstration of endogenous cell mediated innate immune training. We now propose the novel idea that in healthy conditions, resting platelet-monocyte interactions limits monocyte TLR responses, that are overcome by platelet activation. Because platelets are small, they circulate at the interface between endothelial cells and leukocytes, ideally positioning platelets as sensors of both vascular health and tissue injury/infection. How activated platelets induce and amplify immune responses has received an increasing amount of attention, including significant work from our group. However, studies have also shown that normal platelet counts maintain immune homeostasis and our recent manuscript provides a mechanistic framework for how resting platelets immune train monocytes to a tolerant phenotype. We found that circulating monocytes from thrombocytopenic (low platelet count) mice produced more inflammatory cytokines in response to TLR ligands. Resting platelets express CD47 that interacts with circulating monocytes to induce glycolysis dependent changes in histone methylation, epigenetically modifying mono/macs to a more immune tolerant phenotype. Thrombocytopenia (clinically defined as a platelet count less than 150,000/µL) independently associates with dysregulated monocyte responses, increased plasma cytokines, prolonged morbidity, and increased mortality. A decline in platelets may also independently lead to long-term changes in mono/mac responses, for example, some COVID-19 patients have persistent/recurrent thrombocytopenia beyond the infection, and with it, prolonged inflammation. However, little is known about the mechanisms and the long-term implications of thrombocytopenia on immune responses. Hypothesis: Resting platelet-monocyte interactions immune trains monocytes to a quiescent phenotype. Aim #1. To determine the platelet-mediated signaling that leads to monocyte immune programming. Aim #2. To determine the functional outcomes of resting platelet-mediated monocyte immune programming. Aim #3. To determine the functional consequence of a loss of platelet-monocyte interactions in disease contexts.