While platelets are the cellular mediators of thrombosis, platelets are also immune cells. Platelets interact both directly and indirectly with immune cells, impacting their activation and differentiation, as well as all phases of the immune response. Megakaryocytes (Mks) are the cell source of circulating platelets, and until recently Mks were typically only considered as bone marrow (BM) resident cells. However, platelet producing Mks also reside in the lung, and lung Mks express greater levels of immune molecules compared to BM Mks. We therefore sought to define the immune functions of lung Mks. Using single cell RNA-Seq of BM and lung myeloid enriched cells, we found that lung Mks (MkL) had gene expression patterns that are similar to antigen presenting cells (APC). This was confirmed using imaging and conventional flow cytometry. The immune phenotype of Mks was plastic and driven by the tissue immune environment as evidenced by BM Mks having a MkL like phenotype under the influence of pathogen receptor challenge and lung associated immune molecules, such as IL-33. Our in vitro and in vivo assays demonstrated that MkL internalized and processed both antigenic proteins and bacterial pathogens. Furthermore, MkL induced CD4 + T cell activation in a MHC II dependent manner both in vitro and in vivo. These data indicated that Mks in the lung had key immune regulatory roles dictated in part by the tissue environment.

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Platelets have dual physiologic roles as both cellular mediators of thrombosis and immune modulatory cells. Historically, the thrombotic function of platelets has received significant research and clinical attention, but emerging research indicates that the immune regulatory roles of platelets may be just as important. We now know that in addition to their role in the acute thrombotic event at the time of myocardial infarction, platelets initiate and accelerate inflammatory processes that are part of the pathogenesis of atherosclerosis and myocardial infarction expansion. Furthermore, it is increasingly apparent from recent studies that platelets impact the pathogenesis of many vascular inflammatory processes such as autoimmune diseases, sepsis, viral infections, and growth and metastasis of many types of tumors. Therefore, we must consider platelets as immune cells that affect all phases of immune responses.

Objective Reduced blood flow and/or tissue oxygen tension conditions result from thrombotic and vascular diseases such as myocardial infarction, stroke, and peripheral vascular disease. It is largely assumed that while platelet activation is increased by an acute vascular event, chronic vascular inflammation, and/or ischemia, the platelet activation pathways and responses are not themselves changed by the disease process. We therefore sought to determine whether the platelet phenotype is altered by hypoxic and ischemic conditions. Approach and Results In a cohort of patients with metabolic and peripheral artery disease (PAD), platelet activity was enhanced and/or inhibition with oral anti-platelet agents was impaired compared to platelets from control subjects, suggesting a difference in platelet phenotype caused by disease. Isolated murine and human platelets exposed to reduced oxygen (hypoxia chamber, 5% O2) had increased expression of some proteins that augment platelet activation compared to platelets in normoxic conditions (21% O2). Using a murine model of critical limb ischemia (CLI), platelet activity was increased even two weeks post-surgery compared to sham surgery mice. This effect was partly inhibited in platelet specific Extracellular Regulated Protein Kinase 5 (ERK5) knockout mice. Conclusions These findings suggest that ischemic disease changes the platelet phenotype and alters platelet agonist responses due to changes in the expression of signal transduction pathway proteins. Platelet phenotype and function should therefore be better characterized in ischemic and hypoxic diseases to understand the benefits and limitations of anti-platelet therapy.

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