Platelet-immune cell interactions in inflammation and cancer

Abstract

Platelets are well-known for their role in hemostasis and only more recently, they have also been acknowledged for their immune regulatory roles. Despite being small sized and anucleated, they can respond to inflammatory stimuli, secrete a plethora of different inflammatory mediators and interact extensively with other immune cells to initiate and maintain immune responses. In addition to their function in inflammatory conditions, platelets are also involved in cancer development and metastasis. They can not only induce tumor growth, angiogenesis and the survival of circulating tumor cells (CTCs) but also interact with tumor cells directly, which yields &quot;tumor educated&quot; platelets. In this work, I present two independent but complimentary stories that describe novel functions of platelets in driving IL-1 inflammation during innate immune responses and in the regulation of immune checkpoints in cancer.<br /> Multimeric signaling platforms, called inflammasomes, are an intracellular pattern recognition receptor (PRR) that can sense many different activators upon pathogen invasion, including bacterial toxins. Although a variety of different inflammasomes and a multitude of activators and activating mechanisms have been discovered over the last decade, most of these studies were conducted in vitro monocultures of macrophages and monocytes and the regulatory and networking effects of other immune cells were largely neglected. Therefore, the interaction of innate immune cells with platelets during inflammasome activation was investigated in the first part of this work. <br /> My results show for the first time that platelets enhance NLPR3 inflammasome responses in bone-marrow derived macrophages (BMDMs), human monocyte derived macrophages (hMDMs) and human neutrophils, and are crucial for cytokine secretion from NLRP3 activated human monocytes. Platelets alone do not express NLRP3 inflammasome components or secrete IL-1&beta;, excluding a role for platelet-derived NLPR3 or IL-1 cytokines in this effect. Further, the main findings of this study reveal that platelets regulate NLRP3 activation in hMDMs by enhancing NLRP3 and IL-1&beta; transcription, increasing caspase-1 activity and promoting the assembly of ASC specks. This effect is independent of platelet-derived COX1/2 or LOX lipid mediators, nucleosides (ADP and ATP), nucleic acids and platelet &alpha;-granule derived factors, but instead mediated by a platelet-derived, soluble and heat sensitive factor that is partly dependent on extracellular calcium. Finally, RNA sequencing analysis identified genome wide transcriptional changes in hMDMs in the presence of platelets or platelet supernatant pointing to an involvement of metabolic and cancer pathways. Together with a proteomic analysis of platelet supernatant, this study offers an array of new possible candidates that could be involved in the platelet-mediated NLPR3 regulation of hMDMs. Altogether, these results establish platelets as important regulators of the IL-1 inflammatory response in innate immune cells and more detailed studies will be necessary to elucidate the underlying molecular mechanisms.<br /> Since platelets also play important roles in the development of cancer, the potential of platelets as new therapeutic targets in Head and Neck Squamous Cell Carcinoma (HNSCC) patients was investigated in the second part of this work. Despite the unprecedented and durable effects of checkpoint inhibition therapy in HNSCC patients, in total only a small fraction of patients responded favorably and the general mortality rates in this cancer still remain very high. So far, there are no easily accessible biomarkers that could identify suitable patients and predict their response to therapy. <br /> In this work, I show for the first time that platelets and PBMCs from HNSCC and lung cancer patients and smokers express the immune checkpoint protein Programmed cell death ligand 1 (PD-L1). This expression was independent of the disease stage, the occurrence of metastasis or the incidence of the tumor. Further analyses demonstrated that PD-L1 expression on platelets was decreased in response to immunotherapy with the anti-PD-L1 antibody atezolizumab, but reconstituted after 20 days. These findings and the enhanced PD-L1 expression on platelets from smokers highlight the potential of platelets as easily accessible biomarkers for the detection of early cancer development and as predictor of therapeutic success.