Platelets and Their Interactions with Other Immune Cells

Abstract

Platelets are anucleate blood cells, long known to be critically involved in hemostasis and thrombosis. In addition to their role in blood clots, increasing evidence reveals significant roles for platelets in inflammation and immunity. However, the notion that platelets represent immune cells is not broadly recognized in the field of Physiology. This article reviews the role of platelets in inflammation and immune responses, and highlights their interactions with other immune cells, including examples of major functional consequences of these interactions.

Figure 1

(A) Platelets localize to the site of injury, binding to fibrin, and forming a hemostatic plug. (B) Electron micrograph of activated platelets, which spread out over an injured area and extend filopodia. Reprinted by permission from Macmillan Publishers Ltd: Nature Materials (59), 2010.

Figure 2

Ultrastructural features of a discoid platelet showing α granules, mitochondrion, the marginal microtubule band, and open canicular system. Reprinted by permission from Macmillan Publishers Ltd: Nature Reviews Immunology (182), 2011.

Figure 3

Examples of adhesive interactions between platelets (red) and leukocytes (blue). Platelets contain a number of integrins and cell adhesion molecules on their surface which bind to both leukocytes and endothelial cells (green). Major cell adhesion molecule interactions include (platelet-leukocyte) P-selectin-PSGL-1 and GP1bα-Mac-1. Platelets also adhere to endothelial cells and help capture flowing leukocytes from the circulation. Reprinted from reference (85) with permission from Wiley.

Figure 4

Adhesive interactions between activated platelets and neutrophils. (A) EM image of an inflamed mouse cremaster venule, demonstrating platelet-neutrophil-endothelial cell interaction. Image courtesy of Dr. Alan Burns, University of Houston College of Optometry. (B) Platelet (red)-neutrophil (green/blue) interactions in suspension after platelet activation.

Figure 5

Selected examples of pro-inflammatory effects of platelets resulting in functional consequences on leukocytes. Platelets interact with leukocytes through both adhesive mechanisms as well as release of cytokines/chemokines. This results in leukocyte activation and enhanced leukocyte-endothelial adhesion. Through these mechanisms, platelets participate in several normal and pathologic immune functions including microbial killing, leukocyte homing, wound healing, allergic inflammation, and atherosclerosis, among others.