Platelet and Megakaryocyte Roles in Innate and Adaptive Immunity

Abstract

Classically, platelets have been described as the cellular blood component that mediates hemostasis and thrombosis. This important platelet function has received significant research attention for >150 years. The immune cell functions of platelets are much less appreciated. Platelets interact with and activate cells of all branches of immunity in response to pathogen exposures and infection, as well as in response to sterile tissue injury. In this review, we focus on innate immune mechanisms of platelet activation, platelet interactions with innate immune cells, as well as the intersection of platelets and adaptive immunity. The immune potential of platelets is dependent in part on their megakaryocyte precursor providing them with the molecular composition to be first responders and immune sentinels in initiating and orchestrating coordinated pathogen immune responses. There is emerging evidence that extramedullary megakaryocytes may be immune differentiated compared with bone marrow megakaryocytes, but the physiological relevance of immunophenotypic differences are just beginning to be explored. These concepts are also discussed in this review. The immune functions of the megakaryocyte/platelet lineage have likely evolved to coordinate the need to repair a vascular breach with the simultaneous need to induce an immune response that may limit pathogen invasion once the blood is exposed to an external environment.

Keywords: blood platelets; complement system proteins; extracellular traps; immunity; megakaryocyte; thrombosis; toll-like receptors.

Figure 1:. Platelet-TLRs and their contribution to the innate immune response.

Activation of platelet-TLRs induces fast release or surface expression of prepackaged proteins or molecules stored in platelet granules. Surface expression of P-selectin (or CD40L, in some cases) leads to interaction, in the form of heterotypic aggregates (HAGs) with neutrophils and monocytes; HAGs are important for leukocyte activation and tissue transmigration. Platelets can also mediate (TLR7-complement C3 axis) or accelerate (TLR4) the process of NETosis through their TLRs either by direct or indirect interaction with neutrophils. CD40L-CD40 axis may also be involved in NETosis. Activation of platelet-TLR4 can also deliver IL-1β to the vasculature, increasing endothelial permeability and consequently coordinating leukocyte migration in infected or damaged tissues. Finally, TLR-mediated release of ATP or HMGB1 from platelets can serve as a danger-associated molecular pattern (DAMP) signal that can alert the leukocytes to the infection, promote inflammation and tissue transmigration, and mediate regeneration. TLR-toll-like receptors; LPS-liposaccharide; HMGB-1; dsRNA-double stranded RNA; ssRNA-single stranded RNA; CpG-DNA- unmethylated 5’—C—phosphate—G—3’ oligonucleotides (DNA). (Illustration Credit: Ben Smith)

Figure 2:. Platelet TLRs and NODs mostly contribute to thrombosis by amplifying thrombotic signals.

In addition to their interactions with leukocytes, activation of platelet pattern-recognition receptors can also amplify signaling mediated by classically strong (IIa, collagen, AA) or weak platelet agonists (ADP) that lead to platelet aggregation. It is important to acknowledge that pattern-recognition receptors related to sensing bacterial pathogens such as TLR2 are more likely to have direct prothrombotic implications. Activation of platelet thrombotic, adhesive and immune responses in cases of bacteria sensing may be necessary for proper coordination of containing and confining the pathogen while simultaneously inducing activation of innate immune recruitment. Of note, carboxy (alkylpyrrole) protein adducts (CAP) generated during chronic oxidative stress can utilize platelet TLR9 and lead to aggregation in vitro and thrombosis in vivo. TLR9 activation by microbial DNA however is not prothrombotic. TLR-toll-like receptors; LPS-liposaccharide; NOD2-nucleotide-binding oligomerization domain 2 receptor; MDP-muramyl dipeptide; IIa-thrombin; AA-arachidonic acid; pAU- Polyadenylic–polyuridylic acid (dsRNA); pIC- Polyinosylic–polycitosinylic acid (dsRNA); Loxoribine- modified guanosine analog; CpG ODN- unmethylated 5’—C—phosphate—G—3’ oligonucleotides (DNA). (Illustration Credit: Ben Smith)

Figure 3:. Platelets complement the Complement response, bringing TLRs and Complement together.

Platelets contain various molecules that either initiate (C3) and amplify (Factor H, condroitin sulfate (CS)) or inhibit (C1-i) the complement cascade. The alternative pathway is activated by spontaneous hydrolysis of complement C3 that is stabilized by pathogen surfaces and properdin and is amplified by complement factor B and D, or inhibited by complement factor H and I. P-selectin on platelets can bind and activate C3b, further supporting the activation of the complement system during innate immune response. The classical pathway is initiated by antibody complexes IgM and IgG as well as apoptotic surfaces. During infection with viruses such as influenza, platelets can mediate a crosstalk between TLRs and complement in the circulation by releasing C3 from their granules as a function of TLR7 activation. Platelet C3a receptor-increases platelet adhesion, spreading and Ca²⁺ influx and platelet-C5aR-inhibits angiogenesis by inhibiting endothelial function through PF4. The ultimate function of the membrane attack complex, composed of C5b-C6-C7-C8-C9 is to lyse a pathogen (bacteria) or pathogen infected cells (virus). All factors in purple come from platelets. (Created in: BioRender)

Figure 4:. Human platelet FcγRIIa mediates a response to immunocomplexes in the circulation.

FcγRIIa is the activating IgG Fc receptor. IgG immune complexes can trigger platelet integrin activation and granule secretion in a FcγRIIa ITAM- signaling-dependent manner, increasing platelet prothrombotic response. Different IgGs for different pathogens may also exhibit different response on platelets. Of note, IgGs for anti-spike SARS-CoV-2 have to be attached to VWF on endothelial cells in order to induce adhesion/aggregation response of platelets. AA-arachidonic acid; ITAM- immunoreceptor tyrosine-based activation motif on the FcR; vWF-von Willebrand Factor (Created in: BioRender)

Figure 5:. Platelets influence all steps in T cell activation and differentiation.

Platelets interact directly with T cells and regulate antigen presenting cell differentiation. Platelet derived mediators, such as PF4, serotonin (5HT) and TGFβ influence T-helper cell differentiation. APC-antigen presenting cell; MHC-major histocompatibility complex; TCR-T-cell receptor. (Illustration Credit: Ben Smith)

Figure 6:. Megakaryocytes (Mks) are Immune Regulators.

Mks secrete immune molecules to maintain a Mk-HSC niche and HSC quiescence. Mks process and present antigen and produce platelets with processed antigen. APC-antigen presenting cell; MHC-major histocompatibility complex; TCR-T-cell receptor. (Illustration Credit: Ben Smith)