Platelets as components of the circulating immune system : role as scavenging cells

Chacko, Anisha Rose

Abstract

Although platelets are widely known for their role in haemostasis, an increasing body of evidence demonstrates platelets to encompass a role as immune cells. This underappreciated function involves numerous cell surface receptors and a series of intracellular signalling events which ultimately enable platelets to migrate, scavenge and aggregate at the site of infection, thus promoting immunologic responses.

At the same time microorganisms have evolved to develop a variety of virulence factors which enable them to survive and evade host defence systems, in many cases by interfering with the signalling machinery that regulates remodelling of the cytoskeleton (Rho GTPases). One such example is the cytotoxic necrotising factor 1 (CNF1), a toxin secreted by certain Escherichia coli strains and whose effects in platelets remain unclear.

We hypothesise that Rho GTPases (which are key regulators of the actin cytoskeleton), platelet receptors such as FcγRIIA and GPIIb/IIIa (which aid platelet-bacteria and platelet- matrix interactions) and downstream tyrosine kinase (TK) signalling molecules critically influence platelet scavenging and migration properties. In this thesis we aim to adapt, optimise, and make use of the quantification capabilities of assays to gain insight into the molecular mechanisms involved in platelet scavenging and migration properties through the use of a panel of specific inhibitors.

We found inhibition of RhoA, Rac1 and Cdc42 to significantly impair the ability of platelets to scavenge bacteria and migrate on fibrinogen, suggesting that Rho GTPases are critically involved in regulating such platelet immunologic functions. The same was found to be true for various TK signalling molecules (Src, Syk and BTK). These results were complemented with platelet aggregation tests which revealed significant changes elicited by inhibitors in the ability of platelets to aggregate in response to bacteria.

We also hypothesise that CNF1, which targets Rho GTPases by rendering them constitutively active, will lead to major modifications in Rho signalling and consequently affect platelet functions, thus contributing to the pathogenesis of E. coli infection.

We demonstrate indirectly for CNF1 to be taken up and processed by platelets, where it modifies Rho GTPases and induces an increase in F-actin. We found morphological alterations in toxin-treated platelets as well as slightly reduced integrin activation and granule secretion upon stimulation with various agonists.

In summary, our data revealed platelets to encompass immune functions (scavenging and migration) that aid the prevention of infections progressing to conditions such as sepsis, and for this to be tightly regulated by a number of Rho GTPases and TK signalling molecules. However, our data also emphasised how microorganism have evolved and adapted to evade and modify platelet functions to aid their survival in the host.

Citation

Chacko, A. R. (2020). Platelets as components of the circulating immune system : role as scavenging cells. (Thesis). Hull York Medical School, the University of Hull and the University of York. Retrieved from https://hull-repository.worktribe.com/output/4222993

• Thesis Type: Thesis
• Deposit Date: Mar 26, 2021
• Publicly Available Date: Feb 23, 2023
• Keywords: Medicine
• Public URL: https://hull-repository.worktribe.com/output/4222993
• Additional Information: Hull York Medical School, The University of Hull and the University of York
• Award Date: Sep 1, 2020