The Role of Platelets in the Regulation of Pulmonary Infection and Host Defence

Abstract

Background. The implication of platelets in inflammatory disorders, such as asthma, has become increasingly more apparent. Following an inflammatory insult, platelets modulate inflammation by a number of distinct mechanisms, including pulmonary-leukocyte recruitment and release of platelet specific mediators. However, little is known regarding the role of platelets in the regulation of pulmonary infection. Pulmonary colonisation of bacteria, including Pseudomonas aeruginosa (P.aeruginosa) and Staphylococcus aureus (S.aureus) presents a therapeutic challenge due to antimicrobial resistance (AMR). Objectives. In this study, the effect of experimentally induced platelet depletion was investigated on infection and inflammatory parameters in a murine model of pulmonary infection. A new approach in targeting AMR was also tested, using a novel antibiotic enhancer compound, HT61. Methods. C57/B16 mice were experimentally depleted of platelets, 24 hours prior to infection with either sham, P.aeruginosa or S.aureus embedded agar beads. Bronchoalveolar lavage (BAL) fluid and lungs, kidney and spleen were analysed for inflammatory cells and bacterial load respectively. The role of platelet purinergic receptors was also assessed. In vitro assays were performed to investigate the effect of bacteria on platelet function, through measurements of platelet-leukocyte complexes and mediator release. Additional in vivo experiments were performed to test enhancer compound, HT61. Results. Infection induced pulmonary platelet recruitment (p<0.01) in addition to a mild state of thrombocytopenia (p<0.05). Evidence of platelet activation was detected in infected mice through increased levels of platelet-derived mediators Platelet Factor-4 (PF-4) and Regulated upon activation, normal T-cell expressed and secreted (RANTES) in BAL fluid and platelet-neutrophil complexes (p<0.01) in blood. In mice depleted of circulating platelets, there was evidence of systemic bacterial dissemination and weight loss was increased compared to mice with normal platelet levels. Furthermore, pulmonary neutrophil recruitment was significantly reduced 24 hours post infection in mice depleted of circulating platelets. These results were reproduced following inhibition of the P2Y14 receptor and use of the ATP/ADP hydrolysing agent, Apyrase. HT61 as a singular treatment showed no inhibition of P.aeruginosa or S.aureus bacterial load, however, in combination with Tobramycin an enhanced reduction in bacterial load was observed for two strains of P.aeruginosa (p<0.05) when compared to treatment with either drug alone. Conclusions. Here it is demonstrated that infection induces platelet activation and accumulation in the lungs. Mice depleted of circulating platelets or treated with Apyrase or a P2Y14 receptor antagonist demonstrated increased weight loss and increased systemic infection, this therefore suggests that in addition to the role played by neutrophils, platelets also have a distinct role in the host defence against bacterial infection. Furthermore, I demonstrated an enhancement of Tobramycin’s efficacy when administered in combination with HT61, highlighting a potential novel treatment to target AMR.

Date of Award	1 Jan 2019
Original language	English
Awarding Institution	King's College London
Supervisor	Clive Page (Supervisor) & Simon Pitchford (Supervisor)