Regional
The immune characteristics and function of platelets are affected by storage conditions
Ben Winskel-Wood
Research Assistant at Australian Red Cross Lifeblood
Lacey Johnson
Principal research scientist at the Australian Red Cross Lifeblood
Platelets are dynamic cells which are highly sensitive to activation signals. Once activated, platelets rapidly change their structure and function, mobilising receptors to the surface membrane and releasing signalling molecules into the surrounding environment. This activity facilitates the haemostatic function of platelets. However, platelets also contribute to immune function and the control of infections through direct interaction with pathogens and activation of the innate and adaptive immune system1.
Figure 1. Combined brightfield and fluorescence images of a human platelet (CD61; red) bound to a monocyte-like THP-1 cell following co‑culture.
A range of pathogen recognition receptors are present on the platelet surface membrane. These include toll-like receptors (TLRs), which recognise molecular patterns commonly associated with infectious agents (lipopolysaccharide, flagellin, double-stranded RNA)1. Additionally, traditional haemostatic receptors, such as α2β3 in the active conformation, can also adhere to pathogens indirectly through fibrinogen1. Pathogen recognition triggers further platelet activation and the release of signalling and anti-microbial factors (complement, defensins) from internal stores, resulting in pathogen immobilisation and, in specific cases, destruction.
A range of pathogen recognition receptors are present on the platelet surface membrane. These include toll-like receptors (TLRs), which recognise molecular patterns commonly associated with infectious agents (lipopolysaccharide, flagellin, double-stranded RNA)1. Additionally, traditional haemostatic receptors, such as α2β3 in the active conformation, can also adhere to pathogens indirectly through fibrinogen1. Pathogen recognition triggers further platelet activation and the release of signalling and anti-microbial factors (complement, defensins) from internal stores, resulting in pathogen immobilisation and, in specific cases, destruction.
Platelets are also key mediators of leukocyte activity. When activated, P‑selectin is transferred to the platelet surface, which can then bind to leukocyte-expressed PSGL-11. This bond can be stabilised by the interaction of platelet GPIbα with leukocyte-expressed Mac-1, forming a platelet-leukocyte aggregate (PLA). Platelets most commonly form PLAs with monocytes and neutrophils, enhancing their capacity to recognise infectious agents1. Activated platelets can also signal leukocytes remotely by releasing signalling molecules and extracellular vesicles, which can promote a range of effects, including leukocyte activation (CD40L, RANTES, IL-27), maturation (CD40L, IL-1β) and inflammation (IL-1β, RANTES)1.
In general, platelet immune function is beneficial. However, overactivation, which may occur during platelet component collection, manufacture and storage, has been directly linked to the risk of adverse post-transfusion events2, 3. Notably, the accumulation of inflammatory cytokines in platelet components stored at room-temperature (RANTES, CD40L, IL-1β, IL‑27 and OX‑40L) is associated with an increased risk of adverse events2. The severity of adverse events varies from mild (fever, nausea) to severe (acute lung injury). Consequently, understanding how current and new platelet manufacturing and storage technologies affect the immune characteristics and function of platelets is critical.
Refrigeration (4 °C) or cryopreservation (-80 °C, 5% DMSO) of platelets are two alternate storage methods currently under clinical evaluation. While these storage methods enhance aspects of haemostatic function, refrigerated and cryopreserved platelet components contain lower concentrations of inflammatory cytokines than those stored at room‑temperature4. Further, refrigerated platelets exhibit increased sensitivity to bacterial-induced (E. coli and S. aureus) aggregation5. Cryopreservation increases the likelihood of platelet adhesion to a monocyte-like cell-line (THP-1) compared to components stored at room-temperature (Figure 1)6. While it is currently unclear what impact these changes might have post‑transfusion, these findings highlight the importance of assessing the immune function of platelets when evaluating novel storage modes.
References
1. Maouia A, Rebetz J, Kapur R, Semple JW. The immune nature of platelets revisited. Transfus Med Rev. 2020; 34: 209-20.
2. Hamzeh-Cognasse H, Damien P, Nguyen KA, Arthaud C-A, Eyraud M-A, Chavarin P, et al. Immune-reactive soluble OX40 ligand, soluble CD40 ligand, and interleukin-27 are simultaneously oversecreted in platelet components associated with acute transfusion reactions. Transfusion. 2014; 54: 613-25.
3. Mowla SJ, Kracalik IT, Sapiano MRP, O'Hearn L, Andrzejewski C, Jr., Basavaraju SV. A Comparison of transfusion-related adverse reactions among apheresis platelets, whole blood-derived platelets, and platelets subjected to pathogen reduction technology as reported to the National Healthcare Safety Network Hemovigilance Module. Transfus Med Rev. 2021; 35: 78-84.
4. Johnson L, Tan S, Jenkins E, Wood B, Marks DC. Characterization of biologic response modifiers in the supernatant of conventional, refrigerated, and cryopreserved platelets. Transfusion. 2018; 58: 927-37.
5. Winskel‐Wood B, Padula MP, Marks DC, Johnson L. Cold storage alters the immune characteristics of platelets and potentiates bacterial‐induced aggregation. Vox Sang. 2022: 1006-15.
6. Winskel-Wood B, Padula MP, Marks DC, Johnson L. The phenotype of cryopreserved platelets influences the formation of platelet-leukocyte aggregates in an in vitro model. Platelets. 2023; 34: 2206916.
