Is a platelet suntan the answer?

Since the 1970s, platelets have been stored at room temperature to ensure longer circulatory survival for prophylactic treatment of thrombocytopenic donors. With room temperature storage comes the risk of bacterial proliferation in contaminated units and septic reactions in transfused patients. This risk has been partially mitigated by improvements in arm preparation, diversion of an initial aliquot of collected blood, and automated blood culture testing of platelet components. Unfortunately, a very substantial risk remains due to the low bacterial loads present during collection and the first 24 hours of storage, leading to sampling error and false-negative culture test results. Investigators have pursued delayed sampling, increasing sample volumes, and secondary testing as ways to drive down the rate of residual contamination. None of these additional measures have been practiced uniformly in the United States; however, a draft US Food and Drug Administration guidance suggests increased sample volume for aerobic and anaerobic primary culture testing, and some form of secondary testing may be in our future.1 Secondary testing either requires hospitals to perform testing and labeling, and hence participate in shared manufacturing, or presents the following logistical challenges to blood centers if they perform secondary bacterial testing: 1) how to provide enough usable shelf life for 5-day platelet products; and 2) how to create and manage a separate inventory and delayed shipment of those products that qualify for 7-day storage.

As an alternative to primary and secondary testing, platelet units can be rendered sterile by photochemical methods involving ultraviolet (UV) light. One method involving amotosalen and UVA light is cleared for use in the United States; another method utilizing primarily UVB light and riboflavin is in use in Europe but is not cleared for use in the United States. A third method, which is in the research phase of development, does not require the addition of a photochemical but instead uses UVC light to elicit pyrimidine–pyrimidine dimers and photoproducts in nucleic acids of pathogens to sterilize platelet additive solution platelet units. The method is not as efficacious for inactivation of platelets in 100% plasma because plasma proteins and other substances absorb UVC light, preventing light penetration through the unit.

In this issue of TRANSFUSION, Gravemann and colleagues report a study of utilizing UVC light to inactivate 19 species of bacteria as well as several strains of 6 of the species in platelets suspended in 30% to 40% plasma/60% to 70% SSP+. The extent of inactivation varied between 3.1 and 7.1 log with an average of all organisms of greater than 5.0 log. This average level of inactivation by UVC is slightly less than that observed for amotosalen and UVA light (average of all 19 organisms >6.2 log)2 and slightly more than that observed for riboflavin and UVB light (average of all 11 organisms >4.0 log).3 Most researchers believe that platelets immediately after collection are contaminated with low levels of organisms, and 3 to 4 log inactivation should be sufficient to produce sterile units.

However, measuring levels of inactivation is not enough. Inactivation cannot always be performed immediately after collection, and bacterial levels in units may increase because of growth during storage. Therefore, different experiments need to be performed to inoculate platelets with low bacterial inocula and delay treatment until later in storage to see if units are inactivated to sterility.4 Gravemann and colleagues supplemented their already very large study with another study investigating the ability to inactivate units to sterility when inactivation is delayed by 6 or 8 hours after spiking with 1 of 14 bacterial species. They repeated this massive experiment 12 times. Their results indicate that a 6-hour delay between inoculation and treatment uniformly inactivates units to sterility, but in 1 of 12 experiments, each using E. coli and S. pyogenes, where inactivation was performed 8 hours after inoculation, units were not rendered sterile. This compares to a smaller study conducted using amotosalen and UVA where investigators could delay treatment after inoculation for 24 hours using four species and inactivate to sterility, but not for 30 hours for one species,5 and one study with riboflavin and UVB light where viable organisms were observed after low-level inoculation and a 2-hour delay before treatment for five species.6

The pathogen reduction (PR) methods are not without compromise. A trade-off exists between the level of pathogen inactivation and platelet quality, function, and recovery and survival.7-10 While each of the PR methods utilize UV as part of the process, the overall treatment and mechanisms of action vary due to the photochemistry of each method. It is therefore not surprising that the effects on platelet metabolic state, activation level, and recovery and survival differ among the PR methods. UVC treatment was shown to increase platelet glucose consumption and lactate production when plasma carryover is less than 30%11, 12; however, only a trend toward increased platelet metabolism was observed when more than 30% plasma was present.8, 13 The reduced effects on platelet metabolic rate when more than 30% plasma is present are likely due to UVC absorbance by plasma proteins and other plasma constituents. UVB and riboflavin treatment has also been reported to increase platelet metabolic rate,14 while UVA and amotosalen treatment does not accelerate the platelet metabolic process during storage.15

Platelet activation and function have been shown to be negatively influenced by PR.7 UVC treatment at 0.4 J/cm² has been associated with increased platelet CD62P, annexin V, and PAC1 expression with little effect on adenosine triphosphate.8, 11, 12 In some experiments, a reduction in hypertonic shock response was observed when compared to controls.8, 16 The overall platelet activation status appears to be relative to UVC dose, as inactivation with a lower dose of 0.3 J/cm² showed no change in microparticle percentage and CD62P expression when compared to controls.8 UVB and riboflavin treatment is also associated with increased platelet CD62P, a potential loss of mitochondrial integrity, reduced adenosine triphosphate levels, and decreased hypertonic shock response17 in some but not all studies.18 UVA and amotosalen treatment is associated with increased platelet CD62P expression, with no changes in adenosine triphosphate levels or hypertonic shock response.2, 19

Platelet recovery and survival is also impacted by each of the three PR methods. When PR platelet recovery and survival was assessed after 5 days of storage and compared to stored untreated controls, UVC treatment and UVB and riboflavin treatment were comparable with 26% and 25% lower recovery and 29% and 27% lower survival, respectively.8, 10 The extent of reduction of recovery and survival due to UVA and amotosalen treatment was slightly less that that from UVC treatment or UVB and riboflavin treatment (16% lower recovery, 20% lower survival9). It is important to note that the comparison was made to 5-day stored platelet controls. When UVC-treated platelet recovery and survival was compared to fresh platelets, the UVC-treated platelets had 51% recovery and 65% survival. These data would not meet current Food and Drug Administration standards for recovery (not ≥66%) but would meet the current survival standards (≥58%).8

Graveman and colleagues present strong evidence that UVC treatment of platelets inactivates a wide range of bacteria at sterilizing potency. Like a suntan, which has both positive and negative consequences, there are some trade-offs in maintenance of platelet in vitro and in vivo storage properties arising from UVC photochemistry that should be weighed against the promise of eliminating septic reactions from platelet transfusion.