Biochemical changes related to storage in the “stored PRBC” and “stored PLT” were normal for stored blood products (Table (Table44).Table 4Supernatant compositionSoluble CD40L levels were measured because this molecule has been associated with TRALI pathogenesis [29,35]. Storage resulted in accumulation of sCD40L in both “stored PRBC” and in “stored PLT,” with the former having higher levels (Table (Table4).4). However, these levels were much lower than those previously reported in units of Day 42 PRBC or Day 5 PLT [29,35]. To investigate whether heat-inactivation may also have reduced levels of sCD40L, we measured levels in equivalent non-heat-inactivated supernatant pools: PLT (Day 1: 1.85 ng/ml vs. Day 5: 9.25 ng/ml) and PRBC (Day 1: 0.40 ng/ml vs. Day 42: 8.36 ng/ml). This confirmed that heat-inactivation was responsible for the reduced levels of sCD40L evident in both “stored PRBC” and “stored PLT.”In contrast to sCD40L, no significant differences resulting from heat-inactivation were found in the concentrations of EGF, ENA-78, GRO-��, IL-8, IL-16 and MCP-1 (data not shown). As shown in Table Table44 the concentrations of the EGF, ENA-78 and GRO-��, were increased in both the “stored PRBC” and the “stored PLT” compared to the respective fresh product (“fresh PRBC” and “fresh PLT”). However, concentrations of IL-8, IL-16 and MCP-1 were only increased in “stored PRBC” (Table (Table4).4). Hence, there were more storage-related changes in cytokine concentration in “stored PRBC” than in “stored PLT.” Comparison of the two stored products also revealed that “stored PRBC” contained higher concentrations of EGF, IL-8, IL-16 and MCP-1, while “stored PLT” contained higher concentrations of ENA-78 (Table (Table4).4). Together these results demonstrate that there were storage-related changes, which were different depending on the blood product type (that is, “stored PRBC” vs. “stored PLT”).Differing composition did not result in differences in neutrophil primingBecause the neutrophil respiratory burst function plays a key role in the pathogenesis of TRALI [36], we compared the ability of “stored PRBC” and “stored PLT” to prime fMLP-induced activation of the respiratory burst in human neutrophils in vitro. In accordance with previous studies [31,37], “stored PRBC” and “stored PLT” both demonstrated greater priming ability than equivalent fresh supernatants (P < 0.001 in both cases; Figure Figure3).3). However, no difference was observed between the “stored PRBC” and the “stored PLT” (Figure (Figure33).Figure 3Neutrophil priming ability. Ability to prime fMLP-induced human neutrophil respiratory burst function is shown as mean of n = 4 experiments.