Platelet concentrates modulate myeloid dendritic cell immune responses.

Platelet transfusion has been reported to modulate the recipients' immune system. To date, the precise mechanism(s) driving poor patient outcomes (e.g., increased rate of mortality, morbidity, infectious complications and prolonged hospital stays) following platelet transfusion are largely undefined. To determine the potential for platelet concentrates (PC) to modulate responses of crucial immune regulatory cells, a human in vitro whole blood model of transfusion was established. Maturation and activation of human myeloid dendritic cells (mDC) and the specialized subset blood DC antigen (BDCA)3+ DC were assessed following exposure to buffy-coat derived PC at day (D)2 (fresh) and D5 (date-of-expiry). In parallel, to model recipients with underlying viral or bacterial infection, polyinosinic:polycytidylic acid or lipopolysaccharide was added. Exposure to PC had less of an impact on mDC responses than BDCA3+ DC responses. PC alone downregulated BDCA3+ DC expression of co-stimulatory molecules CD40 and CD80. In the model of viral infection, PC downregulated expression of CD83, and in the bacterial model of infection, PC downregulated CD80, CD83, and CD86. PC alone suppressed mDC production of interleukin (IL)-8, IL-12 and tumor necrosis factor (TNF)-α and BDCA3+ DC production of IL-8, IL-12, and IL-6. In the model of viral infection, production of IL-12 and interferon-gamma inducible protein (IP)-10 was reduced in both DC subsets, and IL-8 was reduced in BDCA3+ DC following PC exposure. When modeling bacterial infection, PC suppressed mDC and BDCA3+ DC production of IL-6 and IL-10 with a reduction in TNF-α evident in mDC. This study assessed the impact of PC "transfusion" on DC surface antigen expression and inflammatory mediator production and provided the first evidence that PC transfusion modulates blood mDC and BDCA3+ DC maturation and activation, particularly in the models of infection. Results of this study suggest that patients who receive PC, particularly those with underlying infectious complications, may fail to establish an appropriate immune response precipitating poor patient outcomes.