Rationale: Sepsis-related mortality results in part from immunodeficiency secondary to profound lymphoid apoptosis. The biological mechanisms responsible are not understood.
Objectives: Because recent evidence shows that platelets are involved in microvascular inflammation and that they accumulate in lymphoid microvasculature in sepsis, we hypothesized a direct role for platelets in sepsis-related lymphoid apoptosis.
Methods: We studied megakaryocytes and platelets from a murine-induced sepsis model, with validation in septic children, which showed induction of the cytotoxic serine protease granzyme B.
Measurements and main results: Platelets from septic mice induced marked apoptosis of healthy splenocytes ex vivo. Platelets from septic granzyme B null (-/-) mice showed no lymphotoxicity.
Conclusions: Our findings establish a conceptual advance in sepsis: Septic megakaryocytes produce platelets with acutely altered mRNA profiles, and these platelets mediate lymphotoxicity via granzyme B. Given the contribution of lymphoid apoptosis to sepsis-related mortality, modulation of platelet granzyme B becomes an important new target for investigation and therapy.