Introduction. Platelets are key regulators of intravascular thrombosis and inflammation and have recently been found to express a range of Toll-like receptors (TLRs). This study examined how platelets change the leukocyte response to TLR stimulation.

Methods. PBMCs and granulocytes from 10 healthy volunteers were cultured alone or co-cultured with platelets. Cultures were left unstimulated or stimulated with 1 or 100 ng/mL of either LPS (TLR4 agonist), Pam3CSK4 (TLR2/1) or FSL-1 (TLR2/6) for 4 hours (granulocytes) or 24 hours (PBMCs). Neutrophil activation (CD66b expression), monocyte activation (HLA-DR) and T-cell activation (CD38) were assessed by flow cytometry. Additionally, elastase production by granulocytes and production of IL-6, TNF-α and IL-10 by PBMCs were assayed.

Results. Platelet co-culture significantly decreased neutrophil CD66b expression in response to LPS, Pam3CSK4 and FSL-1. With platelets, monocyte HLA-DR expression in response to high-dose LPS was modestly reduced while CD8+ T-cell CD38 expression in response to low-dose Pam3CSK4 and high-dose FSL-1 was modestly increased. Platelets markedly reduced granulocyte elastase production in response to low-doses of all agonists. In platelet co-cultures and in response to LPS, there was a significant reduction in IL-6 and TNF-α production, and increased IL-10 production by PBMCs. In response to FSL-1 stimulation, platelet co-culture increased IL-6 and IL-10 production, but reduced TNF-α production. Platelet co-culture did not alter PBMC cytokine production in response to Pam3CSK4.

Conclusion. Platelets are complex immunomodulators that regulate agonist- and leukocyte-specific responses to TLR stimulation. The clinical significance of these dynamic interactions is uncertain.