BAFF Is Expressed on Human Platelets and Upregulated by Activation

Activated platelets express CD154, a member of the tumor necrosis factor (TNF) family that plays an important role in the activation of B cells and other antigen-presenting cells (APC). This may facilitate the initiation of immune responses at sites of thrombosis in response to injury. We examined platelets and megakaryocytic cell lines for the expression of another member of the TNF family, B-cell-activating factor belonging to the TNF family (BAFF) (otherwise known as Blys, TALL-1, THANK, TNFSF13B, or zTNF4). The surface expression of BAFF can be detected by flow cytometry on resting platelets from healthy individuals and patients with collagen diseases and hepatitis C virus (HCV) carrier/hepatitis/liver chirrhosis. BAFF can be expressed by activation with thrombin, even if it is not expressed in the resting state. Using sensitive reverse transcriptase polymerase chain reaction (RT-PCR), we detected BAFF mRNA in megakaryocytic cell lines but not in mature platelets isolated from healthy donors. Immunoblot analysis revealed, however, that both megakaryocytic cell lines and mature platelets contained protein of the appropriate size of the membrane-bound form of BAFF that reacted specifically with anti-BAFF antibodies. Our studies reveal that platelets express BAFF, which may imply possible roles of physiologic or pathologic platelet activation at local sites.