Mechanism of cell-mediated lysis of autologous platelets in chronic idiopathic thrombocytopenic purpura

Abstract

To investigate the contribution and mechanism of cell-mediated cytotoxity to the pathogenesis of idiopathic thrombocytopenic purpura (ITP). (1) Mononuclear cells and platelets were prepared from the peripheral blood of 14 ITP patients and 10 healthy controls. Separately, CD 8(+) T cells and NK cells (CD 3(-)CD 16(+)CD 56(+)) were positively selected with magnetic microbeads. (2) As the target cells, the autologous platelets were cultured with CD 8(+) T cells or NK cells for 4 hours and then stained with annexin V. Ratio of platelets expressing annexin V was determined by flow cytometry. (3) The fraction of CD 8(+) T cells expressing FasL, TNFalpha and TRAIL were determined by flow cytometry. (4) The expression levels of perforin and granzyme B mRNA in CD 8(+) T cells were measured by semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) procedure. (1) When the platelets were incubated alone the annexin V positive platelet ratio of the ITP patients was 3.1% +/- 0.9%, not significantly different from that of the controls (3.2% +/- 1.1%, P > 0.05); (2) When the platelets were co-incubated with CD 8(+) cells the annexin V positive platelet ratio of the ITP group (7.6% +/- 2.8%), significantly higher than that of the control group (3.6% +/- 0.9%, P < 0.05); (3) When NK cells were used as effector cells, the annexin V positive platelet ratio of the ITP group was 3.5% +/- 1.1%), not significantly different from that of the control group (3.6% +/- 1.0%, P > 0.05); (4) The expression rates of FasL and TNFalpha on CD 8(+) T cells of the ITP group were 17.5% +/- 4.4% and 11.9% +/- 4.9% respectively, both significantly higher than those of the control group (8.9% +/- 1.5% and 6.4% +/- 2.1% respectively, both P < 0.05), while the expression rate of TRAIL of the ITP group was 16.1% +/- 3.8%, not significantly different from that of the control group (14.0% +/- 3.2%, P > 0.05); (5) The mRNA levels of granzyme B and perforin in the CD 8(+) T cells of the ITP group were 2.20% +/- 0.15% and 2.47% +/- 0.39% respectively, both significantly higher than those of the control group (1.63% +/- 0.22% and 1.80% +/- 0.31% respectively, both P < 0.05). Cytotoxic T lymphocyte (CTL) are activated in ITP and might be involved in the pathogenesis of ITP, while the NK cells have no direct cytotoxic effect on platelets. Apoptosis and perforin/granzyme-mediated cytotoxicity are two important pathways through which CTL destruct auto-platelets.