Objective: To investigate the contribution and mechanism of cell-mediated cytotoxity to the pathogenesis of idiopathic thrombocytopenic purpura (ITP).

Methods: (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.

Results: (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).

Conclusions: 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.