Resting or activated platelets are not able to stimulate allospecific cytotoxicity in vitro.

(A) HLA-A2⁻ PBMCs were stimulated weekly with irradiated PBMCs, platelets, or thrombin-activated platelets from an HLA-A2⁺ donor, as described in Materials and methods. Allospecific cytotoxic activity was tested at day 6 (1 in vitro stimulation) and day 13 (2 in vitro stimulations) against allogenic HLA-A2⁺ PHA-blasts, at the indicated E:T ratios. (B) Allospecific HLA-A2⁻ cytotoxic cell line obtained after 2 stimulations with HLA-A2⁺ PBMCs was split and restimulated weekly either with PBMCs or with resting or activated platelets from the same allogenic donor. As control, medium alone was added without any stimulating cells. Cytotoxicity was tested after 1 restimulation (day 20) or 2 restimulations (day 28), against allogenic PHA-blasts. Data are presented for an E:T ratio of 20.