FIG. 4.

Costimulation-independent proliferation of CD4⁺ thymocytes from Id1 transgenic mice. (A) Sorted CD4⁺ thymocytes were plated in triplicates (2 × 10⁵ cells per well) in a 96-well plate coated with indicated concentrations of anti-CD3ɛ antibody with or without soluble anti-CD28 MAb (2 μg/ml) and incubated for 48 h. Cells were pulsed with 1 μCi of [³H]thymidine per well for the last 18 h of incubation. The amount of [³H]thymidine incorporated by proliferating thymocytes was measured by scintillation counting. (B) Sorted CD4⁺ thymocytes from wild-type and Id1 transgenic mice were stimulated with plate-bound anti-CD3ɛ antibody (10 μg/ml) in a 96-well plate with or without PMA (2.5 ng/ml) for 48 or 72 h. [³H]thymidine incorporation was measured as described for panel A.