CD28 and infection: Prior to infection with HIV-1, autologous CD4+ T cells were (1) unstimulated, (2) stimulated with 5 μg/ml anti-CD3 and 10 μg/ml anti-CD28 for 24 h (strong activation), (3) stimulated with the same concentrations of anti-CD3 and anti-CD28 for 2 h (medium activation), or (4) stimulated with 10-fold lower concentrations of anti-CD3 and anti-CD28 for 2 h (weak activation).