Both CD28 and CTLA-4 are receptors on T cells that are critical for regulating T cell activation.In which one of the following ways does CD28 differ from CTLA-4?
A)Only CD28 binds the costimulatory ligands B7-1 and B7-2 expressed on professional antigen-presenting cells.
B)CD28 counteracts positive,pro-proliferative T cell signals delivered by CTLA-4.
C)CD28 is constitutively expressed on naive T cells,whereas CTLA-4 is expressed on activated T cells.
D)CD28 binds its ligand with 10-fold greater affinity than does CTLA-4.
E)CD28 is important for delivering "signal 1" for T cell activation,whereas CTLA-4 is important for delivering "signal 2."

Question

Quizplus · Accepted Answer

CD28 is constitutively expressed on more than 90% of CD4⁺ T cells and 50% of CD8⁺ T cells,whereas CTLA-4 is expressed only on activated T cells.Both B7-1 and B7-2,expressed on professional antigen-presenting cells (APCs),bind to both CD28 and CTLA-4 receptors on T cells.Binding of B7 molecules on APCs to CD28 delivers "positive" signals to the T cells that stimulate production of growth factors,promote T cell proliferation and differentiation,and induce expression of anti-apoptotic proteins.CTLA-4,however,functions to inhibit T cell activation by counteracting signals delivered by CD28.CTLA-4 also binds B7-1 with 10-fold greater affinity than CD28 binds B7-1;this difference may play an important role in the temporal sequence of T cell activation.

Both CD28 and CTLA-4 Are Receptors on T Cells That