How do CD4 T cells activate CD8 T cells?

By | February 2, 2015

CD4 T cells activate CD8 T cells through an indirect process involving an antigen presenting cell (APC), normally a conventional dendritic cell (DC).

  1. An APC presents a peptide to a naive CD4 T cell via a Class II MHC molecule.
  2. The CD4 T cell becomes active and differentiates into a T helper 1 (Th1) cell through the release of IFN-gamma and IL-12, which are cytokines.
  3. The Th1 expresses CD40L on its surface, which can bind to the CD40 on the APC
  4. The APC is now at a higher level of activation. It now begins to express more Class I MHC, B7, and accessory molecules such as 4-1BB.
  5. A peptide binds to the Class I MHC on the APC.
  6. A CD8 T cell comes along, and its TCR recognizes the peptide on this Class I MHC.
  7. The CD8 T cell now becomes activated by binding to the APC.

Note that the two peptides mentioned had to have originally come from the same pathogen. This is an example of what's called linked recognition.

To summarize, we eventually have a microenvironment in which the APC is bound to a CD4 T cell AND a CD8 T cell.

CD4 T cell activates a CD8 T cell

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