MHC class II surface expression and costimulation
A cell has detected and engulfed antigenic material, which has been chopped up and complexed with MHC class II. Once the MHC-peptide complex has then been transported to the surface of the cell, it is ready to be screened by T cells.
An MHC class II-antigen complex on an APC is recognised by a T cell via its TCR, triggering a first T cell activation signal. This ensures that only a T cell carrying a TCR specific to that specific peptide presented on that specific MHC class II type gets activated.
The first signal instigates a two-way communication, a bit like someone calling you on a mobile phone! Consider what happens to your conversation when the signal is lost. Communication is patchy at best, and can become impossible. Similarly the cells need a stable signal to effect good communication. Using the following video see what molecules are necessary to stabilise the APC:T cell interaction.
The costimulatory molecules are pivotal for the activation of naive T cells, which have never encountered their specific antigen before. Each MHC class II-peptide complex can interact with more than one TCR on a T cell. These multiple interactions help build up a strong enough signal in the T cell that will lead to its activation. The adhesion molecules act like a circle of glue surrounding the main event, allowing the cells to stay in close enough contact to form an interface between the antigen-presenting cell and the T cell. Once activated, the CD4+ helper T cells assist other white blood cells in immunologic processes, such as help B cells mature into plasma cells and help the activation of CD8+ cytotoxic T cells and macrophages, which we will look at in more detail in the next module.