Pathogens such as bacteria and viruses present a daily threat. The activation of an appropriate immune response can control these pathogens to guarantee our survival. Specialized cells – also called antigen-presenting cells - act as guards in our body.
The role of antigen-presenting cells is to find the pathogens. If they meet a pathogen, they capture and metabolise it in their interior. Metabolized products include protein fragments that are also called peptides. Such peptides are presented by self-molecules on the surface of antigen-presenting cells. Thus activated antigen-presenting cells migrate to the draining lymph node to present the antigen to other immune cells, such as T cells.
If an activated antigen-presenting cell meets a T cell which recognises the combination of self molecule plus peptide, a contact surface is formed (see Figure 1).
Figure 1: Schematic display of the reaction
Source: PEI
This contact surface consists of a “sticky” outer ring responsible for adherence between the two cells (yellow), and an inner circle in which self-molecules presenting the peptide and the matching recognition structure on the T cell side are enriched (white). This intense contact between antigen-presenting cell and T cell leads to a first activation signal.
In addition, co-stimulatory molecules are expressed by the antigen-presenting cell, which are also called B7. These recognise a structure on the T cells termed CD28. Interaction of B7 and CD28 results in a second signal that leads to full T cell activation. As a consequence, the activated T cell undergoes massive cell division that creates thousands of copies of the initially activated antigen-specific T cell. Finally, T cells differentiate into effector cells that then move to the site of infection to kill infected cells, or to support other immune cells in producing antibodies.
Thus, for an ordered activation of antigen-specific T cells the first antigen-specific and the second co-stimulatory signal provided by one and the same antigen-presenting cell are required. The CD28 molecule is a marker constitutively expressed by most T cells and by antibody-forming plasma cells. Certain CD28-specific antibodies binding to a defined subregion of CD28 can directly trigger T cell proliferation, independent of an antigen-dependent signal 1.
Updated: 21.11.2019
