Fig. 2.

Antigen processing and presentation in a dendritic cell (DC) that lead to activation of CD4⁺ T-cells (left) and CD8⁺ T-cells (right). (1) Antigen-loaded particles are internalized by DC through cell uptake pathways such as phagocytosis or endocytosis. To activate CD4⁺ T cells, (2a) particulate antigens are processed to peptide fragments by proteases in endosomes, (3a) subsequently loaded onto MHC II molecules, (4a) and the formed MHC II–peptide complex is then trafficked to the cell surface where it presents the antigen to CD4⁺ T cells bearing cognate T cell receptors (TCRs). To activate CD8⁺ T cells (following the cytosolic pathway as depicted), (2b) the antigens are translocated to cytosol and (3b) subsequently processed by proteasome. (4b) The resulting peptide fragments are transported to endoplasmic reticulum (ER), (5b) then loaded onto MHC I molecules in ER, followed by (6b) trafficking of the formed MHC I–peptide complex to cell surface where they can interact with CD8⁺ T cells bearing cognate TCRs. Activated CD4⁺ T cells can differentiate into T helper type 1 (Th1), Th2, Th17 and T follicular helper (Tfh) cells producing various cytokines for signaling B cell activation, whereas activated CD8⁺ T cells can kill infected/mutated cells.