Antigen-presenting cells (APC) stimulate TCR/CD28-mediated signals and activation (blue). Antigen-experienced memory T cells may lose CD28 and require an augmented antigen threshold over time, thus supporting a resistance to classical adaptive stimulatory pathways (grey). Chronic antigen challenge, in turn, may induce the expression of NK cell receptors (NKRs) on some T cell clones (pink), ultimately facilitating the response of antigen-experienced T cells based on acquired NKR signaling . Those mechanisms may compensate for the lost capacity of conventional adaptive pathways (purple).
APC , antigen-present ing cell; TCR , T cell receptor; Ag, antigen; sNKR, stimulatory NK cell receptor; iNKR, inhibiting NK cell receptor ; SL, stimulatory ligand; IL, inhibiting ligand.