Figure 1: A current proposed model of T cell exhaustion in chronic virus infections. Chronic virus infections are associated with T cell exhaustion, that progresses with duration of infection and antigen presence, exemplified by incremental deficiencies in the ability of virus-specific T cells to produce cytokines, proliferate and to survive. PD-1 is involved early on in T cell exhaustion, and is associated with milder deficiencies in human T cell function, whereas Tim-3 is associated with severe T cell exhaustion.