CD8A and viral infectious disease: This data, coupled with previous observations demonstrating a rapid loss of Th1 responses during chronic viral infection (which may possibly reflect clonal deletion) and that Th1 cells rapidly lose their capacity to produce pro-inflammatory cytokines as the infection progresses (Fuller et al., 2004; Brooks et al., 2005; Fahey et al., 2011; Osokine et al., 2014), indicate that Th1 cells do exhibit some cardinal features of the process of CD8+ T cell exhaustion.