Notably, CXCR5+PD-1+CD8+ T cells expressed significant amount of Il21, the critical cytokine for CD4+ Tfh cell function25, and Il33, a cytokine that induces IgA and IgE production and is involved in the development of many autoimmune diseases (Fig. 5a)44,45. Here, PDCD1 is linked to autoimmune disease.