However, this review emphasized the importance of carrying out new assays to elucidate the mechanisms controlling PD-1’s function on CD4+ T cells during the autoimmune response, as it has been suggested that PD-1 could play a role in the differentiation and function of various T-cell subpopulations, such as Tfh, cTfh, and Tph cells, which are associated with autoimmune disorders. This evidence concerns the gene TPH1 and autoimmune disease.