Given that PD-1 is a marker for GC-resident Tfh cells [152] and that cTfh and Tph cells express high levels of PD-1 on their cell surfaces, as discussed above, it could be interesting to assess whether this protein plays a role in the differentiation or function of these cells, which have mainly been implicated in the pathogenesis of SLE and other autoimmune disorders. This evidence concerns the gene TPH1 and autoimmune disease.