TPH1 and systemic lupus erythematosus: 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.