TPH1 and rheumatoid arthritis: Immune checkpoint blockade, particularly in the form of therapeutics that interfere with PD-1:PD-L1 signalling, has revolutionised cancer treatment.23–25 However, by inhibiting the natural mechanisms limiting the magnitude and duration of the T cell response, these inhibitors are associated with a range of immune-related adverse events (irAEs),26 including inflammatory arthritis resembling RA.2 27 Our findings of Tph expansion in early, treatment-naïve RA suggest a role of these cells in driving the initial phase of disease.