TPH1 and systemic lupus erythematosus: Notably, increased circulating PD-1hiCXCR5−CD4+ Tph cells have been observed in patients with IgG4-related disease (11), systemic lupus erythematosus (10), type 1 diabetes (12), IgA nephropathy (13), psoriasis vulgaris (14) and active ulcerative colitis (15), further underscoring their significance.