PDCD1 and systemic lupus erythematosus: Evidence from various studies showed that the levels of circulating Tfh-like cells, which are defined by the expression of CXC chemokine receptor 5 (CXCR5), inducible T cell costimulator (ICOS), programmed cell death 1 (PD-1), IL-21, and B cell lymphoma 6 (Bcl6), are increased in the peripheral blood of patients with SLE, and that the percentages of these cells correlate with disease activity, organ damage, and titers of anti-double-stranded DNA (dsDNA) antibodies and other autoantibodies [15,16,17,18,19].