As previously described, the IL-21R is mostly expressed by mature naïve CD19+ B cells (around 90% of gated IL-21R+ cells are CD19+IgD + CD27-), not only in healthy individuals but also in SLE patients (92.1±3.4% in healthy individuals vs 87.5±7.1% in SLE patients; Figure 5C). This evidence concerns the gene CD19 and systemic lupus erythematosus.