BDNF and systemic lupus erythematosus: The findings showed that the SLE-d group had significantly lower serum BDNF levels than the SLE-nd group (2.9 ± 0.354 vs. 6.92 ± 0.517, P = 0.000), and the HC group had lower serum BDNF levels than the SLE-d group (2.9 ± 0.354 vs. 1.502 ± 0.241, P = 0.001) (Figure 4A).