CD40LG and systemic lupus erythematosus: As shown in Fig. 1, the OD value of anti-PD-1 IgG in sera from SLE patients was significantly higher than in healthy controls (HC, p < 0.001), RA group (p < 0.001), pSS group (p < 0.01), and AS group (p < 0.001) whereas the IgM isotype of PD-1 antibodies showed no significant difference between any two groups.