PDCD1 and systemic lupus erythematosus: In particular, PD-1-deficient C57BL/6 mice spontaneously developed lupus like arthritis, splenomegaly, and glomerulonephritis; furthermore, these animals showed an increased number of B and myeloid cells, and enhanced IgA, IgG2b, and IgG3 levels in the serum (4).