KLRK1 and systemic lupus erythematosus: ELISA assay showed marked increase of perforin and granzyme B derived from the NKG2D+CD4+ T cells co-cultured with lupus serum that stimulated Treg cells, indicating that NKG2D+CD4+ T cells could kill target cells by secreting cytotoxic mediators in vitro (Fig. 4B).