Pathological scoring of immune invasion and joint destruction in wild type (WT) and peptidyl arginine deiminase 4 knockout (PAD4 KO) K/BxN mice. WT (n = 27) and PAD4 KO (n = 29) animals were immunized with pooled K/BxN serum as described, sacrificed at day 10 post injection (p.i.), and their legs prepared for histology. Hematoxylin- and eosin-stained slides were scored blinded by a trained observer for immune cell invasion (0 to 5) and joint erosion (0 to 5). Results are represented as the average of 27 and 29 mice per genotype ± standard deviation. Average scores for joint invasion and erosion were similar in WT and PAD4 KO mice.