Pathological evaluation 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 stained slides were scored blinded by a trained observer for immune cell invasion (0 to 5) and joint erosion (0 to 5). Pictures were taken at 40× magnification and are representative of the average scores for invasion (4) and erosion (2), shown in Table 1. Areas of immune cell invasion into cartilage of joint (closed arrows) and bone reabsorption (open arrows) are highlighted.