Expression of extranuclear HMGB1 protein in lupus nephritis (LN). Representative micrographs illustrate paraffin-embedded kidney biopsies displaying immunohistochemical diaminobenzidine brown staining of HMGB1. (A) A baseline kidney biopsy from a patient with active World Health Organization (WHO) class V LN, magnified in (B), demonstrates that an abundant HMGB1 staining is located predominantly outlining the endothelium but also expressed in the mesangium (arrow). (C) The expression of HMGB1 is less pronounced in a repeat biopsy (still classified as WHO V) of the same patient and is magnified in (D). The staining is now most evident outlining the endothelium but is also found in the brush border of tubular cells. (E) A baseline biopsy from a patient with active class IV LN, magnified in (F), demonstrates that an abundant HMGB1 expression is located outlining the endothelium. (G) A repeat biopsy in a patient with WHO class II after immunosuppressive treatment, magnified in (H), illustrates that HMGB1 is still evident in glomeruli. (I) A baseline biopsy from a patient with WHO class III, magnified in (J), demonstrates that an abundant HMGB1 staining is evident predominantly outlining the glomerular endothelium but also expressed in the mesangium. (K) HMGB1 expression is still evident in a repeat biopsy (still classified as WHO III) of the same patient, magnified in (L), but is not as strong as in the baseline biopsy. (M) A consecutive section from the same patient is stained with irrelevant isotype control antibody. In kidney tissue from a healthy control, HMGB1 expression was predominantly negative or, if present, is restricted to the cell nuclei (N). Original magnifications: 100× (A,C,E,G,I,K,M,N) and 250× (B,D,F,H,J,L). HMGB1, high-mobility group box 1 protein.