Anti-CD40–treated mice develop clinical and biochemical features of hemophagocytic syndrome. (A) Macroscopic picture of representative liver lobes of a control and anti-CD40–treated mouse at 30 hours after antibody treatment (original magnification ×1.25). Ischemic infarcts were detected in the anti-CD40 antibody-treated wild-type mouse (white spots on the liver lobe). (B) Representative photographs of hematoxylin and eosin (H&E)–stained liver tissue sections from a mouse 30 hours after antibody injection. Ischemic infarcts were detected in the area within the blue dashed line. (C) Representative photographs of H&E-stained spleen tissue sections from saline (upper section of the right panel) or anti-CD40–treated mice (bottom section of the right panel). Mice were euthanized 48 hours after antibody treatment. (D) Plasma concentrations of ALT, LDH, ferritin, sCD25 (top panels), and blood count analysis for total leukocytes, monocytes, lymphocytes, and platelet count (bottom panels) in saline (control) and anti-CD40–treated mice (n ≥ 4). (E) Hierarchical clustering analysis of cytokines and chemokines in saline or anti-CD40–treated mice, 12 hours postinjection. Plasma protein concentrations were measured by Bioplex and are color-coded. Red indicates high concentration; yellow low concentration. Each line identifies an individual saline or anti-CD40–treated animal. (F) Gene-set enrichment analysis enrichment score plots for the top 3 most positively enriched hallmark gene sets for the differentially expressed genes in F4/80⁺ liver macrophages of anti-CD40 antibody-treated animals vs saline-treated animals. (G) Top panels, Plasma ALT concentrations in saline (n = 2-3) or anti-CD40 antibody-treated wild-type mice, TNFR1/2 knockout mice, and IFN-γ knockout mice (n = 4). Bottom panels, Plasma ALT concentrations in wild-type mice treated with dexamethasone or neutralizing anti-IFN-γ, with or without anti-CD40 antibody (n = 2-4). (H) Left panels, Representative histograms of liver cell suspensions stained for F4/80 (macrophages) and intracellular TER119 (red blood cells) in saline or anti-CD40-treated wild-type mice. The displayed cells were gated from live CD45^high leukocytes. Data are representative of 3 independent experiments. Right panels, Corresponding pictures generated by Image Stream X flow cytometer showing intracellular TER119^high erythrocytes (yellow) in F4/80^high macrophages (magenta). Individual symbols represent 1 mouse; ****P < .0001; ***P < .001; **P < .01; *P < .05 for all panels.