Biomechanical testing and histology

Biomechanical measurements were conducted using a rheometer (MCR302e; Anton Paar, Graz, Austria) operated with the RheoCompass software (Anton Paar). The spleen samples were positioned in the apparatus under an axial preload of 0.1 N to ensure full surface contact and adhesion. A 100-second relaxation period to allow residual stress dissipation was applied. For slippage minimization, waterproof sandpaper (Type P120, Federation of European Producers of Abrasives standard) was affixed to both the upper measuring plate and the base plate. Oscillatory shear tests were conducted with a peak angular shear strain of 3% at a frequency of 3 Hz. A continuous compressive force of 0.1 N over 50 cycles was applied. Phosphate-buffered saline (PBS) was used for rheometer calibration, and all measurements were performed in PBS at a controlled temperature of 20 °C. After the mechanical tests, all tested samples were put in 4% paraformaldehyde for full fixation and then histologically processed according to a standard protocol. For each sample, one H&E-stained slide was prepared in the circular plane of the tissue cylinder and jointly analysed by J.Z. and B.O. under a light microscope (Olympus BX51, Olympus Corporation, Tokyo, Japan) by consensus. The slides were screened for signs of inflammation (yes/no), blood congestion (yes/no) and fibrosis (no, mild, severe) upon gross inspection at 40x magnification. Representative images were scanned using a commercial software (PreciPoint, PreciPoint GmbH, Garching, Germany) and a digital microscope (M8; PreciPoint).