Pneumocystis murina lesions in lungs of experimentally infected Cd40l–/– mice

The Cd40l–/– mouse is a well-established model of X-linked hyper-immunoglobulin M (IgM) syndrome, an immunodeficiency disorder of human beings characterized by the lack of expression of the CD40 ligand (CD40L) on activated T-cells, predisposing to infections with opportunistic pathogens like Pneumocystis jirovecii. The aim of our study was to describe the pulmonary lesions in Cd40l–/– mice experimentally infected with Pneumocystis murina, in comparison with naturally infected severe combined immunodeficient (SCID) mice. Formalin-fixed paraffin-embedded lungs from 26 Cd40l–/–, 11 SCID, and 5 uninfected Cd40l–/– mice were examined by histology and immunohistochemistry for the presence of the pathogen and for leukocyte populations (CD3, CD4, CD45R/B220, CD8a, Iba-1, Ly-6G, CD206, MHC II, and NKp46/NCR1). Infection was confirmed by immunohistochemistry in 18/26 (69%) Cd40l–/– mice and in 11/11 (100%) SCID mice. Fourteen out of 26 (54%) Cd40l–/– mice had interstitial pneumonia. Twenty-three out of 26 (88%) Cd40l–/– mice had peribronchiolar/perivascular lymphoplasmacytic infiltrates, rich in B-cells and Mott cells. Acidophilic macrophage pneumonia was additionally found in 20/26 (77%) Cd40l–/– mice. Only 4/11 (36%) SCID mice had interstitial pneumonia, but no peribronchiolar/perivascular infiltrates or acidophilic macrophage pneumonia were observed in this strain. This study represents the first description of pulmonary histopathological lesions in Cd40l–/– mice infected with P. murina. We speculate that the singular characteristics of the inflammatory infiltrates observed in Cd40l–/– mice could be explained by the specific immune phenotype of the model.