Acidophilic macrophage pneumonia in laboratory mice

Acidophilic macrophage pneumonia, characterized by an accumulation of characteristic crystalloid-laden alveolar macrophages, was seen in 30/7,500 NMRI, 7/600 T x HT, 2/100 C57BL and in no cases of 1,500 CBA and 1,100 BALB/c mice. Histologically, there was a focal accumulation of large numbers of eosinophilic macrophages, generally associated with granulocytes. Macrophages could be mononucleate or multinucleate and had a crystalline cytoplasm. Free-lying crystals were sometimes observed. Ultrastructurally, macrophages had a cytoplasmic accumulation of needle-shaped and rhomboidal crystals, often showing a clear lattice structure with a repeat of 3-5 nm. The crystalloid inclusions may be derived from the breakdown products of granulocytes and appear similar to inclusions in macrophages in other parts of the hematopoietic system. That these inclusions are probably derived from eosinophils is based on the appearance within macrophages of structures resembling eosinophil granules at various stages of degradation and the similarity between the lattice repeat of the crystalloids and that of the crystalline core of the eosinophil granule. The crystalloid inclusions may be related to the Charcot-Leyden crystals found in human beings.