Signalment:  

Adult, male, CD-1 mouse (Mus muscularis) An approximately 2 year old male sentinel mouse was found dead with no premonitory clinical signs.

Gross Description:  

Gross findings included numerous white, firm masses up to 0.8 cm in diameter in the lung. Additional findings included numerous red masses up to 0.5 cm in diameter in all lobes of the liver and approximately 2 ml blood in the abdominal cavity.

Histopathologic Description:

The lung tumor was peripherally-located, had a glandular and papillary pattern, and was nonencapsulated and expansile resulting in compression of adjacent tissues. Glandular structures were lined with rows of cuboidal to columnar cells that enclosed central lumena and had only slight cellular atypia. Some areas had solid sheets of cells interspersed by cholesterol clefts. The tumor was multicentric (only observed in some submitted sections), with intrabronchiolar growth. 

Areas of lung adjacent to the tumor had multifocal to coalescing inflammatory infiltrates composed of numerous large eosinophilic macrophages and multinucleate cells admixed with eosinophils, neutrophils, and lymphocytes within alveolar and bronchiolar spaces and associated with intra- and extracellular eosinophilic, acicular crystals.

Morphologic Diagnosis:  

1. Lung; Bronchioloalveolar adenocarcinoma, multicentric, mouse
2. Lung; Eosinophilic crystalline pneumonia, moderate, multifocal, mouse

Condition:  

Bronchioloalveolar adenocarcinoma

Contributor Comment:  

This animal had numerous spontaneous neoplasms including multicentric bronchioloalveolar carcinoma and hepatic hemangiosarcoma (the latter not submitted). Bleeding into the abdominal cavity from the hepatic tumors was the cause of death.

Eosinophilic crystalline pneumonia (ECP, formerly referred to as acidophilic macrophage pneumonia³ ) can occur spontaneously or in association with other pulmonary lesions such as infectious processes or tumors² . ECP can be subclinical to fatal and tends to increase in incidence with age and is more prevalent in specific strains of mice (highest incidence in 129S4/SvJae).²

The characteristic crystals in ECP were previously believed to be Charcot-Leyden crystals, a protein found in eosinophils and basophils³. Now it is known, however, that the crystals are composed of Ym1 protein, also referred to as T-lymphocyte-derived eosinophil chemotactic factor.¹ Ym1 is secreted by activated macrophages and is homologous to chitinase². Its normal function is not well-defined, but is believed to be involved in host immune defense, eosinophil recruitment, and cell-cell and cell-matrix interactions consistent with tissue repair.² Macrophages activated by type 2 cytokines produce large amounts of Ym1.²

In addition to resulting in extravasation of eosinophils and recruitment of T cells, Ym1 crystals likely contribute to lung inflammation through mechanical damage and enzymatic degradation.¹

JPC Diagnosis:  

1. Lung: Adenocarcinoma, CD-1 mouse (Mus muscularis), rodent.
2. Lung: Intraalveolar histiocytosis, multifocal, moderate, with abundant intracytoplasmic eosinophilic crystals (eosinophilic crystalline pneumonia).

Conference Comment:  

Eosinophilic crystalline pneumonia is a common background lesion of C57BL/6 background mice. In one report there was an 87% incidence with an overrepresentation of females in 129S4/SvJae mice.² It can be a spontaneous lesion, or associated with pulmonary adenomas, lymphoproliferative disease, allergic pulmonary disease and parasitic or fungal infections.²

There are four types of Ym proteins (Ym1, Ym2, Ym3, and Ym4). Although these proteins are members of the chitinase family of proteins, they do not possess any of the chitinase enzymatic activity.² Ym1 and Ym2 have approximately 95% of the same sequence identity but are expressed in different tissues. Ym1 is expressed in the lung and spleen, but not in the stomach, while Ym2 is expressed in the stomach, but not the lung or spleen.²

Lesions associated with eosinophilic crystalline pneumonia can range from subclinical to severe and fulminating. Three patterns of lung lesions predominate.² The first consists of diffuse interstitial inflammatory infiltrates of macrophages, multinucleate cells, eosinophils, lymphocytes, occasional neutrophils, with moderate to severe lymphoplasmactyic perivascular and peribronchiolar cuffing. The second consists of little to no crystals, with macrophage infiltrates localized to regions of a lung tumor. The third consists of focal to multifocal infiltrates localized around bronchioles with large rectangular crystals in the airways and minimal macrophage infiltrates.² The case presented in this conference appeared consistent with the second pattern of distribution and was unusual in that it had little to no interstitial reaction despite numerous intraalveolar macrophages with abundant eosinophilic crystals. 

Pulmonary adenomas and adenocarcinomas are among the most common primary pulmonary neoplasms in mice. A-strain mice are particularly susceptible, and their development in this strain is usually associated with activation of K-ras within these tumors.⁴ Other less common primary lung tumors within mice are squamous cell carcinoma, papilloma, neuroendocrine carcinomas, and adenosquamous carcinomas.⁴

The cell of origin for pulmonary adenomas and adenocarcinomas are thought to be either type II pneumocytes or Clara cells.⁴ Ultrastructural features of Clara cell differentiation include apical cytoplasmic accumulation of smooth endoplasmic reticulum, which can be admixed with lamellar surfactant granule formation. 

References:

1. Guo L, Johnson RS, Schuh JCL: Biochemical characterization of endogenously formed eosinophilic crystals in the lungs of mice. J Biol Chem 275:8032-8037, 2000
2. Hoenerhoff MJ, Starost MF, and Ward JM: Eosinophilic crystalline pneumonia as a major cause of death in 129S4/SvJae mice. Vet Pathol 43:682-688, 2006
3. Murray AB, Luz A: Acidophilic macrophage pneumonia in laboratory mice. Vet Pathol 27:274-281, 1990
4. Percy DH, Barthold SW: Mouse. In: Pathology of Laboratory Rodents and Rabbits, 3rd ed., pp. 117-118. Blackwell Publishing, Ames, IA, 2007
5. Wilson DW, Dungworth DL: Tumors of the respiratory tract. In: Tumors in Domestic Animals, ed. Meuten DJ, 4th ed., pp. 385-389. Blackwell Publishing, Ames, IA, 2002

A virtual slide is not available for this case.

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