Increased mast cells in the thymus during acute thymic involution in Japanese Black calves with a poor prognosis

Acute thymic involution frequently occurs in calves and is associated with impaired immune function, yet its detailed pathogenesis remains unclear. To investigate the involvement of inflammatory cells in acute thymic involution in calves, this study examined mast cell dynamics in the calf thymus. Thymus samples were collected at necropsy from Japanese Black calves under 300 days of age and classified histopathologically into five stages. Mast cells were specifically identified by toluidine blue staining and immunohistochemistry for tryptase, whereas immunohistochemistry for chymase showed positive reactions in various cell types, including thymocytes. The densities of toluidine blue-metachromatic and tryptase-positive cells were increased in acute thymic involution. These cell densities were consistently higher in the medulla than in the cortex. Tryptase-positive cell density was higher than toluidine blue-metachromatic cell density. In double immunofluorescence for tryptase and PCNA, PCNA-positive nuclei were observed in thymocytes, but not in tryptase-positive cells at any stages of acute thymic involution. Analysis of the publicly available DNA microarray data showed that a cluster of microarray spots annotated to genes encoding mast cell chemoattractants exhibited relatively higher gene expression levels in thymuses with more advanced acute thymic involution than in thymuses with less advanced acute thymic involution. The expression levels of ADM, CCL16, CCL24, and TGFB2 showed positive correlations with the stage of acute thymic involution. These results suggest that mast cells increase in the calf thymus during acute thymic involution without relying on local cell proliferation in the thymus.