Comprehensive phenotypic and clonality analysis of intraepithelial lymphocytes of intestinal T-cell lymphoma and chronic enteropathy in dogs.

Canine intestinal T-cell lymphoma (ITCL) may arise from intraepithelial lymphocyte (IEL) subsets expanded in chronic enteropathy (CE). To investigate this potential cell of origin, we performed flow cytometry, immunohistochemistry, and RNA in situ hybridization on IELs using biopsy samples from 62 dogs, including 6 large-cell lymphomas (LCLs), 9 small-cell lymphomas (SCLs), 31 CEs with increased IELs (IEL+CE), and 16 CEs without increased IELs (IEL-CE). IELs in ITCLs were predominantly CD4-CD8α- (LCL, 5/6; SCL, 4/9), and 4/31 IEL+CEs had a higher proportion of CD4-CD8α- IELs than the other CE cases. Forward scatter values of CD4-CD8α- IELs were higher in IEL+CEs compared with IEL-CEs, correlating positively with the Ki-67 index, indicating proliferation of this population. An increase in IELs lacking T-cell receptor expression was observed in 1/4 LCL, 2/5 SCL, and 1/13 IEL+CE cases. Lack of surface CD3 was observed in 2/9 SCL cases. One of 5 LCL cases partially expressed NKp46, whereas the proportion of NKp46+ IELs was low (< 4%) in 32/33 CE cases examined. In addition, 3/6 LCL cases expressed NKp46 mRNA without detectable protein expression. Clonality analysis of isolated IELs yielded at least 1 clonal peak in 1/2 LCLs, 4/7 SCLs, 12/23 IEL+CEs, and 4/13 IEL-CEs, underscoring the limited diagnostic utility on distinguishing ITCL, particularly SCL, from IEL+CE. These findings suggest that most canine ITCLs have a CD4-CD8α- immunophenotype and that unconventional IELs in IEL+CE may be the cells of origin. Moreover, the distinction between ITCL and CE remains challenging, emphasizing the importance of integrated diagnostic approaches.