There are a number of intestinal diseases of cats characterised by cellular infiltration of the lamina propria; most notably inflammatory bowel disease (IBD) and alimentary lymphoma. The distinction between severe IBD and alimentary lymphoma can be difficult to make by histopathology, but may be clarified by immunostaining. A case with a unique distribution of small intestinal CD4 and CD8 T-cell infiltration distinct from that described in IBD or T cell lymphoma is reported.

The case was a seven-year-old cat, with a chronic history of intermittent diarrhoea and weight loss that had been partially controlled by dietary management and corticosteroids. The cat was euthanased and gastrointestinal tract samples collected at necropsy for immunohistochemistry (IHC) and immunofluorescence (IF).

Gross inflammation was present throughout the intestine but was more severe in the distal ileum and colon. Histopathology indicated ulcerative necrotizing colitis, small intestinal lymphocytic infiltration with loss of normal mucosal architecture and stunting of villi.

CD3, CD4 and CD8 antibodies were used to characterize infiltrating cells by IHC. Two-colour IF was performed with a combination of antibodies raised against feline CD4/CD8 and feline CD45/cross-reactive major histocompatibility complex (MHC) class II.

Small intestinal and colonic tissues showed extensive and diffuse infiltration of the lamina propria with T-cells. In the distal ileum, CD4+ T cells filled the villous lamina, whereas CD8+T-cells infiltrated the cryptal lamina. Infiltrating T cells were positive for CD3 and CD45. MHC Class II positive cells showing dendritic morphology were interspersed amongst the T-cells and epithelial expression by enterocytes was also evident.

The epithelial MHC class II expression is not seen in alimentary lymphoma. Intense focal linear accumulations of CD4+ cells restricted to the villous areas and CD8+ cells in the crypts are not typical of IBD in which there is a more diffuse infiltration. The extensive T-cell infiltration with the unusual distribution pattern may suggest a disease entity distinct from alimentary T-cell lymphoma and severe IBD. Alternatively, a bi-clonal proliferation with each T-cell clone potentially expressing unique homing molecules may account for their distinct regional accumulation.