Co-Expression of T- and B-Cell Markers in a Canine Intestinal Lymphoma: A Case Report

SIMPLE SUMMARY: The diagnosis of chronic gastroenteropathy in dogs can be complex, highlighting the relevance of histopathology allied to immunohistochemistry in the identification of the involved cell types, contributing to the exclusion of differential diagnoses, including special types of intestinal lymphomas. However, rare cases of the co-expression of some immunomarkers in lymphoid cells can make the interpretation of results particularly difficult. This report presents a case of a dog with a diagnosis of intestinal lymphoma, in which the co-expression of CD3 and CD20 in lymphoid cells was identified in immunocytochemistry and immunohistochemistry, making it necessary identify lymphoid clonality using polymerase chain reaction for antigen receptor rearrangement (PARR) for a precise diagnosis. ABSTRACT: An 8-year-old female neutered Labrador retriever was presented for a second opinion consultation due to vomiting and lethargy, having failed to respond to symptomatic therapy. Blood analysis revealed hyperbilirubinemia and hypoalbuminemia, associated with hypocobalaminemia. An abdominal ultrasound identified diffused bowel thickening and hypoechoic hepatomegaly. An ultrasound-guided liver fine-needle aspiration was performed for cytology and also for cell block immunocytochemistry. Gastric and duodenal biopsies were collected by gastroduodenoscopy. Liver cytology showed numerous lymphocytes, suggesting lymphoma at the hepatic infiltration stage, and immunocytochemistry in the cell block of the hepatic aspirate indicated co-expression of CD3 and CD20 in the lymphoid cells present. The histopathology of gastric and duodenal biopsies supported the hypothesis of gastrointestinal lymphoma due to heavy lymphoid infiltration of the gastric epithelium and intestinal mucosa, including the villi. Concurrent immunohistochemistry was performed using CD3, CD20, PAX5, and CD79αcy antibodies. Immunomarking was positive for CD3 and CD20, which overlapped populations of lymphoid cells, and was negative for all other antibodies. In the clonality test, lymphocyte co-expression of CD3 and CD20 was confirmed by monoclonal rearrangement of T-cell gamma receptors. The final diagnosis was type 2 enteropathy-associated T-cell lymphoma with hepatic infiltration. Co-expression was examined in conjunction with the PARR result in the presence of T-cell monoclonal rearrangement.