Background/Objectives: T-cell lymphomas are often histologically indistinguishable from benign T-cell infiltrates, and diagnosis typically relies on slow, complex, and expensive multiplexed PCR reactions, requiring significant training and experience to interpret them. We aimed to raise highly specific antibodies against the two alternatively used and very similar T-cell receptor beta constant regions, TCRbeta1 and TCRbeta2, encoded by the TRBC1 and TRBC2 gene segments, respectively. We sought to demonstrate the feasibility of detecting TCRbeta1 and TCRbeta2 immunohistochemically in routine clinical (formalin-fixed, paraffin-embedded (FFPE)) tissue sections as a novel diagnostic strategy for T-cell lymphomas. Methods: Recombinant rabbit antibodies were validated using Western blotting and FFPE immunostaining of T-cell leukemia lines. The immunostaining of FFPE tissue containing benign and lymphomatous T-cell populations was undertaken, with corroboration by BaseScope^TM high-sensitivity in situ hybridization and quantitative real-time PCR (Q-PCR). An additional Q-PCR literature review and analysis of publicly available RNAseq data was used to determine the TCRbeta2/TCRbeta1 ratio cut-off to separate benign and malignant T-cell populations. Results: Our TCRbeta1/TCRbeta2 antibody pair gave highly specific FFPE tissue staining. All benign samples analyzed (immunohistochemically, by BaseScope^TM, by Q-PCR, and by RNAseq data analysis) had TCRbeta1/TCRbeta2 or TRBC1/TRBC2 ranges well within the previously published flow cytometric benign range (TCRbeta2/TCRbeta1 = 0.18:1–5.7:1), while samples of T-cell lymphoma did not. One out of thirteen (7.7%) lymphoma samples showed some detectable TCRbeta1/TCRbeta2 protein co-expression, and 4 out of 13 (30.8%) T-cell lymphomas showed a TRBC1/TRBC2 transcript co-expression using BaseScope^TM. Conclusions: Analyzing T-cell monotypia immunohistochemically, analogous to B-cell monotypia (kappa: lambda ratio for B-cell and plasma cell neoplasms), could make the diagnosis of T-cell lymphomas cheaper, quicker, and more accurate. Larger studies are needed to validate our antibodies for clinical use.

Background/Objectives: T-cell lymphomas are relatively common in veterinary species, yet current diagnostic tools such as PCR-based clonality assays often lack sensitivity and specificity. In humans, we recently developed two related tissue-based diagnostic approaches based on the differential detection of the mutually exclusively expressed TCRbeta1 and 2 (TCRβ1 and 2) constant region proteins, or the corresponding TRBC1 and TRBC2 transcripts. Analogous to the detection of kappa/lambda light chains for the diagnosis of B-cell/plasma cell neoplasms in human clinical practice, our TCRβ1/2 diagnostic assay has the potential to transform veterinary diagnostic workflows. Methods: We identified and confirmed the sequences of the relevant TRBC1 and TRBC2 sequences in both cats and dogs, focusing on the 3′ untranslated region (UTR), where there is the least sequence homology between TRBC1 and TRBC2. To allow us to design appropriate probe sequences, we confirmed a lack of 3′UTR in either species, and we observed limited 3′ untranslated region UTR sequence polymorphism in the cat but not in the dog 3′UTR. We designed BaseScope™ RNA in situ hybridization probes targeting the 3′ UTR to distinguish between TRBC1 and TRBC2 transcripts in formalin-fixed paraffin-embedded tissues. Results: In normal tissues, we found the TRBC2:TRBC1 expression ratio to be similar to the 1.2:1 ratio in humans, between 1:1 and 3:1, skewing towards TRBC2, in both dogs and cats. These findings were corroborated using quantitative reverse transcription PCR. Applying our in situ hybridization probes to cases of T-cell lymphoma in dogs and cats, we demonstrated that an assay for differential expression of TRBC1 and TRBC2 in T-cell populations could identify clonal T-cell populations, as in human diagnostics. If further studies corroborate this proof-of-concept study, TRBC1/2 detection could obviate the need for slow, complex and expensive multiplexed PCR-based (PCR for antigen receptor rearrangements (PARR)) clonality assays. Conclusions: This study provides proof-of-concept data for a novel diagnostic approach that could simplify and substantially improve the accuracy of lymphoma diagnostics in veterinary medicine, by detecting TRBC1/2 transcripts.