Canine CD4+ T-cell lymphoma identified by flow cytometry exhibits a consistent histomorphology and gene expression profile.

T-cell lymphomas (TCL) are a diverse group of neoplasms with variable diagnostic features, pathophysiologies, therapeutic responses and clinical outcomes. In dogs, TCL includes indolent and aggressive tumours such as T-zone lymphoma (TZL) and peripheral T-cell lymphoma (PTCL), respectively. Delineation of molecular subtypes and investigation into underlying pathophysiologies of aggressive TCLs remains inadequate. We investigate the correlations between flow cytometry and histopathology of 73 cases of nodal TCL. The majority of cases (82.2%) were characterized as CD4 TCL by flow cytometry. Fewer cases were classified as CD8 TCL (6.8%) or CD4 CD8 TCL (11.0%). All cases, regardless of immunophenotype, exhibited conserved histologic features consistent with the WHO classification of PTCL. Histologic subsets of PTCL corresponding to immunophenotypic features were not identified. Neoplastic cell size determined by flow cytometry correlated significantly with mitotic rate. RNA-seq was performed on a subset of CD4 PTCL cases (n = 6) and compared with sorted control CD4 T-cells. The gene expression pattern of CD4 PTCL was similar between all cases regardless of breed. PTCL was enriched in pathways representing G-coupled protein receptor signalling, extracellular matrix remodelling and vascular development, immune signalling and mitotic activity. Furthermore, global gene expression changes were consistent with downregulation of PTEN signalling and upregulation of the MTOR-PI3K-ATK axis. In this study, we evaluated the correlations between flow cytometry, histopathology and gene expression within a large cohort of nodal TCLs. We further demonstrate the ability of flow cytometry to identify a subtype of T-cell lymphoma, CD4+ PTCL, with a uniform histomorphology and gene expression profile.