Proliferation versus inflammation: Immunohistochemical profiling of feline chronic rhinitis and nasal lymphoma.

Lymphoma is the most common nasal cavity tumor in cats. While most recent research efforts have characterized neoplastic cell markers in feline alimentary lymphoma, only few reports are dedicated to immunohistological features of nasal lymphoid tumors beyond T- or B-cell phenotyping. In comparison, feline lymphoplasmacytic chronic rhinitis represents chronic, progressive lymphoplasmacytic inflammation, causing cyclical tissue damage and repair. Sixty-five cases of lymphoplasmacytic rhinitis and 15 cases of feline diffuse, large cell, immunoblastic nasal lymphoma were selected and submitted for immunohistochemistry to detect expression of immune markers of cell origin (CD3, CD20/PAX5, and FOXP3), proliferation (PCNA), signal transducer and activator of transcription 5 (STAT5), DNA repair (MLH1, MSH2, and MSH6), and apoptosis (BCL2). Our results indicate that neoplastic lymphocytes were predominantly CD20+/PAX5+, suggesting a B-cell phenotype. Neoplastic lymphocytes were moderately to strongly STAT5+, MLH1+, MSH6+, MSH2+, PCNA+, and FOXP3+, as well as weakly or moderately BCL2+, in most cases (n = 14/15). In both lymphoma and rhinitis cases, inflammatory lymphocytes were predominantly CD3+, while expression of DNA repair markers MLH1, MSH2, and MSH6 was occasional, varying between weak and moderate. Our results suggest that MLH1, MSH2, MSH6, PCNA, and FOXP3 show potential as diagnostic markers in differentiating lymphoma from lymphoplasmacytic rhinitis, although a diagnostic panel with MLH1, MSH6, and PCNA may suffice for diagnosing most cases where lymphoma is suspected. These findings aid in distinguishing lymphoma from inflammation and identifying potential therapeutic targets. They also suggest a regulatory B-cell phenotype in nasal lymphoma, which may aid in immune evasion.