Clonality assessment and feline leukemia virus protein expression in injection-site sarcoma–associated lymphocytic infiltrates

Injections have been linked to feline sarcomas (feline injection-site sarcoma; FISS) and cutaneous lymphomas (cutaneous lymphoma at injection site; CLIS). Both tumors often exhibit lymphoplasmacytic inflammation ascribed to injected immunogenic material. CLIS is hypothesized to emerge from transformation and clonal expansion of lymphoid cells following persistent immune stimulation with feline leukemia virus (FeLV) reactivation and transformation. To further study whether the lymphocytic infiltrates associated with FISS can represent a suitable niche for the development of CLIS, 34 cases of FISS were examined. Lymphoid cell phenotypes were assessed using CD3 and CD79 immunohistochemistry. For cases with prominent inflammation, FeLV p27 and gp70 immunohistochemistry and PCR for antigen receptor rearrangements were performed. Male domestic shorthair cats predominated. The mean age was 12.2 years (range: 5-17 years). FISS developed in thoracic (8/34, 24%), flank (7/34, 21%), and interscapular (5/34, 15%) regions. Similar proportions of B and T lymphocytes were found in 11/34 (32%) cases; T-cells predominated in 12/34 (35%) cases, and B-cells predominated in 11/34 (32%). At least one FeLV antigen was expressed in lymphoid infiltrates in 10/18 cases (55%), and in neoplastic fibroblasts in 8/18 cases (44%), while both FeLV proteins were expressed in neoplastic cells in 3/18 cases (17%). One cat had clonal T-cell receptor-gamma and was diagnosed with concurrent FISS and CLIS. This case lacked FeLV expression. FeLV amplification from formalin-fixed paraffin-embedded material was unsuccessful. The expression of FeLV p27 and/or gp70 in neoplastic spindle cells and lymphoid infiltrates raises the possibility of FeLV involvement in the tumorigenesis of FISS and CLISs.