Feline Hodgkin-like lymphoma: A morphological, immunohistochemical, and molecular study.

Fourteen feline nodal lymphomas previously diagnosed as Hodgkin-like lymphoma (HLL) were studied through histologic, immunohistochemical, and molecular analyses to further characterize feline HLL. The cohort comprised 12 domestic shorthair and 2 Maine coon cats, with a male/female ratio of 1.3 and a median age of 9.5 years. Reed-Sternberg and Hodgkin cells were observed in 14/14 HLLs, while a minority of cells resembling lymphocyte-predominant cells were observed in 5/14 (36%) cases. Neoplastic cells were embedded in a mixed reactive background mainly composed of T and B lymphocytes and histiocytes. Necrosis was present in 9/14 (64%) cases. Various percentages of neoplastic cells were positive for CD30, PAX5, and MUM1 in 14/14 cases and for CD20 in 12/14 (86%) cases. Polymerase chain reaction (PCR) for antigen receptor rearrangements (PARR) by agarose gel electrophoresis identified clonal immunoglobulin heavy chain (IGH) in 9/14 (64%) cases, clonal T-cell receptor-gamma (TRG) rearrangements in 4/14 (29%) cases, and polyclonal IGH and TRG in 1 case. The predominance of Reed-Sternberg and Hodgkin cells and a CD30+/PAX5+/MUM1+ immunophenotype were consistent findings in this case series and align with the human classic form of Hodgkin lymphoma (HL). However, in contrast to the human tumor, most feline HLLs express CD20. Based on the human HL classification, feline HLLs were further categorized as lymphocyte-rich (6/14, 43%), mixed cellularity (4/14, 29%), nodular sclerosis (2/14, 14%), and lymphocyte-depleted (2/14, 14%). Feline HLL is a complex neoplasm that requires refinement of diagnostic criteria to improve classification and management.