Characterization of Morphologic Changes and Lymphocyte Subset Distribution in Lymph Nodes from Cats with Naturally Acquired Feline Immunodeficiency Virus Infection

Abstract

Lymph nodes were collected at biopsy or necropsy from 18 cats with naturally acquired symptomatic feline immunodeficiency virus (FIV) infection and from 18 seronegative cats. Thirty-five of the cats were domestic shorthairs and one was a Persian cross. The cats ranged from 7 months to 16 years of age and were mainly obtained from California veterinary practitioners, a California cattery, and a Veterinary Teaching Hospital. Based on clinical signs present at tissue collection, ten FIV-infected cats fell into the acquired immunodeficiency syndrome (AIDS)-related complex (ARC) clinical stage and eight in the terminal (AIDS) stage of FIV disease. All cats were FeLV negative by antigen ELISA. Histologic sections of lymph nodes from each cat were examined blindly and were categorized as hyperplastic, involuting, mixed hyperplastic and involuting, depleted, or normal based upon subjective evaluation of follicles and paracortex. The relative abundance of plasma cells was evaluated in methyl green pyronin (MGP) and hematoxylin and eosin-stained sections. Similar numbers of FIV-seropositive and -seronegative cats fell into each lymph node category. The only difference evident between FIV-infected cats and control cats was in the degree of plasmacytosis present; moderate to marked plasmacytosis was present in 13/18 FIV-infected cats but in only 3/18 control cats. Immunohistochemical staining of frozen lymph node sections from six FIV-infected cats and 12 control cats for CD4, CD8, Factor VIII-related antigen, and a B cell/follicular dendritic cell-specific (CD21-like) antigen revealed only a slight increase in the relative proportion of paracortical CD8+ lymphocytes versus CD4+ lymphocytes in lymph nodes from FIV-infected cats. These findings suggest that lymph node changes previously considered to be specific or highly suggestive of lentivirus-induced immunodeficiency disease are actually not specific but occur in FIV-seronegative cats as well. Despite this lack of specificity, lymph node changes seen in FIV-infected cats may have prognostic significance. All cats in the ARC stage had hyperplastic or mixed hyperplastic and involuting nodes, whereas those in the AIDS stage had either involuting or depleted nodes, which suggests that the clinical stage of disease correlates with the category of lymph node change.