Abstract
BackgroundLymphocytosis is relatively common in cats, but few studies describe lymphocyte populations or the clinical course associated with different immunophenotypic expansions.Hypothesis/ObjectivesWe hypothesized that cats frequently develop non‐neoplastic lymphocytosis and that different neoplastic immunophenotypes have variable prognoses. We aimed to characterize the lymphocyte expansions in a large population of cats with lymphocytosis and to assess clinical presentation and outcome in a subset.AnimalsThree cohorts of cats older than 1 year with lymphocytosis (>6000/μL) were examined to define immunophenotypic categories (n = 146), evaluate outcome (n = 94), and determine prevalence of immunophenotypes (n = 350).MethodsRetrospective study of cats with blood submitted for flow cytometry. Medical records (n = 94) were reviewed for clinical data, treatment, and survival information.ResultsFive major immunophenotypic categories were identified: B cell, heterogeneous (≥2 lineages expanded), CD4+ T cell, CD4−CD8− (double negative [DN]) T cell, and CD5‐low‐expressing T cell. B‐cell and heterogeneous phenotypes were more consistent with a non‐neoplastic process, having polyclonal antigen receptor gene rearrangements, younger age at presentation, lower lymphocyte counts, and prolonged survival. The neoplastic phenotypes, CD4+ T cell, DN T cell, and CD5 low T cell, had different median survival times (752 days [n = 37], 271 days [n = 7], 27.5 days [n = 12], respectively). Among CD4+ T‐cell cases, cats with abdominal lymphadenopathy, intestinal involvement, or both and females had shorter survival. Among 350 cats with lymphocytosis, CD4+ T‐cell lymphocytosis was most common, followed by heterogeneous and B‐cell phenotypes.Conclusions and Clinical ImportanceNeoplastic CD4+ T‐cell lymphocytosis is common in cats and has a prolonged clinical course compared to aberrant T‐cell phenotypes. Cats with heterogeneous and B‐cell lymphocyte expansions commonly have non‐neoplastic disease.
Highlights
Lymphocytosis is relatively common in cats, but few studies have examined its flow cytometric features
Absolute lymphocytosis has been documented with a number of infections, including Cytauxzoon felis, Toxoplasma gondii, Aelurostrongylus abstrusus, Leishmania infantum, Mycoplasma haemofelis, Bartonella henselae, feline leukemia virus (FeLV), and feline immunodeficiency virus.[1,2,3,4,5,6,7,8,9]
Peripheral lymphocytosis was reported in several studies of primary immune-mediated hemolytic anemia (IMHA), and bone marrow lymphoid hyperplasia has been described in IMHA and pure red cell aplasia cases.[10,11,12,13,14]
Summary
Lymphocytosis is relatively common in cats, but few studies have examined its flow cytometric features. Clinical findings and outcome have been reported for 18 cats with clinically defined chronic lymphocytic leukemia (CLL).[20] Chronic lymphocytic leukemia was defined as >9000 small, mature lymphocytes/μL with flow cytometry or polymerase chain reaction (PCR) for antigen receptor rearrangements (PARR) confirmation or both and evidence of a peripheral cytopenia or >15% lymphocytes in the bone marrow or both. Almost all these cats had an expanded T-cell population (17/18), and most (16/17) were of CD4+ T-cell origin.[20] The clinical course appeared relatively indolent with most cats treated with prednisone and chlorambucil. Clinical presentation, and outcome on a subset of cases
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