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Research Article: Body condition, age, and sex as modulators of peripheral blood immune-cell populations and lymphocyte proliferation in cats

Date Published: 2026-09-09

Abstract:
Feline obesity is common, and in other species it has been linked to immune dysregulation; yet its immunological consequences in cats, and their modulation by age and sex, remain poorly defined. Whole blood from 78 neutered client-owned cats (37 males, 41 females; 1–16 years) was analyzed by flow cytometry using a panel characterizing T cells (including CD5 + CD4 + and CD5 + CD8 + subsets), B cells (CD21 + ) and natural killer (CD56 + ) cells with their activation (CD80) and proliferation (Ki67) status. Cats were grouped by body condition score into normal-weight (4–5) and overweight (6–9); lymphocyte proliferation was assessed in a subset by a carboxyfluorescein succinimidyl ester assay after concanavalin A stimulation, and serum amyloid A, triglycerides and adiponectin were measured in overweight cats. Across the cohort, overweight cats displayed higher frequencies of proliferating CD21 + Ki67 + B cells than normal-weight cats, while the proliferation assay was descriptively consistent with enhanced T-cell proliferation in both sexes. Increasing age was associated with a decline in CD5 + CD4 + T cells in both sexes and, in females, additionally in CD5 + CD21 + cells alongside rising CD80 expression. Sex differences shifted across life, from B-cell activation in young adults to T-cell subsets in adults (higher CD5 + CD8 + in males, higher Ki67 + fractions in females) and CD56 + NK cells in the oldest cats. Body-condition effects were largely age- and sex-specific and emerged only after stratification: adult overweight males showed higher proliferating CD4 + Ki67 + and CD8 + Ki67 + T cells, young-adult overweight females higher CD5 + and activated CD5 + CD21 + CD80 + cells, and aged overweight females fewer proliferating CD5 + CD21 + Ki67 + B cells. Among overweight cats, higher adiponectin was associated with fewer CD5 + lymphocytes, with no associations for serum amyloid A or triglycerides. Overall, feline obesity was associated with subtle changes in immune-cell composition but more pronounced effects on lymphocyte proliferative responsiveness, both strongly modulated by age and sex.

Introduction:
Overweight and its associated comorbidities represent major challenges in both human and veterinary medicine ( 1 , 2 ). Cats, the most commonly kept companion animals in Europe ( 3 ), are no exception. Epidemiological studies report the prevalence of feline overweight and obesity ranging from 11.5 to 63% across different countries and populations ( 4 – 7 ). Feline obesity is clinically relevant, as it is associated with a range of disorders—including diabetes mellitus, lower urinary tract disease, musculoskeletal…

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