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

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.