Laminar leukocyte accumulation in horses with carbohydrate overload-induced laminitis.

BACKGROUND: While there is evidence of laminar leukocyte infiltration in black walnut extract (BWE)-induced laminitis, there is no such evidence for carbohydrate overload (CHO) laminitis. OBJECTIVE: To assess presence of leukocytes and signs of epidermal stress/injury in the laminar tissue from horses with CHO-induced laminitis. ANIMALS: Twenty-four adult horses. METHODS: Immunohistochemistry for myeloid cell markers calprotectin (CP) and monocyte-specific marker (CD163) was performed on laminar sections obtained from 2 groups of horses in the CHO model: the developmental time point (DTP) group (n = 6) and the onset of lameness (LAM) group (n = 6), and a control (CON) group (n = 8). RESULTS: DTP was characterized by an increase in CP(+) leukocytes (7.8-fold increase versus CON, P < .001), and LAM time point was characterized by a more marked increase in laminar CP(+) (108.5-fold, P < .001) and mild increase in CD163(+) (1.9-fold, P = .007) cell counts. Increased CP epidermal signal (indicating epidermal stress or injury) occurred consistently at the LAM time point, although histological evidence of basement membrane (BM) detachment was minor, only being present in 3/6 horses. CONCLUSIONS AND CLINICAL RELEVANCE: Maximal laminar leukocyte infiltration and epithelial stress occurred at the onset of lameness in the CHO model showing a different temporal pattern from the BWE model, where maximal leukocyte infiltration clearly precedes epithelial stress. Leukocyte infiltration before major histological changes in the CHO model indicates that leukocyte infiltration can be a cause of and not a reaction to BM degradation and structural failure.