Lymphocyte Sub-population in Rectal Biopsies of Horses with Recurrent Colics and Malabsorption (Unhappy Hindgut Syndrome): A Preliminary Study on Morphology and Immune Microenvironment After Recovery

Introduction: Recurrent colic in horses due to pathological changes in the large colon is characterized by mucosal inflammation as a consequence of malabsorption. Rectal biopsies are known useful aids in diagnosis of equine gastrointestinal disorders. Aim of the study is to describe the variation in morphology and immunophenotype of cells infiltrating the epithelium in equine rectal biopsies, as a potential representative of major pathological changes in chronic inflammatory disorders of colon and small intestines. Materials and methods: Seventeen horses (five controls (C), nine with recurrent colic (RC), 3 with non-recurrent colic (NRC)) were included in the study. Sections (5 μm) from formalin fixed, paraffin embedded rectal biopsy samples were stained with haematoxylin-eosin, Masson trichrome and PAS for histopathological evaluation. Immunohistochemistry for T cells (CD3+), B cells (CD20+) and regulatory T cell sub-populations (FOXP3+) was applied. Results: RC and NRC groups showed an increased cellularity of the lamina propria with mild increase of connective tissue and atrophy of goblet cells compared with group C. CD3+ cell numbers were decreased in RC but higher in NRC compared with C. CD20+ cells were more numerous than CD3+ cells in RC and NRC groups. FOXP3 expression did not vary. Conclusions: This preliminary study confirms that examination of a rectal biopsy, despite its limitations, may be of value for equine clinical gastroenterology. Pathological changes in the rectal mucosa, in particular in T and B cell populations, may indicate a chronic residual inflammatory process and be helpful for classification into different diagnostic categories.