Ex Vivo Immune Function and Modulatory Effects of Calcitriol in Dogs with Naturally Occurring Diabetes Mellitus

Human patients with type 1 diabetes mellitus (T1DM) are susceptible to several long-term complications that are related to glycemic control and immune dysregulation. Immune function remains relatively unexplored in dogs with naturally occurring diabetes mellitus (NODM). Calcitriol improves various aspects of immune function in a variety of species, but its effect in diabetic dogs remains unexplored. Therefore, the objectives of this study were to (i) evaluate immune function in dogs with NODM and determine if differences exist based on the level of clinical control and (ii) assess the immunomodulatory effects of calcitriol. Twenty diabetic dogs (clinically controlled, n = ten, not controlled, n = ten) and 20 non-diabetic, healthy control dogs were included in this prospective, case–control study. Whole blood was incubated with calcitriol (10⁻⁷ M) or negative control, after which the samples were divided for phagocytosis and leukocyte cytokine response experiments. The phagocytosis of opsonized <i>Escherichia coli</i> (<i>E. coli</i>) was evaluated with flow cytometry. The samples for leukocyte cytokine response evaluations were stimulated with lipopolysaccharide (LPS), lipoteichoic acid (LTA), or phosphate buffer solution (PBS; negative control), and tumor necrosis factor (TNF)-α, interleukin (IL)-6, IL-8, and IL-10 were measured in supernatant using a canine-specific multiplex bead-based assay. The leukocytes from diabetic dogs produced higher concentrations of IL-10 (<i>p</i> = 0.01), IL-6 (<i>p</i> < 0.0001), and IL-8 (<i>p</i> < 0.0001) than the control dogs while controlling for the intervention and stimulant. Calcitriol decreased the supernatant concentrations of TNF-α (<i>p</i> < 0.001) and IL-8 (<i>p</i> = 0.04) with concomitant increases in IL-6 (<i>p</i> = 0.005). Diabetic dogs had a lower percentage of leukocytes undergoing phagocytosis (<i>p</i> < 0.0001) but a higher number of bacteria phagocytized per cell (<i>p</i> = 0.001) when compared to the control dogs. Calcitriol had no effect on phagocytic capacity. Lastly, the status of clinical control in diabetic dogs did not yield differences in immune function. These results support that dogs with NODM exhibit immune dysregulation and warrant additional investigation.