Introduction: The gut microbiota has several immunoregulatory mechanisms that induce host's immune tolerance against the microbiome throughout life. These organisms use fibers from the diet as nutrients for their metabolic fermentation, generating different types of short-chain fatty acids (SCFAs) as metabolic by-products. Among them, butyrate has been shown to induce colonic Tregs in the gut, and its oral administration also reduces the pathogenesis of different gut inflammatory conditions. In its “native” form, butyrate has low systemic bioavailability, limiting its effects to the gut microenvironment. However, in the form of Tributyrin (Trib), butyrate intestinal absorption increases, boosting its bioavailability and systemic effects. Therefore, we aimed to analyze whether the oral administration of Trib can induce immune modulation and improve graft survival in a minor-mismatched skin transplant model.
Methods: C57BL/6J (B6) mice were daily treated by gavage with Trib or vehicle (control) for 14 days, starting on day -7. On day 0, mice were either immunized with NP-OVA (100 mcg/mouse) and CFA, 7 days after this immunization the draining lymph nodes (dLNs) of these animals were collected and analyzed by flow cytometry (Fig. 1A). To evaluate the effects Trib oral treatment on the survival of minor-mismatched skin grafts, skin grafts from B6 male mice were transplanted onto B6 female recipients. The female recipients were daily treated by gavage with Trib or PBS from day -7 to +7 post-transplant, followed by a 3x/week treatment onwards. Graft survival rates were observed daily (Fig 1E).
Results: Compared to control mice, animals treated with Trib and immunized with NP-OVA showed a reduction in the expression of IFN-γ and granzyme B in total CD8+ T cells as well as a reduction in the population of antigen-specific CD8+ T cells (SIINFEKL tetramer+, Fig 1B-C). These animals also showed an increase in total and OVA-specific Tregs (Fig 1D), with an increase in the expression of Treg activation markers (Ki67, LAG-3 and TIM-3). Furthermore, the oral treatment with Trib promoted an increase in the survival of minor-mismatched skin grafts when compared to controls (Fig 1F).
Conclusions: Our results demonstrate that orally administrated Trib can promote systemic immunomodulation by reducing antigen-specific CD8+ T cell responses and increase the frequency and activation of Tregs. The treatment with Trib also promoted an increase in skin graft survival, indicating that Trib may indeed serve as a new therapeutic agent to tame rejection.