# Tetrahydrobiopterin enhances regulatory T- and mast cell proliferation and alters cytokines expression in a murine heart transplant model

## TL;DR

This study shows that tetrahydrobiopterin (BH4) improves heart transplant survival by boosting regulatory T cells and mast cells and increasing anti-inflammatory cytokines.

## Contribution

The study reveals BH4's novel immunomodulatory effects on regulatory T cells and mast cells in a heart transplant model.

## Key findings

- BH4 treatment increased regulatory T cells and mast cells in secondary lymphoid organs.
- BH4 elevated anti-inflammatory cytokines like IL-10, IL-5, and IL-4 in vivo and in vitro.
- BH4's effects were not mediated by mast cell-derived tryptophan hydroxylase-1.

## Abstract

Administration of tetrahydrobiopterin (BH4) has been shown to attenuate acute allograft rejection in a murine heart transplantation model in a manner similar to that of cyclosporine A. However, its mechanism of action on immune cells remains largely unknown. A fully MHC-mismatched (C3H/He to C57BL/6) mouse heart transplant model was used in this study. The recipients were treated with BH4 or cyclosporine A six days. The degree of acute rejection was assessed by histopathological analysis, splenocytes were analyzed by flow cytometry, and cytokine production was estimated based on the level of protein and RNA in sera and grafts and in vitro in T cell cultures. Proliferation of regulatory T cells and mast cells, suppressor capacity of Tregs, and MLR of T cells were conducted in vitro. Survival curves confirmed the significant improvement observed in the BH4-treated animals. BH4-treatment resulted in a substantial increase in Tregs and mast cells in the secondary lymphoid organs. In vitro assays showed increased proliferation of BH4-treated Tregs and mast cells. Cytokine production in vivo and in vitro in BH4-treated animals revealed an increase in the expression of IL-10, IL-5 and IL-4. BH4-dependent mast cell-derived tryptophan hydroxylase-1 could be excluded as a treatment target in recipient knockout mice. These data suggest that BH4 modulates the innate and adaptive immune systems, resulting in increased proliferation of regulatory T and mast cells accompanied by a modulation of anti-inflammatory cytokines.

The online version contains supplementary material available at 10.1038/s41598-025-20127-1.