*Purpose: Two types of Anti-Thymocyte Globulins (ATG) are commonly used in renal transplantation (Thymoglobuline and Grafalon) and induce severe, dose-dependent and sometimes persistent CD4⁺ T cell lymphopenia which is associated with poor patient outcome. One of the current challenges is to safely reduce the dose of ATG to improve its immunomodulatory effect and control its deleterious side effects.

*Methods: Our aim was to develop an allogeneic coculture model of healthy donor peripheral blood mononuclear cells (PBMC) and interferon-activated human microvascular endothelial cells (aHMEC-1), close to the immunological context during renal transplantation or acute rejection, in order to explore the immunomodulatory effect of reduced concentrations of ATG (from 200 to 100 µg/ml). We used rabbit IgG or anti-CD25 monoclonal antibody as controls. we explored the proportion of regulatory T cell (Treg), the phenotype of endothelial cells (using flow cytometry) and the cytokine microenvironment particularly IL-6, IL-10 and IL-2 concentrations in supernatants (using enzyme-linked immunosorbent assay).

*Results: The first step was to study the effect of ATG on each cellular type with two separate culture models in vitro. At low concentrations (50 to 100µg/ml), ATG induced an increase in CD3⁺CD4⁺CD25^highFoxP3⁺CD127^low Treg proportion without significant CD4⁺ Tcell apoptosis as compared with controls. IL-6 and IL-10 concentrations increased in culture supernatants cultured with Grafalon. We did not find any modification of endothelial cell phenotype or in cytokine concentrations in supernatants among ATG-treated or control aHMEC-1. In a second step, aHMEC were cocultured with PBMC. In this model, only Grafalon-treated cells showed a significant increase in Treg proportion. This increase in Treg was associated with an increased expression by aHMEC-1 of HLA-DR, HLA class I, PDL1 and CD54 and an increase of IL-2, IL-6 and IL-10 concentrations in supernatants. These phenotypîc and functional modifications were not observed with Thymoglobuline or controls.

*Conclusions: In our allogeneic coculture model, only Grafalon increases the aHMEC-1 expression of PDL1, HLA-DR and CD54, and also modulates the release of soluble factors wich may enhance expansion of Treg. Hence Grafalon could play a key role in the cell-cell allointeraction mechanisms. These preliminary results could provide new insights and perspective for the clinical use of low-dose ATG in low immunological risk renal tranplant patient.

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