The extent of HLA‐DR expression on HLA‐DR+ Tregs allows the identification of patients with clinically relevant borderline rejection

Regulatory T cells (Tregs) were shown to be involved into the pathogenesis of acute rejection after transplantation. The suppressive activity of the total regulatory T cell pool depends on its percentage of highly suppressive HLA‐DR⁺‐Treg cells. Therefore, both the suppressive activity of the total Treg pool and the extent of HLA‐DR expression of HLA‐DR⁺‐Tregs (MFI HLA‐DR) were estimated in non transplanted volunteers, patients with end‐stage renal failure (ESRF), healthy renal transplant patients with suspicion on rejection, due to sole histological Bord‐R or sole acute renal failure (ARF), and patients with clinically relevant borderline rejection (Bord‐R and ARF). Compared to patients with only Bord‐R or only ARF, the suppressive activity of the total Treg cell pool was exclusively reduced in patients with clinically relevant Bord‐R. In parallel, the HLA‐DR MFI of the DR⁺‐Treg subset was significantly decreased in these patients, due to a significantly lower proportion of DR^high+‐Tregs, which were shown to have the highest suppressive capacity within the total Treg pool. Our findings clearly demonstrate that the determination of the HLA‐DR MFI of the HLA‐DR⁺‐Treg subset allows a highly sensitive, specific and non‐invasive discrimination between patients with clinically relevant Bord‐R (Bord and ARF) and patients with subclinical rejection or other causes of transplant failure.