# DARC and Anti-Duffy Antibodies in the Line of Fire: The Challenges in Pinpointing the Etiology of Microcirculation Inflammation to a Distinct Entity

## TL;DR

This paper explores how anti-Duffy antibodies may contribute to microvascular inflammation in kidney transplants, offering in vivo evidence of their role.

## Contribution

The study provides in vivo evidence linking anti-Duffy antibodies to microvascular inflammation in kidney transplants.

## Key findings

- DARC and C4d staining overlap strictly in peritubular capillaries.
- MVI was confined to PTC with no glomerulitis or anti-HLA DSA.
- Non-HLA and anti-HLA reactivities complicate MVI assessment.

## Abstract

Antibody-mediated rejection (ABMR) due to non-HLA alloantibodies has gained substantial attention in transplantation research. One candidate for such non-HLA reactivity is the Duffy blood group carrier molecule DARC, which is not only expressed on erythrocytes, but also on kidney microvascular endothelial cells and is postulated as a potential transplantation-relevant alloantigen. However, in vivo observation of anti-Duffy antibodies as trigger of microvascular inflammation (MVI) is lacking. Here we propose a direct relationship between preformed anti-Duffy (anti-Fya) antibodies, complement deposition (C4d) in peritubular capillaries (PTC), and MVI. Double immunofluorescence for DARC and C4d in sequential biopsies revealed a striking overlap of DARC expression and C4d staining that was completely restricted to the peritubular capillaries. Remarkably, MVI was confined to PTC with complete absence of glomerulitis and lack of preformed anti-HLA DSA. Retrospective analysis revealed a self-limiting posttransplant flare of a low-level anti-DQ8 DSA after blood transfusions and a high missing-self KIR ligand constellation. Concomitant occurrence of non-HLA and anti-HLA reactivities next to missing-self constellations substantially complicates the assessment of individual contributions for the development and propagation of MVI. Due to the strictly confined distribution of DARC to PTC our report provides in vivo evidence that anti-Fya alloantibodies may associate with MVI.

Illustration detailing the role of DARC and anti-Duffy antibodies in microvascular inflammation related to kidney transplants. It includes a diagram of DSA-negative peritubular capillaritis, patient pre-treatment status, and a kidney biopsy procedure. A section shows co-localization of C4d and DARC using immunohistochemistry, with evidence supporting the contribution of preformed anti-Duffy antibodies in microvascular injury (MVI).