Microvascular Injury in Very Late Mixed Rejection: A Not So Easy Marker to Be Identified


To the Editor:

We read with great interest the paper by A. Loupy et al. describing the contribution of very late heart transplant rejection (VLR) to the progression of cardiac allograft vasculopathy (CAV) in heart transplantation (HT; Ref. 1). We consider this very important work addressing the hot topic of mixed rejection (MR) in the setting of the recent evolving field of antibody-mediated rejection (AMR) in HT. We recognize the real challenge in correctly attributing long-term changes in heart allografts to cellular and/or antibody-mediated mechanisms. However, the results raise some issues that warrant further discussion.

We know that intravascular macrophages are recognized as one of the key criteria for AMR (2,3). The authors introduce the term microvascular injury (MI) to identify intravascular macrophage accumulation (no further quantification presented) and occasionally endothelial swelling. According to their results, intravascular macrophage accumulation/MI correlated better with the presence of DSA than complement activation detected as C4d/C3d complement fragment deposition on EMBs.

In 65% of their cases of VLR, signs of AMR (MI+ complement activation, 10 cases; complement deposition without MI, 1 case and MI without complement deposition, 2 cases) were detected, with these cases representing the true MR entity. In acute cellular rejection (ACR) grade ≥2R EMBs, the inclusion criteria for this study, we question the possibility to trace intravascular macrophages as marker for AMR in the absence of tissue complement deposition. By definition in ACR, lymphocyte and macrophage infiltrates are present damaging the myocardium (delayed-type hypersensitivity, as an immunological mechanism where autoreactive T cells, with cytolytic T lymphocytes directly killing target cells with class-I MHC associated antigens). Macrophages are recruited as a result of interferon-γ activation.

In our own recent experience from the study of more than 1500 EMBs on which we performed routine C4d on paraffin-embedded tissue, we identified 61 pts that were C4d positive with ACR ≤ 1R. C4d+ EMBs morphological evaluation on hematoxylin–eosin for AMR histologic features, namely intravascular macrophage accumulation and endothelial swelling, detected the presence of intravascular macrophages only in 40% of them. This percentage did not change temporally in the early or late groups at follow-up.

Even in the EMBs of patients with DSA, intravascular macrophages were confined to 30%. Moreover, intravascular macrophages were present in about 40% of DSA negative pts and in 32% of C4d negative EMBs (4). That is why we are puzzled by the statement of the authors that complement positivity may be a too stringent criterion in the diagnosis of AMR whereas intravascular macrophages and DSA may be a more robust prognostic indicators of AMR graft damage. The survival curves did not show any difference between VLR positive and control groups, whereas a dramatic progression to CAV occurred in VLR group with a statistically significant difference for CAV scores. However, there was no difference in CAV grades between VLR/MI positive and VLR/MI negative patients. Accordingly, can these results be interpreted as evidence that MI, as defined in the paper, does not play a major role in CAV progression?


The authors of this manuscript have no conflicts of interest to disclose as described by the American Journal of Transplantation.