When cardiac transplantation was in its infancy, approximately 50 % of patients developed a hemodynamically significant episode of rejection during the first year after transplantation. Currently, at our institution, hemodynamically significant ACR occurs in less than 5 % of patients.

As with other solid organs, ACR in the transplanted heart is mediated by host cytotoxic and helper T-cells targeting graft antigens. One would therefore expect that T-lymphocytes would be the most numerous cells observed in the biopsy during an episode of rejection. In fact, our studies have shown that actually macrophages predominate. Usually, more than 50 % of the infiltrating cells are macrophages [2]. This finding turns out to be useful if IHC studies are performed to assist in the diagnosis of rejection versus QL. In ACR, most infiltrating cells will be macrophages with fewer T-lymphocytes and rare B-lymphocytes. In QL, most infiltrating cells will be B- or T-lymphocytes (depending on whether the top [B-cells] or bottom [T-cells] of the QL is biopsied, with only a relatively small proportion of macrophages. My personal practice is to order CD3, CD20, and CD68 on all biopsies suspected to have more than just mild rejection (grade 2R or 3R). C4d and CD31 (or 34) are also part of the package, as these immunostains are important in the evaluation of AMR to be discussed in a separate section.

In ACR, the first change noted is a perivascular infiltrate of mononuclear cells. These cells then spread out away from small blood vessels to involve interstitial tissue. As the ACR becomes more severe, myocyte injury becomes evident. Myocyte injury takes the form of coagulation necrosis only in very severe rejection. The more typical finding is that of “myocytolysis.” Affected myocytes will lose their sarcoplasmic organelles and the sarcoplasm appears empty. Nuclei enlarge and nucleoli become prominent. Atrophy of myocytes is also observed. These injured myocytes are not necessarily irreversibly injured and they may recover. Interestingly, after injury, the affected fibers demonstrate a fetal phenotype. By IHC, they express vimentin and smooth muscle actin, proteins that are not expressed in normal adult cardiac myocytes.

Probably all transplanted hearts at some time demonstrate some rejection of some degree. The pathologist must not only identify ACR but also grade the findings. The pioneering work in this regard was done by Dr. Margaret Billingham and colleagues [3] at Stanford, who devised the first grading system. There have been several grading systems since, but remarkably, the current system in use to diagnose ACR is very similar to Dr. Billingham’s first system.

Table 2.1 shows the criteria of grading for the current and most recent previous grading systems [4, 5]. The template we use at UCLA for reporting biopsy findings shows the criteria for each grade in both systems (See Appendix). We report both primarily for research purposes. The grade of rejection is based on the pattern of infiltrating cells and whether or not myocyte injury is present. This sounds easy, but artifacts in the tissue and lesions other than rejection can confound the interpretation.

AMR in cardiac transplantation was unrecognized for many years, and underrecognized for many more. Dr. Elizabeth Hammond and colleagues [7] at the University of Utah deserves credit for describing the clinical and pathological findings of AMR and emphasizing the importance of AMR. Before this work, the very existence of AMR had been questioned by prominent cardiac transplant physicians, including pathologists.

In AMR, graft injury results from antibodies, usually directed against antigens on endothelial cells that may initiate complement activation. With the decreasing incidence of ACR, AMR is now more common, affecting 10–20 % of patients, depending on the transplantation population. AMR is more common in “sensitized” patients, such as multiparous females, patients with prior blood transfusions, and those who have had implanted ventricular-assist devices. Typically, AMR is seen soon after transplantation in sensitized individuals, but can occur months or years later, especially in patients with no prior sensitization. AMR can cause hemodynamic dysfunction, accelerated allograft vasculopathy, graft loss, and death.

The histological hallmark of AMR is dilation of capillaries by intraluminal cells that has been described as “endothelial swelling.” Electron microscopic and IHC studies have shown that the majority of these cells are actually macrophages and not endothelial cells [8]. In addition to clarifying the nature of the process, this recognition is of practical importance; demonstrating intravascular macrophages by IHC staining is very helpful in making the diagnosis of AMR. CD31 or CD34 staining highlights capillary dilation and confirms the intravascular location of these cells, as opposed to the predominantly perivascular location of infiltrating cells in ACR.

C4d and/or C3d staining on capillaries that demonstrates complement deposition, can further confirm the diagnosis. Because the histological findings of AMR can be very subtle, sometimes the only evidence of AMR is by IF or IHC studies; therefore, it is recommended that these studies be done routinely early after transplantation or if the patient has unexplained cardiac dysfunction (Tables 2.2 and 2.3) [9].