Acute cellular rejection affects the interstitium, tubules, endothelium, and glomeruli, separately or in combination. The characteristic microscopic features of tubulointerstitial rejection (Banff type I) are pleomorphic interstitial infiltrates of activated lymphocytes and monocytes, accompanied by interstitial edema and tubular injury. The cells invade tubules (“tubulitis”) and infiltrate the cortex (10–25 % in the Banff system is considered suspicious for rejection and >25 % is rejection) (Fig. 20.1). The infiltrating mononuclear cells typically include lymphoblasts, with cytoplasmic basophilia, nucleoli, and occasional mitotic figures, indicative of increased synthetic and proliferative activity. The infiltrate consists of T cells (CD4 and CD8), macrophages, and sometimes B cells. Foxp3+ cells (presumptive T regulatory cells) also accumulate in ACR [11] and may be correlated with a better prognosis in protocol biopsies [12]. TGF-β mRNA is associated with better outcome, perhaps related to promotion of Treg cells [13].

Infiltration of mononuclear cells under arterial and arteriolar endothelium is the pathognomic lesion of Banff type II ACR (“endarteritis” or “endothelialitis”) (Fig. 20.2). When lymphocytes are only on the surface of the endothelium, their significance is less certain. Lymphocytes also commonly surround vessels, a nonspecific feature, unless the cells invade the media. Endarteritis has been reported in 20–50 % of the renal biopsies with acute cellular rejection [14]. Some do not find the lesion as often, which may possibly be ascribed to inadequate sampling, overdiagnosis of rejection, or timing of the biopsy with respect to antirejection therapy. These lesions are more frequent in the larger arteries [14]. In our experience, about 20 % of the arteries are involved so that levels of several arteries in the sample are essential for its detection [14]. Rarely, the lymphocytes invade through the media, so-called transmural endarteritis (Fig. 20.3).

In most cases of acute cellular rejection, the glomeruli are spared or show minor changes, typically a few scattered mononuclear cells (T cells and monocytes) and minimal segmental endothelial damage, termed “glomerulitis” [15]. Occasionally, a severe, diffuse form of glomerular injury is evident and dominates the histologic pattern (Fig. 20.4). In 1981 Richardson and his colleagues [16] drew attention to a distinctive, acute allograft glomerulopathy, characterized by hypercellularity, injury, and enlargement of endothelial cells, infiltration of glomeruli by mononuclear cells, and webs of periodic acid-Schiff (PAS)-positive material. This severe form of glomerulopathy is observed in ~4 % of biopsies taken for allograft dysfunction, typically 1–4 months after transplantation and is believed to be an unusual variant of cellular rejection, sometimes promoted by cytomegalovirus (CMV) infection [15]. T cells are regularly detected in glomeruli immunohistochemically and OKT3 can reverse the lesion [17]. For unknown reasons, rejection becomes focused on glomerular components; florid glomerulopathy may occur with little interstitial inflammation, although cellular endarteritis is common. Glomerulitis is also commonly seen in antibody-mediated rejection and is a sign of activity. Glomerulitis in AMR is predominately due to macrophages rather than T cells which dominate in ACR [18].

Interstitial mononuclear inflammation and tubulitis occur in a variety of diseases other than acute rejection, such as drug-induced allergic tubulointerstitial nephritis and obstruction, and therefore cannot be considered proof that rejection is present. The presence of systemic infection is another well-known cause of tubulointerstitial nephritis and tubulitis. Tubulitis is often present in atrophic tubules from any cause and does not indicate acute rejection. Posttreatment with anti-T-cell antibodies, the inflammation diminishes in most patients, and this is correlated with successful response to therapy. This type of rejection often responds to steroid pulses and rarely causes graft loss [3, 14, 19, 20].

In all large studies, endarteritis has a worse prognosis than tubulointerstitial rejection [3, 14, 19, 20, 21]. Cases with endarteritis are less responsive to steroid pulses, but do respond to OKT3 or antithymocyte globulin (ATGAM), and have a decreased 1-year survival [14]. Later development of intimal fibrosis is correlated prior type II rejection [19].

Endarteritis must not be confused with necrotizing arteritis, characteristic of humoral rejection (type III). Rarely, transmural cellular inflammation causes frank medial necrosis, but this complication of cellular arteritis can be distinguished from necrotizing arteritis by the heavy mononuclear infiltrate. Regrettably, many still do not distinguish these lesions, regarding all “vascular rejection” as predominately humoral. Endarteritis has a much better rate of reversal than necrotizing arteritis (75 % vs. <25 % 1-year graft survival), which alone justifies their separation [19].

Immunofluorescence microscopy shows little, if any, immunoglobulin deposition, but interstitial fibrin is typically present. Fibrin and scant immunoglobulin and C3 deposits may also be found in glomeruli. C4d in peritubular capillaries when present is indicative of a component of antibody-mediated rejection (see below).

Electron microscopy is generally not used for the diagnosis of acute rejection. Studies have shown endothelial injury in peritubular and glomerular capillaries, invasion of lymphocytes into tubules. No immune deposits are detectable. Cases with acute allograft glomerulopathy show markedly reactive endothelial cells, with mononuclear cells in capillaries and often platelet/fibrin aggregates.

Three forms of acute AMR have been recognized [6, 30]: (1) acute tubular injury with little inflammation, (2) peritubular and glomerular capillary inflammation with neutrophils, and (3) necrosis of arteries. Acute humoral rejection (AHR) can be missed in routine sections.

In one series, a component of AHR would not have been recognized in 25 % of cases without the C4d stain: 15 % showed only ACR and 10 % only acute tubular injury [30]. The 10 % acute tubular injury biopsies comprised of two donor-specific antibodies (DSA⁺) in patients with widespread C4d staining of peritubular capillaries (PTCs) that showed predominantly acute tubular injury on initial biopsy. Later biopsies performed in one of these showed typical AHR with abundant neutrophils in PTCs and glomeruli as well as fibrinoid necrosis. Most cases have neutrophils in PTCs (Fig. 20.5). This feature is useful, but not highly sensitive or specific [30]. The histologic features in the Regele et al. series were similarly misleading: 32 % of those with C4d did not meet the Banff criteria for rejection, accounting for about 75 % of the patients with acute AMR [27]. In some cases arterial and arteriolar thrombosis and glomerular thrombosis and necrosis predominate similarly to thrombotic microangiopathy. These lesions must be distinguished from the hemolytic uremic syndrome of calcineurin inhibitor toxicity, which can also have thrombi in arterioles and glomeruli, but does not affect the peritubular capillaries (or have C4d). A minority of cases have fibrinoid necrosis, in which the arterial media shows myocyte necrosis, fragmentation of elastica, and accumulation of brightly eosinophilic material called “fibrinoid” and little mononuclear infiltrate in the intima or adventitia (Fig. 20.6). A scant infiltrate of neutrophils and eosinophils and thrombosis may be present.

Detection of C4d has proved to be useful in the detection of humoral rejection (Fig. 20.7). After antibody binds to antigen, C4 is proteolytically cleaved by activated C1 into C4a and C4b. The cleavage of C4 exposes the reactive and short-lived thiol group in C4b that binds covalently to nearby molecules containing amino or hydroxyl groups, such as proteins and carbohydrates [31]. Bound C4b is proteolytically inactivated into C4d, a 44.5-kd peptide, which contains the thioester site and remains covalently bound at the same site. Thus, if C4d is bound to a structural protein, it is potentially a durable marker of local complement activation by the classical pathway. C4d can disappear as soon as 8 days [32] and after 60 days is usually negative (unless chronic humoral rejection develops) [30]. Cases with fibrinoid necrosis of arteries typically contain immunoglobulin (Ig, usually IgG and IgM), C3, and fibrin in the arterial walls. The criteria for acute humoral rejection proposed by Mauiyyedi et al. [30] and accepted with minor modification by the Banff conference are given in Table 20.2 [6].