Small Bowel Transplant Pathology

Grade of ACR

Apoptotic bodies per 10 consecutive crypts (n)

Epithelial injury

Lamina propria inflammation

No ACR

≤2

None

Normal components

Indeterminate

3–5

Minimal, usually focal

Minimal, usually localized

Mild

≥6

Mild, usually focal

Mild, usually localized

Moderate^a

≥6 and confluent^b

Focal crypt dropout, focal mucosal erosion

Moderate, usually extensive

Severe^a

Variable in residual crypts

Extensive crypt dropout, extensive erosion and/or ulceration

Moderate to severe, extensive

^aA varying degree of arteritis may be present in the setting of moderate and severe ACR, but this feature is not evident in superficial mucosal biopsies

^bConfluent apoptosis involves all or the vast majority of the epithelial cells forming the crypt

Table 6.2

Indeterminate for ACR (Fig. 6.1)

Histological features

Increased crypt apoptotic activity: three to five apoptotic bodies per ten consecutive crypts

May show minimal epithelial injury, usually focal, characterized by reactive changes such as loss of mucin, nuclear enlargement and hyperchromasia, cytoplasmic basophilia, mild villous blunting, and others

Intact surface epithelium

May show a minimal increase in the number of lamina propria inflammatory cells, consisting predominantly of mononuclear cells. Activated lymphocytes may be present

Differential diagnosis

Mild ACR, which may show similar histological findings but with at least six apoptotic bodies per ten consecutive crypts

Infectious enteritis, which may show increased apoptotic activity but may also show more neutrophils and more severe epithelial injury disproportionate to apoptotic activity. Correlation with laboratory studies, such as serologies, stool studies, cultures, polymerase chain reaction, immunohistochemistry, and others, may be necessary

Medication effect (see Sect. 6.1.4)

Potential diagnostic pitfalls

Diagnosis of indeterminate for ACR implies that features of rejection are present but insufficient for diagnosis of mild rejection and, thus, should not be used in the context of nonspecific inflammation or infection

Intraepithelial lymphocytes, eosinophils, or neutrophils should not be mistaken for apoptotic bodies

Table 6.3

Mild ACR (Fig. 6.2)

Histological features

Increased crypt apoptotic activity: ≥6 apoptotic bodies per 10 consecutive crypts

May show mild epithelial injury, usually focal, characterized by reactive changes such as loss of mucin, nuclear enlargement and hyperchromasia, cytoplasmic basophilia, mild villous blunting, and others

Intact surface epithelium

May show a mild increase in the number of lamina propria inflammatory cells, consisting predominantly of mononuclear cells. Activated lymphocytes, eosinophils, and neutrophils may be present

Differential diagnosis

Indeterminate for ACR, which would have similar findings but with <6 apoptotic bodies per 10 consecutive crypts

Moderate ACR, which may show confluent apoptosis but would also show focal crypt dropout and/or focal erosion of the mucosa

Infectious enteritis (see Sect. 6.4)

Medication effect (see Sect. 6.1.4)

Potential diagnostic pitfalls

Biopsies from or near the stoma or anastomosis may show nonspecific inflammation, villous architectural change, and epithelial injury, which should not be used to evaluate for rejection

Intraepithelial lymphocytes, eosinophils, or neutrophils should not be mistaken for apoptotic bodies

Table 6.4

Moderate ACR (Fig. 6.3)

Histological features

Increased crypt apoptotic activity: more prominent than that seen for mild ACR in general. Apoptosis can be confluent to involve all or the vast majority of the epithelial cells in individual crypts

Focal crypt withering and/or dropout

Reactive and regenerative changes

Moderate increase in the number of lamina propria inflammatory cells, usually extensive, consisting predominantly of mononuclear cells. Activated lymphocytes, eosinophils and neutrophils are usually present. Neutrophilic cryptitis may be seen

May show focal mucosal erosion

Differential diagnosis

Severe ACR, which would show more extensive crypt dropout with more extensive erosion or ulceration

Infectious enteritis, especially cytomegalovirus (CMV) enteritis with ulceration

Medication effect (see Sect. 6.1.4)

Potential diagnostic pitfalls

Biopsies from or near the stoma or anastomosis may show mucosal erosion, which should not be used to evaluate for rejection

Focal crypt dropout should not be confused with crushing/squeezing artifact caused by biopsy procedure. Crushing artifact is not accompanied by increased numbers of lamina propria inflammatory cells or increased apoptotic activity in preserved crypts

Table 6.5

Severe ACR (Fig. 6.4)

Histologic features

Extensive crypt dropout

Extensive mucosal erosion and/or ulceration

Increased crypt apoptotic activity in areas adjacent to erosion/ulceration with residual crypts or in concurrent graft biopsies from other sites

Reactive and regenerative changes

Moderate to severe increase in the number of lamina propria inflammatory cells, usually extensive, consisting predominantly of mononuclear cells. Activated lymphocytes, eosinophils and neutrophils are usually present

Severe exfoliative rejection, a form of severe ACR, shows diffuse ulceration with neutrophilic exudates that involves the entire transplanted bowel

Differential diagnosis

Infectious enteritis, especially CMV enteritis with ulceration

Medication effect (see Sect. 6.1.4)

Chronic rejection, which can also show broad areas of ulceration, but will also show additional findings (see Sect. 6.2)

Potential diagnostic pitfalls

Biopsies from anastomotic site can show features of ulceration, which should not be used to evaluate for rejection

Fig. 6.1

(a–d) Examples of indeterminate for ACR. Minimally increased crypt apoptotic activity, <6 apoptotic bodies per 10 consecutive crypts, which does not meet the criteria for mild ACR. Note the differing morphologies of apoptotic bodies, which can be seen as small, pinpoint basophilic fragments of nuclear debris or as larger exploding nuclear fragments with a rim of clear cytoplasm (arrows, a–d)

Fig. 6.2

(a–d) Examples of mild ACR. Mildly increased crypt apoptotic activity (arrows, a–d), ≥6 apoptotic bodies per 10 consecutive crypts, which is accompanied by epithelial reactive changes and increased lamina propria inflammatory cell infiltrates. No crypt dropout or erosion/ulceration is noted

Fig. 6.3

(a–d) Examples of moderate ACR. In addition to increased crypt apoptotic activity (arrows, a), reactive epithelial changes, and increased lamina propria inflammatory cells, moderate ACR is further characterized by more severe mucosal injury including crypt dropout and withering (a), focal surface erosion (b), and confluent crypt apoptosis (c, d)

Fig. 6.4

(a–d) Examples of severe ACR. Extensive crypt dropout and ulceration are seen (a, b), with residual epithelial cells showing reactive and regenerative changes as well as increased apoptotic activity (c, d)

6.1.2 General Approach for Evaluation of Small Bowel Graft Biopsies

Routine H&E sections with multiple serial sections are examined.

Low-power examination

Assess adequacy of biopsy: should be deep enough to include the entire mucosa and have at least ten well-oriented crypt bases (in specimens without crypt dropout or ulceration)
Evaluate overall architecture
- Intact: no ACR, IND, or mild ACR
- Focal crypt dropout or focal erosion: possible moderate ACR
- Extensive crypt dropout or ulceration: possible severe ACR

High-power examination

Count crypt apoptotic bodies in crypt bases
- May be challenging in tangentially sectioned biopsies, but crypt bases will generally contain Paneth cells
- Apoptotic bodies seen in the surface epithelium should not be counted for the purpose of grading ACR
- Choose the area with the highest apoptotic density to count
- Search for confluent apoptosis, which may involve only a single or a few crypts
Evaluate for viral inclusions and other infectious agents

6.1.3 Subclinical Rejection

Histological findings of ACR seen on surveillance/protocol biopsies in the absence of clinical symptoms, or subclinical rejection (SCR), is a relatively well-established entity in other solid organ transplants and has also been described in SBT [7]. SCR has been reported to account for 17–22 % of biopsies, with the majority showing findings of mild ACR. In their analysis, Takahashi et al. [7] demonstrated that SCR occurring within the first 3 months of transplantation was associated with decreased graft survival.

6.1.4 ACR and Its Differential from Medication Effect (Fig. 6.5)

Mycophenolate mofetil (CellCept) is a commonly used immunosuppressive medication following all types of organ transplantation, including SBT. Its use is limited primarily by gastrointestinal (GI) toxicity, which usually manifests as diarrhea. Recent studies have characterized the histological features of CellCept toxicity, which can affect the entire length of the GI tract and resemble graft-versus-host disease (GVHD), inflammatory bowel disease, acute self-limiting colitis, or ischemic colitis. Erosive or ulcerative enterocolitis can be seen [8–11]. Because increased crypt apoptotic activity is a key diagnostic feature for both ACR and CellCept toxicity (especially for CellCept toxicity with a GVHD-like pattern), differentiating between these two entities can be challenging. Clearly, correlation with the patient’s medication history is essential in these situations. In addition, if concurrent biopsies are taken from native bowel (either small bowel or colon), the distribution of histological findings, in particular apoptotic bodies, can be used to help distinguish between ACR and CellCept toxicity. For example, if increased apoptotic bodies are seen in both graft and native bowel biopsies, this would favor CellCept toxicity because medication effect would be expected to affect both transplanted and native bowels. Conversely, if increased apoptotic bodies are seen only in graft biopsies but are not evident in native bowel biopsies, this would favor ACR because it would be expected to affect only transplanted bowel.