**Microcirculation Injury**

**Figure 5.** Diffuse C4d staining (immunoperoxidase method)

452 Current Issues and Future Direction in Kidney Transplantation

**Figure 6.** Diffuse C4d staining (immunofluorescence method)

The term "chronic ABMR" does not relate to a particular time post-transplantation, but rather to architectural remodelling which can affect all compartments of the biopsy. In addition, active ABMR may be superimposed on these changes. (Figure 7)

**Figure 7.** Active chronic ABMR. Severe peritubular capillaritis is seen in the setting of interstitial fibrosis (ci) and tubu‐ lar atrophy (ct)

The hallmarks of chronic ABMR are transplant glomerulopathy (TG) and multilayering of peritubular capillary basement membranes, with or without transplant arteriopathy (TA) and interstitial fibrosis and tubular atrophy, indicating that the microcirculation is the main target of humoral attack. Transplant glomerulopathy manifests as double contours in silver-stained sections and is well demonstrated by electron microscopy (figure 8). There is widening of the subendothelial space by flocculent material and eventual duplication of the glomerular basement membrane. It has been shown in protocol biopsies that ultrastructural changes of endothelial cell injury such as cell activation and loss, can be detected within weeks of transplantation [39], pre-dating more permanent changes like mesangial matrix expansion and glomerular basement membrane duplication.

high ENDAT expression were present; graft loss was even higher when the biopsies showed diffuse C4d positivity as well. Although currently experimental, the detection of high EN‐ DAT expression may prove useful in cases with circulation DSA and C4d-positivity on biop‐

Advances in Antibody Mediated Rejection http://dx.doi.org/10.5772/54855 455

**Figure 9.** Active chronic ABMR. Glomerulitis is superimposed on changes of transplant glomerulopathy.

associated with DSA and microcirculation injury [31].

The lesion of Transplant Arteriopathy TA is characterized by expansion of the arterial in‐ tima by fibrous tissue and a variable amount of inflammation. Originally, TA was attrib‐ uted to chronic T-cell mediated rejection (TCMR) but it more likely reflects generalized scarring seen in the aging kidney allograft, the causes of which include ABMR. A per‐ centage of v1 and v2 lesions, also previously thought to be result of TCMR, may also be

A cluster analysis of 234 indicated renal allograft biopsies by Sis and co-workers [31] revealed an association amongst arteriolar hyalinosis (ah), interstitial fibrosis (ci), tubular atrophy (ct) and transplant arteriopathy (cv). In the past, these features were thought to be the result of chronic calcineurin inhibitor use but it appears that they are non-specific and may be encoun‐ tered in a variety of settings in the renal allograft, including ABMR. Arteriolar hyalinosis, in particular, is commonly encountered in the aging kidney, hypertensive nephrosclerosis and

sy but lacking histologic evidence of tissue damage.

**Vascular lesions**

**Scarring and hyalinosis**

diabetic nephropathy.

**Figure 8.** Electron microscopy showing a widened subendothelial space containing flocculent material (thick arrow). Duplication of the glomerular basement membrane is present (thin arrows).

Despite its close correlation with DSA, TG may not be specific for chronic ABMR, with significant numbers of TG cases reportedly due to hepatitis C and thrombotic microangiopathy [40]. Superimposed active antibody-mediated injury produces endocapillary proliferation and, together with double contour formation, a mesangiocapillary-like pattern in glomeruli (Figure 9). This is not accompanied by immunofluorescence findings typical of that type of glomerulonephritis and no diagnostic deposits are seen by electron microscopy.

The endothelium of peritubular capillaries can also display early ultrastructural evidence of damage before remodelling of the basement membrane occurs. Moderate to severe lamination (>5 layers of basement membrane) is seen in chronic ABMR whereas mild lamination (2-5 layers) may be due to causes other than antibody-mediated rejection in the transplant kidney and is also seen in native renal disease [39].

A study by Sis and co-workers [41] found that approximately 40% of cases with transplant glomerulopathy were C4d negative despite having circulating antibodies and showing high endothelial cell-associated transcript (ENDAT) expression. ENDATs represent altered gene expression due to the effects of alloantibody and are thought to be a sensitive indicator of ABMR. This same study reported a high percentage of graft loss when both antibodies and high ENDAT expression were present; graft loss was even higher when the biopsies showed diffuse C4d positivity as well. Although currently experimental, the detection of high EN‐ DAT expression may prove useful in cases with circulation DSA and C4d-positivity on biop‐ sy but lacking histologic evidence of tissue damage.

**Figure 9.** Active chronic ABMR. Glomerulitis is superimposed on changes of transplant glomerulopathy.

### **Vascular lesions**

basement membrane. It has been shown in protocol biopsies that ultrastructural changes of endothelial cell injury such as cell activation and loss, can be detected within weeks of transplantation [39], pre-dating more permanent changes like mesangial matrix expansion and

**Figure 8.** Electron microscopy showing a widened subendothelial space containing flocculent material (thick arrow).

Despite its close correlation with DSA, TG may not be specific for chronic ABMR, with significant numbers of TG cases reportedly due to hepatitis C and thrombotic microangiopathy [40]. Superimposed active antibody-mediated injury produces endocapillary proliferation and, together with double contour formation, a mesangiocapillary-like pattern in glomeruli (Figure 9). This is not accompanied by immunofluorescence findings typical of that type of

The endothelium of peritubular capillaries can also display early ultrastructural evidence of damage before remodelling of the basement membrane occurs. Moderate to severe lamination (>5 layers of basement membrane) is seen in chronic ABMR whereas mild lamination (2-5 layers) may be due to causes other than antibody-mediated rejection in the transplant kidney

A study by Sis and co-workers [41] found that approximately 40% of cases with transplant glomerulopathy were C4d negative despite having circulating antibodies and showing high endothelial cell-associated transcript (ENDAT) expression. ENDATs represent altered gene expression due to the effects of alloantibody and are thought to be a sensitive indicator of ABMR. This same study reported a high percentage of graft loss when both antibodies and

glomerulonephritis and no diagnostic deposits are seen by electron microscopy.

Duplication of the glomerular basement membrane is present (thin arrows).

and is also seen in native renal disease [39].

glomerular basement membrane duplication.

454 Current Issues and Future Direction in Kidney Transplantation

The lesion of Transplant Arteriopathy TA is characterized by expansion of the arterial in‐ tima by fibrous tissue and a variable amount of inflammation. Originally, TA was attrib‐ uted to chronic T-cell mediated rejection (TCMR) but it more likely reflects generalized scarring seen in the aging kidney allograft, the causes of which include ABMR. A per‐ centage of v1 and v2 lesions, also previously thought to be result of TCMR, may also be associated with DSA and microcirculation injury [31].

#### **Scarring and hyalinosis**

A cluster analysis of 234 indicated renal allograft biopsies by Sis and co-workers [31] revealed an association amongst arteriolar hyalinosis (ah), interstitial fibrosis (ci), tubular atrophy (ct) and transplant arteriopathy (cv). In the past, these features were thought to be the result of chronic calcineurin inhibitor use but it appears that they are non-specific and may be encoun‐ tered in a variety of settings in the renal allograft, including ABMR. Arteriolar hyalinosis, in particular, is commonly encountered in the aging kidney, hypertensive nephrosclerosis and diabetic nephropathy.
