Introduction
Kidney allograft rejection is a common pathological condition that is associated with allograft nephropathy and it is a cause of concern for clinicians. The mechanism of rejection is rather complicated and the factors that are involved in the rejection process and yet to be fully unearthed. However, several explanations have so far been given regarding renal allograft rejection and morphological changes involved.
According to Baboolal et al (2002), chronic allograft nephropathy which leads to allograft rejection is a product of both recipient and donor-related factors. These authors also highlight that renal allograft failure has been occurring at a rate of up to 4 percent in a year following the first year of transplantation regardless of use of immunosuppressive medication used to counter acute rejection. It is however notable that there has been a slight improvement in the success of kidney transplantation but the increase has not been sufficient. A number of clinical characteristics present in kidney allograft failure which undoubtedly alter the morphology of the kidney.
It is also notable that kidney allograft rejection is either humoral or cell-mediated and is further classified as hyperacute, acute and chronic allograft rejection (McKay & Steinberg, 2010). This paper examines the morphology exhibited in kidney allograft failure and further explores the mechanism involved in renal allograft rejection. A number of explanations are given relating to the available knowledge on renal allograft failure, many of which will be considered in this paper.
What is Kidney Allograft Rejection?
Kidney allograft rejection is characterized by transplant dysfunction as evidenced by the allograft losing its function slowly by slowly. Hypertension and protenuria are also important accompanying clinical presentations that signify kidney allograft failure. A biopsy of a failing kidney allograft has several features that are worth noting. In most cases, chronic allograft nephropathy (CAN) occurs due to nephrpotoxicity resulting from long-term accumulation of calcinerium, in-built glomerulonephritis and nephrosclerosis among other conditions (Joosten et al, 2005).
In chronic rejection, fibrointimal thickening occurs and transplant vasculopathy is also seen. Inflammatory cells also infiltrate in the walls of the thickened arteries. Fibrointimal thickening occurs mainly due to the fibroblast migration and the depositing extracellular proteins, which ostensibly come from the recipient as well as precursor cells present in circulation. Membrane layering is also observed in peritubular capillaries’ basement and the condition is known as transplant capillaropathy.
The present glomerular lesions may be in form of wrinkles and collapsed glomerular tuft, expanded mesangial cells as well as enlarged glomerulus and this is often referred to as transplant glomerulopathy. It should be noted that transplant glomerulopathy is characterized by increased graft loss when observed in a renal biopsy. In a majority of patients, chronic allograft nephropathy is witnessed after about ten years from the time the transplant was carried out. This therefore indicates that CAN is a consequence of accumulated kidney damage.
Risk Factors for Kidney Allograft Rejection
Kidney allograft rejection appears to be mediated by a number of risk factors. Chronic allograft rejection has the most number of risks factors involved and these encompass even the existence of previous acute rejection cases. Joosten et al (2005) highlights that among the most serious risk factor in chronic rejection is the existence of one or more acute rejection episodes in the past, though it is not obvious that an acute rejection episode in the past positively predicts the occurrence of a chronic episode in the future. If an acute rejection episode experiences some degree of function loss, the possibility of developing chronic rejection increases unlike when complete functionality of the kidney is achieved after the acute rejection episode.
Kidney allograft rejection is common after IFN-α treatment and the probability of graft recovery is very minimal as reported by Carbognin et al (2006). Interferon alpha is mainly used as a therapy in hepatitis C virus (HCV) infection, hence kidney allograft patients with HCV infection often experience renal allograft rejection which is usually antibody mediated. Cases of graft rejection are more common if the patient had received the IFN-α treatment after receiving the graft compared to allograft rejection experienced if HCV treatment is administered before transplantation is done.
It is for this reason that Carbognin et al (2006) indicate that if patients are receiving a second or subsequent allograft and they contracted HCV after already having a transplantation, it is almost certain that renal allograft rejection will occur. This kind of renal allograft rejection is commonly known as acute-on-chronic renal allograft rejection. For patients who have not had repeat transplantation, it is highly advisable to have the IFN-α treatment administered prior to the kidney transplant.
Sensitization of the recipient’s immune system also tends to increase the risk of chronic kidney allograft rejection. Sensitization leading to production of anti-HLA antibodies can be experienced after pregnancy, unsuccessful transplants in the past or as a result of blood transfusions. A mismatch in HLA molecules also increases the risk of chronic rejection since the mismatch also raises the probability of having an acute renal allograft rejection.
A delay in graft function, possibly due to brain death, causes a cytokine up-regulation which eventually raises graft immunogenicity and the eventual occurrence of acute rejection. The risk of chronic rejection is therefore increased. The younger the recipient is the higher the risk of chronic rejection and blacks also have higher incidences of chronic allograft rejection compared to whites. In addition, inadequate immunosuppresion exacerbates the risk of developing chronic rejection especially if levels of immunosuppresants are low during the early years after transplant.
Forms of Kidney Allograft Rejection
There are three well known forms of kidney transplant rejection as reported by McKay and Steinberg (2010) and these include “hyperacute rejection, acute rejection and chronic rejection” (p. 172). This classification falls under the immunological rejection classification in the Banff 07 classification. The immunological rejection is grouped into both T-cell and antibody mediated rejection. As such, the hyperacute rejection is antibody-mediated while the acute and chronic rejection is either antibody or T-cell mediated. It is also notable that acute T-cell and/or humoral rejection of the allograft can occur and sometimes the chronic kidney allograft injury may be experienced.
It has been indicated that renal allograft rejection occurring very early after transplantation is mainly due to antibody-dependent processes, otherwise known as humoral mechanisms (Rouschop et al, 2006). This is evidenced by the presence of C4d on the surface of peritubular capillaries and this has become one of the diagnostic criteria in acute renal allograft rejection. The Banff 97 classification recognizes that kidney allograft rejection is mediated through a humoral mechanism involving C4d at the peritubular capillaries.
Hyperacute rejection
Hyperacute rejection occurs within a short time posttransplantation, sometimes taking even minutes. This kind of rejection is common among patients who already have prior sensitization and thus they have human leukocyte antigen (HLA) or ABO already present in their circulation. Such patients may also be having any other form of alloantibody which may be sensitive to the donor’s surface antigen. Darkening of the kidney and change into cyanotic is seen during implantation and this is usually associated with renal artery anastomosis. The kidney parenchyma is usually at risk of full hemorrhagic infarction if time goes on without taking the action of removing the kidney.
McKay and Steinberg (2010) describe that an examination of the kidney through a light microscope demonstrates swollen vascular endothelial cells whereas neutrophils are seen to migrate into the capillaries of the glomerular as well as those of the interstitium. Following the migration of neutrophils is the formation of fibrin thrombi in the capillaries of the glomerular and in the arterioles thereafter.
The renal parenchyma then dies off due to the process of ischemic hemorrhage and infarction which is usually widespread. A C4d immunolabeling test shows the peritubular capillaries to be positive due to diffusion of the C4 to the necrotic site. Immunoglobulin M (IgM) can be found when the rejection involves ABO incompatibility. In hyperacute rejection, the chances of the allograft working are very minimal as reported by Fogo et al (2006).
Humoral responses which are primarily involved in chronic kidney allograft rejection are initiated by activation of a cascade of intracellular signals initiated by antigen-B-cell receptor bond. Following antigen internalization, it becomes a component of an MHC class II peptide which then interacts with the right T-cell receptor leading to B-cell activation as well as production of antibody. CD40 and inducible co-stimulator B7h-ICOS are required as costimulators for the humoral response to be generated. There has been evidence from animal studies which suggests that CD4+ cells are pertinent in the development of chronic humoral rejection.
However this does not imply that CD8+ as well as B cells are insignificant in the development of chronic humoral rejection with Joosten et al (2005) reporting that these cells are involved as effector molecules. It therefore means that lack of CD8+ cell or B cells reduces the possibility of developing chronic humoral rejection significantly even if there is the required antigen presentation.
Acute humoral rejection
Acute humoral rejection (AHR) of the kidney allograft is characterized by several clinical features that are of importance. For instance there is allograft dysfunction accompanied by high levels of creatinine with some cases experiencing reduced urine output. AHR is experienced almost any time but it is common in the initial weeks after transplantation or if there is reduced immunosuppression.
Individuals who may be having prior sensitization through transplant, pregnancy or blood transfusion have a higher risk of developing acute cell-mediated rejection. Acute humoral rejection usually presents in form of acute cell injury which is indicated by “acute tubular injury, neutrophil margination, thrombotic microangiopathy, and/or arterial fibrinoid necrosis” (McKay & Steinberg, 2010, p. 175).
The C4d are also present in the peritubular capillaries coupled with presence of HLA among any other antigen that may be present in circulation. Whenever at least two of the above characteristics are identified, it becomes important to be ‘suspicious’ of the presence of acute humoral rejection. On viewing the biopsy via light microscopy, marginating neutrophils are located in the peritubular capillaries, which are already dilated. Endothelial injury as well as glomerular thrombosis not to mention the interstitial capillaries present with injury. A common feature of the larger arteries is the transmural fibrinoid necrosis and infiltration of leukocytes in some cases. It is also possible to witness interstitial edema and hemorrhage with mononuclear inflammation occurring only if acute cellular rejection occurs at the same time.
AHR is classified into class I, class II and class II as per the most current Banff criteria. Class I acute humoral rejection’s biopsy is characterized by acute tubular injury and hardly any inflammation. Capillary glomerulitis is the key characteristic of AHR class II whereas the small arteries experience transmural fibrinoid necrosis in AHR class III. In all the classes of acute humoral rejection, macrophages are commonly found and the C4 complement is also found in the tissue and may last for 7 to 10 days.
The acute humoral rejection is also currently put into two subclasses, peritubular capillary and arterial, since these are the distinctive vascular beds that are involved. Either case, there is antibodies mount immunity against endothelial cells. Cohen (2007) describes that acute tubular injury is a third form of tubular injury that is often encountered. To positively diagnose humoral rejection, it is necessary to identify C4d, which is a product of C4 which is involved in the complement system. After the antibody binds to the antigen, C4 fixation occurs and activated C1 cleaves C4 into C4a and C4b.
Thereafter, the C4b makes a covalent bond with molecules such as proteins that may be in the vicinity causing its activation into C4d and then it remains bound at the site. It is for this reason that the C4d is a complement activation marker. When C4d is found to be positive at the site, anti-HLA antibodies from the donor is the main cause in most cases. This type of humoral rejection is mainly affects the peritubular capillaries and it may be accompanied by acute peritubular injury as may be demonstrated by presence of C4d at the peritubular capillary walls. It requires the carrying out immunohistochemistry in order to make a positive test for C4d in tissue sections.
Fibrinoid necrosis is evident in acute arterial humoral rejection involving muscularis and sometimes the presence of infiltrated leukocytes and rarely, luminal thrombosis. A positive identification of leukocytes is vey rare unlike the presence of eosinophils and neutrophils. It is not uncommon to find parenchymal infarction which may be accompanied by hemorrhage. In case there is poor allograft prognosis, the presence of IgM, IgG, complement factor C3 and fibrin cannot be ruled out on affected arterial walls.
If the acute humoral rejection is of the peritubular capillary form, several abnormalities such as tubular necrosis, inflammation of the glomerular and peritubular capillaries can be observed via the light microscope. Cases of accompanying acute cellular rejection are also possible in acute humoral antibody-mediated rejection. Due to such coexistence of morphologic features that are similar for cellular and humoral rejection when depending on light microscopy, it is advisable to rely on C4d test when diagnosing acute humoral rejection (McKay & Steinberg, 2010).
Chronic humoral rejection
Chronic humoral rejection is characterized by the presence of anti-HLA belonging to class I or class II and these can emanate from the donor or from the recipient. Transplant arteriopathy is also a common finding and it presents in form of narrowed and closure of the medium and large arteries as a result of fibrointimal proliferation. When glomerular injury occurs, it is possible to tell since pronounced proteinuria and mesangial expansion can accompany it.
In addition, transplant glomerulopathy is signified by the presence of HLA antibodies which are specifically from the donor or from the patient if the patient had suffered acute humoral rejection in the past. It is noted that the prognosis of CHR involving transplant glomerulopathy is usually poor and the chance of the graft surviving is reduced by half within the first three years after the transplant. There are three main elements that are used as a diagnosis guide for CHR.
First, chronic injury should be evident on conducting a histologic examination and the injury comes in form of the basement membrane of the glomerular being duplicated, interstitial fibrosis or arterial intimal fibrosis. Second, antibody deposition or activation must be shown, whereby C4d are usually observed in the peritubular capillaries. Finally, a serology test should be positive for anti-HLA or any other antibodies emanating from the donor. It is important to note that suspicion for CHR is established if any two of the above findings are made.
Chronic kidney allograft rejection is the type of rejection whereby renal function is lost gradually and this may start even after three months after transplantation. There are various morphological changes that are seen in this type of rejection, with widespread arterial lesions being very common. Other common abnormalities include interstitial fibrosis, glomerular lesions and pronounced tubular atrophy. Intimal thickening of the arteries accompanied by fibrosis, monocyte accumulation with foam cells, and presence of myointimal cells arranged circularly beneath the endothelium accompanied by calcification causes luminal narrowing.
The possibility of having the internal elastic lamina wit duplications is observed and due to involvement of larger arteries in most cases, it becomes hard to rely on kidney biopsy as a diagnostic approach for chronic allograft rejection. The arterioles present with mild muscular hypertrophy as well as insudative lesions. In the glomerulus, the mesangial matrix increases notably which is also the case with mesangial cells and capillary walls tend to be double-contoured. The double contour is observed to be due to forming of another basemen membrane underneath endothelial cells rather than the expected migration of mesangium peripherally.
Rarely does the glomerular damage extend to the pint of forming mesangiolysis or microaneurysm although it is common to have segmental sclerosis. On performing immunofluorescence, deposits of C3, IgM as well as IgG are located in the capillary walls and in mesangial regions with varying intensity. A combination of the above changes in the glomerular is referred to as chronic transplant glomerulopathy characterizing chronic kidney allograft rejection (Cohen, 2007).
In the tubule-interstitium region, a number of non-specific changes are identified with the most pronounced changes being marked tubular drop-out as well as pronounced atrophy. While ossification is hardly seen, cases of calcified fibrotic interstitium are not uncommon. An examination of the peritubular capillaries shows some level of abnormality with over half of chronic rejection cases showing C4d deposition, a clear indicator of humoral role in chronic rejection.
Peritubular capillaries present with multilayered basement membranes which is a product of new basement membrane that is deposited during the repair process of endothelial cells which are damaged from time to time. The coexistence of chronic transplant glomerulopathy and C4d is sometimes linked to the formation of the multilayered basement membranes.
A relatively unpronounced form of antibody-mediated rejection is the accommodation type of rejection whereby an immunological accommodation is reached. Such an accommodation is achieved if the complement pathway is not fully activated or if the endothelium mounts resistance to attempts of complement elimination. This is most common in kidney allografts characterized by ABO-incompatibility. McKay and Steinberg (2010) report that accommodation may be completely stable, substantial as in the case of chronic rejection, minimal or there may be no stability at all as evidenced in hyperacute rejection.
T-Cell Mediated Kidney Allograft Rejection
In cellular response leading to the kidney graft rejection, the main cells that are involved are the CD4+ helper T cells. The recognition of the antigen-MHC complex by the naïve T cells causes the T cells to be activated by the dendritic cells. In this process, the CD28 is costimulated with CD80 or CD86 that is present on antigen presenting cells. This response is also exacerbated if costimulation occurs through CD40 and CD154. If the response involves Th1 cells (which secretes IL-2, TFN-β and IFN-Y), a cellular immune response ensues while a humoral immune response ensues if Th2 (secrets, IL4-6, IL10and IL13) cells are involved.
Acute cellular rejection
In acute T-cell-mediated kidney allograft rejection, the allograft becomes dysfunctional rapidly, usually in a matter of days, after the transplant as a result of cellular rejection. It is however pertinent to note that reduction of immunosupression can cause the occurrence of acute cellular rejection many years after kidney transplantation. In acute cellular rejection, the T cells react against the histocompatibility antigens of the donor that are usually present in the kidney and this effect is felt in all the parts of the kidney more so in the tubules, blood vessels and the glomeruli.
A characteristic finding in the glomeruli is the increased presence of mononuclear cells in the capillaries and endothelial edema in some cases. Tubulointerstitial rejection is identified by the presence of mononuclear cells in the interstitium with edema, inflammation of the tubules and acute tubular injury. Inflamed tubules are evidenced by presence of mononuclear cells in the tubular basement membrane. For a type I T-cell mediated rejection to be termed as positive, at least a quarter of the interstitial inflammation should be having moderate tubilitis otherwise acute cellular rejected should be suspected.
Type II cellular rejection is characterized by mononuclear cells which are present under the vascular endothelium and caution should be taken not to confuse with the presence of the lymphocytes in the adventitia only. Only in rare cases do edema and of endothelium and vessel’s intima or degeneration is found as a characteristic of type II T-cell rejection. In type III cellular mediated rejection, focal myocyte necrosis is witnessed and this is caused by infiltration of transmural mononuclear cells (Cohen, 2007). There are several locations where acute cellular rejection can be found including the tubules, the interstitium, arteries and the glomeruli.
Tubulo-interstitial rejection
According to Cohen (2007), the commonest location for acute cell-mediated rejection is the tubulo-interstitial rejection and it is also the most frequent form of T-cell mediated rejection. There is notable accumulation of T lymphocytes in the interstitium though it is primarily in the tubular capillaries. The epithelial cells are commonly damaged in addition to destruction of tubular walls. Also common in the acute cell mediated rejection is the presence of the interstitial foci of Tamm-Horsfall protein as well as the subclinical rejection process as presented by transplant biopsies.
Arterial rejection
In arterial rejection type of acute cell-mediated rejection, mononuclear leukocytes especially the lymphocytes accumulate in the arterial walls. It is possible to have the lymphocytes being accompanied by monocytes as well as lymphoblasts. The endothelial cells of the arteries are overcome by the monocytes, a condition referred to as endoarteritis or endotheliatis but it principally affects the larger arteries and not necessarily all arteries.
The possibility of finding necrosis is very minimal but sometimes leukocytes are found in the muscularis as well as presence of lymphocytes at the periphery of the affected arteries. The prognosis of arterial cell-mediated rejection is poor since it hardly responds to corticosteroids. In addition, arterial cellular rejection is found in about half of kidney allografts with tubule-interstitial rejection.
Glomerular rejection
Glomerular rejection form of acute cellular rejection is characterized by serious glomerular inflammation which is accompanied by cellular damage and this happens to be the main feature of glomerular rejection. There is infiltration of monocytes and lymphocytes in the glomeruli unlike the presentation of tubulo-interstitial rejection. Allograft glomerulopathy, which is the dominant feature in most cases, usually coexists with endarteritis and it signifies cell-mediated rejection in its severe form.
Chronic cellular mediated kidney allograft rejection
If cellular immunological response against donor antigen continues for a long time, the allograft experiences injury which is known as chronic cellular injury. The process of chronic cellular injury is a progressive one which can take up to years if not months. Several chronic morphological changes are witnessed in chronic T-cell-mediated rejection and these include atrophication of tubules, duplicated peritubular capillary basement membrane, interstitial fibrosis as well as arterial intimal fibrosis. T cells are also evident in the glomeruli, interstituim and arterial intima as well as any other site suffering from chronic injury. If there is transplant arteriopathy and presence of active mononuclear cells that have crossed the portions that have fibrosis, then the Banff classification regards this as chronic active cellular rejection (Cohen, 2007).
Conclusion
Kidney allograft rejection is a pathological problem that continues to be resolved completely. The pathological problem affects the kidney in various ways due to characteristic immunological responses that are evoked with different morphological changes being observed. Two major classes of allograft rejection are well known including cell-mediated rejection and humoral rejection. Humoral rejection is further classified as either hyperacute, acute, humoral and the unclearly defined accommodation type of humoral kidney allograft rejection. The cellular type of renal allograft rejection is effected by T-cells and it is either acute or chronic types of rejection.
It is important to note that these classes of rejection are based on Banff classification. In both major types of humoral rejection, the glomeruli, the arteries and arterioles, capillary basement membranes and the interstitium are largely affected with loss of function being a remarkable observation. It is evident that despite there being several explanations on the mechanism of kidney allograft rejection much is still unknown about the mechanism thus the continued high prevalence of renal allograft rejection.
References
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