Case 2 -

Hyperacute and Acute Renal Transplant Rejection Arthur H. Cohen, Juan Lechago and Cynthia C. Nast

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Hyperacute renal transplant rejection in an antibody-mediated process, which typically occurs within the first hour post-engraftment, during which time the kidney grossly becomes flaccid, cyanotic and swollen while urine output ceases or never begins [1, 2] . Microscopically there is accumulation of neutrophils in glomerular and peritubular capillaries, arterioles and small arteries. The capillaries and arterioles subsequently develop erythrocyte and fibrin thrombi while the interstitium is edematous and may become hemorrhagic. Tubules undergo coagulative necrosis; there is no inflammation in the interstitium or in the walls of tubules. Immunofluorescence microscopy may disclose IgM, C3 and less often IgG in addition to fibrin within the affected glomeruli and arterioles, but these immune reactants are not always observed. The differential diagnosis includes thrombosis of major arteries and renal vein thrombosis, but widespread intravascular neutrophils and thrombosis of capillaries and arterioles are distinctive features of hyperacute rejection. The majority of cases of hyperacute rejection are caused by presensitization to alloantigens on the surface of graft endothelial cells from blood transfusions, pregnancy and prior transplants [3, 4] . Circulating antibody binds to the alloantigens, fixes complement, and activates platelets and the clotting system resulting in widespread intragraft microthrombosis. A smaller number of hyperacute rejection episodes may be secondary to antibody-dependent cell mediated cytotoxicity against graft endothelium [5]. If only a small number of circulating antibodies are initially present, it may take 8 to 48 hours to produce enough antibody to cause graft damage and loss clinically resulting in a delayed hyperacute rejection. This form of rejection is now quite rare due to the excellent pretransplant screening available. Once it has occurred however, the graft must be explanted. Acute renal transplant rejection occurs in two distinct forms: cell mediated and antibody (humoral) mediated. The cell-mediated variety is far more frequent impacting approximately one-third of all renal transplants, and can be further separated into tubulo-interstitial and vascular types. Acute cell mediated tubulo-interstitial rejection comprises 45-70% of all acute rejection episodes in renal allografts. When a kidney is undergoing this form of rejection it becomes swollen and pale with a congested medulla. Microscopically the interstitium is edematous with a mononuclear inflammatory infiltrate composed of lymphocytes and variable numbers of monocytes. The lymphocytes are primarily activated T cells, both CD4 and CD8. The earliest infiltrates occur at the cortico-medullary junction, then spread throughout the cortex and occasionally into the superficial medulla; as with most renal biopsies unscarred cortex is required for diagnosis. Lymphocytes extend into the tubules between the epithelial cells, a process termed tubulitis and a hallmark of rejection. Tubulitis is found in all parts of the nephron and often begins in the distal tubules [6, 7] . There is variable tubular cell necrosis and tubules may rupture with interstitial extra-tubular Tamm-Horsfall protein. Up to 30% of involved grafts will have some interstitial eosinophils and 25% may have few scattered neutrophils; more than just a few neutrophils raises the possibility of infection or a form of humoral rejection described below. A very small number of biopsies will show large numbers of polyclonal plasma cells without viral infection, termed plasma cell rich acute rejection [8, 9] ; when this occurs it often is later in the course of transplantation and carries a guarded prognosis. In tubulo-interstitial rejection glomerular and peritubular capillaries are unremarkable or have few lymphocytes and monocytes in the lumina. The differential diagnosis includes drug-induced acute interstitial nephritis and viral infections; special stains and, as with all renal biopsies, close clinical correlation may be required for an accurate diagnosis. Acute cell mediated vascular rejection is found in 30-55% of grafts with rejection. It is characterized by T cells and monocytes extending under the endothelium of arteries (interlobular and arcuate) and/or arterioles into the vascular intima. This process has been termed endarteritis, endothelialitis and intimal arteritis, and is diagnostic for acute rejection [10]. All the changes of cell mediated tubulo-interstitial rejection may be seen with vascular rejection, although the latter infrequently occurs as an isolated finding. Associated changes of acute tubular necrosis have been linked to a more guarded prognosis [10a]. A considerable number of lymphocytes may be in the lumina of peritubular and glomerular capillaries, and focal interstitial hemorrhage is a common accompaniment. In a small number of patients with rejection (2-4%) the primary abnormality is acute transplant glomerulopathy in which there are numerous capillary luminal monocytes with endothelial cell swelling, subendothelial lucent zones observed ultrastructurally and occasional mesangiolysis. This may be a form of glomerular vascular rejection, although is still controversial [11], and glomerular monocytes may predict a worse prognosis [12]. Cell mediated rejection occurs when T cells react to donor alloantigens which are expressed in the context of MHC. Tubular cells may be a target due to their ability to produce and express adhesion molecules and chemokines, as well as their capacity to process and present antigen to T cells. Alternatively, endothelium is a primary target as it also can present antigen to T cells, and tubular epithelium may be at risk only due to its proximity to peritubular capillary endothelium. The immunology and immunopathology of transplant rejection is quite complex, and reviews are available for those who desire further details [11, 13, 14, 15, 16] . Clinically, patients present with a rapid rise in the serum creatinine level over several days. In the current immunosuppression era, the classic findings of graft tenderness and fever are seldom observed making renal biopsy a more necessary and oft-used tool for the transplant nephrologist. Tubulo-interstitial rejection is usually treated with pulse steroid therapy whereas vascular rejection is more refractory to therapy often necessitating OKT3 or other more aggressive treatment if there is not severe background chronic renal damage. Pathogenesis of Cell Mediated Rejection Donor alloantigen presentation/ MHC T cell recognition and activation CD28/B7, IL-2 T cell proliferation Further leukocyte recruitment Graft parenchymal damage Acute antibody-mediated (humoral) rejection in its classic form is far less frequent than cell mediated vascular rejection [17]. The classic arterial humoral rejection is characterized by fibrinoid necrosis in the walls of arteries with myocyte necrosis, cellular debris and disruption of the internal elastic lamina, with or without a neutrophil infiltrate [12]. Lymphocytes and monocytes may extend into the walls of arteries and arterial lumina may be thrombosed. Neutrophils infiltrate the lumina of glomerular and peritubular capillaries, often with resulting thrombosis. The interstitium often displays hemorrhage and tubular cells are necrotic; in severe cases there are foci of infarction. Cell mediated tubulo-interstitial rejection may or may not accompany the humoral rejection. Immunofluorescence microscopy discloses fibrin, C3 and often IgG or IgM in the arterial fibrinoid lesions. More recently, a more subtle form of microvascular peritubular capillary acute antibody-mediated rejection has been identified, which generally lacks the typical arterial fibrinoid necrosis of humoral rejection. The most prominent feature in these biopsies is diffuse peritubular capillary staining for the C4d fragment of complement best identified using indirect immunofluorescence on frozen tissue [18, 19] . Renal specimens from these patients show a variety of mrophologic features including variable numbers of peritubular and glomerular capillary luminal neutrophils with or without scattered neutrophilic tubulitis; occasionally the biopsy only shows acute tubular necrosis [20, 21] . C4d staining of peritubular capillaries may be the only feature of acute rejection, although cell mediated acute tubulo-interstitial rejection co-exists with C4d staining in 24-43% of biopsies showing this form of rejection. Up to 45% of specimens showing endarteritis have peritubular capillary C4d staining [22]. Antibody-mediated C4d+ acute rejection may accompany chronic rejection, indicating an ongoing or active rejection process likely inducing or exacerbating the chronic injury [22]. C4d positive rejection is found in up to one-third of renal allograft biopsies in the first few months post-engraftment and to the same degree in all renal transplant biopsies [22, 23] . The long-term prognosis in C4d+ acute rejection appears to be worse than for cell mediated rejection, but not as dismal as for classic humoral rejection with arterial fibrinoid necrosis [19, 22, 24, 25] . Humoral rejection is mediated by antibodies, although it may co-exist with cell mediated rejection. Alloantibodies to MHC class I and class II antigens have been identified in patients with classic humoral rejection and various donor specific antibodies including those against MHC class I have been found in patient with C4d+ biopsies and acute rejection [21, 26, 27] . Anti-endothelial antibodies occasionally can be identified by in vitro testing; the spectrum of alloantigens capable of inducing humoral rejection is likely quite broad [28 ]. However, due to improved pretransplant testing and improved immunosuppression, overall graft loss to classic humoral rejection remains rather low in the 5-7% range. C4d+ acute rejection may be more prevalent, and its impact on overall graft survival remains to be determined . Pathology of Acute Rejection Rejection Glomeruli Tubules Interstitium PTCs* Arteries Hyperacute PMNs, Thrombi Necrosis Edema, Hemorrhage PMNs, thrombi Thrombi - small arteries Cell-mediated tubulo-interstitial Normal Tubulitis Lymphocytes, Edema Normal Normal Cell-mediated vascular ±Monocytes ±Tubulitis ±Lymphocytes, Edema Lymphocytes Lymphocytes, in intima Humoral arterial PMNs, Thrombi, ±Monocytes Necrosis Hemorrhage, Infarction PMNs, Thrombi PMNs, Fibrinoid necrosis Humoral microvascular (C4d +) PMNs ±PMNs Normal or Edema PMNs,C4d + Normal * Peritubular capillaries Pathology of Acute Tubulo-interstitial Rejection and C-I Toxicity   Glomeruli Tubules Interstitium Arterioles Arteries Acute C-Itoxicity Normal Flattened, vacuoles, necrosis, lumina Mild edema Myocyte necrosis, Nodular hyalinization Normal Acute cell mediatedrejection ±Monocytes Tubulitis Lymphocytes,edema Normal or Lymphocytes in intima Normal or Lymphocytes in intima Humoral (C4d+) microvascular acute rejection PMNs ±PMNs Normal or Edema Normal Normal References Williams GM, Hume DM, Hudson RP Jr, Morris PJ, Kanok K, Milgrom F. 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