1 Transplant Immunology & Renal Allograft Rejection Shehzad Rehman, MDAssistant ProfessorDivision of Nephrology, Hypertension and Renal TransplantationUniversity of Florida
2 HLA: IntroductionMain barrier to transplantation is Major Histocompatibility Complex (MHC)MHC in humans is known as Human Leukocyte Antigen (HLA)Short arm of chromosome 6Inherited as 2 haplotypes (half-sets), one from each parent
3 Types of HLA Class I and class II Class I: A, B, C Class II: DR, DP, DQImportant for transplant: A, B, DRNumbered sequentiallyTypical: A2, A23; B7, B13; DR3, DR7
4 Human Chromosome 6 2 types of HLA Class I HLA Class II HLA Slide courtesy Dr. Casey
5 Human Leukocyte Antigen Class I HLAexpressed on all nucleated cellspresents antigen to cytotoxic CD8(+) T cells which induce cell deathClass II HLAexpressed only on antigen-presenting cells (APCs)presents to helper CD4(+) T cells which activate macrophages, B cells, effector T cellsAPC: dendritic cells, macrophages, B-cellsHowever, don’t be confused that B cells have class I and II Ag and T cells only have class I Ag.
6 Mechanisms of Rejection Cellular and antibody-mediated componentsTwo stagesSensitization stageEffector stageTwo kinds of responseAmnestic responseAnamnestic response
7 Sensitization Stage T-Cell receptors (TCRs) recognize foreign antigens Two signals needed:Signal 1: HLA interacts with TCR (CD3)Signal 2: Costimulation: CD28 with CD80 or CD86; or CD40 with CD40LAbsence of costimulation may lead to clonal anergy
9 Sensitization: Direct Pathway Donor APCs carried over on graft present antigens to host T-cellsUsual APCs are dendritic cellsImportant and dominant in early, acute rejection processes
10 Sensitization: Indirect Pathway Host APCs ingest and present allopeptides to host T-cellsLead to T-cell proliferationImportant in late/chronic processesAlso important in antibody-mediated rejection processes
11 Second Messenger Systems Inositol phospholipid → IP3 & DAGThis leads to intracellular Ca2+ releaseCa2+-calmodulin complexes form, triggering second messenger systemsCalcineurin → NFATPKCNFκBThese bind to the IL-2 gene and promote IL-2 transcription via mRNA
12 Effector StageIL-2 and other cytokines promote inflammation and proliferationSignal 3β-chemokines cause macrophage infiltrationGrowth factors cause fibrosis and vascular damageFinal common pathway is cell apoptosis
14 ClassificationAcute rejection can be classified based on various patterns:Time of onset: early vs. latePathogenetic mechanisms: antibody-mediated or cellularClinical/histological features: hyperacute, accelerated, acute, chronic.
15 Hyperacute RejectionIrreversible antibody-mediated rejection, occurring in minutes-to-hours.Preformed donor-specific antibodies.Can also be due to ABO-incompatibility.Immediate antigen-antibody binding with complement activation.Very rare in the post-crossmatch era.Histology: intense engorgement of the glomerular and peritubular capillaries with RBC clumping and fibrin thrombi.
16 Afferent arteriolar thrombosis in a case of hyperacute rejection
17 This small artery in a patient with hyperacute rejection is almost occluded by a fibrin thrombus
18 Accelerated Rejection Usually due to presensitization.Presents several days after transplantation.Usually represents an anamnestic reponse, resulting in rapid production of antibodies after exposure.Rejection starts after antibodies reach a critical level.Can be avoided with attention to crossmatch.
19 Acute Rejection Most commonly seen clinically. Can happen anytime, but 75% are seen within the first 3 months.10-20% of develop AR in first 12 months.T-cell-mediated (cellular), B-cell or antibody-mediated, or both.Classified according to Banff Schema (Solez et al., Am J Transplant, 2008).
21 Acute Cellular Rejection Generally T-cell mediated.Activated via alloantigen-dependent mechanisms (classic antigen-presentation and 3-signal model).Can also be via non-alloantigen-dependent pathway (innate immunity: TLRs and NK cells).
22 Acute Cellular Rejection Borderline changes:Either tubulitis with <25% parenchymal inflammation or infiltration with 1-4 cells/tubuleBanff I: tubulointerstitial inflammation only with >25% parenchymal involvement.IA: moderate tubulitis (5-10 cells/tubule).IB: severe tubulitis (>10 cells/tubule).
23 Moderate interstitial mononuclear inflammation affecting 25-50% of the sampled parenchyma (grade i2)
24 Banff IA: Moderate tubulitis with 5-10 mononuclear cells per tubular cross section
25 Banff IB: Severe tubulitis with greater than 10 mononuclear cells per tubular cross section.
26 Acute Cellular Rejection Banff II: vascular rejectionIIA: Mild-to-moderate intimal arteritis (<25% luminal involvement in at least 1 artery)IIB: Severe intimal arteritis (>25% luminal involvement)Banff III: severe vascular rejectionTransmural arteritis and/or arterial fibrinoid changes and smooth muscle necrosis.
29 Banff III: Severe intimal arteritis with transmural inflammatory infiltration, fibrinoid change and myocyte injury.
30 TreatmentBorderline: based on clinical and pathological picture. Some advocate no treatment.Banff IA: generally steroid pulse.Banff IB: steroid pulse and/or ATG.Banff II/III: ATGBanff II & III may also require treatment for possible concurrent AMR.
31 Antibody-Mediated Rejection Antibodies can target the capillary endothelium, leading to complement fixation.This causes inflammation and endothelial injury capillary endothelialitisComplement activation leads to increased presence of terminal products of complement cascade C4d.Can uncommonly be due to non-anti-HLA antibodies.
32 Antibody-Mediated Rejection According to Banff, diagnosis of AMR requires:Presence of circulating donor-specific antibodies.C4d+Morphologic evidence of acute tissue injury:ATN-like minimal inflammationCapillary and/or glomerular inflammation (PTC/G)Arteritis (v3)If either C4d or DSA are absent, Banff classifies as “suspicious for AMR”.
33 Aggregates of mononuclear inflammatory cells in dilated peritubular capillaries, scored as ptc3
34 Transplant glomerulitis with infiltrating mononuclear inflammatory cells (arrows) within capillary loops
35 Acute humoral rejection in a human renal allograft Acute humoral rejection in a human renal allograft. PAS staining shows the presence of a sparse interstitial infiltrate of neutrophils, together with edema. C4d deposition in peritubular and glomerular capillaries. An immunohistochemical stain using polyclonal rabbit antibodies that are specific for C4d shows C4d deposition.
36 C4d staining: immunohistochemistry vs C4d staining: immunohistochemistry vs. immunocytochemistry/immunofluorescence
37 Treatment Treatment of AMR is not well defined. Options include: ATGIVIGPlasmapheresisRituximabNewer options include agents under study e.g. bortezomib, eculizumab.
38 Chronic RejectionThe term “chronic rejection” was previously used to describe all manner of chronic changes in the allograft, on the assumption that they were immune-mediated.As non-immune factors became known, the term “chronic rejection” has fallen out of favor, to be replaced by chronic allograft nephropathy, and now, interstitial fibrosis/ tubular atrophy.
39 Chronic RejectionSome features remain consistent with chronic immune injury:Transplant glomerulopathy with double-contouringCertain vascular changes (disruptions of the elastica, inflammatory cells)
40 Chronic transplant glomerulopathy with numerous double contours (arrows) in glomerular basement membranes
42 Implications of Rejection Certain clinical features have important implications in acute rejection.Presence of rejection.Grafts have a longer half-life in patients who never had an acute rejection (Hariharan, N Engl J Med, 2000; Flechner, Transplantation, 1996).If fully reversible, may not have a deleterious effect on graft function in the long-term (Opelz, Transpl Proc, 1997; CTS 6-year data):60% vs. 75% for treatment for rejection.73% vs. 75% for those whose S/Cr returned to normal (<1.5 mg/dL)
43 Implications of Rejection Number of rejections (Dickenmann, Transpl Int, 2002; Pascual, N Engl J Med, 2002).Better with single episode than with 2 or more episodes.Timing of rejection (Sijpkens, Transplantation, 2003).10-yr death-censored graft survival 86% for AR <3 months vs. 45% for AR >3 monthsSeverity of rejection.Correlation between Banff grade and outcome.
44 Implications of Rejection Data presented at ATC 2011 suggested interstitial inflammation is a worse prognosticator.Newer data suggested vascular rejection may not carry the same grim prognosis in modern era.Worse prognosis for AMR.
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