The document summarizes the mechanisms of kidney allograft rejection. T lymphocytes recognize donor antigens through direct or indirect presentation by antigen presenting cells, and initiate either antibody-mediated rejection through antibody binding and complement activation or T-cell mediated rejection through T-cell infiltration and cytokine release. The innate immune system also contributes through toll-like receptor activation and complement factors. Histologic findings are used to classify rejection.

1. Rejection of the Kidney Allograft

2. Introduction
• Recent discoveries have clarified how T
lymphocytes, the principal agents of acute
rejection, travel to and recognize the allograft
• Important progress has also been made in
understanding the influences of co-
stimulatory molecules and cytokines and in
elucidating how the innate immune system
participates in graft rejection.

3. Clinical Features of Allograft Rejection
• An increase in serum creatinine points to
rejection
• Subclinical rejection may be apparent only on
biopsy of the organ and, in the absence of
renal dysfunction, can damage the allograft

4. • The histologic findings on biopsy influence the
prognosis and the choice of therapy
• Rejection can be
– hyperacute (occurring within minutes)
– acute (occurring within days to weeks)
– late acute (occurring after 3 months), or
– chronic (occurring months to years after
transplantation)

5. • Also be classified according to
– Pathophysiological changes (cellular-interstitial, vascular,
antibody-endothelial)
– Severity (extent of histologic inflammation and injury)
– Response to treatment (presence or absence of
glucocorticoid resistance)
– Presence or absence of renal dysfunction (indicating acute
or subclinical rejection, respectively), and
– Immunologic mechanisms (adaptive or innate immune
system response)

6. The Innate Immune System
• Up-regulated and aggravate the rejection process either
directly or indirectly through the activation and recruitment
of T lymphocytes
• Injured tissues express ligands of the toll-like receptor
system — damage-associated molecular-pattern (DAMP)
molecules — and other innate danger molecules.
• Toll-like receptors sense the presence of foreign-tissue
molecules and can produce factors that cause the
maturation and activation of dendritic cells. These cells
have an important role in promoting acute rejection

7. • Another element of innate immunity, the
complement system, produces C3a and C5a,
which directly activate intragraft T cells and
antigen-presenting cells.
• An increase in major-histocompatibility-complex
(MHC) class I peptide–related sequence A (MICA)
antigens on endothelial surfaces can activate
natural killer cells and CD8 T cells

8. The Donor
• Certain features of the donor —
– older age,
– Presence of hypotension or hypertension,
– diabetes,
– Renal impairment, donation after cardiac death, and
– prolonged ischemia of the graft
• influence the decision about whether to accept
an organ from a deceased donor or to discard

9. Banff Classification of renal transplant pathology, diagnostic criteria

10. ANTIBODY-MEDIATED REJECTION

11. • Most recipients do not have antibodies
against HLA molecules before transplantation
unless they were sensitized by exposure to
alloantigens through pregnancy, blood
transfusion, or previous transplantation

12. Antibodies against Blood-Group
Antigens
• Kidneys selected for transplantation are
routinely assigned to recipients with a
compatible blood group
• However, ABO-incompatible kidneys have
been successfully transplanted after
perioperative removal of antibodies from the
recipient by means of plasmapheresis or
immunoadsorption

13. • After they have been removed, anti–blood-group
antibodies can rise to pretreatment levels after
transplantation, adhere to the microvasculature,
and activate complement, yet they generally do
not injure the endothelium.
• This anomaly has been attributed to
“accommodation” within the kidney, but the
mechanism responsible for this benign response
is unknown

14. Hyperacute Rejection
• Occurs almost immediately after release of the vascular
cross-clamps
• Kidney appears flaccid and mottled
– reflects the deposition of antibodies against HLA antigens
expressed on the endothelium of the glomeruli and
microvasculature.
• Activation of the classic complement cascade within the
graft is followed by endothelial necrosis, platelet
deposition, and local coagulation.
• Removal of the graft

15. Acute Antibody-Mediated Rejection
• Antibody-mediated rejection often begins within
days after transplantation
• The main feature is rapid graft dysfunction due to
inflammation.
• The main targets of these “recall” antibodies are
MHC antigens displayed by the endothelium of
the donor peritubular and glomerular capillaries

16. • The damaged endothelial cells release various injurious
molecules:
– von Willebrand factor and P-selectin, which promote
platelet aggregation;
– cytokines and chemokines, such as interleukin-1α,
interleukin-8, and chemokine (C-C motif) ligand 2 (CCL2),
which cause leukocytes to adhere to glomeruli
(glomerulitis) or to dilated peritubular capillaries
(margination);
– chemoattractants C3a and C5a.
• C4d, a marker of classic complement activation, is
frequently found in peritubular capillaries

17. • C5b triggers the assembly of the membrane-
attack complex (C5b–C9) which causes
localized endothelial necrosis and apoptosis,
as well as detachment of endothelial cells
from the basement membrane
• Microthrombi, with hemorrhage and arterial-
wall necrosis and infarction, occur in severe
cases

18. Antibodies against donor antigens bind to antigens expressed on endothelial cells in
the graft vessel . The subsequent complement activation and cell adhesion result in
endothelial-cell necrosis, followed by platelet deposition and coagulation

19. Mononuclear cells adhere to the endothelium of the glomeruli , arrows; periodic acid–Schiff stain) and the
peritubular This process is accompanied by C4d deposition in the glomeruli and peritubular capillaries arrows;
C4d immunohistochemical stain) and in the peritubular capillaries between ghost outlines of the renal tubules (,
arrows; C4d immunofluorescent stain).

20. T-CELL–MEDIATED REJECTION

21. • Antigen Presentation
• The most common form of acute allograft
rejection is initiated when donor alloantigens
are presented to the T lymphocytes of the
recipient by antigen-presenting cells (APCs)

22. • Immature dendritic cells within the graft carry
donor antigens from the transplanted organ to
the recipient’s draining lymph nodes and
spleen
• APCs then activate the recipient’s T cells
– These T cells differentiate into various subgroups
and return to the graft, where they take part in
destroying the transplanted organ

23. • T- Cell Subgroups

24. • B cells can also function as APCs by capturing and
presenting antigens with the use of their surface
immunoglobulins and MHC class II molecules.
• Even tubular epithelial and endothelial cells can
present antigen to activated T cells
• Sensitization can occur in the periphery or in
tertiary lymphoid organs that develop within the
transplanted kidney

25. • The Major Histocompatibility Complex
• HLA genes encode glycoproteins (MHC
molecules) that enable the APCs to display
fragments of antigens (peptides) to receptors
on T cells.
• Most of the MHC molecules are either class I
or class II

26. • A major functional difference between them is
that
– class I molecules present peptides derived from
internal proteins (e.g., viral proteins) to cytotoxic
CD8 T cells,
– class II molecules present peptides derived from
extracellular proteins (e.g., bacterial proteins) to
CD4 T cells.

28. • The MHC encodes the HLA system, and
mismatches between donor and recipient HLA
increase the risk of rejection
• Differences of only a few amino acids within
the peptide-binding site of MHC may be
sufficient to provoke graft rejection

29. • Recognition of Alloantigens by T cells
• The responding proportion of T cell
population in transplantation is 1 to 10%.
• Some of these responding T cells are antigen-
experienced and have low thresholds for
cross-reactive activation by MHC antigens.

30. • The recipient’s T lymphocytes can sense
alloantigens by either
– the donor’s APCs (the direct pathway) or
– the recipient’s APCs (the indirect pathway, which
resembles the pathway involved in the recognition
of foreign antigens)

31. • The indirect pathway becomes increasingly
important in long-term immune injury to the
graft, after the donor’s APCs have disappeared
• The recipient’s APCs can also take up membrane
fragments of other cells; these fragments contain
MHC molecules bearing predigested” peptides
derived from the donor’s MHC glycoproteins (the
semidirect pathway).

32. • Costimulation
• Signals required by T cells other than those
engendered by the MHC–peptide complex,
termed costimulatory signals.
• The chief sources of these signals are APCs and
surrounding tissues.
– CD80 (B7-1) and CD86 (B7-2); these two B7 molecules
are ligands for two T-cell–membrane receptors, CD28
and CTLA-4

33. • Binding CD80 or CD86 to CD28 stimulates the
T cell, whereas binding of B7 ligands to CTLA-4
incites an inhibitory signal.
• Other costimulatory molecules include
– CD40, CD154 (the CD40 ligand), and the T-cell
immunoglobulin and mucin (TIM) subgroup

34. T-Cell movement in the Allograft
• T cells use adhesion molecules, including
leukocyte- function–associated antigen 1 (LFA-
1), to roll along and tether to endothelium,
migrate across peritubular capillaries, and
enter the graft

35. • Interstitial mononuclear cells, including CD4
and CD8 T cells, and inflammatory cytokines
and chemokines accumulate in sites of acute
cellular rejection
• Other cells and pathways have a role in acute
rejection

36. • The expression of B-cell genes and CD20
increases in severe cellular rejection, and
eosinophilic infiltrates occur in glucocorticoid-
resistant rejection.
• Activated macrophages, which secrete substantial
quantities of proinflammatory cytokines, tumor
necrosis factor α (TNF-α), and interferon-γ, impair
the function of the graft and intensify T-cell–
mediated rejection

37. • Allografts undergoing rejection produce
chemokines, and some of the cells that
infiltrate the injured graft bear chemokine
receptors

38. Effector T Cells
• T cells mediate allograft injury
– directly through contact with tubular epithelial
cells (cell-mediated cytotoxicity) and through the
effects of locally released cytokines
– indirectly by activating inflammatory or vascular
endothelial cells.

39. • CD8 T cells release
– perforin, which perforates target-cell membranes, and
– granzymes A and B, which enter cells and induce caspase
mediated apoptosis
• The Fas ligand on cytotoxic T cells activates Fas, a receptor
on cells of the graft, and this interaction also induces
caspase-mediated apoptosis.
• CD4 T cells can attack grafted cells
– expressing minor MHC antigens and
– Also by secreting TNF-α and tumor necrosis factor β (TNF-β),
which bind to TNF receptors on endothelial or tubular cells,
causing them to undergo apoptosis.

40. Cellular rejection and transport of cells into the transplant are shown (Panel A).
After the initial tethering, rolling, and arrest of effector T lymphocytes (which
bind selectins and integrins on endothelial cells), lymphocytes and other immune
cells enter the interstitial compartment and invade tubules, causing local tissue
destruction.

41. Histologic features of T-cell–mediated rejection include a dense interstitial
lymphocytic infiltration (Panel B, arrow; periodic acid–Schiff stain), with
mononuclear cells crossing the tubular basement membrane (pink) into the renal
tubules, resulting in tubulitis (Panel C, arrow; periodic acid–Schiff stain).

42. OTHER PATTERNS OF REJECTION

43. Vascular Rejection
• The histologic characteristics of vascular
rejection (also termed arteritis or endarteritis)
include
– the infiltration of vessels by mononuclear cells
– endothelial-cell apoptosis
– synthesis of matrix proteins and collagens by
intimal myofibroblasts

44. • CD4 and CD8 T cells and macrophages invade
the subendothelium and intima of muscular
arteries by means of intercellular adhesion
molecule 1 (ICAM-1) or vascular-cell adhesion
molecules (VCAM) on activated endothelium
and by means of chemokine (e.g., CCL4, CCL5,
and CXCL8)

45. In acute vascular rejection, mononuclear cells adhere to the endothelium of small
muscular arteries (Panel D, arrow; hematoxylin and eosin).
In chronic vascular rejection, neointimal thickening (Panel E, arrow; Masson
trichrome stain) due to myofibroblasts leads to complete vascular occlusion.

46. Late Acute Rejection
• Severe and difficult to reverse, with a high risk of
subsequent graft loss.
• Main features are active immune inflammation
and chronic tubulointerstitial damage
• Develop in graft recipients with high-grade
immunity against the transplant or in those who
receive reduced amounts of immunosuppressive
therapy because of cancer, prior severe infection,
or noncompliance

47. Chronic Rejection
• Failure to maintain sufficient immunosuppression to
control residual antigraft lymphocytes or antibodies.
• Features include
– progressive decline in renal function,
– invasion of the renal parenchyma by T cells, and
– persistent infiltration of the interstitium by T cells and
macrophages.
• Occasionally, smooth-muscle proliferation and hyperplasia
in vessels, forming a neointima; focal destruction of
internal elastic lamina; and finally, vascular occlusion

48. • In chronic antibody-mediated rejection,
undetected preexisting donor-specific
antibodies or antibodies generated after
transplantation deposit on the capillary
endothelium

50. Future Directions
• The alloimmune response remains the primary obstacle to
successful kidney transplantation.
• Rejection of the graft entails much more than T-cell responses.
• Other elements include
– innate immune system of natural killer cells, macrophages, and
complement;
– the adaptive immune system of antigen-specific T lymphocytes and B
cells; and
– cells intrinsic to the graft, such as endothelium.
• Antibody-mediated rejection is increasingly recognized as a
contributor to late graft injury

51. • Current therapies are focused on the initial
stages of T-cell activation, and this strategy
has minimized early acute rejection.
• However, we need to improve our
understanding of the mechanisms underlying
chronic graft dysfunction and develop better
treatments to prevent loss of the graft.

52. • Tests based on the genetic signatures of
lymphocytes or proteomic or metabolomic
patterns, with the use of urine or blood
samples, hold promise for monitoring the
status of the graft

53. • For kidney grafts, levels of mRNA in the urine that
correspond to perforin, granzyme B, FOXP3, or
other molecules appear to be more predictive of
rejection than levels of mRNA from circulating
mononuclear cells.
• Enzyme-linked immunosorbent spot assays that
measure activated lymphocytes and assays of
mitogen-stimulated CD4 T-cell reactivity can
quantify the risks of infection and rejection

54. • The transplant biopsy remains the principal
diagnostic tool, although supplementation by
microarray transcriptome analysis could
improve diagnostic classification and
prognostication

55. • Another barrier to progress the limited
knowledge of the mechanisms underlying the
down-regulation or silencing of the immune
response.
• We do not know why in rare cases recipients
appear to naturally tolerate an allograft, which
functions without immunosuppression.

56. Thank you !!