Antibody-Mediated Rejection

The contract between this chapter and the reader.

1. 1. Define antibody-mediated rejection and the role of donor-specific antibodies.
2. 2. Explain the mechanism of antibody-mediated endothelial injury.
3. 3. Distinguish preformed from de novo DSA and their timing.
4. 4. Apply the Banff diagnostic triad for AMR.
5. 5. Interpret C4d staining and recognise C4d-negative AMR.
6. 6. Detect and quantify DSA with solid-phase assays.
7. 7. Recognise chronic active AMR and transplant glomerulopathy.
8. 8. Treat acute active antibody-mediated rejection.
9. 9. Appraise the limited evidence and the prevention of AMR.

A sixty-second reading. Each bullet stands alone.

• Antibody-mediated rejection is graft injury caused by donor-specific antibodies binding the donor endothelium.
• The antibodies activate complement and recruit cells, injuring the microvasculature.
• Preformed DSA — from prior transplant, pregnancy, or transfusion — cause early AMR; de novo DSA cause late, chronic AMR.
• De novo DSA are often class II (anti-DQ) and arise with inadequate immunosuppression or non-adherence.
• The Banff diagnosis requires tissue injury, evidence of antibody–endothelial interaction, and donor-specific antibody.
• Microvascular inflammation is glomerulitis plus peritubular capillaritis.
• C4d staining in peritubular capillaries marks complement activation, but C4d-negative AMR also occurs.
• Solid-phase single-antigen bead assays detect and semi-quantify DSA by mean fluorescence intensity.
• Chronic active AMR shows transplant glomerulopathy — capillary double contours — with microvascular inflammation and DSA.
• Hyperacute rejection from high-titre preformed DSA is now rare because of the pre-transplant crossmatch.
• Acute active AMR is treated with plasma exchange and IVIG, often with rituximab, and optimised maintenance immunosuppression.
• The evidence for AMR treatment is limited, and chronic active AMR has no proven effective therapy.
• AMR, especially chronic active AMR, is a leading cause of late graft loss and is worse than T-cell-mediated rejection.
• Prevention rests on a negative crossmatch, adequate immunosuppression, adherence, and DSA monitoring.

The medical core. An expert should agree antibody-mediated rejection is fully covered here.

Why it matters at the bedside

Antibody-mediated rejection is the quiet killer of kidney transplants. Where cellular rejection is usually acute and treatable, AMR — especially in its chronic form — smoulders against the endothelium, driven by antibodies we can detect but not reliably suppress, and it is the leading cause of late graft loss. Diagnosing it precisely matters all the more because our treatments are so imperfect.

What AMR is

• Antibody-mediated rejection is graft injury caused by donor-specific antibodies — usually anti-HLA — binding antigens on the donor vascular endothelium. It is a humoral process, distinct from the T-cell-mediated rejection of the next chapter, and it targets the microcirculation: the glomerular and peritubular capillaries.

The mechanism of injury

• Bound DSA injures the endothelium by two routes. It activates the classical complement pathway, leaving the split product C4d covalently fixed in the peritubular capillaries as a footprint of antibody action, and it recruits natural-killer cells and monocytes that damage the endothelium directly (antibody-dependent cellular cytotoxicity). The result is microvascular inflammation — glomerulitis and peritubular capillaritis — and, over time, the chronic changes of transplant glomerulopathy.

Preformed versus de novo DSA

• Timing follows the antibody's origin. Preformed DSA — from a prior transplant, pregnancy, or transfusion — are present before grafting and cause hyperacute or early AMR. De novo DSA develop after transplantation, are frequently class II (anti-DQ), and arise when immunosuppression is inadequate or the patient is non-adherent; they drive the late, chronic form that ends so many grafts.

The Banff diagnostic triad

• Diagnosis is structured: the Banff criteria require, conceptually, three elements together — histologic acute tissue injury (microvascular inflammation, or arteritis, thrombotic microangiopathy, or acute tubular injury without another cause); evidence of current antibody interaction with the endothelium (C4d in peritubular capillaries, or prominent microvascular inflammation, or a validated endothelial gene-expression signature); and serologic donor-specific antibody. Meeting the triad defines active AMR.

C4d and C4d-negative AMR

• C4d in peritubular capillaries is the classic marker of complement-fixing antibody, but its absence does not exclude AMR. C4d-negative AMR — microvascular inflammation with DSA but no C4d — reflects complement-independent, largely cell-mediated endothelial injury, and is the reason the Banff criteria accept microvascular inflammation or molecular signatures as alternative evidence of antibody action.

Detecting and quantifying DSA

• Solid-phase single-antigen bead assays (Luminex) are the workhorse: they detect anti-HLA DSA and semi-quantify them by mean fluorescence intensity, and complement-binding (C1q or C3d) assays gauge pathogenicity. The complement-dependent and flow cytometric crossmatches (the histocompatibility chapter) screen before transplant, and serial testing detects de novo DSA afterward.

Chronic active AMR and transplant glomerulopathy

• Chronic active AMR is the late, indolent form: months to years out, with de novo DSA and the morphologic hallmark of transplant glomerulopathy — double contours of the glomerular basement membrane from chronic endothelial injury and repair — alongside continuing microvascular inflammation. It progresses slowly to graft loss and is the dominant immunologic cause of late failure.

Treating acute AMR

• Acute active AMR is treated by removing and modulating the antibody: plasma exchange to clear circulating DSA, intravenous immunoglobulin to immunomodulate, frequently rituximab to deplete B cells, and corticosteroids, while the maintenance regimen is optimised to address any under-immunosuppression. Refractory or severe cases may prompt complement blockade (eculizumab) or other agents, on weak evidence.

The evidence and chronic AMR

• This is where humility is required. The standard plasma-exchange-and-IVIG approach to acute AMR rests on small trials and observational data rather than robust randomised evidence, and chronic active AMR has no therapy of proven benefit — trials of bortezomib, eculizumab, and tocilizumab have been disappointing or inconclusive. The honest position is that AMR is easier to diagnose than to treat.

Prevention

• Because treatment is imperfect, prevention carries the weight: a negative pre-transplant crossmatch (with desensitization where needed), adequate maintenance immunosuppression, relentless attention to adherence (the commonest driver of de novo DSA), and DSA surveillance to catch the antibody before the graft is lost.

Five fully-built tables.

Table A — Preformed versus de novo DSA

Table B — The Banff diagnostic triad

Table C — The AMR spectrum

Table D — DSA assays

Table E — Treatment of acute AMR

Visualise & Map

Causal chains, each ending in a named action.

Chain 1 — Complement-mediated injury

DSA binds endothelial HLA → classical complement activation → C4d fixed in peritubular capillaries + injury → microvascular inflammation → ACTION: detect DSA and C4d, and treat the AMR.

Chain 2 — The non-adherence pathway

Inadequate immunosuppression or non-adherence → de novo (often anti-DQ) DSA → chronic active AMR → late graft loss → ACTION: secure adherence and adequate immunosuppression; monitor DSA.

Chain 3 — Preformed antibody

Sensitization → preformed DSA → positive crossmatch → hyperacute/early AMR → ACTION: ensure a compatible crossmatch (or desensitize) before transplant.

Chain 4 — Complement-independent injury

DSA recruits NK cells and monocytes → antibody-dependent cellular cytotoxicity → endothelial injury without C4d → ACTION: do not exclude AMR when C4d is negative.

Chain 5 — Chronic endothelial repair

Repeated endothelial injury → basement-membrane reduplication → transplant glomerulopathy (double contours) → progressive loss → ACTION: recognise chronic active AMR and optimise what can be optimised.

Numbered rules. These numbers are the cross-reference handle for the cases and flowcharts.

Clinical Reasoning

Five cases. Each stops you at a decision before it answers it.

Every mechanism from Level 3 earns a bedside consequence and an action.

Exhaustive. Every rule in the chapter is here.

Safety & Evidence

Panel A — Red flags

Panel B — NEVER DO

Anti-patterns clinicians fall into. Each becomes a Level 22 distractor.

The grade reflects strength of evidence, not importance.

Patient Decisions

Outcomes as natural frequencies. Figures are representative; the direction of effect is given where precise numbers are uncertain.

Apply & Test

Copy-paste chart notes that map to the real decisions in this chapter.

Template 1 — AMR diagnosis note

• Indication: graft dysfunction / surveillance; biopsy date ___.
• Histology: glomerulitis (g) ___; peritubular capillaritis (ptc) ___; arteritis/TMA ___; C4d ___.
• Chronic: transplant glomerulopathy (cg) ___; IFTA ___.
• DSA: specificity (class I/II; DQ?) ___; MFI ___; de novo? ___.
• Banff diagnosis: active / chronic active AMR; C4d-negative? ___.

Template 2 — AMR treatment / monitoring note

• Treatment: plasma exchange (sessions) ___; IVIG ___; rituximab ___; steroids ___.
• Maintenance immunosuppression optimised; adherence addressed: ___.
• Response: function and DSA/MFI trend ___.
• If chronic active AMR: limited options acknowledged; plan ___.
• Surveillance plan (DSA, function) and failing-graft planning: ___.

Pre-rounds compression. Rules only.

Active recall. At least one card per objective.

Micro-teaching for rounds. Two scenarios, five steps each.

Metacognition anchored to this chapter's tensions. No answers provided.

1. 1. We can detect DSA with exquisite sensitivity but cannot reliably suppress it; how do you act on a number when the treatment it points to may not work?
2. 2. Chronic active AMR is diagnosable and largely untreatable; how do you balance honest prognostic candour with not abandoning the patient or the graft?
3. 3. De novo DSA so often traces to missed doses; where does the clinician's responsibility for adherence end and the system's begin?
4. 4. DSA monitoring finds antibody before injury, yet earlier detection has not clearly improved outcomes; how do you weigh surveillance that informs but may not save the graft?
5. 5. When the evidence for a treatment is weak but the alternative is watching a graft fail, how do you decide whether to treat at all?

Nine items, each anchored in this chapter. At least one per objective.