Organ Transplant Rejection: Understanding the Challenges & Solutions

Allograft rejection is the recipient's immune system recognizing donor HLA (human leukocyte antigen) mismatches as non-self. It remains the primary barrier to long-term graft survival.

Types of Rejection

Type	Mechanism	Timeline	Histology	Treatment
Hyperacute	Pre-formed anti-HLA/ABO antibodies → complement activation	Minutes–hours	Vascular thrombosis, necrosis, neutrophils	Prevention: crossmatch; irreversible
Acute Cellular (ACR)	Recipient T cells (CD4/CD8) → donor APCs (direct) or recipient APCs (indirect)	Days–months (peak 1–3mo)	Lymphocytic infiltrate, tubulitis (kidney), endothelitis	Pulse steroids; thymoglobulin if steroid-resistant
Acute Antibody-Mediated (AMR)	Donor-specific antibodies (DSA) → complement, FcR, endothelial activation	Weeks–months	C4d+, microvascular inflammation, DSA+	Plasmapheresis + IVIG ± rituximab/bortezomib/eculizumab
Chronic (CAV/CGI)	Alloimmune + non-immune (ischemia, HTN, CNI toxicity, infection)	Months–years	Intimal hyperplasia (vasculopathy), fibrosis, glomerulosclerosis	Risk factor modification; retransplant

Immunosuppression Regimens

Induction (Peri-transplant)

• Basiliximab (IL-2R antagonist) — low risk
• Thymoglobulin (ATG) — high risk, delayed graft function
• Alemtuzumab (anti-CD52) — selected centers

Maintenance (Triple Therapy Standard)

• CNI (tacrolimus > cyclosporine): Calcineurin inhibition → ↓IL-2 transcription. Narrow therapeutic index; nephrotoxicity, neurotoxicity, diabetes, HTN.
• Antimetabolite (MMF/MPA > azathioprine): IMPDH inhibition → ↓lymphocyte proliferation. GI, myelosuppression.
• Corticosteroids (prednisone): Broad anti-inflammatory; metabolic toxicity → steroid-free/avoidance protocols increasing.
• mTOR inhibitors (sirolimus, everolimus): Alternative to CNI (nephrotoxicity sparing); wound healing impairment, proteinuria, pneumonitis.
• Belatacept (CTLA4-Ig): Costimulation blockade; monthly IV; ↑PTLD risk if EBV−; better renal function.

Monitoring & Biomarkers

• Protocol biopsies (kidney: 0, 3, 12 mo; heart: scheduled EMB)
• DSA surveillance (single antigen bead)
• dd-cfDNA (donor-derived cell-free DNA) — non-invasive rejection signal
• Gene expression profiling (AlloMap heart, kSORT kidney)

Infection & Malignancy Trade-off

• Opportunistic: CMV, BK virus (nephropathy), EBV (PTLD), fungal, PJP — prophylaxis standard.
• Cancer: Skin (SCC 65x), PTLD, Kaposi, anogenital — ↓immunosuppression if possible.

Future: Tolerance induction (mixed chimerism, Treg therapy), HLA matching algorithms, xenotransplantation (GalTKO pigs), personalized immunosuppression via biomarkers.