6.5 Transplant & Tumor Immunology and Immunodiagnostic Methods
Key Takeaways
Hyperacute rejection occurs within minutes from preformed antibodies, acute cellular rejection over weeks is T-cell mediated, and chronic rejection over months to years produces vascular intimal fibrosis.
Graft-versus-host disease occurs when immunocompetent donor T cells attack host tissues, classically skin (including palms and soles), liver and gut; irradiating blood products prevents transfusion-associated GVHD.
Tumors evade immunity by losing MHC class I, expressing PD-L1 and recruiting regulatory T cells; checkpoint inhibitors block CTLA-4 or PD-1/PD-L1 and can cause immune-related arthritis, dermatitis and colitis.
ELISA, agglutination, immunofluorescence, Western blot and flow cytometry all rely on specific antigen-antibody binding; a direct Coombs test detects antibody already on red cells, while an indirect Coombs test detects antibody free in serum.
Positive predictive value depends on prevalence: a test with 90% sensitivity and 95% specificity has a PPV of about 67% when disease prevalence is 10%.
6.5 Transplant & Tumor Immunology and Immunodiagnostic Methods
Sections 6.3 and 6.4 covered innate and adaptive immunity, hypersensitivity and immunodeficiency. The Part I outline also lists transplantation and tumor immunology and immunodiagnostic methods. These topics show up in podiatry through transplant recipients with foot infections and skin cancers, allograft use in reconstruction, checkpoint-inhibitor side effects and serologic test interpretation.
Graft Terminology
| Graft | Donor-recipient relationship | Example in foot and ankle care |
|---|---|---|
| Autograft | Same person | Calcaneal or proximal tibial bone graft; split-thickness skin graft |
| Isograft (syngeneic) | Genetically identical twin | Rare |
| Allograft | Same species, different person | Freeze-dried bone allograft, fresh osteochondral talar allograft, acellular dermal matrix |
| Xenograft | Different species | Porcine or bovine collagen matrices |
Recipients recognize donor HLA (MHC) molecules in two ways. In direct allorecognition, recipient T cells react to intact donor MHC on donor antigen-presenting cells. In indirect allorecognition, recipient antigen-presenting cells process donor MHC into peptides and present them on recipient MHC. Freezing or freeze-drying bone allograft kills donor cells and lowers immunogenicity, while fresh osteochondral allografts preserve chondrocyte viability but carry more donor antigen. All tissue allografts are screened to reduce the risk of transmitting disease.
Patterns of Rejection
| Type | Timing | Mechanism | Pathology |
|---|---|---|---|
| Hyperacute | Minutes to hours | Preformed antibodies (ABO or HLA) bind graft endothelium and activate complement (Type II) | Immediate thrombosis and ischemic necrosis; prevented by ABO matching and crossmatch |
| Acute cellular | Weeks to months | CD8+ and CD4+ T cells against donor MHC (Type IV) | Lymphocytic infiltrates; usually reversible with intensified immunosuppression |
| Acute antibody-mediated | Days to months | Donor-specific antibodies | Capillary inflammation with C4d deposition |
| Chronic | Months to years | Mixed cellular and humoral injury | Intimal fibrosis of graft arteries, interstitial fibrosis, obliterative bronchiolitis in lung grafts |
Graft-versus-Host Disease (GVHD)
In GVHD the graft attacks the host. Immunocompetent donor T cells in a hematopoietic stem cell transplant (or, rarely, in a non-irradiated blood product given to an immunodeficient patient) recognize host tissues as foreign. Acute GVHD targets the skin (a maculopapular rash that often begins on the palms and soles), the liver (cholestatic jaundice) and the gut (diarrhea). Chronic GVHD produces sclerodermatous skin and joint contractures. Irradiation of blood products prevents transfusion-associated GVHD.
Consequences of Chronic Immunosuppression
Transplant recipients take calcineurin inhibitors, antimetabolites, mTOR inhibitors and corticosteroids (pharmacology in 12.4). Clinical consequences in the lower extremity include:
- Atypical and opportunistic infections (fungal, nocardial, mycobacterial and viral)
- Blunted inflammatory signs, so infection can present with little redness or fever
- Cutaneous squamous cell carcinoma, which is many times more frequent than in the general population
- Delayed wound healing, particularly with mTOR inhibitors and high-dose corticosteroids
Tumor Immunology
Tumor antigens include:
- Tumor-specific neoantigens from mutated proteins, such as mutant RAS or BRAF peptides
- Tumor-associated antigens that are overexpressed or re-expressed normal proteins, such as oncofetal CEA and alpha-fetoprotein
- Viral antigens in virus-driven cancers: HPV E6/E7, EBV and HHV-8 in Kaposi sarcoma
Immune surveillance relies on CD8+ cytotoxic T cells, which recognize tumor peptides on MHC class I, and on natural killer (NK) cells, which kill cells that have lost MHC class I ("missing self").
Immune evasion mechanisms include:
- Downregulating MHC class I
- Expressing PD-L1, which engages PD-1 on T cells and induces exhaustion
- Recruiting regulatory T cells and secreting TGF-beta and IL-10
- Selecting antigen-loss variants
Checkpoint Inhibitors and Immune-Related Adverse Events
| Target | Drugs | Notes |
|---|---|---|
| CTLA-4 | Ipilimumab | Removes a brake on T-cell priming in lymph nodes |
| PD-1 | Nivolumab, pembrolizumab | Restores effector T-cell activity in the tumor; widely used in advanced melanoma |
| PD-L1 | Atezolizumab, durvalumab | Blocks the ligand on tumor cells |
Releasing these brakes can cause immune-related adverse events: dermatitis and vitiligo-like depigmentation, colitis, hepatitis, hypophysitis, thyroiditis, pneumonitis, and inflammatory arthritis that may begin in the feet. CAR-T cell therapy (for example, anti-CD19 for B-cell malignancies) can cause cytokine release syndrome.
Pathology markers. Immunohistochemical stains help confirm tumor type, for example S-100, SOX10, HMB-45 and Melan-A (MART-1) for melanoma. Serum markers such as PSA, CA-125, CEA, AFP and LDH are used mainly to monitor known cancers, not to screen the general population.
Immunodiagnostic Methods
| Method | Principle | Common uses |
|---|---|---|
| Agglutination | Antibody cross-links particulate antigens (cells or latex beads) into visible clumps | Blood typing; latex rheumatoid factor; Coombs tests |
| Precipitation and immunodiffusion | Soluble antigen-antibody lattices form visible lines | Older fungal serologies; serum protein immunofixation |
| ELISA | Enzyme-linked antibody converts a substrate into a color change proportional to the bound analyte | HIV and hepatitis screening; anti-CCP; many serologies |
| Lateral flow immunoassay | ELISA principle on a test strip | Rapid antigen tests |
| Western blot | Proteins separated by size, then probed with antibody | Confirmation in selected infections |
| Immunofluorescence | Fluorescent antibody binds tissue antigen (direct) or detects patient antibody on a substrate (indirect) | Direct immunofluorescence of skin biopsies (pemphigus, pemphigoid); ANA patterns |
| Flow cytometry | Fluorescent antibodies label cell-surface markers on single cells in suspension | CD4 counts in HIV; leukemia immunophenotyping; DHR test for CGD |
| Complement assays | C3, C4 and CH50 levels | Consumption in SLE and immune-complex disease |
| IGRA | Interferon-gamma released by T cells after TB antigen exposure | Latent TB screening before biologics |
Coombs testing. The direct antiglobulin (direct Coombs) test adds anti-human globulin to the patient's washed red cells to detect antibody or complement already bound to them (autoimmune hemolytic anemia, hemolytic transfusion reactions). The indirect Coombs test detects free antibody in serum (antibody screening before transfusion and in pregnancy).
Interpreting serology. IgM suggests recent infection and IgG suggests past infection or immunity. A fourfold rise in titer between acute and convalescent samples supports acute infection.
Worked Example: Predictive Values
A serologic test has 90% sensitivity and 95% specificity. In 1,000 patients with 10% prevalence (100 diseased):
- True positives = 90 and false negatives = 10
- Of 900 non-diseased, false positives = 45 and true negatives = 855
- PPV = 90 / (90 + 45) = 66.7%
- NPV = 855 / (855 + 10) = 98.8%
At 1% prevalence the PPV of the same test falls to about 15%. Predictive values shift with prevalence; sensitivity and specificity do not.
Two weeks after an allogeneic hematopoietic stem cell transplant, a patient develops a maculopapular rash beginning on the palms and soles, cholestatic jaundice and profuse diarrhea. What is the underlying mechanism?
Immunocompetent donor T cells attacking host skin, liver and gut
Recipient CD8+ T cells destroying the transplanted marrow
Preformed recipient antibodies binding donor endothelium within minutes
Immune complexes from a drug reaction depositing in dermal vessels
A patient receiving pembrolizumab for metastatic melanoma develops new symmetric synovitis of the metatarsophalangeal joints. What is the mechanism of the drug that best explains this adverse event?
Depletion of CD20-positive B cells, causing compensatory synovial inflammation
Inhibition of xanthine oxidase leading to urate crystal deposition
Direct cytotoxicity to synovial fibroblasts from DNA cross-linking
PD-1 checkpoint blockade that unleashes autoreactive T cells
A screening test has 90% sensitivity and 95% specificity. In a population of 1,000 with 10% disease prevalence, what is its positive predictive value?
About 99%
About 67%
About 90%
About 95%
Sections you finish are checked off in the contents.