Transplantation and Tumor Immunology

The histocompatibility barrier

Human leukocyte antigen (HLA) proteins present peptides to T lymphocytes. Class I molecules, encoded by HLA-A, HLA-B, and HLA-C, pair with β2-microglobulin and appear on nearly all nucleated cells. They present peptides to CD8-positive T cells. Class II molecules, encoded by HLA-DR, HLA-DQ, and HLA-DP, are expressed mainly by dendritic cells, monocytes and macrophages, and B lymphocytes. They present peptides to CD4-positive T cells. Inflammatory signals can induce class II expression in other cells.^{11 Abbas AK, et al. Cellular and Molecular Immunology. 2025.}

The HLA genes lie close together on chromosome 6 and are usually inherited as a haplotype. A child receives one haplotype from each parent. Ignoring recombination, any two full siblings therefore have about a 25% chance of sharing both haplotypes, a 50% chance of sharing one, and a 25% chance of sharing neither. HLA is one of the most polymorphic regions in the human genome, so unrelated donor-recipient pairs commonly differ at several loci.^{1,21 Abbas AK, et al. Cellular and Molecular Immunology. 2025.2 Marsh SGE, et al. Nomenclature for factors of the HLA system, 2026. 2026.}

HLA allele names record increasing sequence detail. In HLA-DRB1*13:01:01:02, the first field is the allele group, the second distinguishes the encoded HLA protein, the third records a synonymous coding difference, and the fourth records a noncoding difference. An expression suffix records how the allele is expressed. The suffix N, as in HLA-A*24:09N, identifies a null allele whose product is unexpressed. The w in an antigen name such as HLA-Cw7 distinguishes the HLA-C antigen from complement components. Current allele names use HLA-C without w.^{22 Marsh SGE, et al. Nomenclature for factors of the HLA system, 2026. 2026.}

Other antigen systems also affect compatibility. Minor histocompatibility antigens are polymorphic non-HLA peptides presented by HLA molecules and can sustain graft-versus-host and graft-versus-leukemia responses after an HLA-matched hematopoietic transplant. A and B blood group antigens on vascular endothelium can bind preformed anti-A or anti-B and cause rapid vascular injury. Stress-inducible MICA is a polymorphic, class I-related molecule that can activate natural killer (NK) cells, CD8-positive T cells, and γδ T cells through the activating receptor NKG2D that each expresses. Killer-cell immunoglobulin-like receptor interactions with HLA shape NK-cell alloreactivity in selected hematopoietic transplant platforms. The effect of these NK-cell interactions depends on the donor, recipient, and transplant protocol.^{1,31 Abbas AK, et al. Cellular and Molecular Immunology. 2025.3 Spierings E, et al. Histocompatibility. 2024.}

An autograft is tissue transferred within one person. A syngeneic graft is transferred between genetically identical people, an allograft between genetically different people of the same species, and a xenograft between species. Histocompatibility laboratories focus mainly on allografts. Detailed graft consequences, cellular-therapy processing, and transfusion support are covered in Transplantation Immunology and Laboratory Support.

Allorecognition and graft injury

Recipient T cells can encounter donor HLA through three routes.^{44 Siu JHY, et al. T cell allorecognition pathways in solid organ transplantation. 2018.}

The mixed lymphocyte reaction is the historical laboratory model of direct allorecognition. Experimental systems can show alloreactive precursor frequencies of 1% to 10%. Conventional mixed lymphocyte reactions are slow, difficult to standardize, and weak predictors of clinical compatibility. Molecular HLA typing and antibody-based compatibility testing provide more specific donor assessment for current clinical practice. Direct, indirect, and semi-direct recognition can lead to CD8-mediated cytotoxicity, cytokine-driven inflammation, alloantibody formation, complement activation, and antibody-dependent cellular cytotoxicity. Their relative contributions vary with the graft, timing, donor-recipient pair, and immunosuppression.^{3,43 Spierings E, et al. Histocompatibility. 2024.4 Siu JHY, et al. T cell allorecognition pathways in solid organ transplantation. 2018.}

^{1,3,51 Abbas AK, et al. Cellular and Molecular Immunology. 2025.3 Spierings E, et al. Histocompatibility. 2024.5 Naik RH, et al. Acute renal transplantation rejection. 2026.}

Accelerated rejection is a historical label for early injury in a sensitized recipient. Current organ-specific criteria classify the cellular, antibody, and tissue findings that produce this early injury. Hyperacute rejection has become uncommon because of ABO matching, HLA antibody screening, and pretransplant crossmatching.^{1,31 Abbas AK, et al. Cellular and Molecular Immunology. 2025.3 Spierings E, et al. Histocompatibility. 2024.}

C4d deposition can support antibody-mediated rejection in selected organs and phenotypes. In kidney allografts, the Banff criteria (the international allograft biopsy classification) recognize C4d-positive and C4d-negative antibody-mediated rejection. Other organs use their own criteria. Tissue findings, donor-specific antibody, molecular evidence where validated, and graft function are interpreted together. ^{66 Naesens M, et al. The Banff 2022 Kidney Meeting Report. 2024.}

In graft-versus-host disease (GVHD), mature donor T lymphocytes attack recipient tissue after an allogeneic hematopoietic transplant. Skin, gastrointestinal tract, and liver are common targets. The same donor immune response can attack residual malignant cells and produce a graft-versus-leukemia effect. T-cell depletion can reduce GVHD, with transplant-specific effects on graft failure, infection, relapse, and graft-versus-leukemia activity. NIH categories use clinical phenotype: classic acute, late acute, classic chronic, and overlap chronic GVHD. Classic chronic and overlap chronic GVHD require chronic features. Acute features can occur before or after day 100. ^{77 Jagasia MH, et al. National Institutes of Health Consensus Development Project on… 2014.}

Transplant immunosuppression may combine corticosteroids, antimetabolites, calcineurin inhibitors, mechanistic target of rapamycin inhibitors, and lymphocyte-directed antibody agents. These drugs can cause cytopenias, kidney or liver injury, metabolic changes, and increased susceptibility to infection. Tacrolimus, cyclosporine, sirolimus, and everolimus commonly require therapeutic drug monitoring. Target ranges depend on the specimen, method, transplant protocol, time since transplant, and interacting drugs. ^{1,31 Abbas AK, et al. Cellular and Molecular Immunology. 2025.3 Spierings E, et al. Histocompatibility. 2024.}

HLA typing

The required typing resolution depends on the transplant. Current National Marrow Donor Program guidance begins an allogeneic hematopoietic donor search with high-resolution patient typing at HLA-A, HLA-B, HLA-C, HLA-DRB1, HLA-DQB1, and HLA-DPB1, together with HLA antibody testing and family typing. Solid-organ programs apply organ-specific locus, antibody, and allocation requirements. ^{88 National Marrow Donor Program. Donor selection guidelines. 2025.}

HLA antibody assessment and crossmatching

Pregnancy, transfusion, and transplantation can stimulate HLA alloantibodies. Testing addresses four related questions:

1. Screening determines whether any HLA antibody is detectable.
2. Identification establishes which HLA antigens or shared epitopes react.
3. Donor specificity determines whether antibody is directed against HLA carried by a proposed donor.
4. Cell binding tests whether patient antibody binds donor lymphocytes in a physical crossmatch.

Historical panel-reactive antibody testing used complement-dependent cytotoxicity against a cell panel. The percentage of panel cells killed was reported as percent panel-reactive antibody, or %PRA. Adding anti-human globulin could increase detection of weakly complement-fixing IgG. Current solid-phase assays bind purified HLA molecules to beads. Screening beads detect broad reactivity, phenotype beads narrow the pattern, and single-antigen beads assign specificities. Single-antigen bead signal is semiquantitative and method-dependent. Denatured antigen, shared epitopes, antigen density, complement interference, serum inhibitors, and platform settings can make the fluorescence signal a poor indicator of clinical risk. Interpret the pattern with controls, dilution or pretreatment studies when indicated, the patient's sensitization history, and donor HLA.^{1010 Kongtim P, et al. ASTCT consensus recommendations on testing and treatment of… 2024.}

The US Organ Procurement and Transplantation Network (OPTN) calculated panel-reactive antibody (cPRA) estimates the percentage of deceased donors expected to express one or more antigens that a transplant program has entered as unacceptable for a candidate. The calculation uses the candidate's unacceptable antigens and the OPTN population-frequency model, which accounts for population relationships among HLA phenotypes that multiplying independent antigen frequencies would miss. A higher value indicates a smaller expected compatible donor pool. The histocompatibility laboratory reports cPRA with the underlying antibody and crossmatch findings. The transplant program integrates those results with donor HLA and its organ-specific protocol.^{1111 Health Resources and Services… Calculated panel reactive antibody calculator. 2025.}

In a complement-dependent cytotoxicity crossmatch, recipient serum is incubated with donor T and B lymphocytes, followed by complement and a viability dye. Cell death indicates complement-fixing antibody bound to donor cells. A flow-cytometric crossmatch uses fluorescent anti-human IgG to detect recipient antibody bound to donor T or B cells and is generally more sensitive. The significance of a positive result depends on the organ type, the antibody class and specificity, complement activity, the treatment protocol, and local acceptance rules.^{3,103 Spierings E, et al. Histocompatibility. 2024.10 Kongtim P, et al. ASTCT consensus recommendations on testing and treatment of… 2024.}

After a solid-organ transplant, serial single-antigen bead testing can detect a new or changing donor-specific antibody. Donor-specific antibody is associated with increased antibody-mediated rejection risk and reduced long-term graft survival, with substantial variation among patients. Its significance depends on graft function, histology, timing, and other organ-specific evidence. After an allogeneic hematopoietic transplant, chimerism testing commonly amplifies informative short tandem repeat loci from donor, recipient, and post-transplant specimens. Capillary electrophoresis estimates donor and recipient fractions. Lineage, disease, conditioning, time since transplant, and minimal residual disease findings guide interpretation of a changing donor fraction, so report each chimerism result together with the previous results.^{1212 Bader P, et al. Documentation of engraftment and chimerism after HCT. 2024.}

Tumor antigens

Malignant cells can display targets that distinguish them from most normal adult cells. ^{1313 Paston SJ, et al. Cancer vaccines, adjuvants, and delivery systems. 2021.}

^{1313 Paston SJ, et al. Cancer vaccines, adjuvants, and delivery systems. 2021.}

Tumor markers include proteins, genomic changes, and other substances produced by tumor cells or by the body in response to cancer. Selected markers, interpreted with other findings, support diagnosis, prognosis, treatment selection, response assessment, and detection of recurrence. Analyte-specific tumor-marker measurement and interference are covered in Chemistry. Malignant-cell morphology, immunophenotyping, and molecular classification are covered in Hematology. The antigen categories above explain how an immune response can recognize malignant tissue. ^{1414 National Cancer Institute. Tumor markers.}

^{1515 National Cancer Institute. Infectious agents.}

The proportion of virus-positive tumors varies across these categories. Laboratory evidence may include viral nucleic acid, viral protein, in-situ hybridization, serology, or another validated tumor-specific method.^{1515 National Cancer Institute. Infectious agents.}

Immune control and tumor escape

The Burnet-Thomas immune-surveillance hypothesis proposes that immunity detects and eliminates some transformed cells before a tumor becomes clinically evident. Increased malignancy in transplant immunosuppression and inborn errors of immunity supports this protective role. Dendritic cells can cross-present tumor antigen and prime CD8-positive cytotoxic T lymphocytes. CD4-positive Th1 cells supply costimulation and cytokines such as IL-2 and IFN-γ. NK-cell activity reflects a balance of inhibitory and activating signals. Reduced HLA class I can remove an inhibitory NK-cell signal and also reduce recognition by CD8-positive T cells. Stress ligands can add NK-cell activation. NK cells and cytotoxic T cells kill through perforin and granzymes. Antibody can also support complement activation or antibody-dependent cellular cytotoxicity against an accessible tumor antigen. ^{11 Abbas AK, et al. Cellular and Molecular Immunology. 2025.}

The cancer immunoediting model describes three outcomes of prolonged tumor-immune interaction.

^{1616 Mittal D, et al. New insights into cancer immunoediting and its three component… 2014.}

Escape mechanisms include loss of tumor antigen, reduced HLA class I expression, defective antigen processing, glycan masking, resistance to apoptosis, recruitment of regulatory T cells or suppressive myeloid cells, and secretion of inhibitory cytokines such as TGF-β and IL-10. A tumor can also express checkpoint ligands. PD-1 is an inhibitory receptor on T cells, and PD-L1 or PD-L2 binding reduces effector activity. CTLA-4 is another inhibitory receptor on T cells and competes with CD28 for CD80 and CD86. Checkpoint-ligand expression can therefore weaken tumor-directed T-cell function.^{16,1716 Mittal D, et al. New insights into cancer immunoediting and its three component… 2014.17 National Cancer Institute. Immune checkpoint inhibitors. 2022.}

Practice