Transplantation Immunology and Laboratory Support

Transplantation is used for end-stage organ failure, hematologic malignancy, autoimmune disease, and selected immune deficiencies. Clinical practice also includes vascular composite allografts of the face, uterus, and limb. Alloreactivity may be directed against the graft or the recipient. In rejection, the recipient’s immune system attacks the graft. In graft-versus-host disease, immune cells carried by a hematopoietic graft attack the recipient. The laboratory types donors and recipients, detects and identifies anti-HLA antibodies, performs and interprets crossmatches, measures graft content and chimerism, and selects safe blood components for heavily immunosuppressed recipients.¹

The histocompatibility barrier

HLA proteins are the dominant immunologic barrier to transplantation. Class I molecules, the products of HLA-A, HLA-B, and HLA-C, pair with β2-microglobulin, appear on essentially all nucleated cells, and are surveyed by CD8-positive cytotoxic T cells. Class II molecules, the products of HLA-DR, HLA-DQ, and HLA-DP, are restricted mainly to antigen-presenting cells such as dendritic cells, monocytes and macrophages, and B lymphocytes, and are surveyed by CD4-positive helper T cells. A molecule of either class must bind a short peptide to sit stably on the cell surface, so every HLA molecule displays a sample of the proteins inside or around its cell.¹

The genes of both classes are closely linked on the short arm of chromosome 6 and are inherited together as a haplotype, one from each parent. Mendelian segregation gives any two full siblings a 25% chance of sharing both haplotypes (HLA-identical), a 50% chance of sharing one (haploidentical), and a 25% chance of sharing neither. These inheritance probabilities are the basis of related-donor searches in hematopoietic transplantation.²

The HLA region is the most polymorphic in the human genome. This variation helps populations respond to diverse pathogens and makes virtually any two unrelated individuals HLA-mismatched. Molecular typing resolves tens of thousands of named alleles.^3,4

Allele counts grow with every database release. In HLA-DRB1*13:01:01:02, the fields identify the allele group, protein sequence, synonymous coding variation, and noncoding variation. Expression suffixes carry separate meaning; HLA-A*24:09N is a null allele whose product is unexpressed. This level of resolution can affect a matching decision.^3,4

ABO is the conventional blood group system with the greatest direct impact on solid-organ transplantation, because A and B antigens are expressed on vascular endothelium. Preformed anti-A or anti-B in the recipient can bind a newly reperfused incompatible graft, activate complement, and produce hyperacute rejection within minutes to hours. Donor and recipient therefore follow the same compatibility logic as red cell transfusion: a group A recipient, carrying anti-B, can accept an A or O organ. Selected ABO-incompatible transplants use an antibody-reduction protocol that may combine plasma exchange or immunoadsorption with intravenous immune globulin and B-cell-directed therapy.⁵

Graft types and allorecognition

Most clinical transplants are allografts, and HLA disparity drives much of their rejection risk. Recipient T cells encounter foreign HLA through two routes.⁶

Direct allorecognition. Recipient T cells bind an intact allogeneic HLA-peptide complex displayed on donor antigen-presenting cells carried within the graft. An unusually large fraction of recipient T cells can respond, in part because cross-reactive memory T cells join the alloreactive pool. The mixed lymphocyte reaction is the historical in-vitro correlate of this pathway; routine donor selection now rests on molecular HLA typing and antibody-based compatibility testing.^1,6

Indirect allorecognition. Recipient antigen-presenting cells take up shed donor HLA protein, process it, and present donor-derived fragments on the recipient’s own HLA. This pathway supports alloantibody production and chronic rejection.⁶

Either route can end in direct CD8-mediated cytotoxicity, delayed-type hypersensitivity, or antibody-mediated injury through complement fixation and antibody-dependent cellular cytotoxicity. The dominant mechanism helps determine the timing and pathology of rejection.⁶

Rejection patterns and their prevention

ABO-compatible pairing, periodic anti-HLA antibody screening of waitlisted candidates, and a pretransplant crossmatch help prevent hyperacute rejection. Chronic rejection remains a major cause of late graft dysfunction and loss.^1,6

Graft-versus-host disease reverses the direction of attack: mature donor lymphoid cells within a graft respond against recipient tissue. It occurs with hematopoietic transplantation and, less often, with solid organs that carry substantial lymphoid tissue.⁶

Immunosuppressive agents

Transplant immunosuppression may combine corticosteroids, antimetabolites such as mycophenolate mofetil, calcineurin inhibitors such as tacrolimus, mTOR inhibitors such as sirolimus, and monoclonal or polyclonal antibody agents. These classes interrupt lymphocyte activation, proliferation, and signaling at different points. Combination therapy reduces rejection and increases susceptibility to infection and malignancy.^1,6

Histocompatibility testing

Transplantation requires HLA typing, HLA antibody screening and identification, crossmatching, and post-transplant monitoring, alongside ABO compatibility and infectious disease evaluation of the donor. Allogeneic hematopoietic transplantation uses high-resolution molecular HLA typing for donor selection. Detailed typing, antibody-detection, and crossmatch methods belong to serologic and molecular testing practice.²

Calculated panel-reactive antibody (cPRA) expresses a candidate’s sensitization for organ allocation. The transplant program designates clinically relevant antibody specificities as unacceptable antigens after laboratory and clinical interpretation. The cPRA estimates the proportion of the donor population expected to express at least one unacceptable antigen from donor-population genotype and haplotype frequencies. A higher cPRA means a smaller expected compatible donor pool. Donor-specific antibody and the crossmatch result guide whether a transplant can proceed, requires desensitization, or should use another donor.⁷

Post-transplant monitoring differs by transplant type. Solid-organ recipients are followed for donor-specific antibodies (DSAs), commonly with multiplex single-antigen bead arrays. A DSA can be clinically silent or accompany antibody-mediated graft injury, so the result is interpreted with graft function, histology, and other clinical evidence.¹ Hematopoietic recipients are followed for chimerism by short tandem repeat (STR) analysis. PCR amplification of informative pretransplant donor and recipient STR loci and the post-transplant specimen, followed by capillary electrophoresis, estimates donor and recipient fractions. The desired donor fraction depends on disease, cell lineage, conditioning, and transplant strategy; serial decline can signal graft instability or relapse.⁶

Hematopoietic progenitor cell transplantation

Hematopoietic progenitor cell (HPC) transplantation provides hematopoietic recovery after cytoreductive therapy, replaces defective or destroyed hematopoiesis, and can supply a donor immune graft-versus-leukemia (GVL) effect against residual malignant cells.⁸

Graft sources and donor selection

• Autologous transplantation returns the patient’s collected HPCs after high-dose therapy, an established approach in multiple myeloma and selected lymphomas. Conventional donor-versus-recipient GVHD and donor-derived GVL are absent, and occult malignant cells can return with a contaminated graft.⁸
• Allogeneic transplantation supplies donor hematopoiesis and may provide GVL activity, as needed in selected marrow-failure syndromes, inherited hematopoietic diseases, and leukemias. It introduces graft-failure and GVHD risks. When a suitable family donor is unavailable, unrelated-donor and cord-blood registries broaden the search. Matched unrelated donors undergo high-resolution DNA-based HLA typing.^2,8
• Syngeneic transplantation from an identical twin is rare. Expected HLA and minor-histocompatibility disparities are minimal, so classic alloreactive GVHD and GVL are markedly reduced. An HLA-identical sibling who is not a twin can still differ at minor histocompatibility antigens and produce GVHD or GVL activity.⁸
• Umbilical cord blood can supply a pediatric recipient whose smaller body requires a lower total cell dose. Its low T-cell dose and predominantly naive lymphocytes permit more HLA mismatch and reduce chronic GVHD compared with many adult-derived grafts, although acute GVHD remains possible.^2,6

Donor eligibility under FDA HCT/P rules

Federal HCT/P rules establish communicable-disease eligibility for allogeneic HPC donors. Appropriate FDA-licensed, approved, or cleared donor-screening assays cover HIV-1 and HIV-2 antibodies plus HIV-1 nucleic acid testing (NAT); HBsAg, total anti-HBc, and HBV NAT; anti-HCV and HCV NAT; and syphilis. Viable leukocyte-rich HCT/Ps add anti-HTLV-I/II and CMV antibody testing, and living donors receive West Nile virus NAT. Autologous HCT/Ps are exempt from the federal donor-eligibility determination. Applicable labels identify autologous use and reactive or unevaluated infectious-disease results; program procedures govern any additional testing and storage controls.^9,10

Collection methods

• Bone marrow (HPC, Marrow) is collected by multiple aspirations from the posterior iliac crests into a sterile anticoagulated container using aseptic technique.¹¹
• Apheresis (HPC, Apheresis) collects circulating progenitors after mobilization. G-CSF, especially filgrastim, is the usual mobilizer; GM-CSF is an alternative in selected protocols. Allogeneic donors commonly receive growth-factor mobilization alone. Autologous regimens may use G-CSF alone, chemotherapy plus G-CSF, or plerixafor for predicted or demonstrated poor mobilization. Plerixafor antagonizes CXCR4-mediated retention of progenitors in marrow.^6,11
• Cord blood (HPC, Cord Blood) is collected aseptically by gravity through an umbilical vein into a validated sterile anticoagulant system after cord clamping. Collection may occur before or after placental delivery, with maternal and infant care taking priority.¹¹

CD34 dose and the collection target

CD34 is a transmembrane protein expressed by hematopoietic progenitors and several nonhematopoietic progenitor or endothelial populations. A validated viable CD34-positive count is the operational surrogate for progenitor dose. Targets vary by indication, graft source, recipient, and manipulation; current adult guidance commonly uses a minimum of 2 × 10⁶ CD34-positive cells/kg for autologous collection and 4 × 10⁶/kg for allogeneic collection, with higher preferred ranges. Autologous collection is more vulnerable to disease- and treatment-related poor mobilization. CD34-positive cells express HLA, while A and B antigen expression is limited or heterogeneous and changes with erythroid differentiation. HLA therefore governs donor matching, and product processing and transfusion support manage ABO incompatibility.^6,11

For an 82-kg allogeneic recipient with a protocol goal of 4 × 10⁶ CD34-positive cells/kg, the required total is 82 × 4 × 10⁶ = 3.28 × 10⁸ CD34-positive cells. A first apheresis containing 1.64 × 10⁸ viable CD34-positive cells supplies half of that planned total. The remaining collection plan accounts for measured yield, viability, and expected processing loss.

Laboratory processing

On receipt, the cellular therapy laboratory verifies labeling and container integrity and inspects the product for abnormal appearance or evidence of mishandling or contamination. The laboratory uses aseptic technique, qualified supplies, and a validated environment whenever the system is opened. Product testing includes total nucleated-cell count and viability; HPC products intended to restore hematopoiesis also require a viable CD34-positive count. Additional CBC, differential, platelet, and product assays follow the product specification. After processing, the laboratory performs validated microbial testing, notifies the physician of any positive result, assesses the risk, and documents how the result is managed. Bone marrow is filtered to remove fat, bone particles, and debris. RBC reduction, plasma reduction, mononuclear-cell enrichment, and resuspension use validated product-specific procedures that preserve recovery, viability, and sterility.^6,11

Cryopreservation and thawing

Cryopreservation procedures may reduce RBCs, plasma, and total volume to limit incompatible material, hemolysis, infusion volume, and cryoprotectant exposure. Validated HPC protocols commonly add dimethyl sulfoxide (DMSO) gradually to a final concentration of 5% to 10%, then use controlled-rate freezing and cryogenic nitrogen storage. The product is thawed rapidly by its validated method, commonly near 37 °C, and infused promptly. Washing or dilution can reduce DMSO and hemolysate exposure when clinically indicated, with potential cell loss considered in the decision.^6,11

Graft manipulation

CD34-positive selection may use validated immunomagnetic enrichment or flow-sorting platforms. Tumor-cell contamination of an autologous product is possible, and ex-vivo selection or purging remains disease- and protocol-specific. Donor T-cell depletion or CD34-positive selection can reduce GVHD while delaying immune recovery and increasing infection, graft-failure, or relapse risk in some settings. The outcome depends on the depleted cell subsets, graft source, conditioning, and post-transplant prophylaxis.^6,11

Cord blood testing

The bank uses validated procedures to characterize a cord blood unit for identity and HLA type, ABO/Rh, total nucleated-cell and viable CD34-positive content, viability, hemoglobinopathy status, maternal donor eligibility, and product sterility. Infectious-disease testing follows the FDA requirements for viable leukocyte-rich HCT/Ps. Release and confirmatory testing also verify that the selected unit and recipient records agree.^10,11

ABO incompatibility in HPC transplantation

HLA compatibility carries greater weight than ABO in allogeneic HPC donor selection. ABO-incompatible grafts are therefore common, and laboratory support distinguishes major, minor, and bidirectional mismatch.¹²

Peripheral-blood apheresis products generally contain much less incompatible RBC than marrow and often require no manipulation. Cord-blood and marrow risk depends on the residual RBC and plasma volume, processing method, antibody titer, and program protocol.¹²

During an ABO-mismatched transplant, RBCs are selected to remain compatible with recipient and donor; group O RBCs are the usual interim choice for major or bidirectional mismatch. Plasma should be compatible with both, and group AB plasma lacks anti-A and anti-B. Platelet selection also accounts for ABO group, donor-plasma anti-A/B titers, plasma reduction or platelet additive solution, HLA compatibility, and local policy. An HLA-matched platelet may justify a controlled ABO-incompatible choice. Mixed-field ABO typing can appear during erythroid chimerism. The service moves to donor-type RBC support only after donor erythropoiesis and loss of the relevant recipient isoagglutinins are confirmed under the program’s transition policy.¹²

Engraftment, graft rejection, and GVHD

HPC infusion reactions can resemble transfusion reactions. Conditioning-related liver injury, sinusoidal obstruction syndrome, chemotherapy or radiation lung injury, infection, and drug toxicity can mimic GVHD. Autologous grafts lack conventional donor-versus-recipient alloreactivity, although rare autologous GVHD-like syndromes have been reported.⁶

Engraftment is reported against standard thresholds. Neutrophil recovery is the first of 3 successive days with an absolute neutrophil count of at least 500/µL after the post-transplant nadir. Platelet recovery is the first of 3 consecutive days with a platelet count of at least 20,000/µL after 7 consecutive days without platelet transfusion. Red cell recovery, immune reconstitution, and donor chimerism are separate outcomes. Graft source, viable CD34-positive dose, processing, conditioning, donor and recipient factors, and post-transplant care all influence timing.¹³

Graft rejection is immune-mediated failure of donor-cell establishment; graft failure is the broader clinical category and includes immune and nonimmune causes. Reduced-intensity or nonmyeloablative conditioning can leave more host T- and NK-cell activity and increase rejection risk in some settings. HLA disparity, donor-specific anti-HLA antibody, low cell dose, graft manipulation, and a female-donor-to-male-recipient pairing can also contribute, with their effect depending on the transplant platform.^6,13

Graft-versus-host disease. Donor alloreactive T lymphocytes drive GVHD. Conditioning injury, antigen-presenting cells, and cytokines also contribute to tissue damage. Cord blood produces less chronic GVHD than many adult-derived grafts despite greater HLA disparity, although acute GVHD remains a substantial risk. NIH criteria classify GVHD by clinical phenotype. Classic acute GVHD occurs by day 100 after HCT; persistent, recurrent, or late acute GVHD has acute features after day 100 without chronic features. Classic chronic GVHD has chronic diagnostic features, and overlap chronic GVHD has both acute and chronic features. A donor lymphocyte infusion does not reset the day-100 clock.^14,15

Clinically significant GVHD often requires immunosuppression, which raises bacterial, viral, and fungal infection risk; mild manifestations may use local therapy. Graft T-cell depletion can lower GVHD while delaying immune recovery and raising graft-failure or relapse risk in some settings. For selected relapse after allogeneic HCT, a donor lymphocyte infusion supplies a lymphocyte-rich product, usually from the original donor, to restore donor immune activity and GVL. The product may come from a fresh unstimulated collection or an eligible stored or engineered source under protocol.^15,16

Transfusion support for transplant recipients

Transfusion support during HPC transplantation commonly uses prestorage-leukocyte-reduced and irradiated cellular components. CMV-safe selection depends on recipient serostatus, graft type, component, and program policy.⁶

HLA alloimmunization. Leukoreduction reduces HLA alloimmunization and platelet refractoriness without eliminating either outcome. In the Trial to Reduce Alloimmunization to Platelets, conducted during induction therapy for acute myeloid leukemia, lymphocytotoxic antibodies developed in 45% of controls, 18% of recipients of filtered pooled platelets, 21% of recipients of UV-B-treated pooled platelets, and 17% of recipients of filtered apheresis platelets.¹⁷ Anti-HLA class I antibody is a major immune cause of poor post-transfusion platelet increments. Donor-specific anti-HLA antibody, especially high-level or complement-binding antibody, is associated with poor engraftment or graft failure in several HCT settings and is interpreted by antibody level, locus, assay, and transplant platform.^18,19

CMV transmission. A CMV-seronegative HCT recipient is at risk of primary transfusion-transmitted CMV, while reactivation during immunosuppression is the main concern for a seropositive recipient. CMV-seronegative or leukocyte-reduced cellular components are accepted CMV-safe strategies for seronegative recipients, with pathogen-reduced platelets accepted in relevant settings. Duration varies with recipient serostatus, graft type, immune reconstitution, GVHD, and program policy.²⁰

Transfusion-associated GVHD (TA-GVHD). Viable donor T lymphocytes in a transfused cellular component can proliferate in a susceptible recipient and attack marrow, skin, liver, and gastrointestinal tissue. TA-GVHD is rare and carries a mortality above 90%. Fever, generalized maculopapular rash, diarrhea, liver injury, and marrow aplasia or pancytopenia usually begin 2 to 30 days after transfusion. Irradiation prevents donor T-cell proliferation; the current Circular of Information specifies a central target dose of 2,500 cGy and at least 1,500 cGy to every portion of the component. The transplant program defines which components require irradiation and the duration of support.²¹

References

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