1. Epidemiology and Incidence

Graft-versus-host disease (GVHD) remains a leading cause of non-relapse morbidity and mortality following allogeneic hematopoietic stem cell transplantation (HSCT). The incidence varies significantly based on the donor source, graft type, and intensity of the pre-transplant conditioning regimen.

Incidence by Donor and Graft Source:

• Matched Sibling Donors (MSD): Acute GVHD (aGVHD) occurs in 30–50% of patients, while chronic GVHD (cGVHD) develops in 30–50%.
• Matched Unrelated Donors (MUD): The risk is substantially higher, with aGVHD rates of 60–80% and cGVHD rates up to 70%.
• Haploidentical Transplants: With the advent of post-transplant cyclophosphamide (PTCy), rates have become more favorable, with aGVHD around 30–40% and cGVHD around 25–35%.
• Umbilical Cord Blood (UCB): Generally associated with a lower incidence of severe aGVHD (20–40%), but this benefit is offset by delayed immune reconstitution and higher graft failure rates.

Reduced-intensity conditioning (RIC) regimens lower the immediate toxicity of the transplant procedure but have only a modest effect on reducing overall GVHD rates compared to myeloablative conditioning (MAC).

2. Immunopathophysiology

The development of acute GVHD is a complex, dynamic process classically described in three overlapping phases. It begins with host tissue injury from conditioning and culminates in widespread, donor T-cell-mediated organ damage.

In chronic GVHD, the pathophysiology is distinct, involving B-cell dysregulation, aberrant germinal center reactions, and pathogenic antibody production, leading to fibrosis driven by cytokines like TGF-β.

3. Risk Factors

A combination of immunogenetic and clinical variables modulates the risk of developing GVHD. Pre-existing organ dysfunction can further complicate its presentation and management.

Major Immunogenetic and Clinical Risk Factors:

• HLA Disparity: This is the most significant risk factor. Each human leukocyte antigen (HLA) allele mismatch between donor and recipient increases the risk of severe aGVHD by approximately 10–20%.
• Donor-Recipient Sex Mismatch: A female donor for a male recipient (F→M) increases risk due to potential alloimmunization against male-specific H-Y antigens.
• Conditioning Intensity: Myeloablative conditioning causes more profound tissue injury (Phase I), releasing more damage-associated molecular patterns (DAMPs) and increasing GVHD risk.
• Graft Source: As noted, peripheral blood stem cells confer a higher risk than bone marrow, which in turn is higher than cord blood.
• Recipient Age: Patients over 50 years old tend to have more severe GVHD, possibly due to reduced thymic function and tissue repair capacity.
• Donor Parity: Multiparous female donors (those with multiple pregnancies) may carry a higher risk due to increased exposure to non-inherited paternal antigens.

Impact of Pre-existing Organ Dysfunction:

• Hepatic Impairment: Alters the metabolism of calcineurin inhibitors (CNIs) like tacrolimus and cyclosporine. Dose adjustments are critical, and lower trough targets (e.g., cyclosporine 150–200 ng/mL) may be necessary to avoid toxicity.
• Renal Failure: Reduces the clearance of mycophenolate mofetil (MMF) and certain CNI metabolites, leading to higher systemic drug exposure and increased risk of side effects.

4. Social Determinants of Health

Socioeconomic status, health literacy, and access to care are powerful, non-biologic modifiers of GVHD outcomes. These factors significantly influence a patient’s ability to adhere to complex medical regimens and recognize early warning signs.

• Medication Access and Adherence: High co-pays, “donut holes” in insurance coverage, and logistical challenges can lead to missed doses of critical immunosuppressants, resulting in subtherapeutic levels and breakthrough GVHD.
• Health Literacy: A patient’s ability to understand their condition is paramount. Low health literacy can delay the reporting of early GVHD symptoms like a new skin rash or mild diarrhea, allowing the disease to progress to a more severe, harder-to-treat stage.
• Socioeconomic Barriers: The need for frequent clinic visits, high out-of-pocket costs for medications and transportation, and time away from work create immense stress and can be insurmountable barriers to care for some patients.

5. Controversies and Future Directions

The field of GVHD is rapidly evolving, with a focus on biomarker-guided risk stratification and developing strategies that can uncouple the beneficial graft-versus-leukemia (GVL) effect from the detrimental effects of GVHD.

Balancing GVL and GVHD

The “holy grail” of allogeneic transplant is to eliminate GVHD while preserving the GVL effect. Key strategies include:

• Post-Transplant Cyclophosphamide (PTCy): Selectively depletes alloreactive T-cells while sparing regulatory T-cells and memory T-cells thought to be important for GVL.
• Selective T-Cell Depletion: Ex vivo techniques to remove specific T-cell subsets from the graft before infusion.
• Co-stimulation Blockade: Agents like abatacept, which block the CD28-CD80/86 pathway, have shown promise in preventing severe aGVHD in early clinical trials.

Emerging Therapies

Beyond broad immunosuppression, highly targeted cellular and molecular therapies are being investigated:

• Cellular Therapies: Infusions of regulatory T-cells (Tregs) or mesenchymal stromal cells (MSCs) aim to actively modulate the immune response and promote tolerance.
• Targeted Molecular Blockade: Agents targeting specific inflammatory pathways, such as IL-6 (tocilizumab) or ROCK2 (belumosudil for cGVHD), offer more precise intervention.