What causes gvhd

Overview

Graft-versus-host disease (GVHD) is a serious and potentially life-threatening complication that can arise after an allogeneic stem cell transplant (SCT), also known as a bone marrow transplant. In this procedure, a patient receives healthy stem cells from a donor to replace their own diseased or damaged bone marrow. While the transplanted cells aim to restore the patient's immune system and fight their underlying disease (like leukemia or lymphoma), they also contain active immune cells from the donor. GVHD occurs when these donor immune cells (the 'graft') recognize the recipient's body tissues (the 'host') as foreign and initiate an immune attack against them.

What is an Allogeneic Stem Cell Transplant?

An allogeneic SCT involves transplanting hematopoietic stem cells (which produce blood and immune cells) from a healthy donor into a recipient. This is often a last resort treatment for various hematologic malignancies, aplastic anemia, and certain genetic disorders. The goal is for the donor stem cells to engraft in the recipient's bone marrow, proliferate, and produce healthy blood and immune cells. A crucial aspect of this therapy is the 'graft-versus-leukemia' (GvL) effect, where the donor immune cells also target and destroy any remaining cancer cells in the recipient. However, this same immune system can mistakenly attack healthy recipient tissues, leading to GVHD.

The Mechanism of GVHD

GVHD is fundamentally an immune-mediated process. The donor's T-lymphocytes, a type of white blood cell crucial for immune responses, are the primary culprits. When these T-cells encounter recipient tissues, they recognize differences in human leukocyte antigens (HLAs) – proteins found on the surface of most cells that help the immune system distinguish self from non-self. Even with a well-matched donor (e.g., a sibling with a high HLA match), minor differences can exist. Upon recognizing these foreign antigens, the donor T-cells become activated. These activated T-cells then orchestrate an inflammatory response and recruit other immune cells to attack various organs and tissues in the recipient. This attack can damage healthy cells and tissues, leading to the symptoms of GVHD.

Factors Influencing GVHD Development

Several factors influence the likelihood and severity of GVHD:

• HLA Mismatch: The degree of HLA compatibility between the donor and recipient is a major determinant. The greater the mismatch, the higher the risk of GVHD. While a 10/10 or 9/10 HLA match is ideal, even a single mismatch can increase risk.
• Cell Dose: The number of T-cells infused with the stem cell product can influence GVHD risk. Higher T-cell doses generally increase the risk.
• Donor Type: Different donor sources (e.g., bone marrow, peripheral blood stem cells, umbilical cord blood) and donor relationships (e.g., matched sibling, matched unrelated donor, haploidentical donor) have varying GVHD risks. Unrelated donors and haploidentical donors (where only half of the HLA genes match) often carry a higher risk.
• Recipient Factors: Age and the recipient's overall health status can also play a role.
• Conditioning Regimen: The chemotherapy and radiation used before the transplant to eliminate the recipient's diseased cells and suppress their immune system can also impact GVHD.
• Graft Manipulation: Techniques used to reduce T-cells in the donor graft (T-cell depletion) can lower GVHD risk but may also increase the risk of graft failure and relapse of the original disease.

Types of GVHD

GVHD is broadly classified into two main types based on its timing and clinical presentation:

Acute GVHD

Acute GVHD typically develops within the first 100 days after transplantation, although it can sometimes occur later. It is primarily mediated by the activation and proliferation of donor T-cells. The most commonly affected organs are the skin, liver, and gastrointestinal tract.

• Skin: Manifests as a rash, which can range from mild redness to severe blistering.
• Liver: Causes elevated liver enzymes and jaundice.
• Gut: Leads to nausea, vomiting, diarrhea (which can be bloody), and abdominal pain.

The severity of acute GVHD is graded from I to IV, based on the extent of organ involvement and symptoms.

Chronic GVHD

Chronic GVHD can occur after 100 days, and sometimes even months or years, post-transplant. It is thought to involve a more complex immune dysregulation, potentially including both donor T-cells and recipient B-cells, and can affect a wider range of organs. It often resembles autoimmune diseases.

• Skin: Can cause hardening, thickening, or scarring of the skin, and dry mouth.
• Eyes: Leads to dry eyes or Sjögren's syndrome.
• Mouth: Causes dry mouth and mouth sores.
• Liver: Can result in chronic liver dysfunction.
• Lungs: May cause shortness of breath and lung scarring (fibrosis).
• Musculoskeletal System: Can lead to joint stiffness, contractures, and muscle weakness.
• Gastrointestinal Tract: May cause malabsorption and weight loss.

Chronic GVHD can significantly impact a patient's quality of life and long-term survival.

Prevention and Management

Preventing GVHD is a major focus in stem cell transplantation. Strategies include using well-matched donors, employing T-cell depletion techniques, and administering immunosuppressive medications (like cyclosporine, tacrolimus, methotrexate, mycophenolate mofetil, and steroids) to dampen the donor immune response. For patients who develop GVHD, treatment involves increasing immunosuppression and sometimes using newer therapies targeting specific immune pathways.