What causes chronic gvhd

What is Chronic Graft-Versus-Host Disease (cGVHD)?

Chronic graft-versus-host disease (cGVHD) is a significant and potentially life-threatening complication that can arise after an allogeneic stem cell transplant (also known as a bone marrow transplant). In this procedure, a patient receives healthy stem cells from a matched donor. While this process is designed to replace diseased or damaged bone marrow with healthy cells, the donor's immune system, now present in the recipient's body, can sometimes mistakenly identify the recipient's tissues and organs as foreign. This immune response, where the donor's T-cells attack the recipient's body, is the hallmark of graft-versus-host disease (GVHD).

GVHD can manifest in two forms: acute and chronic. Acute GVHD typically occurs within the first 100 days post-transplant and primarily affects the skin, liver, and gastrointestinal tract. Chronic GVHD, on the other hand, usually develops later, generally between 3 and 12 months after the transplant, although it can manifest even years later. It is a more systemic and often more prolonged condition, affecting a wider range of organs and tissues.

What Causes cGVHD?

The fundamental cause of cGVHD is an immune system mismatch between the donor and the recipient. Specifically, it is caused by the donor's T-lymphocytes (a type of white blood cell) recognizing the recipient's cells as foreign and initiating an immune attack. This occurs because the donor's immune system is essentially 'seeing' the recipient's body as an intruder.

The Role of Immune Incompatibility

The primary drivers of this immune recognition are human leukocyte antigens (HLAs), which are proteins found on the surface of most cells in the body. HLAs act like a cellular ID card, helping the immune system distinguish between the body's own cells and foreign invaders. While stem cell transplant protocols aim for the closest possible HLA match between donor and recipient, perfect matches are not always achievable, especially if a related donor is not available.

Even with a high degree of HLA matching, minor differences can exist. These differences can be enough for the donor's T-cells to detect them and mount an immune response against the recipient's tissues. The donor's immune system, now residing in the recipient's body, is primed to identify and eliminate foreign cells, and unfortunately, it can mistake the recipient's healthy cells for these foreign entities.

Factors Influencing cGVHD Risk

Several factors can influence a patient's risk of developing cGVHD:

• Degree of HLA Match: The less compatible the HLA match between donor and recipient, the higher the risk of cGVHD. Using unrelated donors or partially matched donors increases this risk compared to using a perfectly matched sibling donor.
• Stem Cell Source: Stem cells derived from peripheral blood are associated with a higher risk of cGVHD compared to bone marrow stem cells.
• Donor Type: Allogeneic transplants using stem cells from a donor other than the patient carry the risk.
• Recipient's Age: Older recipients may have a higher risk.
• Previous GVHD: Patients who experienced acute GVHD are also at increased risk for developing chronic GVHD.
• Immunosuppressive Therapy: The type and duration of immunosuppressive drugs used post-transplant can influence the risk, as these medications aim to prevent GVHD but can also affect the immune system's ability to recover.
• Graft-Versus-Tumor (GVT) Effect: While the immune attack is detrimental in cGVHD, the donor's immune cells can also play a beneficial role by attacking any remaining cancer cells in the recipient's body (the graft-versus-tumor effect). Managing cGVHD involves balancing this beneficial effect with the harmful effects of GVHD.

How cGVHD Develops

The development of cGVHD is a complex immunological process. Following the transplant, the donor's T-cells, which are a key component of the adaptive immune system, become activated when they encounter the recipient's tissues. This activation triggers a cascade of inflammatory responses.

Initially, in acute GVHD, there is a rapid release of inflammatory cytokines. In chronic GVHD, the process is more insidious and involves a shift towards a fibrotic and sclerotic response. The donor's immune cells not only attack the recipient's cells but also stimulate the recipient's own cells (fibroblasts) to produce excessive amounts of collagen and other extracellular matrix components. This leads to tissue damage, scarring (fibrosis), and hardening (sclerosis) of affected organs.

The chronic inflammation and tissue damage characteristic of cGVHD can lead to a wide range of symptoms. The presentation is highly variable, but common sites of involvement include:

• Skin: Rashes, dryness, thickening, itching, and sometimes blistering or ulceration.
• Mouth: Dryness (xerostomia), sores, white patches, and difficulty eating.
• Eyes: Dryness (keratoconjunctivitis sicca), redness, light sensitivity, and blurred vision.
• Gastrointestinal Tract: Diarrhea, nausea, vomiting, abdominal pain, and malabsorption.
• Liver: Jaundice, elevated liver enzymes, and impaired liver function.
• Lungs: Shortness of breath, dry cough, and airflow obstruction (bronchiolitis obliterans).
• Musculoskeletal System: Joint stiffness, contractures, and muscle weakness.
• Genitourinary System: Vaginal dryness or narrowing, and urinary symptoms.
• Immune System: Increased susceptibility to infections due to ongoing immunosuppression and impaired immune function.

Managing and Preventing cGVHD

Preventing cGVHD is a primary goal in stem cell transplantation. This involves careful donor selection to achieve the best possible HLA match and the use of prophylactic medications.

Prophylactic strategies often include a combination of immunosuppressive drugs given to the recipient starting before or shortly after the transplant. Common drugs used for prevention include cyclosporine, tacrolimus, methotrexate, mycophenolate mofetil, and sirolimus. Additionally, T-cell depletion (removing T-cells from the donor graft) is sometimes used, particularly when using unrelated or partially matched donors, although this can increase the risk of infection and graft failure.

Once cGVHD develops, treatment focuses on controlling the immune response and managing the symptoms. This typically involves escalating immunosuppressive therapy. Corticosteroids are often the first line of treatment. If steroids are not effective or if side effects are problematic, other immunosuppressants such as calcineurin inhibitors (cyclosporine, tacrolimus), JAK inhibitors (ruxolitinib), and other agents are used. Extracorporeal photopheresis (ECP), a form of light therapy, is also an important treatment modality for refractory cGVHD.

Supportive care is also critical for managing cGVHD. This includes treatments for dry eyes, dry mouth, skin care, nutritional support, physical therapy for joint stiffness, and management of infections. Long-term monitoring by a multidisciplinary team is essential for patients who have undergone a stem cell transplant, especially those with cGVHD, to manage the condition effectively and improve their quality of life.