Graft vs. Host Disease - Marissa Burgess, Marco Yeung

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Pathology and Mechanism

Graft vs. Host Disease (GVHD) is a frequent complication after allogeneic transplants of bone marrow, stem cells, or other organ tissue. Donated tissue (graft) recognizes the recipient's (host) body as foreign, and immunocompetent cells within the graft attack the host, causing systematic tissue destruction, organ failures, and death in severe cases. There are more than 25,000 allogeneic transplants done per year, and projected to increase by more than twice within the next 5 years. [1] Transplants between unrelated people are steadily increasing to about 40% in 2013. GVHD occurs in about 30% in occurrence in transplants between related people, and up to 80% between unrelated people. 20% of the patients with GVHD are staying on prolonged treatment with immunosuppressant, with a wide variety of side effects. Severity are measured in four stages of increasing mortality, ranging from 90% survival in Stage I to 5% survival in Stage IV. [2]

Mechanism of aGVHD signal cascade

There are two types of GVHD identified in patients:

• Acute form of disease (aGVHD): Observed within 100 days from transplant. Pathology often comes with skin symptoms, and about 50% of patients displays liver, and GI symptoms. Degree of mismatch directly correlates with symptom severity and pain. In severe cases, ulceration and blisters can form on skin and surface of GI tracts, in addition to diarrhea, nausea, and anorexia. Damages to GI tract can cause gut microbes product to trigger additional immune response from graft T cells, which can potentially lead up to cytokine storm. [1, 3]
• Chronic form of disease (cGVHD): Observed after 100 days from transplant. Pathophysiology is still poorly understood. Symptoms can manifest all over the body, from jaundice and pericarditis, to thrombocytopenia and anemia. Patients often manage symptoms with extended applications of immunosuppressant, which brings on additional sets of side-effects. [1]

Development of GVHD can be summarized in three sequential phases:

T cell differentiation, and potential target for therapeutics

Activation of Antigen Presenting Cells (APC)

Host tissue damages caused by a wide variety of reasons, such as diseases, chemotherapy (as in the case of leukemia or other cancer), or other underlying conditions, respond by unleashing a variety of signaling molecules, including proinflammatory cytokines, chemokines, and adhesion molecules for antibody binding. Measured concentration of TNF-α seems to directly correlate to the likelihood of GVHD development. GI tract condition is also found to be essential for GVHD propagation, as damages to GI surface can release pathogen-associated molecular pattern (PAMP) from microbial flora, and accelerates cytokine release. Furthermore, previous recent exposure to viral infection, which leaves behind viral PAMPs, will also trigger release various signaling molecules, as indicated by a study involving increased GVHD associated with cytomegalovirus infection. [3-5]

Donor T Cell Activation

In response to stimulation from increased cytokine, donor T cells proliferates and differentiates into a variety of cells, such as cytotoxic T lymphocytes and natural killer cells. These activated immune cells goes through a cascade of intracellular pathways that resulted in transcription of large volume of cytokines and their respective receptors, such as IFN-γ, IL-2 and TNF-α. These cytokine are described to increase sensitivity of monocytes and macrophages, and appropriately accelerate their immune response activity. Various suppressive cytokines, such as IL-10 and TGF-β, has been theorize to be effective against onset of GVHD. [4-5]

Cellular Damage and Inflammatory Response

Additional tissue damage can trigger even more cytokine release, further stimulating additional T cell activation and proliferation. Mechanism includes: perforin, which causes nanoscale pores to form on cell membranes and cause leakage of cellular material and ion concentration upset, and granzyme, which causes apoptosis. Chemokine directs T cells to target organ via inflammation response. Worst case scenario is cytokine storm, where the immune system goes into overdrive into attacking itself due to the positive feedback loop between immune activity and cytokine signaling. [1, 5]

Causes

Billingham described three necessary conditions for GVHD to manifest, summarized below: [X1]

• Graft must contain immunocompetent cells (such as T cells, B cells from donor)
• Graft expresses different antigen than that of the recipient
• Recipient cannot effectively remove the transplanted immune cells from grafts

GVHD is triggered when donor T cells perceives human leukocytes antigen (HLA), encoded by major histocompatibility complex, that is different from its own. Odds and severity of GVHD increases with additional mismatch of HLA between recipient and donor. Furthermore, minor histocompatibility antigens, which are receptors expressed in almost every organ and hematopoietic cells, can triggers immune response from donor T cells. Therefore, genetic difference that lies outside of HLA-similar allografts can also trigger GVHD. Cytokines expressed by the activated donor T cells are implicated by numerous studies to be responsible for triggering immune pathways responsible for GVHD pathology. These includes, but not limited to, Tumor Necrosis Factor (TNF-α), Interleukin 10 (IL-10), and Interferon-γ (IFN-γ). Proteins that are responsible for innate immunity against various infections, such as Keratin 18 receptor, are also found to be responsible. [1, 3]

History

• 1950s: Immunosuppressants and their effects were described
• 1962: Barnes and Loutit first describe GVHD in mice
• 1964: Simonsen correlates host damage to immunologically competent cells from donor grafts
• 1966: Billingham discovered three criterias necessary for GVHD manifestation
• 1980s: T-cell depletion becomes available
• 2000s - 2010s: Various pharmacologic, such as inhibitors, steroids and monoclonal antibodies, targeting immune pathways responsible for GVHD become available
• 2011: New treatment options, such as extracorporeal photopheresis, become available
• Recently: Additional novel treatment options, such as regulatory T cell therapy and mesenchymal stem cell therapy, enters clinical trials

Treatment options

By examining pathology mechanism, researchers have identified different strategies for targeting various specific stages:

• Targeting T cells and B cells proliferation
• Targeting T cells homing onto host tissues
• Regulating T cells signaling/inhibitory pathways, i.e. cytokines
• Modulating intercellular pathways for immune regulations

Most current treatments aim to resolve aGVHD through a combination of the strategies described above, while few options available for cGVHD other than chronic application of immunosuppressant. [2, 5]

Immunosuppressants

Immunosuppressants, including cyclosporine and methotrexate, provide a “blanket” approach where immune response and signaling from both host and donor is shut down. For example, calcineurin, an enzyme essential for T cell activation, can be suppressed with cyclosporine to prevent and alleviate tissue destruction. Patients have to contend with a host of side effects, including significantly increased risk to all types of infection, water retention, weight gain, confusion, muscle weakness, osteoporosis, and avascular necrosis. [2, 5]

Glucocorticoids decrease the number of lymphocytes in the body, including T cells and B cells, because they inhibit nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). NF-κB is a transcription factor that is responsible for synthesizing many mediators (i.e. cytokines) and proteins that promote immune response; thus, inhibiting NF-κB hinders the immune system's ability to respond. [X3] Glucocorticoids also inhibit genes that code for certain cytokines, most importantly IL-2. Reduced production of cytokines decreases T cell proliferation. [X4]

Pharmacologic

A wide range of therapeutics is available for targeting various stages of GVHD mechanism. Various steroids has been shown to significantly lower incidence of GVHD symptoms, attributed to its properties of lymphocyte suppression and anti-inflammatory effect. Steroid treatments have improved survival rate in severe GVHD patient group to up to 50%. Alemtuzumab, a monoclonal antibody therapeutic, binds to CD52 on the surface of lymphocytes, which is then targeted for destruction. This eliminates a control number of immunocompetent cells from the body, and resulting in significantly lower GVHD for both aGVHD and cGVHD. A new drug, named Etanercept, impedes immune signaling pathway by blocking TNF-α and eliminating cytokine effects on T cell activation. Phase II trial indicate a 70% resolution among patient group with aGVHD within 1 month of application. [2, 6]

T cell depletion (TCD)

Since donor T cells are irreplaceable in initialing the sequence of events leading up to GVHD, removal of T cells from graft tissues might achieve alleviation/prevention of host tissue damages. This procedure is referred to as TCD, where donor T cells are deliberately removed from graft tissues to eliminate response from host-released cytokine. Various strategies of removing T cells have been suggested, including 1.) negative selection of T cells ex vivo, 2.) positive selection of CD34+ stem cells ex vivo, and 3.) anti-T cell antibodies in vivo. A small remainder of T cells remain in the graft tissues.

Results from clinical trials indicated that while the patients group experienced drastic reduction in GVHD after bone marrow transplant, survival rate is almost equal to control group due to offset from prevalent graft failures/rejection, increased susceptibility to infections, various lymphoproliferative disorders, and release of malignancy. This is attributed to donor T cells are also responsible for leukemia cancer cell destruction after grafting, referred to as graft-vs.-tumor effect. Selective TCD has been employed in a number of trials, where CD-8 subsets are specifically targeted for removal based on their signal pathway toward aGVHD. Results remain inconclusive on the benefit of selective TCD, however. [2, 7]

Extracorporeal photopheresis

This treatment aims to eliminate controlled number of immunocompetent cells within host, in an attempt to control immune pathway responsible for GVHD. Inflammatory T cells are collected by aphresis and are mixed with DNA-intercalating agent 8-methoxypsoralen. The collection is exposed to UV light, where the white blood cells suffer from irreparable DNA damage and undergo apoptosis. This eliminates the cells primarily responsible for triggering GVHD, and reduces inflammatory effect. This also increases ratio between regulatory T cells to inflammatory T cells, affording greater control of cytokine signaling and preventing cytokine storm trigger. This has resulted in high resolution rate among a large majority of aGVHD patient, with a 50% long term survival rate. While no side effects were mentioned in the article, it's very likely that patient will suffer from increased susceptibility from infection as a side effect. [2]

Future outlook

GVHD remains a challenge for proliferating the usage of tissue/stem cell grafts as treatment for a wide variety of cancers and autoimmune disorders. The exact pathology and mechanism, although shown to be involving donor T cell activation and signaling, are still not known completely. Traditional treatment for GVHD, especially for cGVHD, often comes with significant drawbacks, such as debilitating side effects, immunodeficiency, or graft rejection. Current research involves further understanding various stages of T cell signaling in GVHD, and coming up with new targets for therapeutic treatment. A wide array of new therapy options, such as cellular component therapy, are also entering clinical trial recently. [1, 2]

In 2012, Osiris Therapeutics proclaimed that health regulators in Canada had approved Prochymal for the treatment of acute graft-versus host disease. Prochymal was the first stem cell drug in the world allowed for a systemic disease. [X2]

References

• 1. Ferrara et al. "Graft-versus-Host Disease." Lancet 373.9674 (2009): 1550-561.
• 2. Pasquini et al. 2010 report from the Center for International Blood and Marrow Transplant Research (CIBMTR): current uses and outcomes of hematopoietic cell transplants for blood and bone marrow disorders. Clin Transpl. 2010:87–105.
• 3. Blazar, et al. "Advances in Graft-versus-host Disease Biology and Therapy." Nature Reviews Immunology 12.6 (2012): 443-58.
• 4. Van Den Brink et al. "Cytolytic Pathways In Haematopoietic Stem-Cell Transplantation." Nature Reviews Immunology 2.4 (2002): 273-81.
• 5. Reddy et al. "A Crucial Role for Antigen-presenting Cells and Alloantigen Expression in Graft-versus-leukemia Responses." Nature Medicine 11.11 (2005): 1244-249.
• 6. Welniak et al. "Immunobiology of Allogeneic Hematopoietic Stem Cell Transplantation." Annual Review of Immunology 25.1 (2007): 139-70.
• 7. Vogelsang et al. "Pathogenesis and Treatment of Graft-Versus-Host Disease After Bone Marrow Transplant." Annual Review of Medicine 54.1 (2003): 29-52.
• [X1]. Spiryda, L. B.; Laufer, M. R.; Soiffer, R. J.; Antin, J. A. Biology of Blood and Marrow Transplantation 2003, 9 (12), 760–765.
• [X2] http://www.natlawreview.com/article/world-s-first-stem-cell-drug-approval-achieved-canada