John had been healthy until he was seven years old, when he began complaining of fatigue. His mother noticed that he had become very pale and he had developed small hemorrhages (petechiae) on the skin of his arms and legs. Apart from the pallor and skin petechiae, a physical exam showed nothing unusual. Blood tests revealed that John was anemic: his hemoglobin was 7 g/dl (normal 10-15 g/dl) and his platelet count was 20,000/ml (normal 150,000-250,000/ ml). His white blood cell count was also lower than normal. The pediatrician sent John for a hematology consult, where a bone marrow biopsy was performed.
The biopsy showed that John's bone marrow had very few cells and that red cell, platelet, and white cell precursors were almost completely absent. Aplastic anemia of unknown cause was diagnosed. Aplastic anemia is ultimately fatal but can be cured by a successful bone marrow transplant. Fortunately, John had an HLA-identical 11-year-old brother who could be the bone marrow donor. John was admitted to Children's Hospital and given a course of busulfan (a cytotoxic drug) to eradicate his own leukocytes and make room in the marrow for new stem cells. He was then given 2 x 108 nucleated bone marrow cells per kg body weight obtained from his brother's iliac crests.
Figure 1. Left, surgeons remove bone marrow from the iliac crest (hip bone) of the donor using a large syringe. Right, the recipient receives the bone marrow transfusion i. v.
John did well for three weeks after the bone marrow transplant and was then sent home to recover. However, on the 24th day after transplant he was readmitted to hospital with a skin rash and watery diarrhea consistent with acute GVHD. For a recipient to develop GVHD, the graft must contain immunocompetent cells, the recipient must express major or minor histocompatibility antigens that are lacking in the donor, and the recipient must be incapable of rejecting the graft.
Figure 2. Skin rash of GVHD.
On admission, John had an itchy, patchy red rash on palms and soles, scalp, and neck. He had no fever and was not jaundiced. His lungs were clear and his heartbeat normal. His liver and spleen were not enlarged. John was treated with corticosteroids and the immune suppressive drug tacrolimus (FK506). His skin rash faded, but the intestinal symptoms did not abate and the diarrhea became more profuse. He was given a test dose of antithymocyte serum but developed a fever and shaking chills, so this therapy had to be discontinued. John developed bleeding from the colon and a colonoscopy revealed diffuse GVHD throughout the large bowel.
After six weeks in which the diarrhea did not improve, John was given an anti-CD3 monoclonal antibody by intravenous injection, to try and eliminate the mature donor T cells. He was also given octreotide (an inhibitor of intestinal vasoactive peptide) and another immune suppressive drug, mycophenolate mofetil. This therapy provided no relief of his symptoms, which were finally controlled by injection of a monoclonal antibody to CD2 (another T cell surface marker) every three days. This brought about a 90% decrease in the volume of his stool and the intestinal bleeding stopped. Treatment with anti-CD2 antibody was continued weekly for two months and his symptoms completely disappeared. John was sent home, with continuing treatment with low doses of corticosteroid, after six months hospitalization for GVHD.
Questions
1. Why do bone marrow transplant patients get graft-versus-host disease (GVHD) instead of host-versus-graft rejection?
2. If John and his brother were HLA-identical, why did John get GVHD? [HINT: What is a minor histocompatibility antigen?]
3. What immune cells and molecules are involved in GVHD? Is it primarily a cellular or humoral immune response?
4. Could the transplant center have treated the bone marrow before infusion to remove mature T cells? If so, how could that have been done?
5. Is there a way to test histocompatibility to minor histocompatibility antigens?
6. Why were the skin and GI tract the major sites of GVHD?
7. Why were the anti-CD2 treatments able to be stopped after two months?
8. When bone marrow transplants are given, at least one MHC antigen must be shared between donor and recipient: why? [HINT: review positive and negative selection].
9. When bone marrow transplants are done to cure leukemia, some mature cells in the marrow can have a beneficial effect: why?
10. For what other conditions are bone marrow transplant done?