One step forward in the fight against leukemia
Most of the time scientific knowledge advances by tiny steps, rarely by leaps and bounds. One such example is the tortuous history of the use of transplantation of bone marrow to treat hematological malignancies called leukemia.
How the immune system works in a nutshell.
Let's go slowly and start with a description of leukemia types and a short explanation of our immune system or how we avoid being eaten by other organisms. Hematological means from the blood; therefore, hematological malignancy is a cancer originating from one of the cell types present in the blood. As with any other cancer, they originate when one of the progenitors of a blood cell type loses control over its growth and starts reproducing continuously. There are many types of blood cells (See figure below). They are broadly divided into red and white. Red blood cells or erythrocytes are the transporters of oxygen from the lungs to all parts of the body. Platelets or thrombocytes are transparent and their principal function is to help coagulation. White blood cells or leukocytes are subdivided into myeloid (granulocytes) and lymphoid blood cells (B cells, T cells and NK cells). Granulocytes are direct killers of invaders (bacteria or parasites) and defective cells (dead or cancerous). Lymphoid cells are the sentinels of the immune system which fight infection by two approaches: the innate, carried on mostly by natural killer (NK) cells, and the adaptive responses which are performed by B and T cells.
Human armies use uniforms to distinguish friend from foe. The immune systems of large animals, such as ourselves, use a similar approach; however, the uniform is made up of a collection of certain molecules (proteins, complex carbohydrates or lipids) present on the surface of the cells. Furthermore, some of these surface molecules (HLA class I and II, for example) present pieces of other molecules from the inside of the cell to B or T cells; therefore, the surveillance is not confined to the surface. The B cells, T cells and NK cells are the checkers. Each B cell has a receptor, randomly created, capable of recognizing and binding a particular molecule, the antigen. When a B cell encounters the antigen specific for its B cell receptor either floating or on the a cell's surface, it becomes activated, starts reproducing, becomes a blast cell and produces and release antibodies against its antigen. T cells approach cells and make sure that all the molecules present on their surfaces belong to the animal or person and not to an outsider. NK cells do the same but they check for the lack of certain molecules such as the mayor histocompatibility complex (MHC) proteins (these molecules are called HLA in humans for Human Lymphocyte Antigens). This is what is the recognition of self. But how T cells can know that a protein on the surface of a cell belongs to, let's say John Doe versus John Smith? Well, they are selected before being released on duty. T cells are selected in the thymus, a small gland in our neck, and B cells in the bone marrow. Developing B and T cells that bind too strongly or too weakly to self molecules (those present in John Doe) are deleted or become anergic (inactive), the ones that pass the test are released into the blood and lymph streams. When a T cell encounters a cell that shows a piece of a pathogen (or a piece of John Smith) and this “antigen presenting cell” is excited because it comes from a war zone, the T cell itself becomes also activated and starts reproducing and releasing danger signals in the form of short proteins called interleukins (See figure below).
Developments toward a cure for recalcitrant leukemia.
Treatment for leukemia is based on radiation or chemotherapy or more often a combination of the two. Radiation and the toxic chemicals use for chemotherapy kill cells that are reproducing rapidly. The dosages are carefully calibrated with the hope of killing all the cancer cells and as few as possible of the normal fast growing cells. Normal fast growing cells in the human body are blood cell precursors, epithelia, which are the layers of cells covering the outside of the body (skin) or those on the surface of cavities (mucosa). The epithelial cells that produce hairs in hair follicles and those forming the intestinal mucosa are perhaps the fastest normal growing cells in the body and for that reason after chemotherapy, patients lose hair and suffer bouts of diarrhea.
A serious side effect of the treatment is a reduction in the ability of the immune system to protect against infections. This is a direct consequence of the destruction of the precursors of blood cells, the sentinels and soldiers of the immune system. Damage to epithelia increases this risk even more, especially in the intestine where just one layer of cells separate us from billions of bacteria. Unfortunately, some of the cancerous cells (cancer stem cells) do not grow rapidly and may escape the effect of the treatment; consequently the leukemia may relapse.
From studies in mice, scientists found that a good way to get rid of the leukemia and reduce the risk of infections was to irradiate the animal until all or almost all its bone marrow and blood cells were killed followed by transplantation of bone marrow cells from an identical mouse. The grafted cells would quickly reconstitute the bone marrow, the blood cells and the immune system. If the donor and recipient mice are not quite identical, then the donor lymphocytes will attack the recipient cells and produce graft versus host disease (GVHD). The recipient animal may either die, clear the GVDH or continue with chronic GVDH. The main villains in GVDH are T cells and depletion of T cells in the graft reduces the probability and severity of GVDH.
The technique was applied to humans for the first time in 1959 using bone marrow from the patient's identical twin (see Copelan E.A. for a review New England Journal of Medicine 2006;354:1813-26). The leukemia recurred and the patient died 66 days after the first irradiation treatment. The reason for the cancer recurrence is that cancer stem cells are hard to kill and it is difficult to balance the level of irradiation and chemotherapy to kill all the cancer stem-cells without causing too much tissue destruction. Doctors, nevertheless, continued using transplantation with different levels and/or different chemicals plus using non-identical donors since most patients do not have available homologous donors. After the discovery of the role played by the MHC in the self-recognition mechanism, it was possible to closely match relatives as well as non-relatives to the recipient genotype. Since the HLA genes are present as a cluster in chromosome 6, they are inherited as a group or haplotype. Remember, we each have two chromosomes 6, one from the mother and one from the father. A patient has a 1 in 4 chance of having identical HLA genes to a sibling and a 3 in 4 chance of being haploidentical with only half of the HLA genes being identical. Clearly, it is a lot more likely that a patient has an haploidentical sibling than an identical one.
Very quickly it was learned that it was possible to use allogeneic (non-identical) donors provided that the difference was small and the recipient was decimated of immune cells, to avoid graft rejection, and maintained in an immunosuppressed state after transplantation. It was also found that haploidentical grafts gave fewer cases of cancer recurrence than isogeneic (those from identical twins), that male donors receiving haploidentical grafts from female donors had fewer recurrence rates, but not the other way around, and that the rate of recurrence and complications were proportional to the level of difference between donor and recipient.
The explanation for the lower levels of recurrence among allogeneic transplantations was explained by the fact that GVDH helps to destroy the remnants of recipient blood and leukemia cells by recognition of the major HLA gene products. The lower level of recurrence among haploidentical male recipients from female donors was caused by GVDH recognition of minor histocompatibility products from genes in the Y chromosome. These results indicated that the best conditions for transplant success and complete remission were to use a graft depleted of T cells and a donor with small HLA differences, if possible in minor histocompatibility genes, to induce a weak GVDH. Unfortunately, depletion of graft T cells delays the reconstitution of the immune system and, therefore, increases the chance of infections post transplantation. Because of this, some scientists thought of using regulatory T cells (Treg) to counter donor T cell reaction against the recipient's tissues and avoid the necessity of immunosuppression (see the table above for a summary of relevant factors).
Recently, a group of scientists at the University of Perugia in Italy (Di Ianni et al. Blood 117: 3921-3928. 2011) treated 28 patients, with myeloid or lymphoid leukemia in which other treatments had failed, with a program of irradiation and chemotherapy followed by injection of Treg cells purified from a haploidentical donor's blood and 4 days later with injection of donor's bone marrow stem-cells plus regular donor's blood T cells. There was rapid blood cell reconstitution in 26 of the patients, only 2 suffered severe GVDH and none developed chronic GVDH. This is remarkable considering that no immunosuppressive treatment was used post transplantation. Half of the patients, however, died of infections or organ failure and one suffered a relapse. This proves that Treg cells can be used to control the GVDH while conventional T cells can also help to wipe out the remaining cancer cells. One small step ahead, the next one would be how to optimize the reconstitution of the immune system to improve the chances of success against lurking pathogens, ie. how to also prevent infections.