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Cells – radioprotectors

When I give to my mice a lethal dose of irradiation, sometimes I think what actually kill them without bone marrow transplant? We all know that irradiation kills rapidly dividing (actively cycling) cells – progenitors in the bone marrow and gut. We also know that a significant fraction of organ-specific adult stem cells is non-dividing (quiescent). So, it seems like hematopoietic stem cells (HSC) in bone marrow should survive irradiation much longer than any other cells. This is true – HSC, like other adult stem cells, are extreme survivors [1]. So, a lethal dose of irradiation will kill all of progenitors in bone marrow and mice will die in 6-15 days due to huge deficit of many mature blood cell types. Some cycling HSC are also going to be killed and make a vacant bone marrow niche space. Quiescent HSC will start to divide in order to rescue huge progenitors demand and, at some time point, probably exhaust. I believe that at the time point when mice are dying after irradiation enough HSC have survived to potentially rescue an animal and sustain hematopoiesis. But host HSC become no longer functional. And the reason is radiation-induced toxic bone marrow environment [2], which causes anergy and, eventually, apoptosis of surviving HSC.

A little quote [3] about bone marrow cells sensitivity to irradiation:

… total body exposure at doses more than 7–8 Gy total body irradiation (TBI) in human corresponds to medullar eradication. Under this threshold spontaneous recovery from residual hematopoietic stem and progenitor cells may be expected within 30–50 days but going through cytopenic phases of granulocytic, megakaryocytic and erythrocytic lineages. HSCT should be considered if the victim’s HSC pool is essentially irreversibly damaged. Interestingly, even after TBI, intrinsically radioresistant stem cells have been detected [4] in distinct bone marrow (BM) areas comprising a residual hematopoietic stem and progenitor cell pool.

I was always curious how can we possibly rescue lethally irradiated mice without bone marrow transplant? If there are at least one or two surviving HSC in bone marrow, it becomes a real possibility. The most convincing evidence is coming from the observation that after bone marrow transplant we frequently see not only predominant donor-derived blood chimerism (90-97%), but also, some host-derived cells (1-5%). Percent of host-derived cells in bone marrow usually is much higher than in the blood of rescued animals, because our extreme survivors – HSC reside there. So, donor’s BM cells are able to rescue residual host HSC to some degree.

Back in 2002, Koichi Akashi’s group demonstrated [5] that some donor-derived BM progenitors can rescue lethally irradiated mice:

… rare hematopoietic stem cells survive myeloablation that can eventually repopulate irradiated hosts if myeloerythroid-restricted progenitors transiently rescue ablated animals through the critical window of bone marrow failure.

Another cell type, which can protect host HSC from the damaging effects of lethal irradiation is mesenchymal stromal stem/ progenitor cells (MSC). Because MSC is making a niche in bone marrow, it was proposed that their co-transplantation will increase donor’s HSC engraftment. Clinical trials are still on the way [6], but it becomes clear that donor’s MSC are able to increase HSC performance without engrafting in bone marrow. Recent study [3], which tested radioprotective function of MSC, clarified the potential mechanism:

Our data revealed that systemically administered MSCs provoke a protective mechanism counteracting the inflammatory events and also supporting detoxification and stress management after radiation exposure. Further our results suggest that MSCs, their release of trophic factors and their HSC-niche modulating activity rescue endogenous hematopoiesis thereby serving as fast and effective first-line treatment to combat radiation-induced hematopoietic failure.

So, donor’s MSC are able to clean radiation-induced toxic bone marrow environment via paracrine mechanisms and make the residual host HSC happy. More than that, it was reported in the ISCT 2010 annual meeting [7], that besides restoring endogenous hematopoiesis, allogeneic MSC can promote regeneration of damaged gut epithelium:

A single injection of 6 millions C57Bl/6 MSC given 24 hours following 9Gy total body irradiation protected all treated mice while 50% mortality was recorded in the PBS control group.
… these results clearly demostrated the potential of MSC as a complementary treatment for radiotherapy or for the treatment of accidental radiation exposure.

From cells to genes and proteins
Well, if the mechanisms of cell-radioprotectors is due to release of growth factors, why don’t we identify them and use them instead of cells? More than a decade ago people started to experiment with hematopoietic cytokines and growth factors [4] in order to rescue lethally irradiated mice without bone marrow transplant. it was shown that early injection of hematopoietic cytokines rescues host HSC [8] in lethally irradiated mice.
Very interesting and promising approach is targeting PUMA gene [9]. PUMA is p53 up-regulated mediator of apoptosis.

In the absence of Puma, HSCs were highly resistant to Puma-null mice or the wild-type mice reconstituted with Puma-null bone marrow cells were strikingly able to survive for a long term after high-dose
… This study demonstrates that Puma is a unique mediator in radiation-induced death of HSCs. Puma may be a potential target for developing an effective treatment aimed to protect HSCs from lethal radiation.

From the experimental concept to “off-the-shelf” commercial products
I don’t know what is the current status of using individual hematopoietic cytokines for treatment of radiation exposures. I hope we can therapeutically target PUMA soon. Using cell products sounds more promising to me, because cells can release an unique “natural mixture” of cytokines and growth factors, many of which we don’t know yet. Using of unmatched allogeneic “off-the-shelf” cells could be more advantageous in comparison with conventional bone marrow transplant. The feasibility of using allogeneic frozen hematopoietic progenitors for treatment of acute radiation syndrome was tested by Colleen Delaney’s group and reported at ASH 2010 [10]:

Infusion of Notch-expanded and cryopreserved cells into lethally irradiated mismatched recipients demonstrated that short-term engraftment without manifestations of GVHD can be achieved across major H-2 barriers and resulted in significantly enhanced survival in a dose dependent manner.

Cellerant Therapeutics [11] have got a cell product in development for treatment of acute radiation syndrome. They claim that “CLT-008 Will Keep People Alive After a Nuclear Attack! [12]” CLT-008 is expanded allogeneic myeloid progenitors “off-the-shelf”.

In conclusion, I have some reasons for hope that we will get radioprotective “off-the-shelf” cell products or biologicals soon and be ready for the next Chernobyl or Hiroshima.