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Acceleration of tissue regeneration by recruiting bone marrow-derived

mesenchymal stem cells

Katsuto Tamai, M.D., Ph.D.

Division of Gene Therapy Science, Osaka University Graduate School of Medicine,

2-2 Yamadaoka, Suita, Osaka, Japan.

Bone marrow cell transplantation (BMT), either locally or systemically, might be an

option as therapies for various diseases with tissue damage. To be successful, however,

transplanted bone marrow cells would have to efficiently migrate to the target tissues and

provide appropriate cell types capable of correcting the tissue damage. In this context, BMT is

clearly an option for treating diseases of hematopietic and mesenchymal tissues, but for skin

diseases, BMT might not at first appear to be possible or rational because the skin is an

epithelial tissue which has not been shown previously to contain an adequate number of bone

marrow-derived keratinocytes (BMDKs), either in normal or injured conditions.

Epidermolysis bullosa (EB) refers to a group of inherited blistering skin diseases caused

by genetic defects of structural molecules at the cutaneous basement membrane zone. One of

the clinically most severe forms of EB, known as recessive dystrophic EB, results from loss

of the anchoring fibril protein, type VII collagen. In affected individuals, trauma to the skin

leads to extensive blister formation with detachment of the epidermis, including keratinocyte

stem cells, from the underlying dermis. Detachment of the epidermis followed by

regeneration of new epidermis occurs throughout the lives of EB patients, evoking a

fundamental question of just how EB skin is able to regenerate the epidermal structures when

faced with the continuous loss of keratinocytes stem cells in the blister roofs.

We recently examined the capacity of bone marrow to contribute to the regeneration of

skin by searching for evidence of BMDKs in EB skin. The mouse model for dystrophic EB,

which completely lacks type VII collagen gene and protein expression, shows extensive

blister formation and dies a few days after birth. Therefore, we grafted full-thickness skin

from this mouse onto a syngeneic wild type mouse, which had been transplanted with green

fluorescent protein (GFP)-transgenic bone marrow cells via its tail vein following lethal dose

irradiation and observed for 8 weeks to ensure successful BMT prior to the skin graft. The

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engrafted EB mouse skin initially exhibited blisters with separation of the epidermis from the

underlying dermis, akin to the knockout mouse and patients with dystrophic EB. However,

subsequent assessment of the engrafted mice showed that the severity of the skin blistering

became less or ceased, suggesting functional improvement of the EB skin on the GFP-BMT

mouse. To assess the phenotypic improvement, we examined the EB skin histologically at

day-24 after the engraftment. Surprisingly, GFP-positive keratinocytes expressing

skin-specific cytokeratins were clearly evident in the regenerated epidermis of the EB skin.

Even more remarkable was the detection of type VII collagen at the cutaneous basement

membrane zone in the EB mouse skin. Of note, EB mouse skin engrafted onto the

GFP-transgenic mouse with prior wild type BMT did not show any GFP-positive BMDKs.

These data strongly suggest that the functional BMDKs providing type VII collagen in the EB

mouse skin are exclusively derived from bone marrow cells.

At present, there is no effective therapy for patients with EB. Current management

often involves skin grafts, either derived from unaffected skin sites or from cultured

keratinocytes. Our observations in this mouse model may therefore have implications for

interpreting how skin grafts in patients with EB work: rather than simply adding keratinocytes

to the wounds, the benefits might actually be explained by migration of bone marrow cells to

provide robust BMDKs in the grafted skin. A further important consideration of BMT in

patients with EB is immune tolerance following restoration of type VII collagen to the

basement membrane zone. BMT is known to induce central immune tolerance against

molecules expressed in the transplanted marrow-derived cells in the thymus. Since EB

patients may not have immune tolerance against type VII collagen, BMT may not only rescue

epidermal tolerance but also may induce immune tolerance in advance of supplementary

therapies such as protein, cell or gene therapies.

We have proved for the first time that transplanted bone marrow cells can ameliorate

the pathological abnormalities underlying a genetic skin disease. Although clinically-relevant

sources of transplantable bone marrow cells may be limited to either HLA-matched donor

cells or self-marrow cells with prior gene correction, our findings provide novel insight into

BMT as a potential treatment for intractable diseases affecting epithelial tissues such as the

skin.

Very recently, we have found that grafted skin would generate bone marrow-derived

mesenchymal stem cell-recruiting signals, and referred this factor as “KOI-KOI signal”, since

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“KOI-KOI” means “Come here” in Japanese. This KOI-KOI signal may be applied as a drug

to recruit mesenchymal stem cells to the damaged tissues, hopefully resulting in accelerating

functional scar-less healing (Figure).

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