2380? The?Journal?of?Clinical?Investigation http://www.jci.org Volume 117 Number 9 September 2007
8. Lipton, A.J., et al. 2001. S-Nitrosothiols signal the ven-
tilatory response to hypoxia. Nature. 413:171–174.
9. Palmer, L.A., et al. 2007. S-nitrosothiols signal hypox-
ia-mimetic pathology. J. Clin. Invest. 117:2592–2601.
10. McMahon, T.J., et al. 2005. A nitric oxide processing
defect of red blood cells created by hypoxia: deficien-
cy of S-nitrosohemoglobin in pulmonary hyperten-
sion. Proc. Natl. Acad. Sci. U. S. A. 102:14801–14806.
11. Diesen, D., and Stamler, J.S. 2007. S-Nitrosylation
and PEGylation of hemoglobin: toward a blood
substitute that recapitulates blood. J. Mol. Cell.
12. Kaelin, W.G., Jr. 2002. Molecular basis of the VHL
hereditary cancer syndrome. Nat. Rev. Cancer.
13. Hagen, T., Taylor, C.T., Lam, F., and Moncada, S.
2003. Redistribution of intracellular oxygen in
hypoxia by nitric oxide: effect on HIF1alpha. Science.
14. Barrett, B.J., and Parfrey, P.S. 2006. Clinical prac-
tice. Preventing nephropathy induced by contrast
medium. N. Engl. J. Med. 354:379–386.
15. Hildebrandt, W., Alexander, S., Bartsch, P., and Droge,
W. 2002. Effect of N-acetyl-cysteine on the hypoxic
ventilatory response and erythropoietin production:
linkage between plasma thiol redox state and O(2)
chemosensitivity. Blood. 99:1552–1555.
16. Demedts, M., et al. 2005. High-dose acetylcysteine
in idiopathic pulmonary fibrosis. N. Engl. J. Med.
17. Fish, J.E., et al. 2007. Hypoxia-inducible expression of
a natural cis-antisense transcript inhibits endothelial
nitric-oxide synthase. J. Biol. Chem. 282:15652–15666.
18. Ward, M.E., et al. 2005. Hypoxia induces a func-
tionally significant and translationally efficient
neuronal NO synthase mRNA variant. J. Clin. Invest.
b Cell transplantation and immunosuppression:
can’t live with it, can’t live without it
Klaus H. Kaestner
Department of Genetics and Institute for Diabetes, Obesity, and Metabolism, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
Although insulin therapy for diabetes has
come a long way since the hormone’s dis-
covery in the 1920s, insulin therapy cannot
cure the disease and prevent its devastating
complications in the long term. Beginning
in the 1970s, whole-organ pancreas trans-
plantation showed that replenishment
of the b cell complement could fully nor-
malize glucose homeostasis in patients.
Transplantation of cadaveric islets into the
liver via the portal vein was not success-
ful until 2000, when a glucocorticoid-free
immunosuppressive regimen was used and
shown to result in insulin independence
even in formerly brittle (that is, metaboli-
cally unstable) diabetic recipients (1). This
advance increased the hopes for diabetics
all over the world for a healthy life and led
to dramatic policy changes in biomedical
research funding. Because available organ
donors would never be sufficient for the
treatment of the millions of type 1 diabet-
ics in the world, the NIH and private foun-
dations funded major initiatives to develop
new sources of b cells for transplantation,
for instance through the National Insti-
tute of Diabetes and Digestive and Kidney
Diseases–funded Beta Cell Biology Consor-
tium (BCBC; http://www.betacell.org).
Multiple sources of cells have been con-
sidered as substrates for the development
of this cell-based therapy. Among these
are embryonic stem cells, hepatocytes,
and putative resident endocrine stem cells
within the pancreas; yet the success of
these efforts has been limited thus far (2).
Recent work has refocused the attention of
diabetes researchers onto the mature b cell
itself, because it was shown using genetic
lineage tracing that under normal circum-
stances in healthy rodents, new b cells are
mostly derived from existing b cells (3).
Additional support for this concept came
from successive labeling of proliferating
cells using two thymidine analogs, which
showed that adult pancreatic islets do not
contain specialized progenitors that turn
over rapidly, but rather that most or all b
cells have the potential to proliferate (4).
What these models had not established is
whether b cell proliferation can also occur
in the diabetic setting.
A mouse model for b cell
regeneration in the diabetic state
To address this question, Nir and col-
leagues developed, and describe in this
issue of the JCI, a new model for the condi-
tional and controlled ablation of pancre-
atic b cells (5) (Figure 1). Using an elegant
genetic trick, they were able to express
diphtheria toxin specifically in b cells at a
time of their choosing, which resulted in
the elimination of b cells through apop-
tosis. The treated mice were severely dia-
betic, consistent with an 80% reduction
in their b cell mass and pancreatic insulin
content. Unlike commonly used chemi-
cal agents employed for the killing of b
cells such as streptozotocin, there was no
bystander effect of the diphtheria toxin,
and the degree of b cell ablation was repro-
ducible. The beauty of the new system is
that it allowed the investigators to record
the spontaneous recovery of b cell func-
tion after ablation by simply removing the
inducing agent. Remarkably, within a few
weeks after diphtheria toxin expression
ceased, b cell mass and glucose homeosta-
sis were normalized. This was accompa-
Conflict?of?interest: The author has declared that no
conflict of interest exists.
Citation?for?this?article: J. Clin. Invest. 117:2380–2382
?The?Journal?of?Clinical?Investigation http://www.jci.org Volume 117 Number 9 September 2007
nied by a dramatically increased rate of b
cell proliferation. The fact that b cell mass
staged a near-full recovery even in the face
of severe hyperglycemia (greater than 500
mg/dl) contradicts the widely held notion
that glucotoxicity is a major impediment
to b cell proliferation or survival (6, 7).
Next, Nir and colleagues (5) addressed
the question: What is the origin of the b
cells that reappear even in a diabetic set-
ting? The new b cells most likely did not
arise from a reactivation of the fetal devel-
opmental program, as the neurogenin 3
gene, a marker of fetal endocrine precur-
sors, was not reactivated in their model.
Next the authors used genetic pulse-label-
ing of differentiated b cells, followed by
labeling of those cells that had reentered
the cell cycle, in order to assess the origin
of the new cells. These data showed that
the cells that rebuild the destroyed islets
are largely if not exclusively derived from
existing b cells, and not from neogenesis or
from expansion of non–b cell precursors.
As the authors point out, it is of course pos-
sible that in a setting of even more severe
injury to the endocrine pancreas, other
cells, including pancreatic duct or acinar
cells or even hematopoietic progenitors,
are being recruited to the b cell lineage. In
this regard, the endocrine pancreas might
be similar to the liver, in which hepatocytes
are able to repopulate the organ under con-
ditions of mild or medium injury, but acti-
vation of oval cells or even hematopoietic
stems cells can contribute to the recovery
when hepatocyte proliferation is blocked.
Immunosuppression: a necessary
evil in islet transplantation
Beyond proving the regenerative capac-
ity of the mature rodent b cell, even in the
face of diabetes, what are the potential
applications of this new model? Nir et al.
(5) provide the first example of a practical
application in which they are able to follow
the regenerative response of the b cell in a
precisely defined setting by evaluating this
response in the presence of the immuno-
suppressive regimen used in human islet
transplantation. Using the typical clinical
doses of Sirolimus (also known as Rapa-
mycin, a mammalian target of rapamycin
inhibitor) and Tacrolimus (also known as
FK506, a calcineurin inhibitor) during the
recovery phase in their mice, they show
that b cell regeneration is dramatically
blunted in the presence of these agents.
Because b cell proliferation is limited and
b cell mass cannot expand, hyperglycemia
persists. This raises the possibility that the
immunosuppressive regimen required for
the allogenic islet transplant is at the same
time inhibiting the residual proliferative
capacity of these transplanted human islets.
Likewise, might it be possible to extend the
normoglycemic life diabetics enjoy after an
islet transplant using different immuno-
Using the conditional b cell ablation model
described here (5), researchers will be able
to test the impact of immunosuppressive
drugs on b cell regeneration as a comple-
ment to testing their immunoregula-
tory efficacy. In addition, the new mouse
model offers other clinical applications.
For instance, agents and drugs suspected
of being able to promote b cell expansion
can be tested in a diabetic setting in vivo.
Immunosuppressants inhibit b cell recovery after targeted ablation. (A) Normal b cell mass and glucose homeostasis in transgenic mice before appli-
cation of doxycycline, the inducer of b cell death, in the transgenic animal model used by Nir et al. in this issue of the JCI (5). DTA, diphtheria toxin
A. (B) Doxycycline treatment activates the expression of diphtheria toxin A specifically in the b cells of the pancreas, reducing b cell mass by 80%
and causing hyperglycemia. (C) Regeneration of b cell mass through b cell proliferation and normalization of glucose homeostasis after doxycycline
withdrawal. In the presence of Sirolimus and Tacrolimus, the immunosuppressants used in human islet transplantation, the regenerative response
of the b cells is inhibited, and hyperglycemia persists.
2382? The?Journal?of?Clinical?Investigation http://www.jci.org Volume 117 Number 9 September 2007
Increasing existing b cell mass is a prom-
ising avenue for the treatment of type 2
diabetes, and initial positive results have
been obtained with one such agent, exen-
din 4, an analog of glucagon-like peptide 1
(8–10). Likewise, the new model will allow
researchers to uncover negative effects
of commonly used drugs in b cell regen-
eration. Finally, these mice will also be
instrumental in elucidating the molecular
mechanisms that govern the proliferative
response of the mature b cell.
Address correspondence to: Klaus H. Kaest-
ner, Department of Genetics and Institute
for Diabetes, Obesity, and Metabolism,
560A CRB/6140, University of Pennsylva-
nia, Philadelphia, Pennsylvania 19104, USA.
Phone: (215) 898-8759; Fax: (215) 573-5892;
1. Shapiro, A.M., et al. 2000. Islet transplantation in
seven patients with type 1 diabetes mellitus using
a glucocorticoid-free immunosuppressive regimen.
N. Engl. J. Med. 343:230–238.
2. Bonner-Weir, S., and Weir, G.C. 2005. New sources
of pancreatic beta-cells. Nat. Biotechnol. 23:857–861.
3. Dor, Y., Brown, J., Martinez, O.I., and Melton, D.A.
2004. Adult pancreatic beta-cells are formed by
self-duplication rather than stem-cell differentia-
tion. Nature. 429:41–46.
4. Teta, M., Rankin, M.M., Long, S.Y., Stein, G.M.,
and Kushner, J.A. 2007. Growth and regeneration
of adult beta cells does not involve specialized pro-
genitors. Dev. Cell. 12:817–826.
5. Nir, T., Melton, D.A., and Dor, Y. 2007. Recovery
from diabetes in mice by b cell regeneration. J. Clin.
Invest. 117:2553–2561. doi:10.1175/JCI32959.
6. Brownlee, M. 2003. A radical explanation for
glucose-induced b cell dysfunction. J. Clin. Invest.
7. Wajchenberg, B.L. 2007. beta-cell failure in diabetes
and preservation by clinical treatment. Endocr. Rev.
8. Brubaker, P.L., and Drucker, D.J. 2004. Minireview:
Glucagon-like peptides regulate cell proliferation
and apoptosis in the pancreas, gut, and central ner-
vous system. Endocrinology. 145:2653–2659.
9. DeFronzo, R.A., et al. 2005. Effects of exenatide
(exendin-4) on glycemic control and weight over
30 weeks in metformin-treated patients with type
2 diabetes. Diabetes Care. 28:1092–1100.
10. Kendall, D.M., et al. 2005. Effects of exenatide
(exendin-4) on glycemic control over 30 weeks
in patients with type 2 diabetes treated with
metformin and a sulfonylurea. Diabetes Care.
Deepening our understanding
of immune sentinels in the skin
Frank O. Nestle1 and Brian J. Nickoloff2
1St. John’s Institute of Dermatology, Division of Genetics and Molecular Medicine, King’s College London School of Medicine, London, United Kingdom.
2Department of Pathology and Department of Microbiology and Immunology, Oncology Institute, Cardinal Bernardin Cancer Center,
Loyola University, Chicago, Illinois, USA.
The skin is one of the largest organs of the
body and has a variety of different func-
tions including providing the first line of
defense against invading pathogens. Upon
casual inspection, normal human skin
does not appear to be a major reservoir
of immune sentinels and effector cells.
However, the absence of inflammation
belies the significant number and pheno-
typic complexity of tissue-resident T cells,
macrophages, and DCs that are becoming
more widely appreciated by immunolo-
gists interested in immune surveillance,
autoimmunity, chronic inflammatory
diseases in the skin, and beyond (1). Once
the primary barrier function of skin (e.g.,
disruption of the outermost dead layer
of cells forming the stratum corneum) is
breached, immune sentinel and effector
cells are poised to provide rapid and effi-
cient immunological backup to restore
tissue homeostasis. For example, it is not
widely appreciated that there are approxi-
mately 2 × 1010 skin-resident T cells, i.e.,
twice the total number of T cells in the
circulation (2). A frequent misconception
regarding cutaneous immunity is that
“skin” equals “epidermis” (3). Once one
probes beneath the basement membrane
zone, there is a multitude of cells, includ-
ing T lymphocytes, DCs, and macro-
phages, that are regarded as components
of the dermal immune system and com-
plement the overlying epidermal compo-
nents to form the skin immune system.
The dermal immune system provides a
highly reactive immunologically based
compartment that is crucially involved
in the majority of chronic inflammatory
skin disorders including psoriasis and
atopic dermatitis (4). Here, we primarily
focus on the dermal-based immune sys-
tem under normal or noninflammatory
of the immune system
Skin-resident sentinels of the immune
system include a spectrum of cells, rang-
ing from motile DCs highly specialized in
sensing danger and presenting antigens to
tissue-resident macrophages specializing
in phagocytosis, thereby clearing poten-
tially dangerous substances and mediating
tissue remodeling. Langerhans cells (LCs)
are dendritic, antigen-presenting sentinels
of the epidermis. LCs renew locally in epi-
dermis and only recruit blood-borne pre-
cursors following injury (5). Even though
LCs were described 140 years ago, their in
vivo function still remains elusive. Recent
scientific evidence obtained using LC-
depleted genetically modified mice adds
Nonstandard?abbreviations?used: BDCA, blood
DC antigen; DC-SIGN, DC-specific ICAM-grabbing
nonintegrin; DDC, dermal DC; FXIIIa, factor XIIIa; LC,
Langerhans cell; PDC, plasmacytoid pre-DC.
Conflict?of?interest: The authors have declared that no
conflict of interest exists.
Citation?for?this?article: J. Clin. Invest. 117:2382–2385