Increased Telomere Length in NK Cells Generated from Human Induced Pluripotent Stem Cells
George Scaria, MD, PhD1, Trevor Argall1, Shyam S. Jose2, Laura Bendzick BS1, Dan S.
Kaufman, MD, PhD1
1. Department of Medicine and Stem Cell Institute, University of Minnesota Medical School,
Minneapolis, MN, USA
2. St. Anne's University Hospital - International Clinical Research Center, Brno, Czech Republic
Natural Killer (NK) cells function as key effector cells of the innate immune system by providing
a key role in host defense against malignancy and viral infection. NK cells also represent a
useful tool for cellular therapy due to their ability to kill target cells in a human leukocyte antigen
(HLA) non-restricted manner without prior sensitization. Our lab has pioneered methods for the
production of NK cells from human embryonic stem cells (hESCs) and induced pluripotent stem
cells (iPSCs). These hESC and iPSC-derived NK cells effectively kill diverse human tumor cells
both in vitro and in vivo. Current limitations in therapeutic use of NK cells include the
persistence and expansion of transplanted NK cells in vivo. In order to overcome these barriers,
various means for the ex-vivo expansion of large numbers of NK cells prior to administration to
patients have been devised. One method involves propagation of NK cells generated from
hESC/iPSC sources with artificial antigen presenting cells (aAPCs). However, aAPC-mediated
expansion leads to telomere erosion. Telomere length gradually shortens over time as cells
divide. When telomeres become critically short, the cells enter a state of senescence. Therefore,
aAPCs have been engineered to express membrane bound IL-21 (mbIL21), which promotes
proliferation of NK cells and activates telomerase, to ameliorate this effect. We have performed
studies using real time PCR to measure the telomere length of NK cells generated from iPSCs
and expanded with aAPCs expressing mbIL21 and compared them to aAPC expanded NK cells
isolated from peripheral blood. Telomere length in NK cells derived from IPSCs is initially five to
six times greater (200-300kb/genome), compared to those expanded from peripheral blood
(<50kb/genome). To better characterize the role that telomeres play in NK cell development, we
have generated iPSCs from fibroblasts of telomerase deficient patients which have mutations in
the TERC RNA component. The reprogramming of fibroblasts to the pluripotent state results in
initial lengthening of telomeres. Telomerase deficient fibroblasts have telomere lengths of 130150kb/genome compared to normal human fibroblasts with telomeres of ~250kb/genome.
Somewhat surprisingly, iPSCs generated from telomerase deficient fibroblasts can have
markedly increased telomere lengths, as high as 2000kb/genome. We are currently generating
NK cells from these telomerase-deficient iPSCs. We predict that NK cells with longer telomeres
from hESC/iPSC sources will persist longer post-transplantation than those expanded from NK
cells from the peripheral blood and will be more effective at killing tumor targets. We are
currently investigating this using in vivo xenograft models.