A trip down memory lane: Engineering stemness to enhance T-cell therapy
Department of Microbiology and Immunology
Medical University of South Carolina
Bion Howard and Zihai Li
Hollings Cancer Center
First Written 2014
Adoptive T cell transfer is poised to revolutionize the treatment of cancer. Methods to optimize these therapies should account for data which show that less differentiated “memory” subsets of the T cell lineage mediate more effective antitumor responses. One possible explanation for this phenomenon is that common pathways for “stemness” are activated in more naive T cells, enabling proliferation and persistence which promotes antitumor function. Recently, serial transfer of single cells from the naive and central memory CD62L+ T cell compartments was shown to enable full reconstitution of immune response to Listeria over multiple generations, (Graef 2014) proving that these subtypes exhibit self-renewal and pluripotency which are hallmarks of stem cells. The goal of this essay is to look forward to a new era in which genetic engineering techniques will be used to enhance the stemness traits of adoptive cell therapies, thus promoting the successful application of cancer immunotherapy.
The process of differentiation in multicellular organisms bears a striking analogy to the mathematical beauty of music theory. While each of our cells inherently possesses the same set of blueprints, these blueprints are not singular, they are divisible: they consist of many thousands of individual genes (notes) which are activated (played) in careful combinations (chords, scales) to produce the dynamic functions distinguishing heart and skin and brain and blood. While each differentiation state plays a different set of songs under different circumstances, there must be numerous areas of overlap, where different subsets of cells play similar notes with shared invariant relationships. Theoretically, these similarities in the music of gene expression would most likely be found amongst highly conserved cellular functions, like the replication and self-renewal exemplifying the phenotype known as “stemness.” It would not make sense for multicellular organisms to “reinvent the wheel” and evolve new ways to implement these stemness functions repeatedly in different cell types. Instead, there must be a smaller number of “notes” which cells can play if a stemness phenotype is required. It is interesting that these same important traits of endless self-renewal are often found in cancer; what organs have a higher evolutionary drive for stemness than tumors?
Stemness is a trait of extreme importance for biomedicine. It would not be possible for animals to live many years if they lacked the capability to renew certain normal tissues, and loss of this rejuvenation over time may be a key component of ageing processes. Recently, Graef et al proved that the CD8 memory T cells responsible for decades-long adaptive immunity in mammals are stem cells. They accomplished this proof by showing that multiple generations of mice lacking any immune cells could still mount a successful defense against an intracellular pathogen, Listeria, if they received just a single memory cell! From a pathologic perspective, cancer stem cells (CSCs) and memory T cells are both proposed to resist therapies and thus enable drug resistance and disease recurrence. (Sakariassen 2007, Turtle 2009) Perhaps some of this stemness-resistance link is mediated by tumor microenvironment, but there must also be overlap in molecular pathways, implying that we could imbue cell therapies with resistance and thereby enhance their combinatorial efficacy with traditional treatments. Finally, adoptive T cell therapy with tumor infiltrating lymphocytes or chimeric antigen receptors has been shown to be more effective when the T cells used are CD62L+ memory cells, (Klebanoff 2012) probably because these cells are more capable of multiplying to outnumber cancer cells, persisting for long periods of time to prevent recurrence, and continuing to function without reaching an exhausted or anergic phenotype. There is a definite overlap between stem cells and drug resistant cells: both cancer stem cells and memory stem cells express high levels of drug efflux pumps and anti-apoptotic proteins. Chemo and radio therapies both select for stemness because stemness and treatment resistance are linked. Perhaps by playing this stemness song in our own cell therapies (by turning the right genes on or off), we can level this playing field between CSCs and T cells and thus better enable transferred cells to multiply, overwhelm, and eliminate difficult tumors.
Thus, it is crucial for us to investigate the systems biology of stemness in great detail: these crucial molecular pathways likely function as a double-edged sword, enabling the continual production of normal tissue like blood and skin and hair but also the persistent self-replication of tumors and metastases. If these functions are important in sickness and in health, they may also be useful in engineering, because we could imbue our cell therapies with the same proliferation and persistence. New tools in synthetic biology allow us to turn on, turn off, and edit genes, and so we should survey the landscape of stemness to identify key nodes in the network that might be gainfully reprogrammed.
We should investigate the effects of modulating these proteins on the success of T cell therapies. Genes which restrain stemness might be the best targets because it is a smaller technical hurdle to deactivate anti-stemness genes with RNAi or genome editing than it is to activate a pro-stemness molecules with gene therapy or artificial transcription factors. The most important concept will be to test combinations of these changes alongside biosafety engineering, like inducible apoptosis or metabolic insufficiency to prevent uncontrolled replication and secondary iatrogenic harm; if we seek to fight fire with fire, we must consider methods to prevent getting burned. Further, we must consider that there may be mutual exclusion effects between certain stemness pathways and T cell effector function. It will be important to assess the effector functions of modified cell therapies and seek a proper balance between stemness and anti-tumor immunity. Another good idea would be to personalize the stemness programming of our cell therapies so that the therapeutic cells achieve stemness in a different manner than the cancer cells; this way, we can combine our stem cell immunotherapy with targeted drugs that block the tumor’s stemness program. Finally, it may be worthwhile to identify specific stemness pathways which can promote therapeutic cell migration toward cancer stem cells. Only by carefully testing the comparative efficacy of many different target combinations can we hope to fully understand the best ways to promote stemness in cell therapy in a beneficial way. Maybe we should automate the process of testing these combinations using cloud-based labs like “arcturus” or liquid handing high-throughput robots like “opentrons.” Regardless of technical details, making cells play the proper notes in the proper harmony may make all the difference in immunotherapy.
Works Cited
Gattinoni et al. Paths to stemness: building the ultimate antitumor T cell. Nature Reviews Cancer. 2012.
Gattinoni et al. Memory T Cells Officially Join the Stem Cell Club. Immunity. 2013.
Graef et al. Serial Transfer of Single-Cell-Derived Immunocompetence Reveals Stemness of CD8+ Central Memory T Cells. Immunity. 2014.
Hu et al. A Genome-wide Regulatory Network Identifies Key Transcription Factors for Memory CD8+ T Cell Development. Nature Communications. 2013.
Klebanoff et al. Sorting through subsets: Which T cell populations mediate highly effective adoptive immunotherapy? Journal of Immunotherapy. 2012.
Leinert et al. Synthetic biology in mammalian cells:Next generation research tools and therapeutics. Nature Rev Molecular Cell Biol. 2014.
Sakariassen et al. Cancer Stem Cells as Mediators of Treatment Resistance in Brain Tumors: Status and Controversies. Neoplasia. 2007.
Turtle et al. A distinct subset of self-renewing human memory CD8+ T cells survives cytotoxic chemotherapy. Immunity. 2009.
Hong et al. RECK inhibits stemness gene expression and tumorigenicity of gastric cancer cells by suppressing ADAM-mediated Notch1 activation. Journal of Cell Physiology. 2014.
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