Memory T cell populations are heterogeneous in phenotype and function and many questions remain as to the mechanisms mediating their long term persistence. Recent research by several groups have described populations of antigen-experienced T cells within human peripheral blood mononuclear cells (PBMC) that exhibit stem cell-like characteristics: increased self-renewal capacity and the ability to derive the more differentiated central and effector memory and effector populations in vitro and in vivo, and may thus be the cell type mediating memory T cell persistence.
In 2011, Gattinoni et. al. identified a population of stem cell-like memory T cells (TSCM) with surface markers characteristic of naive T cells in human PBMCs. TSCM cells were CD45RO−, CCR7+, CD45RA+, CD62L+, CD27+, CD28+ and IL-7Ra+. The TSCM population comprised 2-3% of CD8+ and CD4+ T cells in healthy donors. These TSCM cells could be differentiated from naïve T cells by high expression of CD95 and IL-2Rb, markers which are also expressed by memory T cells. Furthermore, the TSCM population exhibited a gene expression profile that was intermediate between naïve (TN) and central memory (TCM) cells.
Like memory T cells, these TSCM cells were antigen-experienced and exhibited rapid effector activity upon T cell receptor (TCR) stimulation. Importantly, they also exhibited the stem-like property of self-renewal in the presence of homeostatic IL-15 signals. Following TCR stimulation, TSCM cells could differentiate into TCM and effector memory (TEM) T cell subsets, and the authors demonstrated a progressive differentiation pattern of TN à TSCM à TCM à TEM, where no differentiation in the opposite direction was observed following TCR stimulation of sorted TSCM, TCM, and TEM populations. Human TSCM cells also survived significantly longer and produced more progeny in vivo then either TCM or TEM populations in a NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ (NSG) mouse xenograft adoptive transfer model.
One of the most exciting clinical implications of this work however, was the demonstration that TSCM cells exhibited profoundly superior anti-tumor activity than either TCM or TEM populations in a xenograft mouse tumor model where mesothelin-specific human T cell populations were transferred into NSG mice bearing human mesothelioma M108 tumors. Thus, TSCM cells may be the most effective cellular subset for use in adoptive T cell therapy in cancer patients.
Subsequent to this finding, a paper was published by Lugli et. al., describing a protocol for identifying and isolating human TSCM cells from PBMCs as well as their in vitro expansion. The flow cytometry staining panel proposed for TSCM cell identification includes antibodies targeting CD3, CD8, CD4, CD45RO, CCR7, either CD62L, CD27, CD28 or CD45RA, and CD95. CD58 and CD122 (IL-2Rb) were also proposed as additional markers for better differentiation of TSCM cells from naïve populations which express these at lower levels.
Interestingly, the naïve-like TSCM population described by Gattinoni et. al. is not the only memory T cell population demonstrated to have “stem-like” characteristics.
In a quest to understand the mechanism by which patients who have undergone multiple rounds of cytotoxic chemotherapy induced lymphopenia maintain resistance to viral infections, Turtle et. al. described a population of human PBMC CD8+ T cells within the central and effector memory populations that were distinguished by expression of high levels of IL-18Rα and the natural killer (NK) cell receptor CD161. These cells exhibited a hematopoietic stem cell-like capacity to efflux chemotherapeutic agents mediated by expression of ABCB1, survive chemotherapy, and replenish the virus-specific memory T cell pool in acute myeloid leukemia (AML) patients.
Human memory TH17 cells also have stem cell-like characteristics. Despite their effector memory-like surface marker phenotype being CD45RO+ CCR7- CD62L-, compared with TH1 and TH2 subsets, TH17 cells were shown to have increased capacities for proliferation, in vivo persistence, resistance to apoptosis, and higher expression levels of stem-cell associated genes HIF1a, Notch, Bcl2, OCT4, and Nanog. TH17 cells were able to differentiate into TH1 and TREG subsets. TH17 cells also express CD161 and thus may overlap with Turtle et. al.’s CD161+ABCB1+ stem-like memory cells.
However, conflicting evidence has been presented as to the identity of CD161+ IL-17 expressing cells and whether or not these cells are in fact Vα7.2+ mucosal associated invariant T cells (MAITs) which are selected by nonpolymorphic MHC class Ib molecules. MAIT cells however are not known to be virus-specific whereas the CD161+ABCB1+ stem-like memory population identified by Turtle et. al. included influenza and EBV-specific populations. Thus much remains to be clarified regarding the overlap between CD161+ABCB1+ stem-like memory populations, IL-17 expressing CD4+ (TH17) and CD8+ (TC17) cells which also express CD161, and CD161+ IL-17 expressing MAIT cell populations.
In summary, the ability to identify various stem-like memory CD4 and CD8 human T cell populations in human PBMC using flow cytometry allows for many questions to be addressed about the phenotype, functions, and clinical applications of these cells.
A human memory T cell subset with stem cell-like properties. Gattinoni L, Lugli E, Ji Y, Pos Z, Paulos CM, Quigley MF, Almeida JR, Gostick E, Yu Z, Carpenito C, Wang E, Douek DC, Price DA, June CH, Marincola FM, Roederer M, Restifo NP. Nat Med. 2011 Sep 18;17(10):1290-7.
Identification, isolation and in vitro expansion of human and nonhuman primate T stem cell memory cells. Lugli E, Gattinoni L, Roberto A, Mavilio D, Price DA, Restifo NP, Roederer M. Nat Protoc. 2012 Dec 6;8(1):33-42.
A distinct subset of self-renewing human memory CD8+ T cells survives cytotoxic chemotherapy. Turtle CJ, Swanson HM, Fujii N, Estey EH, Riddell SR. Immunity. 2009 Nov 20;31(5):834-44.
Human TH17 cells are long-lived effector memory cells. Kryczek I, Zhao E, Liu Y, Wang Y, Vatan L, Szeliga W, Moyer J, Klimczak A, Lange A, Zou W. Sci Transl Med. 2011 Oct 12;3(104):104ra100.
Human MAIT cells are xenobiotic-resistant, tissue-targeted, CD161hi IL-17-secreting T cells. Dusseaux M, Martin E, Serriari N, Péguillet I, Premel V, Louis D, Milder M, Le Bourhis L, Soudais C, Treiner E, Lantz O. Blood. 2011 Jan 27;117(4):1250-9. doi: 10.1182/blood-2010-08-303339. Epub 2010 Nov 17.
CD161-expressing human T cells. Fergusson JR, Fleming VM, Klenerman P. Front Immunol. 2011;2:36. doi: 10.3389/fimmu.2011.00036.