A T memory stem cell (TSCM) is a type of long-lived memory T cell with the ability to reconstitute the full diversity of memory and effector T cell subpopulations as well as to maintain their own pool through self-renewal. First described in mice in 2009 [1] then in humans [2] represent a cell type that has reshaped the landscape of immunology and medicine because of their superior ability to self-renew and persist in the setting of cancer and infectious disease. Developmentally, TSCM are an intermediate subset between naïve (Tn) and central memory (Tcm) T cells, expressing both naïve T cells markers, such as CD45RA+, CD45RO-, high levels of CD27, CD28, IL-7Rα (CD127), CD62L, and C-C chemokine receptor 7 (CCR7), as well as markers of memory T cells, such as CD95, CD122 (IL-2Rβ), CXCR3, LFA-1. [3] [4] [5] . These cells represent a small fraction of circulating T cells, approximately 2-3%. [3] . Like naïve T cells, TSCM cells are found more abundantly in lymph nodes than in the spleen or bone marrow; but in contrast to naïve T cells, TSCM cells are clonally expanded. Similarly to memory T cells, TSCM are able to rapidly proliferate and secrete pro-inflammatory cytokines (IFN-γ, IL-2, and TNF-α) in response to antigen re-exposure, but show higher proliferation potential compared with Tcm cells; their homeostatic turnover is also dependent on IL-7 and IL-15.[ [4] ]
Longitudinal studies on TSCM dynamics in patients undergoing hematopoietic stem cell transplantation (HSCT) have shown that donor-derived TSCM cells were highly enriched early after HSCT, differentiated directly from Tn, and that Tn and TSCM cells (but not central memory or effector T cells) were able to reconstitute the entire heterogeneity of memory T cell subsets including TSCM cells. [6] Together with the transcriptome analysis of differentially expressed genes reflecting the relatedness of TSCM and Tn cells, these data were in sharp contrast to the existing hierarchical model of human T cell differentiation: naïve T cells (Tn) → effector T cells (Teff) → effector memory T cells → central memory T cells (Tcm)/.
After primary antigen exposure and elimination, antigen-specific TSCM preferentially survive among memory T cells and stably persist for a long term throughout the human lifespan. [7] Multiparametric flow cytometry and TCR sequencing studies showed that more than 30% of naïve T cells primed by antigen directly differentiate into TSCM cells. [3] Current observations allow to suggest that TSCM is a population which plays an essential role in maintaining a long-term memory in vivo. [4] Long-term studies on T cells in a cohort of patients vaccinated against yellow fever revealed that vaccine-induced CD8+ TSCM cells specific to yellow fever antigens were stably maintained for 25 years, capable of self-renewal ex vivo, and preserved surface markers and mRNA profiles closest to naïve T cells. [8] In another longitudinal study on leukaemia patients who had undergone HSCT, it was reported that genetically modified TSCM could be detected up to 14 years after infusion. [9] Complex analysis of TSCM dynamics under physiological conditions including stable isotope labeling, mathematical modeling, cross-sectional data from vaccinated individuals, and telomere length analysis revealed that there are at least 2 distinct TSCM subpopulations with different longevity and turnover rates: 1) short-lived, with an average half-life of 5 months, 2) long-lived, with a high degree of self-renewal and the half-life of approximately 9 years, which is consistent with the long-term maintenance of the recall response to antigen (8–15 years). [4]
Analysis of TCR β repertoire of TSCM and Tm revealed that TSCM have higher TCRβ diversity compared with Tm, that TCR sequences of TSCM were antigen-experienced and their composition differed with those of naïve T cells. It also revealed that in type I diabetes patients there was an enrichment of self-reactive clonotypes in TSCM rather than in Tm, suggesting that TSCM might serve as a pool of autoreactive T cells. [10]
Pathogen-specific TSCM cells have been identified in a number of studies of human acute and chronic infections caused by viruses, bacteria and parasites. The presence of TSCM might be essential for the control of persisting infections, in which effector T cells undergo exhaustion and need to be restored; this was supported by the evidence of a negative correlation between the severity of chronic viral (HIV-1) and parasitic (trypanosome) infections and the frequency of circulated TSCM cells. [3]
TSCM are considered as a promising approach in immune cell therapy in cancers due to their high proliferation capacity, longevity and increased survival as well as more potent antitumor effects compared with Tcm and Tem in vivo. Studies on adoptive cell therapy in mouse melanoma model revealed a significant linear correlation between the differentiation status of infused T cells and the strength of tumor regression in the order TSCM >TCM > TEM; TSCM infusion led to a more sustained reduction in tumor growth and correlated with a significant increase in overall survival of treated mice. Previous works on humans and mice also demonstrated that less differentiated T cells show greater proliferative capacity and ability to persist after cell transfer compared with their more differentiated counterparts; in humans, the ability of infused T cells to persist has been positively correlated with response to adoptive cell therapy. [5] [11]
However, the clinical exploitation of TSCM cells is impeded due to their paucity in the peripheral blood and due to the current lack of unified protocols for generating and maintaining TSCM in vitro for clinical manufacturing. Among current efficient strategies, there is a combination of IL-7 and IL-15, which have been successfully used to generate tumor-redirected TSCM cells from naive cell precursors, with yielding cells having a gene signature of naturally occurring TSCM cells and enhanced proliferative capacity compared to other T cell subsets. This strategy can be particularly suitable for generating virus-specific TSCM cells for adoptive cell therapy to prevent or treat viral infections after transplantation or in other immunocompromised patients. Another strategy promoting the efficient generation of tumor-reactive TSCM cells relies on the activation of naïve-like T cells in the presence of IL-7, IL-21 and TWS119 which is an agonist of Wnt-β signaling. It has been found that CAR-modified TSCM cells generated this way are phenotypically, functionally and transcriptomically equivalent to naturally occurring TSCM cells; moreover, they had metabolic features which are specific for long-lived memory T cells, such as high spare respiratory capacity and low glycolytic metabolism (predominance of oxidative phosphorylation). Such CAR-modified T cells can be redirected efficiently against required tumor antigens, and have been shown to generate durable anti-tumor responses. [3]
One of the hardest challenges in application of T cell therapies in treatment of solid tumors is the problem of CD8+ T cells exhaustion resulting from their repeated exposure to tumor antigens and immunosuppressive tumor microenvironment sending inhibitory signals through the cytokines and cell surface receptors. Exhausted T cells are characterized by the expression of large amounts of inhibitory molecules such as PD-1, CTLA-4, LAG3, Tim-3, CD244/2B4, CD160, and TIGIT; they do not respond to TCR stimulation and have reduced capacity to secrete anti-tumor cytokines such as IFN-γ and TNF-α. [12] On a transcriptional level, recent studies have found that transcription factors which play key role in T cells exhaustion include TCF-1, T-bet, Eomes, PRDM1, NFAT, NR4A, IRF4 and BATF. According to the current differentiation model of T cells exhaustion, T cells stepwise lose their “stemness” while acquiring “exhaustion”. Therefore, approaches that would avoid T cells exhaustion and would “reinvigorate” exhausted T cells have a potential to significantly improve the efficacy of cancer immunotherapies. [5]
Studies of the recent years revealed that TCF-1+ T cells, which represent early memory T cells including TSCM cells, play important roles in T cells persistence and efficacy in cancer immunotherapy. Flow cytometry analysis of tumor-infiltrating antigen-presenting cell (APC) populations in human kidney, prostate and bladder tumors revealed a significant correlation between the presence of dendritic cells (but not macrophages) and the number of TCF1+ stem-like CD8+ T cells in the tumor. [13] Subsequent immunofluorescence staining showed that TCF1+ stem-like T cells were found only in regions with high density of MHC II+ cells; in contrast, the TCF1- population of terminally exhausted CD8+ T cells was distributed across the tissue with no preference for APC dense zones. Expanded analysis of large sections of tumor tissues confirmed that tumors had many regions with dense APC zones, and TCF-1+ stem-like CD8 cells preferentially resided there. These data suggest that regions highly enriched with APC serve as an intratumoral niche for stem-like CD8+ T cells, which give rise to terminally differentiated T cells and thus sustain the anti-tumor immune response. Furthermore, immunofluorescence analysis of large regions of tumor tissue from 26 patients with kidney cancer revealed that patients with controlled disease had significantly more MHC-II dense regions where TCF1+ CD8 T cells resided; further stratification of patients showed that patients with low MHC-II+ cell density in such regions experienced significantly impaired progression-free survival. A focused study of patients with stage III kidney cancer, around 50% of whom progress after surgery, revealed that there were >10-fold fewer immune niches in patients who progressed. [13]
Despite some variations depending on tumor type and therapy, most studies agree that tumor-infiltrating lymphocytes (TIL) in patients responding to checkpoint-blockade therapy, such as anti-PD1 therapy, contain more TCF1+ early memory T cells, while fewer T cells with exhausted phenotype compared with TILs in non-responders. A study performed on the preclinical model of colon cancer has shown that PD-1 blockade induced a shift from naïve-like to memory precursor-like subsets, which are maintained by the transcriptional regulator TCF-1. The effectiveness of CAR-T cell therapy in chronic lymphocytic leukemia has also been reported to depend on the number of early memory T cells and T cell exhaustion. [12]
T cells are one of the important types of white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell receptor (TCR) on their cell surface.
A cytotoxic T cell (also known as TC, cytotoxic T lymphocyte, CTL, T-killer cell, cytolytic T cell, CD8+ T-cell or killer T cell) is a T lymphocyte (a type of white blood cell) that kills cancer cells, cells that are infected by intracellular pathogens (such as viruses or bacteria), or cells that are damaged in other ways.
The T helper cells (Th cells), also known as CD4+ cells or CD4-positive cells, are a type of T cell that play an important role in the adaptive immune system. They aid the activity of other immune cells by releasing cytokines. They are considered essential in B cell antibody class switching, breaking cross-tolerance in dendritic cells, in the activation and growth of cytotoxic T cells, and in maximizing bactericidal activity of phagocytes such as macrophages and neutrophils. CD4+ cells are mature Th cells that express the surface protein CD4. Genetic variation in regulatory elements expressed by CD4+ cells determines susceptibility to a broad class of autoimmune diseases.
Immunotherapy or biological therapy is the treatment of disease by activating or suppressing the immune system. Immunotherapies designed to elicit or amplify an immune response are classified as activation immunotherapies, while immunotherapies that reduce or suppress are classified as suppression immunotherapies. Immunotherapy is under preliminary research for its potential to treat various forms of cancer.
Cancer immunotherapy (immuno-oncotherapy) is the stimulation of the immune system to treat cancer, improving the immune system's natural ability to fight the disease. It is an application of the fundamental research of cancer immunology (immuno-oncology) and a growing subspecialty of oncology.
Memory T cells are a subset of T lymphocytes that might have some of the same functions as memory B cells. Their lineage is unclear.
CD3 is a protein complex and T cell co-receptor that is involved in activating both the cytotoxic T cell and T helper cells. It is composed of four distinct chains. In mammals, the complex contains a CD3γ chain, a CD3δ chain, and two CD3ε chains. These chains associate with the T-cell receptor (TCR) and the CD3-zeta (ζ-chain) to generate an activation signal in T lymphocytes. The TCR, CD3-zeta, and the other CD3 molecules together constitute the TCR complex.
In immunology, a naive T cell (Th0 cell) is a T cell that has differentiated in the thymus, and successfully undergone the positive and negative processes of central selection in the thymus. Among these are the naive forms of helper T cells (CD4+) and cytotoxic T cells (CD8+). Naive T cells, unlike activated or memory T cells, have not encountered its cognate antigen within the periphery. After this encounter, the naive T cell is considered a mature T cell.
Immunosenescence is the gradual deterioration of the immune system, brought on by natural age advancement. A 2020 review concluded that the adaptive immune system is affected more than the innate immune system. Immunosenescence involves both the host's capacity to respond to infections and the development of long-term immune memory. Age-associated immune deficiency is found in both long- and short-lived species as a function of their age relative to life expectancy rather than elapsed time.
Cancer immunology (immuno-oncology) is an interdisciplinary branch of biology and a sub-discipline of immunology that is concerned with understanding the role of the immune system in the progression and development of cancer; the most well known application is cancer immunotherapy, which utilises the immune system as a treatment for cancer. Cancer immunosurveillance and immunoediting are based on protection against development of tumors in animal systems and (ii) identification of targets for immune recognition of human cancer.
Programmed death-ligand 1 (PD-L1) also known as cluster of differentiation 274 (CD274) or B7 homolog 1 (B7-H1) is a protein that in humans is encoded by the CD274 gene.
Gamma delta T cells are T cells that have a γδ T-cell receptor (TCR) on their surface. Most T cells are αβ T cells with TCR composed of two glycoprotein chains called α (alpha) and β (beta) TCR chains. In contrast, γδ T cells have a TCR that is made up of one γ (gamma) chain and one δ (delta) chain. This group of T cells is usually less common than αβ T cells. Their highest abundance is in the gut mucosa, within a population of lymphocytes known as intraepithelial lymphocytes (IELs).
Programmed cell death protein 1(PD-1),. PD-1 is a protein encoded in humans by the PDCD1 gene. PD-1 is a cell surface receptor on T cells and B cells that has a role in regulating the immune system's response to the cells of the human body by down-regulating the immune system and promoting self-tolerance by suppressing T cell inflammatory activity. This prevents autoimmune diseases, but it can also prevent the immune system from killing cancer cells.
Hepatitis A virus cellular receptor 2 (HAVCR2), also known as T-cell immunoglobulin and mucin-domain containing-3 (TIM-3), is a protein that in humans is encoded by the HAVCR2 (TIM-3) gene. HAVCR2 was first described in 2002 as a cell surface molecule expressed on IFNγ producing CD4+ Th1 and CD8+ Tc1 cells. Later, the expression was detected in Th17 cells, regulatory T-cells, and innate immune cells. HAVCR2 receptor is a regulator of the immune response.
Adoptive cell transfer (ACT) is the transfer of cells into a patient. The cells may have originated from the patient or from another individual. The cells are most commonly derived from the immune system with the goal of improving immune functionality and characteristics. In autologous cancer immunotherapy, T cells are extracted from the patient, genetically modified and cultured in vitro and returned to the same patient. Comparatively, allogeneic therapies involve cells isolated and expanded from a donor separate from the patient receiving the cells.
Nicholas P. Restifo is an American immunologist, physician and educator in cancer immunotherapy. Until July 2019, he was a tenured senior investigator in the intramural National Cancer Institute of the National Institutes of Health at Bethesda, Maryland. Nicholas was an executive vice president of research at Lyell based in San Francisco.
Cytokine-induced killer cells (CIK) cells are a group of immune effector cells featuring a mixed T- and natural killer (NK) cell-like phenotype. They are generated by ex vivo incubation of human peripheral blood mononuclear cells (PBMC) or cord blood mononuclear cells with interferon-gamma (IFN-γ), anti-CD3 antibody, recombinant human interleukin (IL)-1 and recombinant human interleukin (IL)-2.
Eftilagimod alpha is a large-molecule cancer drug being developed by the clinical-stage biotechnology company Immutep. Efti is a soluble version of the immune checkpoint molecule LAG-3. It is an APC Activator used to increase an immune response to tumors, and is administered by subcutaneous injection. Efti has three intended clinical settings:
T-cell depletion (TCD) is the process of T cell removal or reduction, which alters the immune system and its responses. Depletion can occur naturally or be induced for treatment purposes. TCD can reduce the risk of graft-versus-host disease (GVHD), which is a common issue in transplants. The idea that TCD of the allograft can eliminate GVHD was first introduced in 1958. In humans the first TCD was performed in severe combined immunodeficiency patients.
Cellular adoptive immunotherapy is a type of immunotherapy. Immune cells such as T-cells are usually isolated from patients for expansion or engineering purposes and reinfused back into patients to fight diseases using their own immune system. A major application of cellular adoptive therapy is cancer treatment, as the immune system plays a vital role in the development and growth of cancer. The primary types of cellular adoptive immunotherapies are T cell therapies. Other therapies include CAR-T therapy, CAR-NK therapy, macrophage-based immunotherapy and dendritic cell therapy.