Different models are proposed on the conversion of T_SCM into effector cells. Even though some literature acknowledges the linear model of T_SCM cell differentiation (6, 8, 9), circular (on-off-on) and asymmetric models are also suggested (6). Herein, we briefly explain each model and discuss which one is more plausible at least in the context of autoimmune diseases like type 1 diabetes (T1D) (10).

In the linear model, the cell differentiation in each phase depends on the T-cell receptor (TCR) potential signal and extent and persistence of antigenic stimulation on the cells. This model explains that while T cells are directed straightly toward the memory and effector phases, they gradually lose their memory strength and develop remarkably effector capabilities, which is also termed as the “decreasing potential” model (Figure 1A). However, it seems that this model is in conflict with the primary definition of T_SCM cells in regard to the preservation of memory capacities over long periods of time (6).

The “circular” or “on-off-on” model is opposite the linear one, as it explains that once T cells are exposed to an Ag, they differentiate into effector T cells, and upon the response contraction, the participated effector T cells dedifferentiate into different memory subsets until reencountering with the cognate Ag, by which the cells are able to remember and redifferentiate into the effector T cells (Figure 1B). This model could be relatively accepted in the context of some infectious diseases (6).

The third model proposes that the formation of memory and effector T cells is predetermined from the first division via asymmetric distribution of critical transcriptional and epigenetic modulators between two daughter cells whereby one acquires memory ability and another one develops its effector potency. This pattern, called the “asymmetric division model,” can be more conceivable in autoimmune diseases such as T1D in which the full activation signal is received by one daughter cell, while the weak stimulation signal is picked up by another one (Figure 1C) (6, 10).

Likewise, in other immune cells, immunophenotyping is a helpful technique to identify the T_SCM cell subset (3). The subset is characterized in humans and NHPs by expressing the combination of effector and memory T-cell markers, including CD45RA⁺, CD45RO^–, CD27⁺, CD28⁺, CCR7⁺, CD62L⁺, CD95⁺, CD122⁺, and CD127⁺ (1, 2, 4). Nevertheless, CD45RA⁺, CD45RO^–, CD27⁺, CD28⁺, CCR7⁺, CD62L⁺, and particularly CD95⁺ are considered distinctive markers between naive and T_SCM cells (Figure 2) (1, 2, 4). The equivalent markers of T_SCM cells in mice are known as CD62L⁺, stem cell marker (Sca-1)⁺, CD122⁺, antiapoptotic marker molecule (Bcl-2)⁺, CCR5⁺, and CXCR3⁺ (Figure 2). Given that memory T cells are typically identified by CD45RO⁺ and CD27⁺, contrary to effector T cells expressing CD45RA (1–4), the markers CD27⁺ and CD45RA⁺ by T_SCM cells indicate both memory and effector abilities of this subset (4). CCR7 (through binding to CCL19 and CCL21) along with CD62L drive T_SCM cells to SLO homing. Not only does CD57 expression reflect telomere shortage (cell senescence) due to repetitive proliferation, but it also enhances either degranulation ability or inflammatory cytokine secretion of T_SCM cells (1, 2, 4). Cell imaging techniques showed that normal T cells that express CXCR3 have higher proliferation, multipotency, and polyfunctionality alongside highly released cytokines tumor necrosis factor-alpha (TNF-α), IFN-γ, and IL-2, which are also seen in T_SCM cells (2, 4). Obviously, the chemokine receptor helps T_SCM cells in lymph node homing (2, 4). Moreover, the expression of CD122 (IL-2R) and CD127 (IL-7R) on T_SCM cells indicates that IL-2, IL-15, and IL-7 play critical roles in boosting proliferation and survival of the cells (1–4). The function of T_SCM cell human markers has been expressed in Table 1.

Although it has been demonstrated that human T_SCM cells are capable of immediately releasing TNF-α, IFN-γ, perforin, and high amounts of IL-2, Zhang et al. demonstrated that murine T_SCM cells are not able to produce cytotoxic molecules and IFN-γ (1, 5). Gene expression profiling studies revealed alterations in the genes expressed in human and mouse T_SCM cells. Among them, higher expression levels of the transcription factors including TCF-1/LEF, Forkhead box protein O1 (FOXO-1), the inhibitor of DNA binding-3 (Id-3), and B-cell lymphoma-6 (BCL-6) were reported in T_SCM cells. In contrast, the levels of T-bet, B lymphocyte-induced maturation protein 1 (BLIMP-1), signal transducer and activator of transcription (STAT)-4, inhibitor of DNA binding-2 (Id-2), Eomes, and zinc finger E-box binding homeobox 2 (ZEB-2) genes were revealed to be partially low (Figure 3A). Notably, TCF-1/LEF, Eomes, and Id-3 are regarded as master regulators of the wnt-β-catenin signaling pathway (1, 2, 4, 13).

Among the molecules involved in the development of T_SCM, it has been suggested that inhibition of lactate dehydrogenase (LDH) along with IL-21 could be effective in this process. Notably, pieces of literature reported that T_SCM cells can be generated via the IL-10-IL-21-STAT-3 signaling pathway (Figure 3B.1) (4, 14); however, it is unclear how the IL-10-IL-21-STAT-3 signaling pathway plays a proliferative role in this cell subset production. Although IL-2 is defined as the most prevalent growth factor for T cells, it has been demonstrated that high concentrations of IL-2 lead to the expansion of effector T-cell subsets while decreasing early memory T-cell generation through abating T_CM cell populations (1, 4). Some studies reported that lower levels of IL-2 in the presence of IL-21 could improve the early memory T-cell proliferation and also could be effective in boosting T_SCM cell populations (Figure 3B.2) (1, 4). Additionally, IL-7 can accelerate the proliferation of T_SCM cells through different mechanisms (Figures 3A, B.3). These mechanisms consist of 1) inhibition of programmed cell death-1 (PD-1) and forkhead box p3 (FoxP3) expressions, 2) epigenetic modification through histone acetylation of gene promoters of effector T cells in order to convert to “naive-revertant cells” that can be phenotypically considered T_SCM cells, and, more importantly, 3) maintenance of T_SCM cell’s phenotype by IL-7 and IL-15 supplementation in the cell culture (Figure 3B.4). Similar to IL-7, IL-21 can directly and indirectly enhance T_SCM cell populations (4). According to Chen et al., IL-21 coincidently upregulates T-bet and suppressor of cytokine signaling gene expression and downregulates Eomes and GATA binding protein 3 to promote T_SCM cell proliferation (Figure 3A) (4, 14). Another study has also demonstrated that IL-21 linked to anti-PD-1 antibody (Ab) can prompt T_SCM cell development (Figure 3B.3) (4, 15).

Human CD8⁺ T_SCM cells express not only the surface antigens related to naive T lymphocytes (CD45RO⁻CD62L⁺CCR7⁺) but also markers exclusive for the memory subclass, including CD95, CXCR3, lymphocyte function-associated antigen 1 (LFA-1), and the β chain of IL-2 and IL-15 receptors (16). These cells enjoy similar superior potential for self-renewal and multipotentiality realized in CD4⁺ T_SCM (17). CD8⁺ T_SCM cells also show a robust response to IL-7 and possess a memory function that causes instant cytokine release after TCR stimulation (18). Meanwhile, from a comparative point of view, in vivo studies show that CD4⁺ T_SCM cells favorably respond to IL-7, while CD8⁺ T_SCM cells are amplified by both IL-7 and IL-15 (19). Gattinoni et al. (20) found that the glycogen synthase kinase-3β (GSK-3β) inhibitor TWS119 or Wnt3a prompts the formation of CD8⁺ T_SCM cells through the Wnt/β-catenin/TCF-1 signaling pathway. However, CD8⁺ and CD4⁺ T_SCM cells proliferated more efficiently once cocultured with anti-CD3/anti-CD28 conjugated beads alongside low concentrations of IL-7 and IL-15 in comparison with TWS119 exposure (20). Likewise, the generation of CD8⁺ T_SCM cells can be improved by IL-21 through the Janus kinase 2 (JAK-2)/STAT-3 pathway (14). Nevertheless, suppressors of cytokine signaling (SOCS) can restrict the impacts of these cytokines. For instance, upon activation of SOCS1, the formation of T_SCM cells from naive T cells via IL-21 induction is intensely inhibited (21).

Despite sporadic literature on the characterization of the CD4⁺ subset, a study revealed that CD4⁺ T_SCM cells are sensitive to the external environment. That is, during aging and chronic infections, the numbers and the functions of CD4⁺ T_SCM cells defect in the circulation. Inflammation can also affect CD4⁺ T_SCM cells via the induction of Wnt/β-catenin signaling, which culminates in an improved CD4⁺ T_SCM cell proliferative rate. Thus, the T_SCM differentiation by promotion of the Wnt/β-catenin pathway with a high concentration of agonist drives the acquisition of a CD4⁺ T_SCM phenotype (22).

Naive T cells are inactive in the peripheral blood and have low metabolic supplies. Thus, they principally use oxidative phosphorylation (OXPHOS) to produce ATP. However, differentiated T cells employ glycolysis to multiply, whereas memory T cells tend to benefit from fatty acid oxidation-dependent OXPHOS to provide ATP, which aids in performing prolonged immune response and heightened longevity (23, 24). Another study on T1D reported that in vitro formation of T_SCM cells from naive T cells occurred under IL-7 stimulation through overexpression of the glucose transporter GLUT1 to sustain glycolysis and subsequent oxidation of pyruvate in the mitochondria. Thus, targeting glucose metabolism by means of the selective inhibitor for GLUT1 (WZB117) can efficiently diminish the T_SCM differentiation in T1D subjects (25). Pilipow et al. (26) displayed that circulating T _SCM cells possess an important reservoir of reduced glutathione (GSH) ex vivo and limiting ROS with antioxidants in activated CD8⁺ T cells stops terminal differentiation while permitting the generation of long-lived T_SCM cells. In this case, N-acetylcysteine was capable of inducing CD8⁺ T_SCM cells from naive T precursors in vitro (26). In the research conducted by Kondo et al., (27) it has been observed that coculturing T cells with stromal OP9 cells expressing the NOTCH ligand professionally differentiated conventional human T cells into T_SCM cells through mitochondrial metabolic reprogramming. NOTCH signaling along with its downstream target, forkhead box M1 (FOXM1), stimulated mitochondrial biogenesis and fatty acid synthesis during T_SCM formation, notifying that NOTCH/FOXM1 pathway might be a beneficial target for T_SCM cell formation via metabolic alternations (27).

It is stated that imperative transcription factors and cytokines, accompanied by some inhibitors during the process of T-cell differentiation, stimulate T_SCM cell production through regulating T cell-related metabolic enzymes (28, 29). Good et al. (30) have declared that blocking the mTOR pathway such as inhibitors of Bruton’s tyrosine kinase (BTK) and IL-2-inducible T-cell kinase (ITK) can direct T cells to T_SCM cell differentiation (31). Scholz et al. (31) achieved that inhibition of mTOR complex 1 (mTORC1) by rapamycin or TWS119 in activated human naive T cells eventuates in the induction of T_SCM cells via T-cell metabolism alternation toward fatty acid oxidation. As rapamycin is routinely used in the treatment of autoimmune diseases (32), it seems that the usage of rapamycin may adversely exaggerate autoimmune diseases in the long run. Additionally, recent evidence indicates that T_SCM cells can be induced by Mek1/2 inhibitor (Meki) through regulating the metabolism regardless of affecting TCR-mediated activation (33). These studies point out that the regulation of metabolism and glycolysis is the fundamental factor in prompting the T_SCM formation. Hence, targeted metabolic checkpoints can bring about T cells differentiating into memory and afford more fresh T cells for immunotherapy.

Acquired AA is a rare condition of bone marrow failure syndrome in which hematopoietic stem/progenitor cells (HSPCs) are destroyed by mechanisms unrelated to the inherited syndrome (42). Despite the exact pathophysiology mechanism of AA still being blurred and the specificity of some recognized auto-Ags such as diazepam-binding related protein-1 not being proven in vivo, the immune attack to allogeneic hematopoietic cells by autoreactive T cells is considered as an underlying mechanism of autoimmunity in AA (43). Some features of autoreactive T cells in AA encouraged Hosokawa et al. (34) to explore the role of T_SCM cells in the immunopathogenesis of AA. It has been shown that the recognition of HSPC-restricted Ags through major histocompatibility complex (MHC) class I or II by oligoclonal CD8⁺ autoreactive T cells leads to pro-inflammatory cytokine secretion like IFN-γ against HSPC cells. Following immunosuppressive therapy (IST) with anti-thymocyte globulin (ATG) and cyclosporine A (CsA), the regeneration of oligoclonal T cells and even the new ones can occur (34). Accordantly, Hosokawa et al. (34) displayed that, in comparison to healthy individuals, the rate of CD8⁺ T_SCM cells in AA patients was higher at the onset of diagnosis and after IST in responder (complete and partial response) and non-responder patients, respectively. This indicates a favorable response to IST when CD8⁺ T_SCM cell frequency is high at the time of diagnosis in responders and inversely bringing about disease aggravation in non-responders to IST. Moreover, their intracellular staining revealed that both CD4⁺ and CD8⁺ T_SCM cells have more elevated levels of IL-2 and IFN-γ than those in healthy controls (34).

AU is an organ-specific disorder in which immune cells and, in particular, CD4⁺ and CD8⁺ Ag-specific memory T cells reside within the ocular tissue (44). The study of Hosokawa et al. (34) on AU patients showed that CD8⁺ T_SCM cell frequencies in these patients are significantly higher than those in healthy individuals. They also displayed that IST (combination of prednisolone and anti-TNF-α antibody) can be effective in reducing the CD8⁺ T_SCM cell population in patients with AU (34), suggesting CD8⁺ T_SCM cells as a potential marker associating with a better response to IST following lower frequency of the cells.

Systemic lupus erythematosus (SLE) is a complex autoimmune disease affecting multiple organs. Despite that the pathogenesis of SLE is still not fully understood, it is believed that factors, including environmental, hormonal, genetic, and immunological, serve a role in SLE development. Among others, the role of immunological dysregulation seems more prominent. In fact, immunological dysregulation can disrupt the balance of T helper (TH)1/TH2 and TH17/T regulatory (Treg) cell and ultimately shift them toward autoreactive T cells (45). T follicular helper (TFH) cells also interact with B cells, to boost their autoantibody (auto-Ab) production (38, 45). Auto-Abs and autoreactive T-cell activity are considered the major hindrance to achieving complete remission in SLE patients, despite long-lasting IST that indicates the emergence of the T_SCM cell population in SLE patients (38, 45). Recently, Hosokawa et al. (34) demonstrated that the CD4⁺ T_SCM cell population in SLE patients is lower than that in healthy controls due to receiving IST during sampling (34). Furthermore, Lee et al. (38) found that CD4⁺ and CD8⁺ T_SCM cell frequencies in SLE patients were remarkably elevated than those in the controls. They also observed that the CD4⁺ T_SCM cells of SLE patients can differentiate into TFH cells through BCL6, CXCR5, PD1, ICOS, LEF1, TCF-1, and IL-21 (as TFH cell inducer) gene overexpression, and BLIMP-1 gene encoding downregulation, leading to pathogenic auto-Ab formation (38). Strikingly, they declared that the CD4⁺ T_SCM cell population in SLE patients participates in the inflammatory process by producing higher levels of TNF-α, IFN-γ, IL-2, and IFN-α than those in normal individuals (38).

ITP is an acquired autoimmune disease that is characterized by platelet devastation due to T-cell dysfunction (46). Indeed, autoreactive T cells, including TH1 and TH17, give rise to auto-Ab production against platelets, and cytotoxic CD8⁺ T cells invade the surface glycoprotein GPIIb/IIIa of platelets. Additionally, the number and function of CD4⁺CD25⁺Treg cells decrease, resulting in the development and progression of ITP (46). In the investigation conducted by Cao et al., (41) it was disclosed that the population of CD8⁺ T_SCM cells in ITP patients outnumbered that of controls. Moreover, they found that prednisolone prescription, an IST, reduces the CD8⁺ T_SCM cell frequency and alleviates platelet destruction in responder groups (complete and partial) (41). They suggested that the higher frequency of this T-cell memory subset can lead to ITP exacerbation (41).

T1D is another chronic autoimmune disease in which autoreactive T cells attack β-cell auto-Ags such as glutamic acid decarboxylase 65 (GAD65), (pro)insulin, and islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) (25). Some autoreactive T cells in T1D patients exhibit distinguishable characteristics, including memory marker expression like CD95 and the presence of IL-7, which prolongs the survival and maintenance of autoreactive T cells, particularly T_SCM cells. More importantly, similar to other autoimmune diseases, a high dosage of IST during and after pancreas transplantation fails to eliminate autoreactive memory T cells in these patients (25). The mechanism of T_SCM cells in T1D is explainable by the “asymmetric division model” by which activation of T cells through β-cell auto-Ag presentation in an immune synapse between DC and naive T-cell undertakes mitosis simultaneously in the immune synapse. After polarization, the daughter cell receiving a continuous strong signal from the immune synapse becomes fully activated and consequently differentiates into CD25^hi CD127^low effector T cell (6). Another daughter cell with a faint signal expressing CD25^low CD127^hi (T_SCM cell) migrates via chemokine receptor CXCR-4 upregulation linking to CXCL-12 (stromal derived factor-1) into bone marrow (BM) where the stromal cells immensely produce IL-7 (6). A hemostatic cytokine IL-7 not only upregulates CXCR-4 on T cells but also assists T_SCM cells in self-renewal. It is postulated that the T1D autoreactive T cells arrested in BM and decreased their turnover by IL-7 can reconstitute specific effector and memory autoreactive T cells (Figure 4) (6). Vignali et al. (25) measured the frequency of autoreactive CD8⁺ T_SCM cells against GAD65, insulin, and IGRP in new-onset (<6 months T1D) and long-term (>20 years T1D) patients. They observed that the frequency of circulating specific CD8⁺ T_SCM cells against GAD65 and insulin in the new-onset patients was significantly higher than that in healthy controls (25). They also found that IL-7 can considerably amplify their frequency by GLUT-1 upregulation in T1D patients compared to the controls (25). Our study revealed that in new-onset (<1 year T1D), the frequency of CD4⁺ T_SCM cells is noticeably higher than that in long-term (>5 years T1D) and normal individuals (35). Therefore, both studies stated that T_SCM cell subsets can lead to disease progression (25, 35).

RA is a chronic inflammatory autoimmune disease in which synovial fluid (SF) is predominantly targeted by autoreactive T cells and leads to arthropathy (47). Besides autoreactive CD4⁺ T cells serving a key role in the immunopathogenesis of RA through their TH1, TH17, and TFH cell subsets, autoreactive CD8⁺ T cells are observed in the SF of new-onset patients (40). The study performed by Takeshita et al. (40) showed that the frequency of CD4⁺ and CD8⁺ T_SCM cells elevated in RA patients’ peripheral blood and SF. Moreover, they found that among immunosuppressant drugs, not only does methotrexate (MTX) decline T_SCM cell frequency, but it also reduces other T-cell subsets (except for TFH cells), resulting in MTX’s suppression effect on T-cell proliferation pathways including E2F, IL-2-STAT5, and mTORC1 (40). Consistent with this study, Cianciotti et al. (39) displayed that the citrullinated vimentin (Cit-vimentin)-specific CD4⁺ T_SCM cell population is higher in the circulation of RA patients than that in controls. They also found that TNF-α blockade can attenuate this subset frequency and prohibit differentiation of circulating TH17 from CD4⁺ T_SCM cells through blocking TNFII receptor (TNFRII) signaling, suggesting TNF-α as a prosurvival factor for T_SCM cells. Thereby, both mentioned studies implicated that the frequency of T_SCM cells can be a beneficial marker for disease development and response prediction (39).