Revisiting the role of human memory CD8+ T cells in immune surveillance

Virtual memory T (T_VM) cells were first described 13 years ago in mice as a specialized subset of CD8+ T cells that are semi-differentiated, antigen-naïve, and characterized by a memory phenotype and functionality [1]. A putative T_VM cell subset has been identified in humans for its presence in cord blood, indicating that it does not necessarily have to be antigen experienced [2]. Although phenotypically different (CD44^hi CD62L⁺ CD49d^low in mice and CD45RA+ killer-cell immunoglobulin-like receptor (KIR)/NKG2A+ in humans), T_VM cells exhibit similar functionality in mice and humans; that is, they are cytokine-responsive and exert rapid cytotoxicity [3]. Likewise, there are several studied CD8+ T-cell subpopulations that are functionally reminiscent of T_VM cells, such as innate-like CD8+ T cells, natural killer (NK)-like CD8+ T cells, and memory phenotype CD8+ T cells. The molecular conditions underlying their development, definition, and immunological properties have been previously discussed [3]. Here, we mainly focus on the origin, activation, and effector functions of T_VM cells, their clinical relevance, and the potential application of T_VM cells in developing novel therapeutic approaches.

The fate of developing thymocytes is determined by the interaction of T-cell receptors (TCRs) with self-peptide–MHC complexes tightly regulated by clonal selection. Self-reactive T (T_SR) cells are normally contained under physiological conditions. A recent study by Li et al. identified a KIR+CD8+T_SR cell subset in humans that can selectively suppress pathogenic self-reactive CD4+ T cells in autoimmune or infectious diseases [4]. It is worth noting that T_VM cells favor TCR clonotypes with a high affinity for self-antigens in mice [5]. In humans, more than 90% of T_VM cells are positive for KIRs [6] and NK-cell receptors, which are expected to be upregulated on CD8+ T_SR cells [7]. In addition, T_VM cells share some other features with the newly identified KIR+CD8+ T_SR cells in humans (Fig. 1): (i) they are present in the peripheral blood at a frequency of ~5% among total CD8+ T cells in healthy adults and accumulate with age, (ii) they express high levels of cytotoxic and NK-cell-associated molecules, (iii) they mainly exhibit a terminally differentiated phenotype, and (iv) they are sensitive to IL-15 but not TCR stimulation. Notably, T_VM cells are less potent in provoking autoimmune diseases than true memory cells [5], as downstream TCR signaling might be strictly inhibited in T_SR cells [8]. Therefore, it is necessary to better clarify the features of CD8+ T_SR cells and unify the nomenclature. Integration of existing findings on T_VM cells may facilitate understanding of self-tolerance and immune dysregulation in physiological and pathological conditions.

Fig. 1.

The role of human TVM cells in immune surveillance. Human T_VM cells are self-reactive and express CD45RA+KIR/NKG2A+ and NK-associated molecules (e.g., NKG2C and NKG2D) with low exhaustion and high survivability. They are sensitive to cytokine stimulation, exert rapid effector functions, and rely on glycolysis and OXPHOS. Under certain physiological or pathological conditions (such as aging, autoimmune diseases, cancer, and infectious diseases), T_VM cells can sense and suppress altered autologous cells through several proposed mechanisms, including a bystander activation through cytokine receptors (IL-15, IL-12, and IL-18), b overcoming KIR-mediated inhibition due to HLA-I downregulation in target cells, and c TCR-mediated recognition of cognate antigens presented by classic and nonclassic HLA-I molecules. T_VM, virtual memory CD8+ T; KIR, killer-cell immunoglobulin-like receptor; OXPHOS, oxidative phosphorylation. The figure was drawn using Biorender.com

Recent studies have highlighted that the phenotype and functionality of T_VM cells are influenced by their distinctive metabolic characteristics. T_VM cells show a unique effector-memory mixed metabolic profile and are capable of relying on both glycolysis and oxidative phosphorylation [9]. Compared with naïve and true memory cells, T_VM cells exhibit the highest spare respiratory capacity (SRC, which reflects the mitochondrial capacity that a cell holds in reserve). SRC levels correlate with IL-15 sensitivity and Bcl-2 expression and are therefore associated with the function and survival capacity of T_VM cells [10]. Moreover, these metabolic features are retained during aging, resulting in quantitative accumulation with the functional senescence of T_VM cells in aged individuals [9–11]. However, most of these findings come from investigations conducted in mice, and whether human T_VM cells share the same or have distinct metabolic features requires further study.

T_VM cells have both innate adaptive immunity characteristics and can respond to various cytokine stimulations, secrete high levels of IFN-γ and GZMB, exert quick effector functions, and adapt to tissue damage and other conditions [1]. These features allow them to play an important role in various disease states.

Our previous study found that T_VM cells were significantly expanded in association with the elevation of IFN-α and IL-15 levels in chronic HIV-1-infected patients. Interestingly, there was a significant inverse correlation between the frequency of T_VM cells and the size of the viral reservoir in patients undergoing antiretroviral treatment (ART), suggesting an opportunity for assessing residual HIV replication in T_VM cells in ART patients [6]. Such a direct anti-HIV effect was verified by direct coculture of purified T_VM cells with autologous HIV-positive CD4+ T cells from ART-treated patients. Accordingly, a population of cells with high functional overlap with T_VM cells was found to eliminate SIV-infected CD4+ T cells in SIV-infected rhesus monkeys [12]. We further investigated the effector function of T_VM cells in controlling HIV-1 in ART patients and demonstrated that the CCL5-secreting ability of T_VM cells was inversely correlated with HIV-1 DNA and cell-associated unspliced RNA levels. In an in vitro viral inhibition assay, T_VM cells inhibited HIV-1-infected cells in a CCL5-dependent manner [13]. Thus, T_VM cells are expanded and functionally intact in ART patients, exerting critical immune pressure on residual HIV replication. Furthermore, in the posttreatment controllers, in whom the function of HIV-specific CD8+ T cells was significantly weakened, T_VM cells may have accounted for key effectors of CD8+ T-cell-mediated control of viral rebound [6]. This hypothesis deserves further study, and what remains to be clarified in the future is whether there are regulators that inhibit T_VM cell function under HIV infection and how to use the exclusive function of T_VM cells to achieve the goal of further controlling the viral reservoir and functional cure of HIV.

The accumulation of T_VM cells was particularly pronounced in the elderly group. Accompanied by a decrease in naïve and antigen-specific memory CD8+ T cells, the proportion of T_VM cells significantly increased [11]. In aged individuals, the response of T_VM cells to TCR is weakened, and apoptosis occurs under the stimulation of antigenic peptides, while it can be avoided under stimulation with cytokines, such as IL-15; eventually, T_VM cells play a significant protective role. Some studies have demonstrated that T_VM cells retain asymmetric division ability during aging, indicating that their proliferation ability compared is preserved to naïve copartners [9]. In addition to their killing function, T_VM cells also have immunomodulatory functions that can inhibit T-cell responses and proliferation. Simultaneously, in controlling autoimmunity, only pathogenic T cells are targeted, and immune cells that recognize exogenous antigens are not harmed [4]. A major role of T_VM cells may be to control activated autoreactive T cells during infection, complementing Treg-mediated peripheral tolerance. Thus, given the findings of multiple studies on autoimmune diseases, the role of T_VM cells may be bilateral: they can limit tissue damage caused by cytotoxic immune cells, while excessive activation of T_VM cells may hinder the effective clearance of pathogens [4].

Several studies have focused on the role of T_VM cells in antitumor immunity. Various tumor cells downregulate MHC-I expression, limiting the killing ability of antigen-specific CD8+ T cells, but T_VM cells can specifically kill MHC I-downregulated tumor cells because of the reduced TCR response and their stronger natural immune response [2]. Recently, a class of innate killer T cells identified in tumor tissues was found to be highly analogous to T_VM cells, with enhanced tissue residency and capacity to respond to IL-15 expressed by cancer cells leading to activation of antitumor function [8].

Based on these studies, how T_VM cells recognize target cells remains a critical question. There are several potential explanations for this observation (Fig. 1). (i) Cytokine-driven bystander effects. Bystander activation of T_VM cells by cytokines (such as IL-15, IL12, and IL-18) can play a role in various types of microbial infection [14]. (ii) Rescue of KIR-mediated control. We have demonstrated that KIRs on CD8+ T cells are not only markers but also functional receptors for T_VM cell-mediated HIV infection. In particular, HIV replication downregulates MHC-I on CD4+ T cells, which sensitizes these cells to T_VM cells due to the suppression of KIR-mediated control. As a result, KIR blockade was able to enhance the viral inhibitory effect on T_VM cells [6]. (iii) Sensing of cognate antigens. Li et al. showed that the KIR+CD8+ T-cell-mediated killing of self-reactive pathogenic CD4+ T cells was dependent on both classic and nonclassic MHC-I molecules in a cell–cell contact manner [4], highlighting the potential recognition of cognate self-antigens. These antigens might be completely different from neoantigens. One possibility is that peptides derived from alarmins activate their cognate TCRs. Another explanation could be the ectopic expression of mosaic antigens. The production and presentation of self-antigens in medullary thymic epithelial cells (mTECs) are strictly regulated by AIRE together with lineage-defining transcription factors [15]. Since AIRE is also reported to be activated in peripheral tissue and organs, it is possible that altered cells can ectopically express mosaic antigens that should be restricted to mTECs, resulting in antigen-driven activation of T_VM cells. Therefore, a better understanding of the mechanism of T_VM cell activation is needed.

In summary, T_VM cells are self-reactive, functionally conserved, and rely on various specific effector mechanisms to protect against multiple disease states. Future studies are needed to better characterize T_VM cells together with their analogous subsets under various conditions, such as aging, autoimmune diseases, tumors, and infectious diseases.

Author contributions

Y-YW, WH, and CZ wrote the manuscript and constructed the figures. F-SW revised the manuscript and figures.

Competing interests

The authors declare no competing interests.