Re-defining T-Cell Exhaustion: Subset, Function, and Regulation

Abstract

Acute viral infection or vaccination generates highly functional memory CD8 T cells following the Ag resolution. In contrast, persistent antigenic stimulation in chronic viral infection and cancer leads to a state of T-cell dysfunction termed T-cell exhaustion. We and other have recently identified a novel subset of exhausted CD8 T cells that act as stem cells for maintaining virus-specific CD8 T cells in a mouse model of chronic lymphocytic choriomeningitis virus infection. This stem cell-like CD8 T-cell subset has been also observed in both mouse and human tumor models. Most importantly, in both chronic viral infection and tumor models, the proliferative burst of Ag-specific CD8 T cells driven by PD-1-directed immunotherapy comes exclusively from this stem cell-like CD8 T-cell subset. Therefore, a better understanding of the mechanisms how CD8 T-cell subsets are regulated during chronic viral infection and cancer is required to improve the current immunotherapies that restore the function of exhausted CD8 T cells. In this review, we discuss the differentiation of virus-specific CD8 T cells during chronic viral infection, the characteristics and function of CD8 T-cell subsets, and the therapeutic intervention of PD-1-directed immunotherapy in cancer.

Keywords: Immunotherapy; PD-1; Stem cell-like CD8 T-cell subset; T-cell exhaustion.

Figure 1. Differentiation pathway of Ag-specific CD8 T cells during chronic viral infection. (A) Upon acute viral infection, naïve CD8 T cells activate and differentiate into memory precursors (MP) and terminal effectors (TE). Terminal effectors die by AICD and memory precursors survive and become memory CD8 T cells (M) after the clearance of viral infection. Similarly, naïve CD8 T cells (N) are activated and differentiate into a stem cell-like subset (SL) and terminally differentiated cells (TD) upon chronic viral infection. Analogous to terminal effectors, terminally differentiated cells also die by AICD. Different from the acute infection, sustained antigenic stimulation during chronic viral infection resulted in the continual differentiation of stem cell-like CD8 T cells into terminally differentiated CD8 T cells. (B) TCF1⁺CXCR5⁺ stem cell-like CD8 T cells maintain their population by slow self-renewal. Upon antigenic stimulation, these stem cell-like CD8 T cells differentiate into CD101⁻Tim-3⁺ transitory population. This CD101⁻Tim-3⁺ subset possesses in vivo proliferative potential after antigenic stimulation, can differentiate further into terminally differentiated CD101⁺Tim-3⁺ CD8 T cells, and contributes to viral control with the highest cytolytic activity. With upregulation of CD101, terminally differentiated CD101⁺Tim-3⁺ CD8 T cells lost in vivo proliferative potential and possessed impaired cytolytic function.

Figure 2. Migration of CD8 T-cell subsets during chronic viral infection. Both stem cell-like and terminally differentiated CD8 T cells are circulating after the onset of viral infection. After the establishment of T-cell exhaustion, stem cell-like cells become resident and mainly reside in the lymphoid organs, especially in the white pulp of the spleen. A fraction of the transitory CD101⁻Tim-3⁺ CD8 T cells circulate via the blood and migrate other inflamed tissues corresponding to the expression of a fractalkine receptor CX3CR1. In the red pulp of the spleen and non-lymphoid tissues, CD101⁻Tim-3⁺ CD8 T-cell subset differentiates into terminally differentiated CD101⁺Tim-3⁺ CD8 T cells and become resident.

Figure 3. Transcriptional network to regulate differentiation of CD8 T cells during chronic viral infection. TCR stimulation by persistent Ags initiates a set of transcription factors, including NFAT2 followed by BATF and IRF4. TOX is also activated by NFAT2 via separate route. TCR-driven factors generally contribute to PD-1 upregulation in both stem cell-like and terminally differentiated CD8 T cells undergoing exhaustion process. Although IRF4 together with BATF directly represses TCF1 expression and upregulates BLIMP1, highly upregulated TCF1 in stem cell-like subset increases BCL6 expression, which also accelerates E2A expression. This TCF1-BCL6-E2A axis altogether or independently induces the genes related to stem cell-like CD8 T cell signature, such as Cxcr5, Sell, and Il7r. In the other hand, terminally differentiated CD8 T cells shows a typical feature with high level of BLIMP1 and low level of TCF1. Repressed level of TCF1 are not enough to upregulate BCL6, which allows the induction of ID2 and subsequent suppression of E2A. Both BLIMP1 and ID2 contribute to the enhancement of Gzmb expression, leading to the differentiation of terminally differentiated CD8 T cells. Straight red lines with arrow symbol indicate stimulation or activation. Dotted blue lines with block symbol indicate inhibition or suppression. Thickness of lines defines a strength of signal. Straight black lines with arrow symbol and orange lines with arrow symbol exhibit a binding to upstream of the corresponding genes and their transcription, respectively.

Figure 4. Proposed mechanism of PD-1-directed immunotherapy during chronic viral infection and cancer. (A) After the establishment of T-cell exhaustion, PD-1/PD-L1 axis plays a role in 2 different ways. First, PD-1/PD-L1 interaction inhibits the proliferation and differentiation of stem cell-like CD8 T cells into the more differentiated CD8 T-cell subsets. CD28 is a major target for PD-1-mediated suppression. Next, PD-L1 on infected cells or tumor cells impairs the cytolytic function and cytokine production of PD-1⁺ Ag-specific differentiated CD8 T-cell subsets. (B) In the absence of PD-1-mediagted signals, the proliferation and differentiation of stem cell-like CD8 T cells into transitory and terminally differentiated CD8 T cells are accelerated and this happens in a CD28/B7-dependent manner. The increased population of transitory subset showing the highest anti-viral/tumor activity is critical for the viral clearance and tumor regression. In terms of the quality, the cytolytic function and cytokine production of transitory and terminally differentiated CD8 T-cell subsets might be increased after PD-1 blockade.