Introduction
Hepatocellular carcinoma (HCC) and intrahepatic cholangiocarcinoma (ICC) are the two major subtypes of primary liver cancers (PLCs) [
1]. The HCC accounts for up to 85%, while the ICC accounts for 10 to 15% of all PLC cases [
2]. Although some early-stage PLC patients can benefit from surgical treatment and systematic anti-tumor therapies, including target therapy and immunotherapy, those with advanced stage can only receive limited benefits from few multi-kinase inhibitors [
2,
3]. Currently, immunotherapeutic strategies, such as tumor vaccines, adoptive T-cell transfer therapy, immune-checkpoint inhibitors, and even immunotherapeutic strategies combined with conventional strategies, have been increasingly investigated in managing HCC and ICC. However, some are still in clinical trials [
4‐
6]. It is worth mentioning that understanding tumor heterogeneity and the diverse landscapes of the tumor microenvironment (TME) provides a solution to unraveling how to improve the sensitivity of immunotherapies [
7‐
10]. Novel biomarkers are identified and used to predict clinical outcome for PLCs such as surface markers, tumor mutation burden, and microsatellite instability [
11‐
13]. Recent studies on single-cell RNA sequencing (scRNA-seq) analysis from HCC and ICC have sparked growing interest within the past few years. It has been reported that besides specific drug-resistant subpopulations, such as cancer stem cells (CSCs), the scRNA-seq can also track the development of tumor-infiltrating CD8
+T cells. Blockade of the inhibitory molecules on the reversible exhausted CD8
+T cells at early-stage may pave the way for developing novel immunotherapeutic strategies [
14‐
17].
Tissue-resident CD8
+T cells, identified as CD103
+CD8
+T cells, are the essential effector cell population of anti-tumor immune response in tissue regional immunity [
18]. E-cadherin is the important ligand of CD103 (integrin alpha E, ITGAE) [
19]. In the TME, the epithelial cancer cells can express E-cadherin, interact with CD103 on CD8
+T cells, and then maintain the interaction of cancer cells and CD8
+T cells, leading to the residence of tumor antigen-reactive CD8
+T cells and the persistent anti-tumor effect in tumor tissues [
20]. Results from our and other groups have confirmed that transforming growth factor-β (TGF-β) signaling or IL-33 induces CD103 expression on tumor-infiltrating CD8
+T cells. Moreover, IL-33 can promote the proliferation and effector function of tissue-resident CD103
+CD8
+T cells [
18,
21‐
23]. Furthermore, the co-expression of CD103 and CD69 on CD8
+T cells is used to identify the distinct subset of tissue-resident memory CD8
+T cells (T
RM) [
24,
25]. Unlike central memory and effector memory CD8
+T cells, the CD8
+T
RM cells can express unique chemokine and tissue-homing receptors but lack the lymph node homing molecules CD62L and/or CCR7, and the homing of T
RM cells to peripheral tissues is chemokine dependent [
26,
27].
As an important surface marker of tissue-resident CD8
+T cells, CD103 enables antigen-specific CD8
+T cells to reside within the epithelial tissues by binding to the epithelial cell marker E-cadherin and provides first-line defense against infection in peripheral nonlymphoid tissues [
24,
28,
29]. Moreover, in the TME, the tissue-resident CD103
+CD8
+T cells can secret granzyme B and perforin and trigger cytotoxic activities, enhancing the anti-tumor immune system response [
30,
31]. We have previously reported that the higher intensity of tissue-resident CD103
+CD8
+T cells in human colorectal cancer tissues predicts a better overall survival (OS) of the patients [
32]. Notably, the prognostic value, immunolocalization, and involvement of tissue-resident CD8
+T cells in human HCC and ICC remain elusive. In our present study, multicolor immunohistochemistry (mIHC) and imaging analysis were used to characterize the immunolocalization of tissue-resident CD8
+T cells in the tissue microarray (TMA) slides of human HCC and ICC. The single-cell transcriptomics analysis was performed to reveal the cellular functions and cell communications in TME of these two malignancies.
Discussion
The dynamics of CD8
+T-cell-mediated immune surveillance play an essential role in the continuous resistance and the outcome of intracellular infections and cancer [
36]. It has been suggested that in the tumor model of transplantable cutaneous melanoma, the CD8
+T
RM cells can promote a regional and durable melanoma-immune equilibrium within the skin [
24]. Notably, the tissue-resident CD103
+CD8
+T cells in the TME are found to upregulate the inhibitory checkpoints, such as PD-1, CTLA-4, TIM-3, LAG3, TIGIT, and CD39. Therefore, these resident CD8
+T cells are also considered tumor antigen-reactive T cells [
37‐
39]. Furthermore, tissue-resident CD103
+T cells are increased and respond to anti-PD-1 therapies in early-phase clinic trials [
28]. Many retrospective studies have shown that tissue-resident CD103
+CD8
+TILs can be quantified as a significant predictor for the patient’s survival in many human solid tumors [
37].
Our present study evaluated the expression of CD103 and immunolocalization of tissue-resident CD8
+T cells in HCC and ICC patients using mIHC. HCC tissues exhibited a higher percentage of infiltrating CD8
+T cells and CD103
+ immune cells compared with the adjacent normal tissues. However, the percentage of CD103
+CD8
+T cells did not reveal any differences between HCC/ICC tissues and normal tissues. Some studies have demonstrated that CD103
+CD8
+T
RM cells represent highly activated T-cell subsets and provide tumor reactivity [
40,
41]. We next evaluated the expressions of CD8 and CD103 to identify the clinical outcomes of liver cancer. Our results indicated that the intensity of infiltrating CD8
+T cells had a prognostic value in human HCC and ICC. Furthermore, the HCC patients with CD8
highCD103
high expression were associated with a better OS.
Moreover, the ICC patients expressing CD8
lowCD103
low T cells had a lower OS rate than the ICC patients with CD8
lowCD103
high and CD8
highCD103
high expression. There was an improvement in OS regarding the patients with CD8
lowCD103
high and CD8
highCD103
high expression in HCC or ICC, which might be related to the spatial heterogeneity of TILs in the immune microenvironment of HCC and ICC [
42]. It was confirmed that in human HCC and ICC, the abundance of tumor-infiltrating CD8
+T cells and CD103
+CD8
+T cells could be a prognostic predictor for patients’ OS. Furthermore, the percentages of CD103
+CD8
+TILs were also linked to the OS of HCC and ICC. Therefore, CD103 might be targeted for enhancing tumor immunity of CD8
+T cells, and the infiltration of CD103
+CD8
+T cells in the liver TME might be considered as a biomarker to predict a better prognosis of the patients. The correlations between the numbers of CD8
+TILs, CD103
+ immune cells, and tissue-resident CD103
+CD8
+T cells and clinical parameters in HCC and ICC were investigated. We found that more CD8
+T cells expressed CD103 in HCC and ICC tissues, which was negatively associated with the advanced pathological stage.
The tissue-resident CD103
+CD8
+T cells show their importance in anti-tumor immunity [
43]. Further analysis is required to understand more immune checkpoint genes closely related to immune cell infiltration. The scRNA-seq data of CD3
+TILs from human lung cancer reveal that the CD8
+T-LAYN (PD-1
highCTLA-4
high) is the predominant subpopulation of CD103
high CD8
+T cells, and these cells are the major subset in response to anti-PD-1 therapy [
34]. Moreover, the patients with higher infiltration of tissue-resident CD103
+CD8
+T cells will receive greater survival-related benefits from anti-PD-1 therapy [
37,
38]. In our present study, we also reanalyzed the published scRNA-seq data of human HCC and ICC. We found that the biological functions of CD103
+CD8
+TILs in human HCC or ICC tissues were highly consistent with those cells in human lung cancer [
14,
17,
34]. Besides, some other inhibitory immune checkpoint molecules, such as
LAG3,
TIGIT, and
HAVCR2, were also highly expressed in CD103
+CD8
+TILs in human HCC or ICC tissues, suggesting that those CD103
+CD8
+TILs were also the vital population in response to the immune checkpoint blockade therapy targeting LAG3, TIGIT or TIM3, or even in combination with anti-PD-1. In addition, based on the cell–cell communication analysis between CD103
+CD8
+TILs and other types of cells in human HCC or ICC tissues, some other cellular components played essential roles in regulating the biological functions of CD103
+CD8
+TILs. For example, various cell-derived TGF-β, and TAM-derived TNF-α, contributed to the formation of tissue-resident CD103
+CD8
+TILs in the TME. Moreover, the tumor cell-derived CXCL6, CXCL2, and CCL20, TAM-derived CXCL9/CXCL10, and the stromal cell-derived CXCL12 also contributed to the accumulation of tissue-resident CD103
+CD8
+TILs in the TME. Therefore, future clinical treatment targeting these molecules and cells may improve the effectiveness of the CD103
+CD8
+T-cell-mediated anti-tumor response in the TME.
In addition, there are also some limitations of our present study. First, the sample sizes of both HCC and ICC tissues need to be further extended and may improve the prognostic evaluation of tissue-resident CD103+CD8+T cells in these two malignancies. Second, due to the limited fluorescence channels could be selected in the multicolor immunohistochemistry assay, it would be better if we can add more functional markers to evaluate the anti-tumor response of tissue-resident CD103+CD8+T cells in the TME of both HCC and ICC. Third, due to the limitation of detailed therapeutic information of all the HCC and ICC patients involved in the present study, it is of great importance to evaluate the predictive value of tissue-resident CD103+CD8+T cells for therapeutic efficacy of target therapy or immunotherapy against HCC or ICC. Resolving these three limitations would help the clinical application of the examination of tissue-resident CD103+CD8+T cells in HCC or ICC tissues for the prediction of therapeutic efficacy and the understanding of immune regulation mechanism of these two malignancies.
Taken together, our present study indicated the prognostic values of infiltration intensity of tissue-resident CD103+CD8+TILs in human HCC and ICC. Moreover, we also revealed that the tissue-resident CD103+CD8+T cells were enriched in genes associated with T-cell activation, proliferation, cytokine function, and even T-cell exhaustion by using single-cell transcriptomics analysis.
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