Introduction
Renal cell carcinoma (RCC) is one of the ten most common cancers; 73,750 new cases of RCC and 14,830 RCC deaths were reported in the U.S. in 2020 [
1]. Kidney clear cell carcinoma (KIRC) is the main pathologic subtype of RCC [
2]. Despite our great advances in diagnosis, screening, surgery, and treatment, the clinical outcomes of KIRC are still unsatisfactory [
3]. The prognosis for advanced KIRC is extremely poor due to its inherent resistance to radiotherapy and chemotherapy, and the challenge for clinical management lies in treating the poor prognosis caused by radiotherapy and chemotherapy resistance [
4]. KIRC has an immunogenic tumor microenvironment (TME) that contains a variety of tumor-infiltrating T lymphocytes [
5]. Features of the tumor microenvironment heavily affect disease biology and may affect responses to systemic therapy [
6‐
9]. In recent years, advances in immunotherapy, particularly immune checkpoint blockade (ICB) and engineered T cells have revolutionized cancer treatment [
10]. ICB or ICB plus TKIs targeting programmed cell death 1 (PD-1), programmed cell death ligand 1 (PD-L1), and cytotoxic T-lymphocyte-associated antigen 4 (CTLA4) are now the standard of care for RCC [
11,
12]. In the era of ICB, understanding immunogenic TME will help to find new therapeutic strategies in KIRC management.
CD72 is a type II membrane protein expressed mainly in B cells and is a member of the C-type lectin superfamily [
13]. CD72 contains a C-type lectin-like structural domain (CTLD) and an immunoreceptor tyrosine-based inhibitory motif (ITIM) [
14,
15]. CD72 overexpression can negatively regulate BCR signaling in B cell lines by recruiting phosphatase 1 (SHP-1) of the SH2 structural domain to phosphorylated ITIM [
16]. CD72 functions similarly to the inhibitory co-receptor CD22 in down-regulating B-cell receptor (BCR) signaling and functioning as a molecular switch determining whether apoptosis or proliferation occurs in antigen-stimulated B cells [
14]. Furthermore, CD72 and SEMA4D (CD100) interaction enhances the activation of B cells and dendritic cells (DCs) [
17]. Recent studies have shown that CD72 is closely associated with developing various immune-related diseases. For example, CD72 is strongly associated with developing systemic lupus erythematosus (SLE) [
18‐
20] in autoimmune diseases. CD72 is also closely associated with the tumor microenvironment [
21]. Several studies have shown that CD72 is a marker for progenitor cell B-cell leukemia and a new marker for detecting microscopic residual disease in acute lymphoblastic leukemia [
22,
23]. In addition, CD72 has been identified as a prognostic gene in the tumor microenvironment of colorectal cancer [
24,
25]. CD72 may be an independent predictor of prognosis in nasopharyngeal carcinoma patients [
26]. However, there are no studies on the role of CD72 in immune infiltration in KIRC. Therefore, this study aimed to investigate the relationship between CD72 and immune infiltration and prognosis of KIRC and to provide an important molecular basis for the early non-invasive diagnosis and immunotherapy of KRIC.
Discussion
KIRC, as the most common renal cell carcinoma, is challenging to diagnose at an early stage, surgery has limitations, and postoperative metastasis and recurrence are the main reasons for its high mortality rate. With the emergence of tyrosine kinase inhibitors targeting vascular endothelial growth factor receptors, rapamycin-targeted protein inhibitors, and immune checkpoint inhibitors, the choice of second-line treatment is expanding, and renal cancer treatment has entered a new era [
43]. However, KIRC has high intrinsic heterogeneity and lacks early diagnostic and prognostic biomarkers [
44,
45]. In addition, KIRC is one of the most immune-infiltrated tumors [
46,
47]. Therefore, searching for new diagnostic and prognostic biomarkers and therapeutic targets and developing new anti-tumor drugs and immunotherapies are essential ways to improve the survival of KIRC patients.
In this study, we obtained clinical and RNA data of KIRC patients from the TCGA database and downloaded immune-related genes from the ImmPort database. Then we used the "limma" package, "survival" package, and Venn overlaps analysis to obtain differentially expressed mRNAs (DEmRNAs) related to the prognosis and immunity of KIRC patients, and we finally chose CD72 as the target gene.
Our study was analyzed using the UCSC XENA and TCGA database, and the results showed that in KIRC, compared with paracancerous tissues, the expression level of CD72 was significantly increased, which correlated with poor patient prognosis. KIRC patients with high expression of CD72 were less likely than those with low expression of CD72 in terms of either OS, DSS, or PFI. These results suggest that CD72 is associated with the occurrence and development of KIRC. In this study, CD72 expression was significantly correlated with the Pathologic T stage, Pathologic stage, Pathologic M stage, Pathologic N stage, Histologic grade, Laterality, and OS event, suggesting that CD72 may play an essential role in the biological function of tumor cells. Play an important role in the biological function of tumor cells. These results suggest that CD72 may be a useful diagnostic molecular marker for KIRC and can predict the outcome of KIRC patients. In addition, based on the ROC diagnostic curve (AUC = 0.954) and logistic analysis, it was further shown that CD72 could be used for KIRC diagnosis.CD72 is expected to be a potential biomarker for determining the prognosis, suggesting that patients with KIRC may benefit from using CD72 for diagnosis and prognosis.
To further understand the molecular mechanism of CD72 in tumorigenesis and development, functional enrichment analysis of GO, KEGG, and GSEA was performed using CD72 and its related differentially expressed genes. GO-based enrichment analysis showed that CD72 and its co-expressed mRNAs were enriched for several molecular functions (e.g., microtubule binding, cytoskeletal motor activity, histone kinase activity, G protein-coupled receptor binding, long-chain fatty acid binding), biological processes (mitotic nuclear division, mitotic sister chromatid segregation, organelle fission, chromosome segregation).
Abnormal expression of microtubule-binding proteins can cause cytoskeletal changes. Abnormal cytoskeletal motility activity may disrupt critical processes such as cell proliferation, migration, and metastasis [
48,
49], leading to disorganization of cellular structure and abnormal proliferation of tumor cells [
50]. Aberrant activation of histone kinases may lead to disrupted chromatin structure and aberrant gene expression, thereby affecting tumor cell proliferation and transcriptional regulation [
51,
52]. Aberrant activity of G protein-coupled receptors may trigger aberrant activation of cell signaling pathways, promoting the proliferation and survival of tumor cells [
53,
54]. RCC is essentially a metabolic disease characterized by a reprogramming of energetic metabolism [
55‐
58]. In particular the metabolic flux through glycolysis is partitioned [
59‐
61], and mitochondrial bioenergetics and OxPhox are impaired, as well as lipid metabolism [
59,
62‐
64]. In addition, aberrant expression of long-chain fatty acid binding proteins may lead to disturbances in intracellular energy metabolism and alterations in tumor cell growth [
65].
In tumor development, mitotic nuclear division, sister chromatid segregation, and abnormal chromosome segregation lead to genomic instability, increase the risk of chromosomal abnormalities and mutations, and promote tumor formation and development [
66,
67]. The correct alignment of the mitotic spindle during cell division is critical for cell fate determination, tissue organization, and development. Changes in the dynamics and control of microtubules that impair the mitotic spindle lead to chromosomal instability, leading to the generation of tumor cells [
68,
69]. The molecular mechanism of CD72 in the development of KIRC may be related to its role in cell cycle regulation, cell growth, and cell migration.
KEGG analysis also showed that CD72 function is associated with signaling pathways such as cell cycle, PPAR signaling pathway, p53 signaling pathway, coagulation factor signaling pathway, and cytokine–cytokine receptor interactions. Abnormal cell cycle regulation is essential in tumorigenesis and progression, leading to unlimited cell proliferation [
70]. Abnormalities in PPAR and p53 signaling pathways are associated with the progression of multiple tumors [
71‐
75]. Abnormalities in chemokine pathways are associated with tumor infiltration, metastasis, and neovascularization [
76‐
78]. Cytokine-receptor interactions are critical for regulating cell growth, differentiation, and immunity, and abnormalities can lead to tumor immune escape phenomena [
79,
80].
Based on GSEA enrichment analysis, CD72 and its co-expressed mRNAs are enriched in signaling pathways such as Pd 1 Signaling, CTLA4 Pathway, Th17 Cell Differentiation Pathway, B Cell Receptor Signaling Pathway, Costimulation By the CD28 Family, Primary Immunodeficiency, Cancer Immunotherapy By Pd1 Blockade, Modulators of Tcr Signaling and T Cell Activation, Costimulation By the CD28 Family, Primary Immunodeficiency, Cancer Immunotherapy By Pd1 Blockade, Modulators of Tcr Signaling and T Cell Activation, and other Signaling Pathways associated with tumor immunity and tumorigenesis.
Correlation analysis showed a significant association between CD72 and PD-L1 (CD274) and CTLA4. Pd 1 and CTLA4 are two key immune checkpoint molecules with essential roles in regulating T-cell function and activating [
81,
82]. When Pd-L1 (ligand for Pd 1) and CTLA4 are upregulated, tumor cells can evade the immune response and promote tumor growth. Activated T cells express PD-1 encoded by the PDCD1 gene, while PD-L1 encoded by the CD274 gene is overexpressed on the membrane surface of tumor cells. PD-1 binds to PD-L1, inhibits T-cell activation and causes their death, and then assists in the immune escape of tumor cells [
83]. Abnormal B-cell receptor (BCR) signaling pathway activation may also lead to malignant transformation of B cells, which drives tumorigenesis and progression [
84]. In addition, through activation of the Th17 cell differentiation pathway, increased numbers of tumor-infiltrating immune cells, promotion of neovascularization, and alterations in the tumor microenvironment can promote tumorigenesis and progression [
85]. Co-stimulatory pathways of the CD28 family impact tumor growth and immune surveillance by influencing T-cell activation, proliferation, and anti-tumor immune responses, as well as modulating immune cell function [
86]. The above results suggest that CD72 is essential in tumorigenesis and progression.
The results of immune infiltration showed that the degree of infiltration of T cells, T helper cells, Treg, Th1 cells, cytotoxic cells, aDC, B cells, macrophages, TFH, Tcm, CD8 T cells, CD56bright cells, Tem, eosinophils, DC, and neutrophils showed a significant positive correlation with CD72 expression. Mast cells, NK cells, and Th17 infiltration degree were significantly negatively correlated with CD72 expression. The transition from Th1/Th2 balance to Th2 dominance is critical during tumor progression. Th2 cells are detrimental to cellular immune anti-tumor effects. Restoration of Th1/Th2 balance is vital in tumor therapy [
87]. Tregs are usually enriched in the tumor microenvironment, and many Tregs have a poor prognosis [
88]. Lack of NK cell numbers and defective NK cell function promote the escape of tumor cells from immune surveillance [
89]. Tumor-Associated Macrophages (TAMs) Promote Tumor Growth and Metastasis by Suppressing Tumor Immunosurveillance [
90]. In addition, immunomodulatory interactions may be altered between lymphocytes and non-lymphocytes, leading to immune escape, immunosuppression, tumor growth and progression promotion, and concomitant suppression of tumor immunotherapy in KIRC [
90]. All these results suggest that up-regulation of CD72 expression can suppress the anti-tumor immune response in KIRC patients.
In this study, correlation analysis showed a significant association between CD72 and PD-L1 (CD274), CTLA4, PD-1 (PDCD1), and immune cells (e.g., T cells, T helper cells, TReg, Th1 cells, Cytotoxic cells, Mast cells, NK cells, Th17 cells, etc.) were significantly associated with each other. In addition, in KIRC patients, compared with the CD72 low-expression group, the CD72 high-expression group showed a significant correlation between immune cells (T cells, T helper cells, Treg, Th1 cells, Cytotoxic cells, aDC, B cells, Macrophages, TFH, Tcm, CD8 T cells, CD56bright cells, Tem, Eosinophils, DC, Neutrophils, etc.) were more enriched. Our results suggest that CD72 is closely associated with immune infiltration and immunosuppression in the microenvironment of KIRC tumors.