CTLA4 expression profiles and their association with clinical outcomes of breast cancer: a systemic review

As of 2020, there were 2.3 million women diagnosed with breast cancer (BRCA) and 685,000 deaths globally, which means it has become a major cancer threat to female health [1]. BRCA can be divided into different types based on whether it has molecular markers for estrogen receptors (ER), progesterone receptors (PR), and human epidermal growth factor 2 (HER2) [2]. BRCA is a highly heterogeneous tumor with a wide range of etiologies and clinical symptoms [3]. BRCA patient prognosis is significantly affected by immunity [4].

Cancer immunotherapy is a new and interesting area of cancer treatment, with the primary objective of using one’s own immune system to identify and eliminate tumor cells [5]. The immune system is remarkably equipped to recognize cancer neoantigens as foreign and can develop antitumor responses with the potential to eliminate the tumor [6]. Immune checkpoints are an essential component of the immune system [7], which also are cell surface receptors that are expressed on immunological cells [8]. Immunotherapy is based on the use of immune checkpoint inhibitors to prevent the interaction of immunological checkpoints and activate the immune response against tumor cells [9]. In recent years, immune checkpoint inhibitors have become the treatment option for many recurrent or spreading cancers, especially the targeted cytotoxic T-lymphocyte-associated protein 4 (CTLA4) checkpoint blocking therapy, which has achieved remarkable efficacy in the treatment of different cancers [10, 11, 12].

CTLA4 is a protein receptor that is a member of the immunoglobulin superfamily that is expressed by activated T cells and transmits an inhibitory signal to T cells [13]. CTLA4 is an important co-inhibitory molecule that suppresses the functions of T cells and transmits an inhibitory signal to T cells [14, 15]. CTLA4 is a target for monoclonal antibody-based drugs that enhance anticancer immunity, such as ipilimumab, which was the first CTLA4 inhibitor to be developed and the only one to date that has been approved by the U.S. Food and Drug Administration [16]. CTLA4 expression is associated with the development of BRCA [17].

As an immunological checkpoint, CTLA4 can inhibit the immune system from tolerating tumors, offering a potentially beneficial new approach to the treatment of cancer. Herein, we analyzed the expression pattern, function, and prognostic value of CTLA4 in BRCA using various online databases and annotation tools, and thoroughly evaluated the importance of CTLA4 expression in BRCA. The findings will aid in the understanding of CTLA4’s prognostic value in human BRCA.

The relationship between CTLA4 expression profiles and clinical outcomes of BRCA was investigated in this study. BRCA is one of the most frequent cancers in women, and it is also the leading cause of cancer mortality in women [29]. Immune checkpoint inhibitors have been shown to increase survival in a variety of solid tumors and are now a standard part of treatment for many cancer types [30, 31, 32, 33]. A previous study reported on targeted CTLA4 checkpoint blocking therapy, which has achieved remarkable efficacy in the treatment of different cancers [12]. It has been reported that high expression of CTLA4 correlates with poor survival in patients with melanoma, renal cell carcinoma, and colorectal cancer, and a significantly higher expression of it is present in patients with BRCA [34]. However, the mechanism of CTLA4 in BRCA progression is unclear. CTLA4 as a checkpoint predominantly functions early in the life cycle of the immune response, during T-cell priming and activation, and it enhances the immunosuppressive activity of Tregs [35]. Identifying aberrantly expressed genes in tumors is important for the development of individualized treatments, which can improve therapeutic outcomes [36]. Therefore, we first analyzed the expression of CTLA4 in BRCA tissues and normal breast tissues, finding that CTLA4 expression in BRCA tissues was higher than in normal tissues (P < 0.01). Then, we compared the expression of CTLA4 in different subtypes of BRCA and the expression of normal breast tissue using the UCLan (University of Central Lancashire) platform, and the results indicated that the expression of CTLA4 in BRCA subtypes Luminal, HER2 positive, and TNBC tissues was significantly higher than in normal tissues (P < 0.001).

Previous studies reported that high expression of CTLA4 correlates with poor survival of patients with melanoma, renal cell carcinoma, and colorectal cancer [37, 38]. However, it’s unclear what effect higher CTLA4 expression has on the OS of BRCA. In this study, we found that overexpression of CTLA4 mRNA levels was related to better OS (P < 0.05). It may be related to the fact that CTLA4 can relieve inhibitory signals of T-cell activation and thus enhance an effective antitumor response [39]. The previous study indicated that cancer cells express PD-L1 and CTLA4, which prevent the immune system from recognizing and destroying tumor cells, and PD-1 and CTLA4 blocking antibodies can activate the antitumor response, leading to tumor regression [40]. CTLA4 protein plays a key role in activating the antitumor response against cancer cells [41].

A study has reported that CTLA4 expression was associated with multiple lesions, larger tumor size, advanced lymph node stage, lymphovascular infiltration, and skin invasion in BRCA [42]. Our results indicated that overexpression of CTLA4 was related to age, tumor size stage, ER status, PR status, and PAM50 subtypes in BRCA patients (P < 0.01).

In this study, to comprehensively explore the mechanism of CTLA4 in BRCA, we investigated the associations between CTLA4 expression and immune cell infiltration, and the results showed that the expression of CTLA4 in BRCA was positively correlated with the infiltration of multiple immune cells. GO analysis found that the high expression group and the low expression of CTLA4 in BRCA were mainly involved in antigen binding, immunoglobulin complex, and lymphocyte-mediated immunity. KEGG pathway analysis showed that the high expression group and the low expression group mediated by CTLA4 were enriched in cytokine-cytokine receptor interaction. CTLA4 has 30 edges and 11 nodes in the PPI network, according to our findings, with the top 5 hub genes: CTLA4, LCK, FOXP3, CD80, and PTPN11. LCK (or lymphocyte-specific protein tyrosine kinase) is a member of the Src family and regulates the activation of T cells [9]. Previous studies have shown that LCK is expressed in various cancer types, such as BRCA, colon cancer, and lung carcinoma [43, 44, 45]. LCK has been discovered to be expressed in BRCA and has been identified as a crucial participant in the HER2-enriched subtype network [44]. FOXP3 is a protein involved in immune system responses [46], which is expressed in BRCA and was associated with poor prognosis [47]. A member of the FOX protein family, FOXP3 appears to function as a master regulator of the regulatory pathway in the development and function of regulatory T cells [48, 49]. In addition, FOXP3 is the most specific and reliable marker of regulatory T cells, and elevated FOXP3 lymphocytes are thought to contribute to tumor-immune evasion in BRCA [50]. The cluster of differentiation 80 is a B7 type I membrane protein that regulates the immune system through an inhibitory interaction with CTLA4 [51]. PTPN11 has significant relevance in BRCA, and it encodes phosphatase SHP2, involved in BRCA progression [52].

CTLA4 may play an important regulatory role in the tumor-immune microenvironment of BRCA, which may serve as a potential indicator of prognosis and immunotherapy response for BRCA. Furthermore, CTLA4 may influence BRCA prognosis through antigen binding, immunoglobulin complexes, lymphocyte-mediated immunity, and cytokine-cytokine receptor interaction. These findings help us understand how CTLA4 plays a role in BRCA and set the stage for more research.