The Importance of the Immune System and Molecular Cell Signaling Pathways in the Pathogenesis and Progression of Lung Cancer

Lung cancer is a disease that in recent years has become one of the greatest threats to modern society. Every year there are more and more new cases and the percentage of deaths caused by this type of cancer increases. Despite many studies, scientists are still looking for answers regarding the mechanisms of lung cancer development and progression, with particular emphasis on the role of the immune system. The aim of this literature review was to present the importance of disorders of the immune system and the accompanying changes at the level of cell signaling in the pathogenesis of lung cancer. The collected results showed that in the process of immunopathogenesis of almost all subtypes of lung cancer, changes in the tumor microenvironment, deregulation of immune checkpoints and abnormalities in cell signaling pathways are involved, which contribute to the multistage and multifaceted carcinogenesis of this type of cancer. We, therefore, suggest that in future studies, researchers should focus on a detailed analysis of tumor microenvironmental immune checkpoints, and to validate their validity, perform genetic polymorphism analyses in a wide range of patients and healthy individuals to determine the genetic susceptibility to lung cancer development. In addition, further research related to the analysis of the tumor microenvironment; immune system disorders, with a particular emphasis on immunological checkpoints and genetic differences may contribute to the development of new personalized therapies that improve the prognosis of patients.


Introduction
Lung cancer is one of the diseases with the highest mortality rate in the world. According to the World Health Organization, in 2020 the number of deaths caused by this cancer amounted to 1.80 million people, which accounted for 25% of the recorded deaths due to cancer [1,2]. The American Cancer Society lung cancer statistics for 2022 show that approximately 236,740 new cases of lung cancer were diagnosed in the United States alone (117,910 in men and 118,830 in women) and approximately 130,180 people died from lung cancer (68,820 men and 61,360 women) [3]. In addition to gender and age differences (diagnosed patients are >65 years of age), many environmental, genetic and lifestyle factors are involved in the development of lung cancer (Figure 1) [4][5][6][7][8][9][10][11]. However, more and more scientific studies highlight the extremely important role of genetic changes in the development and progression of this type of cancer [12][13][14][15].  [4][5][6][7][8][9][10][11]).
Genetic aberrations and epigenetic changes occurring in the human body have a significant impact on proto-oncogenes (genes that stimulate cell growth, which may be transformed into oncogenes as a result of mutation) and suppressor genes (genes encoding proteins, whose task is to inhibit cell growth and differentiation and to maintain cell stability; mutations in these genes lead to uncontrolled cell proliferation), facilitating the development and migration of cancer cells. However, this is only one aspect of the process [16][17][18][19][20]. Under conditions of homeostasis, the immune system has an extremely important potential for monitoring, recognizing and destroying cancer cells. However, these cells have developed several mechanisms that allow them to escape the surveillance of the immune system [21][22][23][24]. Therefore, all deregulations in the human body related to the functioning of the immune system should be considered as the second potential process involved in the pathogenesis of cancer, including lung cancer [25].
Despite many studies on the risk factors, classification and pathogenesis of lung cancer, very little data are devoted to disorders of the immune system and signal transduction pathways, which, as a result of genetic changes, determine the development of cancer cells. The aim of this publication is to review the literature on the importance of disorders of the immune system and the accompanying molecular implications in the pathogenesis of lung cancer. We paid particular attention to the importance of cancer antigens and neoantigens, as well as the role of immune checkpoints and abnormalities in cell signaling pathways that contribute to the multifaceted and multistage carcinogenesis of lung cancer.
In the case of NSCLC lung cancer, three main histological subtypes can be distinguished: adenocarcinoma (ADC), accounting for about 40% of all lung cases; squamous cell carcinoma (SCC), which is strongly associated with a smoking history and accounted for about 20% of all cases lung cancer; and large cell carcinoma (LCC). As indicated by histological studies, lung cancer is characterized by high heterogeneity, which is also reflected at the molecular level. Thanks to the development of genomic research, it was possible to determine the molecular patterns of each of the subtypes of lung cancer, which, according to research data, also differed depending on the histological subtype [33][34][35][36].
The TSG analysis for selected lung cancer subtypes based on the COSMIC Cancer Gene Census showed that there are certain genes in which mutations occur much more often. We can indicate both genes that are not specific to a specific subtype, such as EGFR or TP53, as well as those that are characteristic of a specific subtype of lung cancer (Table  1). Table 1. List of the 20 most mutated genes by lung cancer subtype (adapted from [37]).  [30][31][32]).

AC
In the case of NSCLC lung cancer, three main histological subtypes can be distinguished: adenocarcinoma (ADC), accounting for about 40% of all lung cases; squamous cell carcinoma (SCC), which is strongly associated with a smoking history and accounted for about 20% of all cases lung cancer; and large cell carcinoma (LCC). As indicated by histological studies, lung cancer is characterized by high heterogeneity, which is also reflected at the molecular level. Thanks to the development of genomic research, it was possible to determine the molecular patterns of each of the subtypes of lung cancer, which, according to research data, also differed depending on the histological subtype [33][34][35][36].
The TSG analysis for selected lung cancer subtypes based on the COSMIC Cancer Gene Census showed that there are certain genes in which mutations occur much more often. We can indicate both genes that are not specific to a specific subtype, such as EGFR or TP53, as well as those that are characteristic of a specific subtype of lung cancer (Table 1). Table 1. List of the 20 most mutated genes by lung cancer subtype (adapted from [37]).
Most of the genes indicated in the literature and databases encode proteins involved in the signal transduction process (kinases, transcription factors, receptors), which, as a result of mutations, introduce disturbances in the signal transduction pathway between cells.
Specific proteins could be used for differential diagnosis, facilitating the diagnosis of a specific type of lung cancer. Examples of proteins that have been tested and assessed for differential diagnosis include p63 in correlation with the TTF-1 transcription factor [48]. MUC5B was also correlated with the transcription factor TTF-1 [49]. The first protein was studied in the context of small-cell lung cancer compared to poorly differentiated non-keratinizing squamous cell carcinoma [48]. In these studies, the researchers demonstrated the utility of evaluating a panel of p63 and TTF-1 antibodies as useful markers in distinguishing SCLC from poorly differentiated non-keratinizing SCC on surgical, biopsy and cytology sections. The researchers based their conclusions on the positive expression of the p63 protein, in the negative correlation of the reaction for TTF-1, shown in subjects with differentiated non-keratinizing SCC, while the opposite results were obtained for SCLC. Similar conclusions were obtained in studies conducted by Veena et al., and in the summary of their research the researchers indicated that obtaining positive results for TTF1 and negative results for p63 can help in the diagnosis of adenocarcinoma, while samples that are positive for p63 and negative for TTF1 negative can be indicated and help in the diagnosis of squamous cell carcinoma [50]. In turn, another study conducted by Nagashio et al. indicated the usefulness of MUC5B protein determination in correlation with TTF-1, which may be useful for differential diagnosis between AC and SCC. In addition, it has been suggested that an assessment of the expression of the above factors could be a prognostic indicator for the prognosis of patients with pulmonary AC [49]. This means that any abnormalities of these genetic factors or disorders in the functioning of the pathway itself should be carefully analyzed in terms of their involvement in the development and progression of lung cancer.

Disorders of Cell Signaling Pathways as an Element of Lung Cancer Development and Progression
The development of new research techniques in the field of genomic and proteomic research has contributed to the increased interest of many researchers in the interactions between cancer cells, their unique microenvironment and intercellular communication pathways [51]. Analyses have been launched to demonstrate the occurrence of genetic aberrations and epigenetic changes that may determine an increased risk of lung cancer, but also affect the selection of therapeutic therapies and their therapeutic success. In recent years, several papers on this issue have appeared in the literature, which have allowed the classification of the observed disorders into three characteristic pathways, which, as the researchers indicated, can direct cells towards a malignant phenotype. These include signal transduction pathways that stimulate cell growth, pathways associated with tumor suppressor genes and pathways associated with cell evasion of apoptosis [52][53][54]. Detailed characteristics of these three groups of pathways with the most important genes/proteins involved in the development and progression of lung cancer are presented in Table 2.

•
The main regulator of the cell proliferation process; • Responsible for endocrine and paracrine regulation of cell growth and proliferation.
• Acts as a strong oncogene in the case of deregulatory changes; • Its dysregulation has been observed in 40% of NSCLC and ADC cases; • Its increased expression is associated with a lower prognosis of patients.

Epigenetic Changes Involved in the Pathogenesis of Lung Cancer
All the cells of the human body have almost identical genetic information, yet the cells of different tissues are very different from each other. This remarkable diversity is due to the different use of the same genetic information through epigenetic mechanisms. These mechanisms allow the emergence of some kind of permanent gene expression patterns for a given differentiated cell, with a simultaneous short-term and reversible change in their expression. Research on the epigenome (i.e., a set of DNA and histone protein modifications) developed in recent years has led to the determination of a number of molecular mechanisms that are epigenetic modifications in the course of lung cancer [106][107][108]. Such mechanisms include the following: (1) A change in DNA methylation status within the CpG islands of tumor suppressor genes; (2) Covalent modifications of histone tails; (3) Regulation of genes by micro-RNAs (miRNAs).
The first mechanism can be considered in two ways: in chemical terms (the process in which the methyl group -CH 3 is transferred between two molecules) and in biological terms (i.e., the effect of methylation on the functioning of DNA). In response to various external factors, the DNA of each person undergoes changes that are designed to adapt the functioning of the body to changing conditions [109]. Therefore, the correct course of this process plays a key role for the efficient functioning of cells and organs because it determines the expression and activity of individual genes. As a result of disorders of methylation pathways in the human body, there may be abnormalities related to immune reactions or the development of cancer (genetically determined or resulting from spontaneous mutations) [110,111]. Thus, methylation has become the subject of many intensive studies aimed at elucidating its detailed role in the pathogenesis of neoplastic diseases. According to the available literature, three types of abnormal methylation may occur in the development of this type of disease: global hypomethylation, hypermethylation of suppressor genes, and regulation of miRNA activity by methylation [112,113]. Hypomethylation is a process observed in the course of many diseases, especially those of a chronic nature, such as autoimmune diseases and cancer. According to some scientists, this process can reduce the stability of chromosomes and activate proto-oncogenes, and, in non-promoter regions of genes, it can weaken the stability of the genome by demethylating transposons. The phenomenon of global hypomethylation does not depend on local changes in DNA methylation in regulatory regions of genes. Genetic and cellular data support the idea that global DNA methylation is responsible for genome integrity and can also lead to chromosomal aberrations [114][115][116].
Hypermethylation, i.e., an increased degree of methylation, causes silencing of DNA repair genes and tumor suppressor genes (anti-oncogenes). The result of this process is that there is no limit to cell proliferation and there is a high probability that the abnormal cell will not be directed to the apoptotic pathway. In addition, the hypermethylation of DNA repair genes turns off genes that code for proteins responsible for repairing abnormal genes [117]. Since hypermethylation of promoter regions results in a loss of gene function, it has a major impact on cell functioning and can inactivate genes and key biological pathways [118]. In the course of neoplastic diseases, we can observe both the loss of methylation in non-coding regions of the genome, which in turn leads to disturbances in its stability, or the methylation of CpG islands in the tumors themselves, which results in a loss of expression of tumor suppressor genes. In lung cancer, studies have indicated that many genes have been silenced by methylation, including RARB (retinoic acid receptor beta), CDKN2A (cyclin dependent kinase inhibitor 2A), TIMP3 (TIMP metallopeptidase inhibitor 3), MGMT (methylguanine methyltransferase), DAPK (death associated protein kinase 1), CDH13 (cadherin 13) and CDH1 (cadherin 1) [119][120][121][122][123][124][125][126][127][128][129][130]. Researchers suggested that restoring the expression of silenced genes in the course of lung cancer may be a new therapeutic target.
The second mechanism of epigenetic changes involves modifications of histones and their implementing enzymes in response to internal and external stimuli [131]. Disturbances in their functioning can lead to many unfavorable changes, such as chromatin compaction, changes in nucleosome dynamics and abnormalities at the level of transcription. All changes contribute to the imbalance of gene expression, which in the case of the development of cancer, is observed as a gain or loss of function, overexpression, suppression by promoter hypermethylation, chromosome translocation or mutations of histone-modifying enzymes/complexes and even histone modification sites [132]. In lung cancer, scientists have observed a specific combination of histone markers resulting from epigenetic changes that include the deacetylation of H3 and H4 histones, loss of lysine 4 trimethylation of histone H3 and an increase in the trimethylation of H3K9 and H3K27. In 2007, Barlési et al. showed that the acetylation and trimethylation states of H2 and H3 can be a kind of prognostic marker in the course of NSCLC. They showed that the predictive value of epigenetic changes involved many histones, and of particular note were H2A (H2AK5ac) and H3 (H3K4me2, H3K9ac), which were higher in the early development of NSCLC [133].
The team of Van Den Broeck et al. showed in 2008 that there is increased H4K5/H4K8 acetylation as well as a loss of H4K20 trimethylation in NSCLC and pre-invasive bronchial dysplastic lesions. In addition, they found that the loss of H4K20 trimethylation was significantly associated with the early stage I ADC subpopulation, and their data indicated shortened survival [134]. Similar conclusions were also drawn by researchers from the team of Seligson et al. in 2009, who showed that the cellular levels of H3K4me2 and H3K18ac were relatively low, and the identified histone modification trends were independent predictors of prognosis in the course of ADC [135]. Studies presented in the literature suggest that reduced cellular levels of specific histone modifications may be a kind of predictive factor related to the prognosis of patients with lung cancer.
The third mechanism of epigenetic changes concerns the regulation of miRNAs. This is a family of small non-coding RNAs (21-25 nucleotides long) that can negatively affect the translation of messenger RNA (mRNA) and promote its degradation by base pairing with complementary mRNA target sites [135]. Thanks to the development of such a mechanism, miRNAs can change gene expression after the transcription process. The literature describes over 500 miRNAs found in humans, which can function both as oncogenes, but also as tumor suppressor genes [136]. As a result of intensive research in recent years, scientists have managed to classify some miRNAs involved in the pathogenesis of lung cancer into eight groups related to signaling pathways and cellular processes: sustaining proliferative signaling; evading growth suppressors; enabling replicative immortality; activating invasion and metastasis; inducing angiogenesis; deregulating circular energetics; resisting cell death; avoiding immune destruction and tumors promoting inflammation. Detailed data are presented in Table 3. The scientists indicated that the use of miRNAs in the future may allow the identification of histological subtypes of lung cancer, which will increase the diagnostic possibilities and their participation as molecular biomarkers in screening tests, as well as determine whether to distinguish primary from metastatic tumors [137]. In addition, researchers hope that miRNAs will play an important role in determining patient prognoses and tailoring personalized therapies for lung cancer in the future [138].   [189,190] miR-15a 13q14.2 CDK/Cyclin Evading growth suppressors [191][192][193] miR-16 13q14 CDK/Cyclin Evading growth suppressors [194][195][196][197]

The Importance of the Immune System in the Pathogenesis of Lung Cancer
In the condition of immune homeostasis, monitoring, recognition and destruction of foreign antigens and cancer cells take place [299]. The latter group is capable of an uncontrolled proliferation and spread throughout the human body, as well as attacking healthy tissues or cells. Tumor development and progression is a multistage process and involves eight characteristic processes that are also observed in the development of lung cancer (Figure 3) [300].
Understanding the role of the immune system in the development and progression of many types of cancer has become one of the most intensive research objects of recent decades.
Based on this type of research, cancer immunotherapies are being developed that target the immune microenvironment of tumors. Some researchers have indicated that type 1 and type 2 immune responses are important in the progression of cancer. Type 1 immune responses are considered important in fighting cancer, and immunotherapies are aimed at increasing the activity of this type of immune response [301]. Such a conclusion seems correct because in NSCLC tumors Stankovic's team identified major different subpopulations of T cells, and the second most common type of disease cell in NSCLC tumors was B cells [302].
Still, there are many cases of research in which the scientists have been able to discover only a small part or aspect of how the immune response works in fighting cancer cells.
That is why in order to better understand the interactions between cancer cells and the immune system, researchers looked at the contributions of components of the adaptive and innate immune responses, the involvement of tumor antigens, the importance of the tumor microenvironment and immune checkpoints [25,53]. Understanding the role of the immune system in the development and progression of many types of cancer has become one of the most intensive research objects of recent decades.
Based on this type of research, cancer immunotherapies are being developed that target the immune microenvironment of tumors. Some researchers have indicated that type 1 and type 2 immune responses are important in the progression of cancer. Type 1 immune responses are considered important in fighting cancer, and immunotherapies are aimed at increasing the activity of this type of immune response [301]. Such a conclusion seems correct because in NSCLC tumors Stankovic's team identified major different subpopulations of T cells, and the second most common type of disease cell in NSCLC tumors was B cells [302].
Still, there are many cases of research in which the scientists have been able to discover only a small part or aspect of how the immune response works in fighting cancer cells.
That is why in order to better understand the interactions between cancer cells and the immune system, researchers looked at the contributions of components of the adaptive and innate immune responses, the involvement of tumor antigens, the importance of the tumor microenvironment and immune checkpoints [25,53].

Importance of Tumor Antigens and Neoantigens,
Immune surveillance assumes that cancer cells appear in the human body more often than the clinical manifestation of cancer, but their development is controlled by appropriate immune mechanisms that allow their detection and destruction [303,304]. This process is carried out by both the innate and adaptive immune systems and is assisted by cancer-specific antigens (tumor markers) that can be recognized by the immune system. Tumor markers are specific substances, often proteins, that are produced by the cancer tissue itself or sometimes by the body in response to the appearance of cancer. The presence or elevated levels of tumor markers in the blood or urine can help detect certain types of cancer, including lung cancer [305]. One group of antigens is known as tumor-associated antigens (TAAs), which are antigens that are overexpressed in cancer cells but can also be found in normal tissues, while the other group is tumor-associated antigens (TSAs). Both groups of antigens have become the subject of many studies aimed at supporting the diagnostic process and therapy of patients with cancer. In the case of lung cancer, the majority of the literature has focused

Importance of Tumor Antigens and Neoantigens
Immune surveillance assumes that cancer cells appear in the human body more often than the clinical manifestation of cancer, but their development is controlled by appropriate immune mechanisms that allow their detection and destruction [303,304]. This process is carried out by both the innate and adaptive immune systems and is assisted by cancerspecific antigens (tumor markers) that can be recognized by the immune system. Tumor markers are specific substances, often proteins, that are produced by the cancer tissue itself or sometimes by the body in response to the appearance of cancer. The presence or elevated levels of tumor markers in the blood or urine can help detect certain types of cancer, including lung cancer [305]. One group of antigens is known as tumor-associated antigens (TAAs), which are antigens that are overexpressed in cancer cells but can also be found in normal tissues, while the other group is tumor-associated antigens (TSAs). Both groups of antigens have become the subject of many studies aimed at supporting the diagnostic process and therapy of patients with cancer. In the case of lung cancer, the majority of the literature has focused on the search for tumor antigens that may be specific biomarker molecules in the course of NSCLC.
Research The researchers showed that the expression of cancer genes in the testis, both cumulative and single, showed a significant relationship with the clinical characteristics of patients, i.e., with male sex, smoking history, cancer stage or the presence of metastases. In addition, the scientists' Cox regression analysis showed that the expression of NY-ESO-1 and MAGE-A3 was correlated with a poor patient prognosis [306]. Another study carried out by Grunwald et al. in 2006 showed that the expression of seven genes (MAGE-A3, NY-ESO-1, LAGE-1, BRDT, HOM-TES-85, TPX-1 and LDHC) was associated with the development of NSCLC. Researchers have shown that 81% of NSCLC patients tested expressed at least one of the genes tested and 50% of the sample at least two of the genes. Additionally, the researchers showed that all but one of the genes are regulated by genomic methylation, and not all of them are co-expressed [307]. In 2020, the team of Palat et al. conducted research in which 12 selected cancer antigens were determined (CEA, MAGE-A1, MAGE-A3, MAGE-A4, PRAME, hTERT, HER2, MUC1, Survivin, STEAP1, SOX2 and NY-ESO-1), and their involvement in the pathogenesis of lung cancer was studied, with a particular emphasis on NSCLC subtypes [308]. Based on the analyses carried out, the researchers showed a higher expression of TAA SCCs in relation to ACs. In addition, they showed that the patients' T-cell response to stimulation was significantly lower in patients with SCCs than those with ACs. Researchers have suggested that there are specific differences in T-cell function between NSCLC subtypes [308]. Some researchers have pointed out that cancer proteins circulating in serum or plasma, released from cancer cells, are few and their sensitivity, especially in terms of diagnostics, is low, especially in the early stages of cancer. Therefore, in recent years, scientists have used the fact that TAs are capable of inducing an immune response and stimulating the formation of autoantibodies associated with them (TAAb). They are created not only before, but also during the formation of the cancer itself, which means that they can be potential biomarker molecules that will be involved in the diagnostic process [309]. The researchers pointed out that TAAbs have several significant advantages that outweigh their use over TAs: the response of TAAbs to TAs is often enhanced by immune responses, allowing them to be more easily detected; TAAbs are relatively stable in body fluids, unlike TAs, and, therefore, according to the researchers, they are more stable; and TAAbs are highly specific and easily detectable in small sample volumes [310][311][312].
More and more attention has been paid by scientists to the development of combinations of biomarker molecules found in the blood or sputum of lung cancer patients that could potentially contribute to the early detection of neoplastic changes. According to the data available from the literature, these molecules are usually associated in several combinations and classified into one of four groups: autoantibody-based marker combinations, metabolites, protein-based biomarker combinations and mixed panels of markers ( Figure 4) .
Lung cancers carry a high mutational load (point mutations, misfolding, overexpression, abnormal glycosylation, shortening or abnormal degradation), which leads to the formation of unique peptides capable of inducing an immune response in the body, i.e., neoantigens. The formation of abnormal proteins as a result of their mutations can lead to their recognition by the host's immune system, more specifically by the histocompatibility system (MHC) class I (MHC-I) or MHC class II. Neoantigens in the tumor cytosol are presented on MHC-I and recognized by CD8+ cytotoxic T cells, while neoantigens released into the tumor microenvironment can be taken up by circulating antigen-presenting cells [337,338]. In the literature, we can find studies indicating that neoantigens are specific to the patient and not to the tumor itself, which, according to the researchers, suggests that tumors avoid destruction due to the immune system. These properties make neoantigens increasingly recognized as key mediators of tumor-specific immune activation and they have been identified as potential targets for personalized cancer therapies (including cancer vaccines) [312,[337][338][339]. Thanks to the development of molecular biology techniques, including next-generation sequencing, and the participation of bioinformatics, it is possible to use data to predict neoantigens, most often on the basis of their affinity to MHC-I. However, research in recent years has resulted in a much broader approach to the detection of neoantigens in lung cancer (especially NSCLC), taking into account both the stability of binding to MHC-I, as well as the level of gene expression or aggretopicity [340][341][342][343]. According to the literature, the median number of predicted neoantigens in NSCLC is quite variable. In 2016, Karasaki et al. conducted research aimed at identifying individual and specific somatic mutations occurring in NSCLC, which could be a new method of neoantigen-based immunotherapy. The researchers found from 13 to 659 neoantigens for ADC (median 46) and from 10 to 145 neoantigens for SCC (median 95.5) [344]. Other reports in the literature state that the number of neoantigens detected in NSCLC ranged from 63 to 214 neoantigens per tumor. Researchers have observed that increased numbers of neoantigens were noted among patients who had molecular signatures associated with cigarette smoking and increased expression of programmed death ligand 1 (PD-L1) [339,345,346].

Importance of Immune Checkpoints
Immune checkpoints are systems of protein receptors and ligands exposed on the surface of cells of the immune system (especially T lymphocytes) that modulate the body's immune responses [347][348][349]. Many studies have shown that they are involved in altering the ability of the immune system to recognize malignant cells. In conditions of immune homeostasis, immune checkpoints provide self-tolerance by maintaining T lymphocyte activation, which causes inhibition of the immune response, but in the case of cancer, they are inhibited and the immune system is reactivated so that it is able to launch an attack on cancer cells [350,351]. In the literature, we found data on the detailed mechanisms of action of almost 20 significant modulators of immune checkpoints (Table 4), such as LAG-3, TIM3 and TIGIT, as well as the programmed cell death receptor 1 (PD-1) and a protein associated with cytotoxic T-4 lymphocytes (CTLA-4) ( Figure 5), which have found their greatest application in clinical practice, including the treatment of lung cancer [350][351][352][353][354][355].

PD-1/PD-L1 Pathway
The PD-1 molecule together with its PDL-1/PD-L2 ligands as immune checkpoint molecules are responsible for suppressing the effector phase of activated T lymphocytes, reducing the inflammatory response and, consequently, autoimmunity [356]. These molecules can exist in the form of being bound to the cell membrane and soluble in serum and blood plasma [357]. Forms associated with the cell membrane occur, among others, in cells such as T and B lymphocytes, Treg lymphocytes, NK cells and APC cells [356]. In turn, the soluble molecule PD-L1 is detected in plasma as well as in body fluids, such as pleural effusion of lung cancer patients [357].
In a properly functioning body, after the elimination of pathogens or abnormal infected cells, the PD-1/PD-L pathway has a suppressive effect on T lymphocytes, causing their exhaustion and apoptosis, which prevents autoimmunity. Unfortunately, this natural immunosuppression is used by cancer cells to evade surveillance by the immune system [356].
Therefore, monoclonal antibodies directed against PD-1/PD-L1 are currently used as one of the forms of therapy and treatment of cancer, including lung cancer [352].
In clinical trials, the effectiveness of various models of therapy have been observed, in which preparations such as pembrolizumab, nivolumab, atezolizumab and durvalumab have been used [357].

PD-1/PD-L1 Pathway
The PD-1 molecule together with its PDL-1/PD-L2 ligands as immune checkpoint molecules are responsible for suppressing the effector phase of activated T lymphocytes, reducing the inflammatory response and, consequently, autoimmunity [356]. These molecules can exist in the form of being bound to the cell membrane and soluble in serum and blood plasma [357]. Forms associated with the cell membrane occur, among others, in cells such as T and B lymphocytes, Treg lymphocytes, NK cells and APC cells [356]. In turn, the soluble molecule PD-L1 is detected in plasma as well as in body fluids, such as pleural effusion of lung cancer patients [357].
In a properly functioning body, after the elimination of pathogens or abnormal infected cells, the PD-1/PD-L pathway has a suppressive effect on T lymphocytes, causing their exhaustion and apoptosis, which prevents autoimmunity. Unfortunately, this natural immunosuppression is used by cancer cells to evade surveillance by the immune system [356].
Therefore, monoclonal antibodies directed against PD-1/PD-L1 are currently used as one of the forms of therapy and treatment of cancer, including lung cancer [352].
In clinical trials, the effectiveness of various models of therapy have been observed, in which preparations such as pembrolizumab, nivolumab, atezolizumab and durvalumab have been used [357].
In clinical trials in the first-line treatment of advanced NSCLC, nivolumab demonstrated an improvement in overall survival outcomes for advanced squamous NSCLC compared to docetaxel (a cytotoxic anticancer drug used in cancer chemotherapy). Similar observations were also obtained but with better results in relation to docetaxelem. In the case of Atezolizumab, its positive impact on patient outcomes was also assessed. The obtained results indicated a beneficial therapeutic path using PD-1/PD-L1 antibodies in patients with advanced NSCLC. In addition, such therapy has a more body-friendly safety profile compared to chemotherapy [358].

CTLA4 Pathway
The CTLA-4 molecule, a member of the immunoglobulin superfamily, was expressed primarily by activated T cells. It is an important surface protein and co-inhibitor, localized to activated CD4+/CD8+ T cells to reduce the activity of these cells by binding CD80/CD86/CD28 molecules [359,360].
One of the main tasks of CTLA-4 is to inhibit the function of CD28, a co-stimulatory receptor for T cells. Previous studies suggested that CTLA-4 expression increases upon activation of T cells, while CD28 appears mainly on the surface of naive T cells. Therefore, this molecule seems to have a key role in the regulation of activated T cells, as the literature reports that a lack of CTLA-4 causes unregulated T-cell proliferation [360].
In turn, the deletion or inhibition of T-cell-specific CTLA-4 reduces their ability to control both antitumor and autoimmune responses. Therefore, it is suggested that in the treatment of tumors with a high percentage of T cells that respond to tumor antigens, inhibition of CTLA-4 may be beneficial by restoring anti-tumor immunity by controlling overactive T cells [360].
Currently, as the first-line treatment for lung cancers, such as inoperable malignant pleural mesothelioma (MPM) or NSCLC (with PD-L1 expression in the tumor ≥1% and no EGFR/ALK aberration), treatment with nivolumab and ipilimumab (CTLA-4 inhibitor) is proposed. These formulations, when injected intravenously, have produced favorable treatment results for patients. At the same time, blocking the CTLA-4 molecule using ipilimumab may cause side effects such as colitis and enteritis [361]. Therefore, it is important to find optimal treatments with checkpoint inhibitors so that the benefits outweigh the negative effects. At the same time, current clinical data and basic research give hope for the development of an effective anticancer therapy based on checkpoint inhibitors, because their use can shift the immune balance towards promoting tumor killing and enhancing the immune attack on pathological cells [359]. Interacts with both co-stimulatory (CD28) and coinhibitory receptors (cytotoxic T lymphocytes, antigen 4 (CTLA-4)) and regulates the immune response; • The low level of CD80 expression serves as a tumor escape mechanism due to the higher affinity and therefore preferential binding of CTLA-4 to CD80 compared to CD28; • In contrast, CD80 overexpression promotes T cell activation and tumor rejection, and CD80 deficiency also increases the immunogenicity of tumor cells.

•
Binding to CD155 results in increased secretion of IL10 and decreased secretion of IL12B and inhibition of T cell activation by promoting the production of mature immunoregulatory dendritic cells [431][432][433][434] Abbreviations

Search Strategy
This study was performed on the basis of a meta-analysis of observational studies in accordance with the epidemiological guidelines and the PRISMA statement. To maximize sensitivity, a broad search strategy was driven using the international databases Web of Science, Scopus and PubMed/MEDLINE. The search strategy covered publications from 2002-2022 and used the following keywords: "immune system"; "immune response"; "genetic association"; tumor microenvironment"; "signal transduction", "immune checkpoint". Only articles in English were included in the analysis. After removing duplicates, the abstracts of the publications were reviewed by two independent researchers in accordance with the PRISMA abstract list. All disputes were resolved by a third investigator. The detailed search strategy is shown in Figure 6.

Search Strategy
This study was performed on the basis of a meta-analysis of observational studies in accordance with the epidemiological guidelines and the PRISMA statement. To maximize sensitivity, a broad search strategy was driven using the international databases Web of Science, Scopus and PubMed/MEDLINE. The search strategy covered publications from 2002-2022 and used the following keywords: "immune system"; "immune response"; "genetic association"; tumor microenvironment"; "signal transduction", "immune checkpoint". Only articles in English were included in the analysis. After removing duplicates, the abstracts of the publications were reviewed by two independent researchers in accordance with the PRISMA abstract list. All disputes were resolved by a third investigator. The detailed search strategy is shown in Figure 6.

Assessment of (Quality) Bias Risk and Data Synthesis Strategy
The remaining articles were then reviewed according to the PRISMA statement. They were then systematically reviewed and assessed against the inclusion and exclusion criteria. All steps were carried out by two independent investigators, and disputes were resolved through discussion with the third investigator.

Conclusions
The aim of our literature review was to present the importance of immune system disorders and the accompanying changes at the level of cell signaling in the pathogenesis of lung cancer. The information presented by us indicates one of the causes of lung cancer development, which are changes in the tumor microenvironment. Moreover, in our review, we indicated the relationship between deregulation of the immune system and disturbances in cell signaling pathways that contribute to the multistage and multifaceted carcinogenesis of this type of cancer. We hope that the data presented by us will attract the attention of researchers and clinicians and will allow the development of more effective therapies. Despite the numerous scientific studies available in the literature on the etiopathogenesis of various types of lung cancer, it seems important for scientists to undertake an interdisciplinary and multifaceted approach, combining molecular, immunohistochemical and histological methods in order to expand the existing knowledge, which will contribute to faster and more effective diagnosis in the early stages of the disease.

Conflicts of Interest:
The authors declare no conflict of interest.