Colorectal cancer (CRC) is one of the main causes of cancer-related deaths around the world. The prognosis of patients relies on histopathological criteria of tumor invasion according to the American Joint Committee on Cancer (AJCC) and Union for International Cancer Control (UICC) TNM classification system and on features of tumor cell differentiation [1, 2]. This approach provides useful but incomplete information to predict prognosis. There has been increasing interest in predicting CRC prognosis, focusing on tumor cells, molecular pathways, mutation status, inflammation and immune cell infiltration [3, 4].

Human immunity has a complex and multifaceted role in cancer, affecting all aspects of the disease, from tumorigenesis to treatment [5]. Immune cells can act both as suppressors of tumor initiation and progression and as promoters of proliferation, infiltration and metastasis [6]. In the tumor microenvironment, various immune cells, both innate immune cells and adaptive immune cells, have been reported in all tumor types depending on the tumor origin, location, and individual characteristics. The Immunoscore was confirmed to predict clinical outcome in patients with early- [7] and advanced [8]-stage CRC. The Immunoscore, a derived immune score, is a scoring system that summarizes the density of CD3+ and CD8+ T cell effectors within the tumor and its invasive margin. It has been suggested that the use of the Immunoscore in combination with the AJCC/UICC stage could lead to a better determination of tumor prognosis [3].

Blood indexes, such as tumor markers, systemic inflammation and lipid metabolism, are also correlated with cancer prognosis. There is a growing consensus that inflammation is involved in the development of malignancy and that an ongoing systemic inflammatory response is associated with a worse prognosis [9]. These factors include lactate dehydrogenase (LDH) levels; C-reactive protein (CRP) levels; albumin (ALB) levels; the numbers of neutrophils, lymphocytes, monocytes, and platelets; the NLR (neutrophil-to-lymphocyte ratio); the PLR (platelet-to-lymphocyte ratio); and the LMR (lymphocyte-to-monocyte ratio). The tumor markers in CRC, carcinoembryonic antigen (CEA) [10] and carbohydrate antigen 19–9 (CA 19–9) [11], can predict the prognosis of CRC. Additionally, several studies have demonstrated the importance of lipid metabolism regulation in the promotion of migration [12], invasion [13], and angiogenesis [14]. Lipid metabolism is associated with cancer survival and has been proposed as a prognostic marker [15].

In light of these recent findings, both the immune state and the above blood indexes could impact prognosis. Do they have any relationship? The present study aimed to investigate the association between CD3+ and CD8+ T immune cells in the tumor microenvironment and clinicopathologic characteristics and blood indexes, including tumor markers, inflammatory indictors and lipid metabolism factors, for patients with CRC. We hope to offer evidence to monitor the immunity status of the CRC microenvironment by basic indexes.

The present study investigated the relationship between clinicopathologic characteristics and blood indexes with CD3+ and CD8+ T cells in the CRC tissue microenvironment. We observed that older patients had low expression of CD3+ and CD8+ T cells in the IM and a low Immunoscore. The expression of CD8+ T cells in the IM and the Immunoscore were lower in right-sided tumors than in left-sided tumors. The higher expression of CD8+ T cells in CT was found in the group of patients in T4 stage. The higher the CEA level in the blood, the fewer CD8+ T cells were in the CT. Either fewer monocytes or a higher LMR in the blood, the larger number of CD3+ T cells in the CT. The higher ApoA1 was in the blood, the more CD3+ T cells were in the CT and in the IM. The results from this study suggest that noninvasive peripheral blood analysis of some markers could be distinctly helpful in assessing the immunity status in the tumor microenvironment.

Elder patients had different immunity from the younger patients. We observed fewer T cells in the tumor tissue of elderly patients than in that of younger patients. When the patient’s age was older than 75 years, both the CD3+/CD8+ T cells in the IM and the Immunoscore became less statistically significant. At the same time, the CD3+/CD8+ T cells also decreased in the CT despite no significant difference. Aging resulted in deteriorating health and an increased risk of cancer accompanied by progressive, multidimensional, physiological degeneration with an immune system decrease thought to play a key role in regulating these declines [16], which is known as immunosenescence [17]. Additionally, the tumor immuno-microenvironment may be altered during aging as a result of age-related immune dysfunction. Provinciali et al. found that mammary tumors in elderly mice had reduced numbers of infiltrating CD3+ and CD8+ T cells compared to younger mice [18]. Many reasons could explain why T cells may be significantly decreased in older patients. Thymic output decreases with age, resulting in lower proportions of T cell populations, contributing to an inability to mount T cell responses to novel tumor-associated antigens [19]. Moreover, T lymphocytes from older individuals present a significant reduction in the activation of nuclear factor-kΒ, which is responsible for the expression of proinflammatory cytokine genes [20]. Thus, T cells would become deactivated in older people. This study found 34 patients over 75 years old with decreased expression of CD3+/CD8+ T cells in CRC tissue compared to the 206 younger patients. It would be interesting to further investigate this change in large number of older patients with CRC or other cancers.

The data from this study revealed that right- and left-sided CRCs had obviously different Immunoscores and T cell concentrations in cancer tissue. The CD8+ T cells in the IM and the Immunoscore were both low for right-sided tumors. CD8+ T cells are cytotoxic T cells and play a central role in anticancer immunity; therefore, our results could be useful for explaining the poor prognosis in right-sided CRC. Recently, Jonna Bernstsson et al. reported that CD8+ T cell infiltration differed significantly according to the tumor side in CRC, with denser collection in a left-sided tumor than in a right-sided tumor [21], which is in line with our result. Many studies suggest differences, including epidemiology, tumor characteristics and prognosis, between right-sided and left-sided CRCs [22]. Moreover, right-sided tumors demonstrate diverse genetic and molecular characteristics compared to left-sided tumors [23]. These differences in biological behavior may induce different responses to chemotherapy and targeted agents [24]. In the present study, we found a difference in the immuno-microenvironment between proximal and distant CRCs, which provides further evidence that anatomical subsites may represent distinct disease entities.

Interestingly, we observed a higher density of CD8+ T cells in the CT in T4 stage. T4 stage indicates that tumor cells invade through the visceral peritoneum or invade or adhere to adjacent organs or structures. We assume that it would expose more antigens that could lead to inflammation. Cancer is closely related to inflammation. Many cancers arise from sites of infection, chronic irritation, and inflammation. We speculated that T4 tumors might induced obvious inflammation and then attracted more immune cells. Whether those patients are more likely to benefit from immunotherapy is another appealing topic since T4 has more CD8+ T cells in the CT.

The present study elucidated that the immune concentration in cancer tissue is closely related to inflammation. The more CD3+ T cells were in the CT, the fewer monocytes and the higher LMR were in the blood. Monocytes and their progeny within the tumor microenvironment can produce factors promoting the growth, migration, invasion, and survival of tumor cells [25]. Nevertheless, another study showed that a dense number of macrophages in the blood indicated a good prognosis for CRC patients [26]. Tumor-associated macrophages (TAMs), which are derived from circulating monocyte populations, play a key role in the tumor immuno-microenvironment, encouraging metastasis and tumor progression [27]. The association between systemic inflammation and the poor prognosis of cancers could be explained by the effects on the tumor immuno-microenvironment. It has already been shown that the levels of tumor-infiltrating lymphocytes predict better survival in CRC patients [28]. However, little mechanistic evidence linking the poor prognosis of cancer patients with systemic inflammation exists, and hardly any study has focused on the relationship between the immuno-microenvironment in tumor tissue and systemic inflammation. Our study performed a preliminary exploration and found that monocytes and the LMR are significantly connected with the number of immunity cells in CRC tissue.

We observed that the higher the CEA level in the blood, the fewer CD8+ T cells were in the CT. CEA is the most commonly used tumor marker in patients with CRC. CEA is implicated in cell adhesion, cell-to-cell interactions and signal transduction in cancer cells [29]. CRC patients with abnormal CEA values have been shown to have lower overall survival [30]. It is still unknown how circulating CEA, released by CRC cells, inhibits the migration of CD8+ T cells to the tumor center, which is valuable for studying intensive molecular mechanisms.

The present study demonstrated that the higher the circulating ApoA1 level, the more CD3+ T cells were in both the CT and the IM. ApoA1, a predominant protein component in HDL, transports excess CHO from peripheral tissues to the liver and has anti-inflammatory, antiapoptotic and antioxidant functions [31]. Studies have confirmed that ApoA1 could alter TAMs from a protumor M2 to an antitumor M1 phenotype [32]. It also modulates regulatory T cells [33]. Thus, ApoA1 is situated at the nexus of a number of physiologically significant immune functions. Furthermore, decreased serum ApoA1 levels have been reported to correlate with poor CRC outcomes [15]. We inferred that CD3+ T cells aggregated in both the CT and the IM for CRC with high ApoA1, consequently, ApoA1 was strongly positively correlated with CRC patient survival.

Cancer immunotherapies that inhibit negative immune feedback, such as those targeting programmed cell death 1 (PD1)/programmed cell death-ligand 1 (PDL1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA4), have proven efficacious against several tumor types [34, 35]. However, not all cancer patients benefit from immunotherapies, and to date, PD1/PDL1 have been approved in CRC only with MSI-H by the Food and Drug Administration [36]. No other definitive biomarker exists to easily predict the outcomes of this immune system activity. Immune infiltration of cancers has been suspected to be a positive factor for patient outcome since the early 1900s [37]. However, these immune cells do not have a major classification for clinical decision making. Franck Pagès et al. confirmed that the Immunoscore had the highest relative contribution to the risk of all clinical parameters, including the AJCC/UICC TNM classification system [8]. The Immunoscore represents a standardized immune-based assay for the classification of cancer. On the basis of our identification of an association between markers in the blood and the densities of immune cells in the tumor microenvironment, we proposed that the CEA level, number of monocytes, LMR, and ApoA1 may be used to ascertain the immunological status in the tumor microenvironment. Importantly, the CEA level, number of monocytes, LMR, and ApoA1 could be easily calculated in the blood, eliminating the need for invasive procedures and complex processes to evaluate the tumor immuno-microenvironment.

Nevertheless, this study has a few limitations. First, as a retrospective study, it is still possible that patients had potential systemic inflammation not related to cancer. Second, we did not analyze the specific subtypes of T cells excluding CD3+ and CD8+ immune cells, although there are different roles and prognoses that other immune cells may play in the tumor microenvironment. Third, the underlying reason explaining why blood markers were associated with the density of CD3+ and CD8+ immune cells requires further investigation.

Our results demonstrate that age, T stage, tumor location, CEA level, number of monocytes, LMR and ApoA1 are associated with immune cell densities in the tumor microenvironment. This study suggests the possibility that the presence of cancer-changed immune cells in the tumor microenvironment could be evaluated noninvasively with markers in peripheral blood samples and clinicopathologic characteristics. In the next step, it will be significant to establish a nomogram model including these indexes to predict the immunity status in the tumor microenvironment, and to explore the predictive and prognostic role of Immunoscore in CRC. It is also attractive to explore why these blood markers are associated with the density of CD3+ and CD8+ immune cells in tumors.