Immunotherapy is emerging as a promising anticancer strategy for many tumors[31]. Immunotherapy for GC has achieved fruitful results and changed treatment procedures[32], showing great potential to improve the prognosis of patients[33]. Recent breakthrough studies on ICIs represented by programmed death-1 (PD-1) and PD-L1 have opened new avenues for immunotherapy of GC[18, 34]. However, although anti-PD-1 antibody is a promising approach for AGC patients, its efficiency is still limited[18]. Furthermore, immunotherapy drugs are expensive, and easy to develop drug resistance and hyper-progressive disease, which limits their wide application in clinical practice[35–37].
In order to optimize the comprehensive anti-cancer regimen of immunotherapy, it is urgent to study how biomarkers accurately identify the population with immunotherapy advantages, and thus achieve as much precision and predictability as possible[38]. However, existing prediction biomarkers are still insufficient, and prediction methods need to be improved[37]. Peripheral blood inflammation composite indicators such as LIPI, NLR, PLR and Hb level have proved to be convenient and promising biomarkers for GC prognosis[21–27]. Considering the high heterogeneity of tumor tissue in GC, the accuracy of tumor biomarkers to evaluate treatment prognosis is not high, which will inevitably hinder the precise diagnosis of patients and the selection of targeted therapy[28]. In contrast, the accuracy of the composite index, which combines multiple indicators, in screening beneficiaries of immunotherapy improved significantly.
However, the mechanism of correlation between these peripheral blood inflammatory complex indicators and tumor prognosis is complex and needs to be further explored through basic experiments and clinical trials. Recent studies reported that chronic inflammation may be a trigger for gastrointestinal cancer, which may be one of the reasons for the correlation between peripheral blood inflammation indicators and tumor prognosis[39]. Apart from the direct immune response to kill tumor cells, these biomarkers are also related to tumor immunostimulatory signals and the activation of effector cells.
Neutrophils are traditionally defined as short-lived myeloid cells with unique crack. As a type of white blood cells, they exist in the form of nuclear, and rank the top in terms of importance and quantity in the blood circulating. They are usually the first responders under autoimmune physiological or pathological conditions of sterile injury, infection and inflammation, and act as the first line of defense to protect the host from tissue injury and infection[40, 41]. Tumor-associated neutrophils (TAN) accumulate in local areas and can be triggered by external stimuli in the tumor microenvironment (TME), switching between an antitumorigenic phenotype and a pro-tumorigenic phenotype[42]. Neutrophils that promote tumor cell growth and metastasis have the following functions: direct cytotoxicity, secretion of reactive oxygen species (ROS), nitric oxide (NO) and proteases, regulation of reticulocytosis, autophagy and other immune cells[43]. These neutrophils can activate CD8+T cells and DCS, and may even present tumor antigens[44]. Antitumor neutrophils kill tumor cells through direct cytotoxic effects as well as indirect effects by the activation of adaptive immune responses[45]. Moreover, increased numbers of neutrophils can inhibit the immune effect ability of lymphocytes[46]. Lymphocytes are important cells in the body's immune response, especially responsible for adaptive immunity, providing antigen-specific responses regulated by class I major histocompatibility complex (MHC)[46,47]. Studies have shown that lymphocytes and their subsets (CD8+T cells and CD3+ T cells) are related to the good prognosis of some tumors [48–50]. Lymphocytes infiltrated in TME are usually an indicator of the body's immune state, which can prevent the proliferation and migration of tumor cells [51].
The occurrence of tumor-related inflammation can inhibit the synthesis of albumin[52]. Meanwhile, with the decrease of serum albumin, tumor patients will suffer from decreased immunity and malnutrition[52,53]. Due to the lack of sufficient serum albumin combined with drugs, if patients with cancer develop hypoproteinemia, Chemotherapy drugs may have high residues in the blood and have high toxicity [53].
Studies have reported that low albumin concentration reflects cancer-induced malnutrition, and may have negative impact on prognosis[54]. In recent years, more and more researchers have paid attention to the combination of inflammatory cells representing systemic inflammatory state and albumin reflecting nutritional status.
Glasgow Prognostic Score (GPS), a composite biomarker of albumin and c-reactive protein (CRP), is reported to be a sensitive prognostic marker for GC[55]. Zhang J et al. found that a composite biomarker of serum CEA and fibrinogen/albumin ratio could also be used as a positive indicator to predict tumor progression and prognosis for GC patients[56]. As mentioned above, higher neutrophil levels and lower albumin levels are likely to cause a poor prognosis for cancer patients.
PNI, a composite index formed by ALC and albumin, has been proved to be related to the prognosis of advanced head and neck cancer treated with ICI[57]. PNI was initially established to evaluate the relationship between baseline nutritional status of tumor patients undergoing gastrointestinal surgery and postoperative complications[58]. However, the correlation between the prognosis of PNI and ICI treatment is unclear. Ul M et al. retrospectively analyzed 99 patients with advanced stage IV head and neck tumors who received ICI treatment in Johns Hopkins Hospital from 2014 to 2020, and found that PNI and BMI were related to the prognosis of ICI treatment [58]. According to the optimal cut-off value of PNI of 45, patients were divided into low PNI(< 45) group and normal PNI(≥ 45) group. Compared with normal PNI, lower PNI is significantly correlated with shorter OS (P = 0.014) and PFS (P = 0.016). After multivariate adjustment, PNI was still an independent prognostic factor of OS (P = 0.041) and PFS (P = 0.011)[58]. A retrospective study by Shoji et al. on 102 consecutive patients with NSCLC treated with ICI showed that the baseline PNI level was significantly correlated with PFS (P = 0.0013) and OS (P = 0.0053)[29]. The PFS and OS of low- PNI group (< 45.5) were significantly shorter than those of high- PNI group (≥ 45.5)[29]. In addition, Peng et al. also proved that PNI is related to the prognosis of NSCLC patients receiving ICI treatment[59].
In our study, the average PNI of 44.11 is taken as the best cut-off value, which is close to the best cut-off value taken by uller M et al., Shoji et al. and Peng et al[29, 58, 59]. According to the optimal cut-off value determined by PNI, patients were divided into high-level group and low-level group. Our study shows that the prognosis of PNI treated with ICI in AGC patients is basically consistent with uller M et al, Shoji et al and Peng et al. The PFS and OS of high-level group were longer than those of low-level PNI group. Due to our included subjects used different immunotherapy regimens in different therapeutic lines, we further conducted subgroup analysis to explore the influence of PNI on OS and PFS in different lines and different immunotherapy regimens. Subgroup analysis showed that patients with high-PNI could benefit from OS whether they used immunotherapy in the 1st line or the multi-line, but PFS could only benefit from immunotherapy in the 1st line. Moreover, patients in the high- PNI group can benefit from OS and PFS, both in immunotherapy alone and in combination with other regimens. Our research also found that AGC patients with pleural fluid and subsequent ICIs can not benefit from PFS and OS after immunotherapy and with dosage of immunotherapy ≥ 200 mg can not benefit from OS.
There are some limitations in this study, including but not limited to: 1) the accuracy of the results may be undermined by retrospective bias such as selection, recall and measurement; 2) Patients included in the criteria received different drugs in different treatment lines; 3) The study subjects were confined to the same hospital; 4) The collected peripheral blood results could not reflect the actual dynamic changes; 5) The exploration scope of this study was limited to peripheral blood, which was widely used in clinical practice and easy to operate, and did not involve genomics and radiomics, which might provide more valuable information and enrich the contents.