Immunotherapy has been rapidly adopted as a standard treatment for NSCLC. Only a limited number of patients are benefitted from ICIs; therefore, biomarkers for identifying this population are critical. We evaluated the role of PLC in predicting the response of ICIs in NSCLC; an elevated PLC after ICI treatment was associated with prolonged PFS and OS in patients with NSCLC.
Besides, post-treatment PLC was a more statistically significant biomarker than the pre-treatment PLC, in terms of PFS, OS, and ORR. This may be attributed to the release of neoantigen into peripheral blood, post tumor cell death . This was coherent with results from previous studies that reported that ICIs used in combination with radiotherapy were more effective than ICIs alone . Radiotherapy shows an additive effect with ICIs; the increased release of neoantigens after tumor lysis due to radiotherapy stimulates systemic anti-tumor immunity . Therefore, if patients had a good response to ICIs, peripheral activated effector T-cell expansion may occur due to the emission of neoantigens after tumor cell lysis; this may be reflected as lymphocytosis after treatment. Therefore, lymphocytosis after immunotherapy indirectly refers to the successful treatment of NSCLC.
High-density engagement of TILs, including effector T cells and NK cells, stimulated by tumor-associated antigens (neoantigens), is important for achieving a suitable response to immunotherapy . The amount of TILs is also assessed through tumor biopsy, similar to the analysis of TMB. Lee et al. demonstrated that the amount of CD8 + TILs was associated with PLC in patients with breast cancer . In melanoma patients with tumor-reactive and specific lymphocytes, PD-1 positive CD8 + lymphocytes were enriched in the peripheral blood. Based on these results, elevated pretreatment PLC could mean increased TIL levels in the tumor microenvironment. This may attribute to patients with increased pretreatment PLC having a favorable prognosis in our study, although only the Q4 group had a significantly increased PFS among the pre-treatment quartile groups.
Our study showed that old people responded better to immunochemotherapy. This result contradicted results from the CheckMate 227 trial . This may be associated with the difference in the number of patients > 75 years in the two studies; 13 (9%) patients were > 75 years in the CheckMate study, while 37 (15%) patients were > 75 years in ours. A recent systematic review showed that elderly patients achieved better outcomes than younger patients; however, the study did not focus exclusively on NSCLC (five NSCLC and four other solid tumors were included) . Changes in the gut microbiome  or the balance between regulatory and cytotoxic T cells  with age are some proposed theories in this regard, although not specific to lung cancer. Age is commonly known as a poor prognostic factor affecting survival in lung cancer. Interestingly, elderly patients aged > 75 years had a better OS than those aged < 65 years. This result could be explained in three ways. First, the OS in this study was not the rate calculated at the time of diagnosis, but the survival rate determined from the date the ICIs were first administered. Therefore, old age cannot be generalized as a good prognostic factor for lung cancer. Second, ICIs were used as the first-line treatment for elderly patients with lung cancer compared with patients aged < 65 years (25.0% vs. 17.3%, p = 0.001). Third, elderly patients used ICIs for the first time; therefore, the probabilities of bone marrow failure or side effects due to toxicity from previous systemic chemotherapy were minimal. Consequently, elderly patients showed longer survival. Therefore, our results confirm that older patients had a better response than younger patients.
Our study also showed that ever smokers had a significantly lower progression rate than never smokers. This finding was consistent with two previous studies, one that showed the effect of nivolumab in advanced NSCLC and another being a meta-analysis. This might be due to the high TMB in ever smokers compared with never smokers among patients with lung cancer. In addition, patients with EGFR mutations had a more rapid progression rate those without. This result is well-known and supported by a lot of clinical and experimental evidence. Therefore, the data from these patients were excluded from analyses, although we did not exclude them from the randomized controlled study.
We did not find an association between irAEs and lymphocytosis both pre- and post-treatment. The association between PLC and irAEs remains controversial. Diehl et al. showed that patients with a PLC > 2000 cells/µL at baseline were associated with an increased risk of irAEs. Kamran et al. found that lymphopenia is a predictor for irAEs. Therefore, we were unable to identify if lymphocytosis or lymphopenia was associated with irAEs.
This study has some limitations. First, this study was retrospective and conducted at a single institution. Second, the mechanism behind lymphocytosis influencing TIL and neoantigen levels is still ambiguous. Third, patients with EGFR mutations could not be excluded from the study. Lastly, we could not suggest a clinically useful cutoff for lymphocytosis because we used quartile analysis. Despite these limitations, this is the first study to determine whether PLC, not NLR, could be used as a biomarker and to evaluate the cutoff value of PLC that showed significance in a larger cohort compared with previous studies. In addition, the present study shortened the duration of response assessment significantly, from 6–12 weeks to 4 weeks. Therefore, the effectiveness of ICI based treatment in patients with NSCLC can be determined soon after initiation of therapy.
In conclusion, PFS was significantly longer with elevated post-treatment PLCs in patients with advanced NSCLC treated with ICIs. Lymphocytosis pre- and post-treatment was not associated with irAEs. However, further studies are necessary to support these findings.