In this study, we compared the clinicopathologic characteristics and overall survival of Indonesian primary nodular melanoma cases with different mRNA levels of PD-L1 and NKG2A. Two important findings were observed in our study: (1) melanoma cases with PD-L1 overexpression had significantly lower survival rates compared with those with normal PD-L1 expression and (2) PD-L1 and NKG2A levels were strongly correlated.
We observed that patients with tumors showing upregulated PD-L1 had significantly lower overall survival, with an approximately threefold higher HR, compared with those with tumors showing normal PD-L1. The PD-L1 molecule interacts with PD-1 receptors on T cells, causing anergy, exhaustion, and even apoptosis . Melanoma cells can thus escape the immune system by increasing their PD-L1 expression. Experiments using melanoma cell lines showed that cells with upregulated PD-L1 demonstrate highly invasive and aggressive behavior . In a study on melanoma patients treated with surgery and dacarbazine adjuvant chemotherapy, patients with positive PD-L1 on IHC staining had lower median survival time compared with the subgroup with negative or indeterminate PD-L1 status (9.7 months vs. 11.6 months) .
Our results seem to contradict those of Gupta et al., who observed that higher PD-L1 mRNA levels reflect a better prognosis for patients with melanoma treated with anti-PD-1 agents . This incongruity may have stemmed from differences in the treatments administered to the subjects. Patients with high expression of PD-L1 respond well to anti-PD-1 antibodies, thus explaining the increase in progression-free and overall survival . The results suggest that PD-L1 expression is a negative prognostic factor in patients with melanoma in general. However, when treated with anti-PD-1 antibodies, patients with high levels of PD-L1 respond well and have good outcomes. Therefore, the choice of therapy also affects the performance of PD-L1 as a prognostic factor.
PD-L1 expression can occur as an independent phenomenon or a secondary reaction to the presence of TILs. TILs can secrete interferon-gamma, which induces the expression of PD-L1 in tumor cells . Studies have reported the reactive pathway as the predominant background for PD-L1 expression in melanomas because of the strongly positive correlation between PD-L1 and TILs . However, in our study, the group without TILs had higher average PD-L1 mRNA levels than the group with TILs. Combined with the lower survival of PD-L1 overexpressers, this finding suggests that the cases with increased PD-L1 in our study likely have no TILs and are independent (constitutive) expressers.
Constitutive and reactive expression of PD-L1 may have different prognostic implications. When previous studies divided patients based on PD-L1 expression and the presence of TILs, patients with constitutive PD-L1 expression without lymphocyte infiltrates showed the poorest outcomes, followed by reactive PD-L1 expressers, those with PD-L1(-) without TILs, and, finally, those with PD-L1(-) and TILs [16, 19]. In our results, both groups with upregulated PD-L1 showed poorer prognoses than the groups with normoregulated PD-L1. However, survival did not differ significantly when the cases were divided further based on TIL status, likely because none of the patients were treated using immunotherapy, in which the presence of TILs predicts improved response and outcomes . Another factor that could explain this lack of significance is the limited sample size.
Patients with upregulated NKG2A mRNA did not differ significantly from normo-regulated patients in terms of survival. NKG2A is an inhibitory receptor found on NK cells that plays a major role in the immune response against tumors . Cancer cells can attempt to evade the immune system by upregulating HLA self-molecules that activate NKG2A receptors and impair the function of NK cells. Trials in mouse models indicate that monalizumab is ineffective as a single therapy but highly effective when used together with other immunotherapy agents that promote activated TILs, such as anti-PD-1 or cancer vaccines . One escape strategy used by cells to escape cytotoxic TILs is downregulation of MHC I expression, which renders them targets for NK cells . This finding may explain the role of anti-NKG2A as an adjunct treatment for other immunotherapies. Our results reinforce the idea that NKG2A may not be an independent therapeutic and prognostic factor but may play a role when combined with other factors.
The mRNA expressions of NKG2A and PD-L1 were strongly correlated. This finding indicates that tumors with high PD-L1 expression would also likely express NKG2A strongly and, thus, respond well to anti-NKG2A agents. When NKG2A expression was combined with the TIL parameter, the distribution of survival curves obtained resembled the curves for PD-L1 combined with TILs. NKG2A upregulation with and without the presence of TILs may have different pathogeneses and prognostic implications, similar to PD-L1.
The lack of correlation between the expression of NKG2A and PD-L1 and clinicopathologic characteristics in this work resembles the findings of several previous studies [6, 18]. The small sample size of this study may have contributed to the low statistical significance found.
The findings of this study must be interpreted with caution because of the small sample size of patients who did not receive adjuvant immunotherapy. However, our results support the findings of several studies that show that mRNA profiles may serve as a prognostic factor in melanoma cases [12,24]. Our interesting results suggest that further research and clinical trials are warranted to ascertain the roles of PD-L1 and NKG2A in the prognosis and therapy of Asian patients who had not previously received immune checkpoint inhibitors.