The study was conducted to investigate the association between 26 immune gene sets (listed in Table S1) and immune checkpoint proteins expressed in HNSCC and their effect on patient survival. The 26 immune gene-set signatures tested in the present report were reported in a previous publication to be associated with the response to ICI in Triple Negative Breast Cancer [9]. In the current study, we assessed these signatures in HNSCC, as 20 of the 26 gene sets were also found to be modulated in HNSCC [12]. In particular, in the present analysis we assessed the effect of the 26 immunosignature on both the programmed cell death protein ligand − 1 (PDL-1) axis and the T-lymphocyte associated protein 4 (CTLA-4) axis, controlling for potential confounders and effect modifiers [13]. The 520 participants of the TCGA sample were equally distributed for age, gender (males/females) and smoking. 19% (N = 97) of the patients were HPV positive and only 4% (N = 20) had a stage 1 lesion at diagnosis. Patients had cancer lesions characterized by mutated TP53 (70%) and mutation of CDKN2A (22%), FAT1 (22%) and PIK3CA (18%) genes (Table S2). The 26 immune gene expression signatures, the related analysis of the genes that comprise these signatures, and the analysis of the 15 immune cell types, are described in Table S1.
STEP 1 – phase A: Regression and survival analysis
In Fig. 2, panel A, we aimed to identify subgroups of HNSCC patients with a significant difference of IS levels. The first set of results are represented by the forest plot with Odds ratio with 95% CI of demographic and prognostic predictors of the immune signature expression by using regression models in the HNSCC dataset from TCGA. All the analyses were performed at univariate level considering the effect of only one independent variable (demographic or prognostic variables) on the signature (dependent variable). We observed that only the HPV status, with HPV negative versus the positive lesions, lymphnode status (N0 vs N+) and TP53-mut plus additional mutations versus TP53-mut-only lesions, were statistically associated with a higher immune signature expression. Details of regression analyses on clinical factors for each immune gene set are shown in S-Fig. 1 and S-Fig. 2. Notably, building a multivariable regression model, TP53 mutational status resulted in the main clinical factor significantly associated with the immune signature (Table S3). To provide clinical meaning to the described results, in Fig. 2, panel B, we reported overall-survival (OS) and progression-free-survival (PFS) curves assessed in the TCGA cohort of 520 patients. In that cohort, high immune signature expression was significantly associated with both overall and progression-free survival.
Phase B: Immunosignature in TP53 mutated (i) and TP53 co-mutated patients (ii) and cell types enrichment (iii).
Because the mutational TP53 status was so important, we assessed the expression distribution of the immune signature by three groups of patients with TP53-WT status, TP53-mut status, and TP53-mut in combination with one of the other three most frequent mutations observed in HNSCC cancer patients (FAT1, CDKN2A, and PI3K genes), hereinafter denoted as TP53-mut+. TP53-WT patients were characterized by a higher IS expression level compared with TP53-mut and TP53-mut+ patients (Fig. 2C). Surprisingly, TP53-mut+ patients had a significantly higher IS score in comparison to the TP53-mut patients (Tukey’s post-hoc test, p < 0.05) though significantly lower than WT patients (Tukey’s post-hoc test, p < 0.01) (Fig. 2C).
In line with the importance of the TP53 status for the response to ICI, an analysis of cell type composition, performed with Xcell software [10], revealed distinct immune cell composition across the three TP53 groups (Fig. 2D). In support of a fundamental difference between TP53-mut tumors with vs. Without additional mutations, the abundance of 7 immune cell types was statistically different between TP53-mut and TP53mut + patients (Fig. 2E). To further detail the functional link between TP53 gene mutations with gain of function activity and immune signature, we assessed the role of the TP53 mutated-dependent MYC signature identified in our previously work [14]. In Fig. 3, we assessed the expression of PDL1 and CTLA4 in TCGA patients with high or low expression of MYC-related signature (Fig. 3A and B) and the expression of the immune gene sets in both TCGA and GEO cohort (GSE195832) (Fig. 3C and D) patients. Again, lower expression level of the TP53 mutated-dependent MYC signature was significantly associated with higher levels of the IS score, PDL1 and CTLA4. Furthermore, in TCGA cohort we had sufficient clinical information and sample size to adjust those modulation for potential confounding factors. The multivariate models reported at the bottom of panels A, B and C confirmed that those associations, between genes or IS and the MYC-dependent signature, were independent from other clinical factors. To validate these results, we performed qRT-PCR analysis of PD-L1 (Fig. 3E) and CTLA4 (Fig. 3F) in Cal27 cells, a head and neck cancer cell line carrying a TP53 mutation, treated with JQ-1. The latter is a small-molecule that inhibits the activity of the BET family proteins by masking their bromodomain acetyl-lysine-binding pockets [15]. JQ-1 has been demonstrated to act as an antineoplastic agent by mainly inhibiting c-MYC functions. Both genes showed increased expression after treatment when compared to their controls, strengthening the potential role of MYC in this immunogeniccontext (Fig. 3E and F).
Step2- - Phase I and II: Analysis of NCI-60 cell lines and in vitro validation. Analysis of the immune gene sets and MYC dependent signature in a cohort of HNSCC patients treated with PDL1-inhibitors.
We further investigated the role of the immune gene sets and of the TP53 mutated-dependent MYC dependent gene signature in their well characterized cohort of HNSCC patients under treatment with PD-L1 inhibitors obtained from the GEO database (accession ID: GSE159067).
In Fig. 4A, we report results of our analysis on the prognostic value of the tested immune gene sets in both OS (left panel) and PFS (right panel). These results corroborate our findings from the TCGA cohort (Fig. 2B), demonstrating that our expression-based IS score is indeed associated with improved survival across clinical datasets.
The immune gene sets, we used to define our immune score, was also strongly correlated to the classification (“COLD” and “HOT”) introduced by Foy and colleagues (S-Fig. 3). Notably, In Fig. 4B, low levels of the immune gene sets are significantly associated with stable or progressive disease during immunotherapy. Furthermore, low level of the TP53-dependent MYC signature was significantly associated with the immunologically “HOT” type (Fig. 4C). These results are in line with our finding on TCGA data and cell lines about the potential role of MYC in an immunogenic context. In NCI-60 cell lines, indeed, we found that WT-TP53 HNSCC cell lines exhibit higher expression of CTLA4 and PDL1 when compared to cell lines harbouring TP53 mutations (S-Fig. 4A). Higher levels of PDL1 were also found in cell lines carrying TP53 co-mutations, such as Detroit-562 and FaDu, when compared to those carrying only TP53 mutation (S-Fig. 4B). We also observed that depletion of either mutant p53 protein or its co-factor YAP released PDL1 expression in HNSCC cell lines (S-Fig. 4C). Enhanced expression of PDL1 was also obtained in CAL-27 and Detroit-562 after treatment with alpelisib, a selective inhibitor of p110α-subunit of PI3K (S-Fig. 4D). We have previously identified that mutated p53 and YAP favour c-Myc stability and its transcriptional activity in HNSCC cell lines [14]. In that contest the use of alpelisib has been found to partially impair this pro tumorigenic axis [14].
Finally, we used specific marker genes of the cell types identified in the cell enrichment analysis of TCGA data (Fig. 2C) to evaluate their quantitative expression on immunotherapy treated patients. Six out of the seven investigated cell types resulted strongly up-regulated in HNSCC patients characterized by complete or partial response to the treatment (Fig. 4D). The same cell types showed a significant reduced abundance in patients with low Immune Score (S-Fig. 5).
To study the potential cause of the difference in the response to ICI between TP53-mut and TP53-mut + HNSCC patients, we considered the aneuploidy score of TCGA HNSCC patients. In general, aneuploidy is strongly associated with TP53 mutations, and is negatively correlated to several immune signatures across various cancers [16, 17]. In line with previous evidence, we observed the negative correlation between aneuploidy scores and our IS (Fig. 5A). As expected, aneuploidy levels were significantly higher in the TP53-mut and TP53-mut+ patients (Fig. 5B). Interestingly, however, aneuploidy levels were significantly lower in the TP53-mut+ group relative to the TP53-mut group. Therefore, aneuploidy levels may underlie the difference in ICI response between the two groups. To establish the association between TP53 mutation, co-mutation and aneuploidy levels in immune gene prediction set, we built multivariate regression models, adjusting the TP53 mutational and co-mutational status for the aneuploidy scores. In the multivariate models TP53 mutation, TP53 co-mutation and aneuploidy were found to be independent predictors of the immune signature (Fig. 5C and D).