The assessment of TIL subsets using mIHC and computer-assisted imaging quantitative analysis provides a robust tool for the accurate identification of TIL subsets and objective quantification of the immune context in tumors. In this study, the fluorescent mIHC methods were established based on PerkinElmer Opal™ Multiplex technology, which was developed specifically for FFPE tissue and showed high sensitivity in detection of low-abundance targets due to tyramide signal amplification [18, 29]. Our three-day fluorescent mIHC staining protocols can be easily conducted, even in resource-limited settings. After stained TMA sections were imaged, we used free and open source image analysis software for image editing and high-throughput analysis. First, the grayscale images from 5-plex mIHC were processed automatically by a pipeline established with CellProfiler [32] to overlay each biomarker with segmentation markers and to export as RGB images. Second, based on international TILs assessment guidelines [8], the unwanted areas were excluded using ImageJ. Third, for high-throughput automatic imaging quantitative analysis, we developed measurement pipelines for each biomarker using CellProfiler. The pipeline automatically quantified TILs in both the intratumoral (iTILs score) and stromal areas (sTILs score). The potential nonspecific staining signals could be excluded by relating biomarker signal with nuclei. The individual TIL subset was quantified using both positive cell count and positive cell density. We found that both measures showed similar associations with breast cancer survival in this study. Hundreds of images were processed at one time, and quantitative data were exported automatically for statistical analysis. Hence, we successfully established a feasible computer-assisted high throughput automatic analysis method to evaluate the spatial distribution of TIL subpopulations in breast cancer, which is useful for further large population-based studies and for other in situ biomarker studies.
Although TILs have been widely observed in breast tumors, the prognostic potential of these markers and their predictive roles have only been investigated in the last decade [5–7]. Increasing evidence suggests a linear relation between high TIL levels and improved outcome in the patients with TNBC and HER2-positive breast cancer [1, 33, 34] but an inverse association with survival outcomes in patients with ER + breast cancer [7, 33]. TILs were also considered biomarkers for pathological complete response (pCR) in breast cancer patients treated with neoadjuvant therapy, with the positive correlation between pCR rate and TIL level [35, 36]. The prognostic value of TILs, especially cytotoxic CD8 + T cells or immunosuppressive FOXP3 + Tregs, have been previously investigated in breast cancer; however, results remain inconsistent across different subtypes of breast cancer. In the current study, we found that the spatial distribution of TIL subsets was associated with overall survival among breast cancer patients (i.e., increased iCD56 + cells within the tumor bed area was associated with worse OS). Stratified analysis indicated that higher expression of sPD-L1 and higher sCD8+/sFOXP3 + ratio correlated more favorably with OS in TNM stage III-IV breast cancer patients, whereas increased sFOXP3 + and total FOXP3 + cells were associated with better OS among grade III breast cancer patients. Furthermore, three T cell markers (CD3, CD8, and FOXP3) significantly correlated with each other, and PD-L1 positivity significantly correlated with all other markers, especially CD3+, CD8+, and CD56 + cells in stromal and intratumoral areas (Table 2). These findings are generally consistent with the potential biological impact of selected biomarkers on breast cancer cells, as described below.
CD56 + NK cells are historically considered as the first line of host defense against tumor cells, and infiltration of tumors with NK cells is a prognostic marker for several malignancies, including breast cancer. A recent study showed that the frequency of NK cells increased significantly in poorly differentiated (grade III) breast cancer and in tumor draining lymph nodes, suggesting a suppressive phenotype for these cells in breast cancer [37]. Another study showed CD56 + NK cell infiltration was inversely correlated with PR and ER receptor expression status (p = 0.021 and p = 0.007, respectively), two well-established biomarkers of better prognosis in breast cancer [38]. In agreement with those findings, our study found that total CD56 + NK cells and iCD56 + cell density was inversely associated with OS. There are two major NK subpopulations that have been described in humans: a CD56dim subset that predominates in blood and exhibits a high cytotoxic potential, and a CD56bright subset that is more abundant in secondary lymphoid tissues and functions as regulatory and tolerant subsets [39]. The distinct suppressor subsets of NK cells in the TME could suppress antitumor immune responses in solid tumors through inhibitory receptors on the surface of NK cells to downregulate activating signals, leading to a decline of the antitumor immune response and resulting in poor outcomes for cancer patients. The CD56 + NK cells detected in this study might belong to the subset of immunosuppressive NK cell phenotypes.
Previous pooled studies indicate that CD8 + T cells are associated with better disease-free survival (DFS) and breast cancer specific survival (BCSS) [2]. A recent meta-analysis with 37 studies also found that a higher CD8 + TIL level was associated with better DFS, although no significant association was found with OS among TNBC patients [40], which is largely consistent with the results of our study. The prognostic role of Tregs, defined as FOXP3 + T cells, remains controversial [12, 13]. A meta-analysis of twenty-five published studies with 22,964 patients reported that high levels of Treg lymphocytes were associated with poor DFS and OS, but not with poor BCSS [2]. In contrast, Bottai et al. reported that FOXP3 + cells were significantly associated with better RFS and OS among early stage TNBC patients. However, the prognostic value of FOXP3 + cells was not significant after adjusting for CD8+ [41]. An updated meta-analysis of thirty-seven studies with 10,258 patients showed that higher FOXP3 + TIL level was associated with better DFS but not OS in TNBC patients [40]. Furthermore, a higher CD8/FOXP3 ratio was found to be related to higher pCR rate and improved DFS and OS among advanced stage breast cancer patients [42]. In our study, we saw positive associations between stromal FOXP3 + cells and the CD8+/FOXP3 + ratio with better OS, especially in grade III or stage III-VI patients. Our results suggest that the CD8+/FOXP3 + ratio may be a better prognostic biomarker than CD8 or FOXP3 alone for advanced stage or high-grade breast cancer.
Immune check-point molecules programmed cell death 1 (PD-1) and PD-L1 are key immune response modifiers through regulating T-cell activation and immune surveillance [43]. Expression of PD-L1 is not only related to the response of immune checkpoint therapy but also correlated with prognosis in many cancer types [44]. A recent meta-analysis with a total of 14,367 primary breast cancer patients showed that PD-L1 expression on tumor cells was associated with shorter DFS and OS, while PD-L1 + TILs was related to better DFS and OS [45]. In this study, we found that higher sPD-L1 + cells correlated more favorably with OS among TNM stage III-IV breast cancer patients, indicating that PD-L1 + TILs may be an indicator for favorable prognosis in breast cancer patients with advanced stage disease.
Our study has several strengths. Our newly established reliable and sensitive fluorescent mIHC and automatic computer-assisted quantification methods using free and open source imaging software are easy to implement, even in resource-limited settings. The mIHC workflow provided a cost-efficient complete solution from staining to imaging to analysis protocol and can facilitate high throughput analysis that is suitable for large clinical trials or epidemiologic studies. The positive cell percentage and density of TIL markers in intratumoral and stromal compartments were quantified automatically and consistently through whole study samples. In addition, the results generated from analysis of our TMA sections are consistent with the potential biological roles of our selected biomarkers on breast cancer cells and on the prognosis of breast cancer patients, which also supports that three 1 mm cores for each breast cancer case for TIL and other biomarker studies is sufficient [46–53].
Our study also has some limitations. Information on cancer recurrence and cause of death was not available in the NBHS cohort study, so we were unable to conduct disease-free and breast cancer-specific survival analysis. The small sample size of this pilot study meant low statistical power to detect moderate associations between TILs and breast cancer outcomes and prohibited additional analyses by breast cancer subtype. Further studies with larger populations are needed to elucidate the association between TILs and breast cancer outcomes.
In conclusion, we developed a high throughput workflow for fluorescent mIHC and automatic quantification analysis that enables investigation of the expression and spatial distribution of different TIL subsets and immune biomarkers in large scale studies and can also be used in other tissue biomarker assays. Results from our pilot study indicated that increased iCD56 + cells were associated with worse OS of breast cancer patients. Increased sPD-L1 + cells and high sCD8+/FOXP3 + ratio were associated with more favorable OS in TNM stage III-IV breast cancer patients, whereas increased sFOXP3 + and total FOXP3 + cells indicated better OS among grade III breast cancer patients. CD8+/FOXP3 + ratio may be a better prognostic biomarker than CD8 or FOXP3 alone.