Single-cell protein analysis in support of biomarker evaluation for breast cancer immunotherapeutics.
10 Background: Triple-negative breast cancer (TNBC) is being tested for PD-1/PD-L1 checkpoint inhibitors combination therapy. The relationship between tumor expression of PD-L1 and patient outcomes has been established using immunohistochemistry (IHC) biomarker assays across various malignancies. PD-L1 is expressed in tumor and immune cells within the tumors in TNBC, but only immune-cell PD-L1 is associated with response to anti-PD-L1 (atezolizumab) so differentiating tumor vs. immune PD-L1 expression is essential. The currently available IHC biomarker assays that relay on tissue biopsies struggle to provide clinically meaningful identification of responders vs. non-responders. Poor sensitivity and specificity is partially attributed to tumor heterogeneity that is not well-represented in these biopsies. In addition, PD-L1 expression may vary over time due to disease progression or treatment. Since tissue biopsy collection is often an invasive procedure, it is restricted. Needle biopsies can mitigate some of these issues as a minimally invasive method to probe multiple regions within several cancer lesions across several time points. MILO, a new single cell Western Blot (WB) technology allows quantitative measurements of multiple protein biomarkers within one cell in small samples. Methods: To evaluate the capability of MILO to differentiate between PD-L1 in immune and tumor cell populations, we interrogated BC cell lines as well as peripheral blood mononuclear cells (PBMCs) mixed in known ratios. We used MDA-MB-231 that shows high PD-L1 and low HER2 expression and BT-474, which shows the inverse expression profile for these biomarkers along with PBMCs as PD-L1+ immune cells. Primary breast tumor tissues are currently interrogated to establish utility in needle biopsies. Results: MILO demonstrated heterogeneity at the single-cell level of HER2 and PD-L1 expression in BC tumor cells and enabled differentiation between PD-L1 expression in immune vs. tumor cells using cell-specific markers and low cell numbers. Conclusions: MILO can improve patient selection and prediction of response to immune checkpoint inhibition by promoting the utility of needle biopsies to complement IHC of core biopsies.