Immunoprofiling analysis of FFPE tumor tissue samples has become an essential component of immuno-oncology studies, particularly for the discovery and validation of novel predictive biomarkers for cancer immunotherapy in the context of clinical trials1,2. Chromogenic IHC, using chemical chromogens such as diaminobenzidine, remains the standard technique in diagnostic pathology for the immunolabeling of biopsy tissue3. Standard IHC can also be used for cancer tissue immunoprofiling, including the quantitation of subpopulations of tumor-associated lymphocytes and the assessment of expression levels of immune checkpoints such as programmed cell death ligand 1 (PD-L1)4,5. Standard IHC is limited, however, in that only one antigen can be labeled per tissue section. Because immunoprofiling studies typically require the analysis of the combined expression of several markers, the use of standard IHC would require the staining of multiple tissue sections, each stained with a single marker, and would, therefore, be substantially limited for the analysis of small tissue samples such as core needle biopsies. Standard IHC methods are also limited for the assessment of markers that are coexpressed by diverse cell populations, as is common with immune checkpoint markers such as PD-L1, which is expressed by both tumor-associated macrophages and cancer cells. This limitation has been reported in, for instance, the use of standard monoplex IHC by pathologists for the quantitative analysis of an IHC marker expressed by diverse cell types6. The development of multiplex chromogenic IHC techniques employing diverse colored chromogens on the same tissue section represents an advancement over the standard IHC monoplex method,7 although they remain limited by the immunolabeling of just a few markers and also present an important technical challenge for the proper evaluation of markers expressed in the same subcellular compartments of the same cell populations.
The aforementioned caveats regarding tissue availability from clinical samples, as well as the limitations of multiplex chromogenic IHC techniques, have given rise to the need to develop improved multiplex methods for immuno-oncology studies based on fluorescent labeling combined with imaging systems that can effectively separate the signals of multiple fluorophores from the same slide. One such technique is based on tyramide signal amplification (TSA) combined with multispectral microscopy imaging for efficient color separation8. A commercially available TSA-based kit employs fluorophores optimized for multispectral imaging8 (see Table of Materials). A critical advantage of this system is its compatibility with the same unlabeled primary antibodies that have already been validated and optimized for standard chromogenic IHC9,10,11. This allows not only faster optimization but also flexibility in the optimization and panel modifications incorporating new targets. Furthermore, the multiplex immunofluorescence (mIF) TSA method can be optimized for commercially available automated IHC stainer systems, allowing for a straightforward transfer from monoplex chromogenic IHC to mIF.
Here we present a protocol for an mIF panel for immuno-oncology studies that is based on automated mIF TSA staining and uses a multispectral scanner for imaging. This protocol can be adapted and modified by any laboratory user with access to the described instrumentation and reagents. The protocol includes a panel of six primary antibodies for the immunoprofiling of carcinomas: PD-L1, PD-1, CD68 (as a pan-macrophage marker), CD8 (T-cytotoxic cells), Ki-67, and AE1/AE3 (pan-cytokeratin, used as an epithelial marker for the identification of carcinoma cells). A recent study describes the optimization of a manual TSA mIF protocol by using chromogenic IHC as a standard reference to validate the multiplex staining12. The updated method presented here has been developed by using a commercially available, seven-color TSA kit optimized in an automated stainer, drastically shortening the staining time from 3–5 days to 14 h, while also improving the consistency of the staining. In addition to the detailed main protocol presented here, a Supplemental Materials section includes the "drop-control" method, an additional quality control process to evaluate a new mIF panel, as well as technical notes for the optimization, troubleshooting, and development of new multiplex panels to help the laboratory user to set up and optimize the mIF TSA method for customized mIF panels.