By Anthony M Magliocco MD
Next generation sequencing NGS has begun to revolutionize the capabilities of molecular pathologists to analyze the molecular foundation of cancer specimens offering improved diagnosis, classification and treatment selection.
Not to be outdone, similar remarkable advances are impacting tissue pathology as well. Immunohistochemistry is a powerful technique that allows pathologists to visualize and measure protein in routine formalin fixed, paraffin embedded tissue sections.
Moffitt Cancer Center is pushing the boundaries of tissue analysis using digital image analysis and multispectral immunohistocemistry in the Morsani Molecular Laboratories directed by Dr Magliocco. The Advanced CLIA Analytical Microscopy laboratory at Moffitt is led by Susan McCarthy, an expert in molecular analysis and histotechnology. The fact that the laboratory is also certified by CLIA and CAP is important, as the new complex imaging methods can then easily be translated to patient care under a CLIA LDT regulatory mechanism.
Classically, Immunohistochemistry involves applying a primary antibody which will bind to an antigen and then detecting it with a tagged secondary antibody (essentially an antibody against the first antbody). Typically this binding is visualized using a brown dye (or chromogen) such as DAB.
The result allows the pathologist to assess the presence of a target protein and approximately its concentration. This method is very common and hundreds of different proteins can be analyzed. in the example above, an invasive breast cancer is stained with an antibody against estrogen receptor. The intense brown staining of the nuclei of cancer cells indicate that the cancer is strongly expressing the estrogen receptor and is likely being driven by estrogenic processes. The blue staining cells in the background are mostly lymphocytes stained with a blue counter stain to allow visualization of the tissue. Breast cancers like this are considered to be estrogen receptor positive. It is known that estrogen receptor positive cancers are most likely to respond to hormonal treatments such as tamoxifen or aromatase inhibitors.
While single marker IHC is a powerful tool it presents challenges in that only one marker at a time and accurate quantification tools are lacking. Further, the assays are usually pushed to enable high sensitivity without proper dynamic range, so it can be difficult to accurately quantify expression in strongly expressing cells.
“Next Generation IHC” or multiplex IHC is a technique where more than one antibody can be applied to a tissue section at the same time and then visualized with secondary antibodies attached to different fluorescing tags. This produces a multiplex image which can be “deconvoluted” into separate channels for direct analysis
In this image the separate proteins Progesterone receptor and Cytokeratin can be isolated using filters to isolate the red and green fluorescence (the colors are actually false but help visualization for the human eye) the DAPI is a stain the stains nuclei.
Once the various channels of image are acquired using a digital scanning microscope such as Aperio FL, computer software such as AQUA can isolate components of the images to define areas of interest on the tissue also called “masks”. These areas could be tumor nuclei, tumor cytoplasm, lymphocytes, etc. Once masks are defined the signal from the target protein can then be accurately measured.
This ability to accurately localize and measure the protein semi-automatically produces very reproducible and precise data. Much like running an elisa on the slide
Some assays like estrogen receptor can be problematic to quantify, and this can lead to misclassification and mistreatment of patients causing serious risks to patients. A region of Canada unfortunately experienced problematic testing for many years which affected the quality and treatment of hundreds of breast cancer patients
ALso different approved method to measure hormone receptor in breast tissue may produce different results
Perkin Elmer is pushing multispectral imaging and quantification even further with their Vectra imaging systems and new Opal chemistries facilitating high multiplex staining.
This elegant system allows 6 or more separate antibodies to be evaluated on a single tissue section.
This powerful, and stunningly beautiful, method enables multiple cell types to be visualized in a single tissue section.
In the above image of a lung cancer produced by Susan McCarthy at Moffitt the orange areas are PDL1 and PD1 expression, the green are CD3 cells, the yellow CD8 and the red FOXP3. The cancer cells are highlighted in light blue and the dark blue identify nuceli.
Analysis like this preserve the rich complexity of the tumor microenvironment and allow direct visualization of not only the types of cells present, but their localization in respect to cancer cells and other immune cells. This method has produced excitement amongst oncologists and immunologists seeking improved tools and clues into which tumors may be most susceptible to immune checkpoint treatment.
Challenges still remain in bringing assays like this into routine care. Its critical that the assay can meet quality and CLIA standards which include development of standard operating procedures, and delineation of the performance characteristics of the assay such as sensitivity, precision, performance, detectable range, and robustness.
Another layer of complexity is approach to the digital analysis of the data or “dry lab” as there can be many approaches such as counting cells or measuring areas of signal. Sources of variation could include different approaches to segmenting images and normalization. Despite these challenges I remain optimistic that these technologies will provide powerful new insights into the biology of cancer, and provide pathologists with compelling new way to microscopically examine human tissue.