Immunohistochemical Stains in Tumor Diagnosis
Reading a tumor's lineage from the proteins it keeps
Why Stain a Tumor?
Immunohistochemistry (IHC). Labeled antibodies detect specific proteins in tissue sections, revealing a tumor's lineage when its microscopic appearance does not. Lineage drives therapy: an undifferentiated tumor may be a carcinoma, lymphoma, melanoma, or sarcoma, each treated differently. Transcription-factor markers (TTF-1, myogenin, OCT3/4, WT1, CDX2) stain nuclei; most others stain cytoplasm or membrane.
Intermediate filaments. Tumors usually keep the intermediate filament of their cell of origin, which makes these proteins the first clue to lineage.
| Filament | Normal cells | Tumors |
|---|---|---|
| Cytokeratin | Epithelium, mesothelium | Carcinomas; mesothelioma |
| Vimentin | Mesenchymal cells | Sarcomas; also renal cell carcinoma, endometrial carcinoma, meningioma |
| Desmin | Smooth, skeletal, and cardiac muscle | Leiomyosarcoma, rhabdomyosarcoma |
| GFAP | Astrocytes and other glia | Astrocytoma, glioblastoma, ependymoma |
| Neurofilament | Neurons | Neuroblastoma and other neuronal tumors |
Mesenchymal, Neural Crest, and Neuroendocrine Markers
Muscle. Desmin marks both smooth and skeletal muscle tumors; smooth muscle tumors also express smooth muscle actin and h-caldesmon. Nuclear myogenin and MyoD1, transcription factors of skeletal muscle differentiation, confirm rhabdomyosarcoma, the most common soft-tissue sarcoma of childhood (classic sites: head and neck, including the orbit, and the genitourinary tract).
Vessels. CD31 (PECAM-1), CD34, ERG, and von Willebrand factor mark endothelium in hemangioma, angiosarcoma, and Kaposi sarcoma; Kaposi sarcoma also shows nuclear HHV-8 latency-associated nuclear antigen (LANA). CD34 is not vessel-specific: it also marks hematopoietic stem cells, GIST, solitary fibrous tumor, and dermatofibrosarcoma protuberans.
GIST. Gastrointestinal stromal tumors arise from the interstitial cells of Cajal, the gut's pacemaker cells, and carry activating KIT or PDGFRA mutations. They stain for CD117 (c-KIT) and DOG1 and often CD34, whereas leiomyomas are desmin-positive and CD117-negative. Imatinib inhibits the KIT tyrosine kinase. Mast cells (mastocytosis) and seminoma are also CD117-positive.
Neural crest. S-100 stains melanoma, schwannoma, and Langerhans cell histiocytosis. It is sensitive but not specific for melanoma, so HMB-45 (a premelanosome protein) and Melan-A (MART-1) confirm melanocytic origin. Langerhans cell histiocytosis is S-100+, CD1a+, and langerin (CD207)+, with Birbeck ("tennis racket") granules on electron microscopy.
Neuroendocrine. Chromogranin A and synaptophysin, proteins of neurosecretory granules and vesicles, identify small cell lung carcinoma (often with SIADH), carcinoid tumors, pheochromocytoma, neuroblastoma, medullary thyroid carcinoma, and Merkel cell carcinoma. Electron microscopy shows membrane-bound dense-core neurosecretory granules.
Carcinoma, Mesothelial, and Germ Cell Markers
TTF-1. This nuclear transcription factor marks lung adenocarcinoma and follicular-derived thyroid tumors, as well as most small cell carcinomas. Napsin A supports lung adenocarcinoma and thyroglobulin supports thyroid; squamous cell carcinoma of the lung is TTF-1 negative but p40/p63 positive.
Mesothelioma versus adenocarcinoma. Both are keratin-positive. Mesothelioma adds calretinin, WT1, and D2-40 and lacks the adenocarcinoma markers CEA, TTF-1, napsin A, MOC-31, and Ber-EP4. Electron microscopy shows long, slender microvilli in mesothelioma versus short, stubby microvilli in adenocarcinoma.
Prostate. PSA (or prostatic acid phosphatase, NKX3.1) in a metastasis, such as a bone lesion, confirms prostatic origin.
Germ cell tumors. Seminoma is PLAP+, OCT3/4+, and CD117+ but CD30−. Embryonal carcinoma shares OCT3/4 but is CD30+ and strongly keratin-positive; yolk sac tumor makes AFP, and choriocarcinoma makes hCG. Sex cord-stromal tumors (Leydig and Sertoli cell) express inhibin and calretinin instead.
Hematolymphoid Markers
CD45 (leukocyte common antigen). Marks hematolymphoid cells and separates lymphoma (CD45+, keratin−) from carcinoma. CD20 (with CD19) marks B cells and is the target of rituximab; CD3 marks T cells.
Reed-Sternberg cells. In classic Hodgkin lymphoma they are CD15+ and CD30+ but usually CD45−. The popcorn cells of nodular lymphocyte-predominant Hodgkin lymphoma instead express CD20 and CD45 and lack CD15 and CD30.
Hairy cell leukemia. These B cells express annexin A1 (the most specific marker), CD103, CD11c, and CD25, stain for tartrate-resistant acid phosphatase (TRAP), and nearly all carry BRAF V600E. Its mimics (splenic marginal zone lymphoma, hairy cell leukemia variant) lack annexin A1, and unlike CLL or mantle cell lymphoma (CD5+) or follicular and Burkitt lymphoma (CD10+), it is typically CD5− and CD10−.
Approach to the Unknown Primary
- Lineage screenCytokeratin (carcinoma), CD45 (lymphoma), S-100 (melanoma), vimentin (sarcoma)
- Carcinoma patternCK7/CK20 profile narrows the organ of origin
- Site-specific markerTTF-1 (lung, thyroid), PSA (prostate), CDX2 (intestine), GATA3 (breast, urothelium), PAX8 (kidney, thyroid, müllerian tract)
| Pattern | Typical primaries |
|---|---|
| CK7−/CK20+ | Colorectal adenocarcinoma; Merkel cell carcinoma (perinuclear dot) |
| CK7+/CK20− | Lung adenocarcinoma, breast, thyroid, endometrial and serous ovarian carcinoma, mesothelioma |
| CK7+/CK20+ | Urothelial carcinoma; many pancreatic and mucinous ovarian tumors |
| CK7−/CK20− | Prostate, hepatocellular, renal cell, and adrenocortical carcinoma |
A CK20+/CK7− adenocarcinoma in the liver is colorectal until proven otherwise; nuclear CDX2 adds support.