Cytokeratin 13 (MSVA-613M)

Cytokeratin 13 (CK13), also termed keratin 13 (KRT13) is a type I acidic high molecular weight keratin protein encoded by the KRT13 gene located at 17q21. It dimerizes with the type I keratin 4 and forms intermediate filaments that primarily shape the cytoskeleton of specific epithelial cells. In these cells, KRT13 is part of the cytoskeletal scaffold which contributes to the cell architecture and provides the cells with the ability to withstand mechanical stress. Mutations in CK10 have been linked to the autosomal dominant disorder “White Sponge Nevus”.

Cytokeratin 13 staining pattern in Normal Tissues with antibody MSVA-613M (images are shown in our “Normal Tissue Gallery”)

These findings are largely consistent with the RNA and protein data summarized in the Human Protein Atlas (Tissue expression Cytokeratin 13) 

Suggested positive tissue control: Tonsil: All squamous epithelial cells of the surface squamous epithelium (except the basal layer) and a fraction of squamous cells of the crypts should show strong KRT13 staining. 

Suggested negative tissue control: Tonsil: All lymphocytes and blood vessels must not show any KRT13 staining.

Normal tissue gallery

KRT13 immunostaining almost exclusively occurs in squamous cell carcinomas of various sites of origin. At frequencies <10% KRT13 expression can also be seen in adenocarcinomas of the pancreas, stomach, ovary, lung, endometrium, breast, esophagus, and the prostate, as well as in nonseminomatous germ cell tumors.

The TCGA findings on Cytokeratin 13 RNA expression in different tumor categories have been summarized in the Human Protein Atlas.

Cancer tissue gallery

Cytokeratin 13 (MSVA-613M) publication summary

Relevant publication:  Lennartz et al. “Cytokeratin 13 (CK13) expression in cancer: a tissue microarray study on 10,439 tumors” Published in the Journal of Pathology, Microbiology and Immunology – the APMIS journal 2022 Oct 21. doi: 10.1111/apm.13280. Epub ahead of print. PMID: 36269681.

A total of 9,156 tumors from 131 different tumor categories were successfully analyzed by using the following protocol: Heat-induced antigen retrieval for 5 minutes in an autoclave at 121°C in pH 7,8 Target Retrieval Solution buffer. MSVA-613M at a dilution of 1:150 at 37°C for 60 minutes. Visualization of bound antibody by the EnVision Kit (Dako, Agilent). This protocol was also used for all stainings depicted in our tumor and normal tissue galleries.

Overall, 42 (32,1%) of 137 tumor categories showed detectable CK13 staining in at least one case and 24 (18,3%) tumor categories included at least one case with strong CK13 positivity. The highest rate of positive staining was found in subtypes of urothelial neoplasms (52.1–92.3%) including Brenner tumor of the ovary (86.8%) and in squamous cell carcinomas from different sites of origin (39.1–77.6%), Warthin tumors of salivary glands (66.7%), adenosquamous carcinomas of the cervix (33.3%), thymomas (16.0%), and endometroid carcinomas of the ovary (15.3%). Twenty other neoplasms showed a mostly weak CK13 staining in less than 15% of cases. In a fraction of these rarely CK13-positive tumor entities , focal CK13 staining was seen in areas of squamous differentiation. The distribution of positive staining results is shown in an “organ-systematic” (Figure 1) and in a “ranking order” figure (Figure 2) below (images based on data from Lennartz et al). Data on associations with histopathological and clinical parameters of tumor aggressiveness in several cancer types are also summarized below (Figure 3; based on data described by Lennartz et al).

Authors conclusions on diagnostic utility of CK13 IHC with respect to the distinction of different tumor entities (Lennartz et al.):

• CK13 is a marker for squamous and urothelial cell differentiation but CK13 expression can also be seen in other tumor entities.

Authors conclusions on the prognostic role of CK13 immunostaining results (Lennartz et al.):

• Reduced CK13 staining was linked to high grade in squamous cell carcinomas (p = 0.0050).
• CK13 positivity was associated with invasive tumor disease in urothelial neoplasms (p<0,0001).

Figure 1. Cytokeratin 13 staining in cancer (“organ-systematic”; according to Lennartz et al.)

Figure 2. Cytokeratin 13 staining in cancer (“ranking list”; according to Lennartz et al.)

IHC users have different preferences on how the stains should look like. Some prefer high staining intensity of the target stain and even accept some background. Others favor absolute specificity and lighter target stains. Factors that invariably lead to more intense staining include higher concentration of the antibody and visualization tools, longer incubation time, higher temperature during incubation, higher temperature and longer duration of the heat induced epitope retrieval (slide pretreatment). The impact of the pH during slide pretreatment has variable effects and depends on the antibody and the target protein. Accordingly, multiple different protocols can generate identical staining results.

All images and data shown here and in our image galleries were obtained by the manual protocol described below. Other protocols resulting in equivalent staining are described as well.

Manual protocol

Freshly cut sections should be used (less than 10 days between cutting and staining). Heat-induced antigen retrieval for 5 minutes in an autoclave at 121°C in pH7,8 Target Retrieval Solution buffer. Apply MSVA-613M at a dilution of 1:150 at 37°C for 60 minutes. Visualization of bound antibody by the EnVision Kit (Dako, Agilent) according to the manufacturer’s directions.

Agilent / Dako – Autostainer Link 48

Pretreatment in PT-Link for 30 minutes at 95°C (pH high); FLEX peroxidase blocking for 5 minutes (room temperature), MSVA-613M 1:150 for 20 minutes (room temperature), FLEX+ mouse/rabbit (LINKER) for 15 minutes (room temperature), horseradish peroxidase (HRP) for 20 minutes (room temperature), FLEX DAB+Sub-Chromo for 10 minutes (room temperature), FLEX hematoxylin for 5 minutes (room temperature).

These images reflect stainings by the protocol described above. It is of note that a comparable staining result can also be obtained by different protocols. In general, a longer pretreatment, a longer incubation time of the primary antibody, a higher antibody concentration, and a longer incubation time of FLEX+LINKER result in stronger staining, potentially at the cost of more background staining. Modifications of the protocol with a strengthening effect on staining intensity in combination with changes of other parameters that result in lower staining intensity can result in a comparable result as shown above.

Leica – BOND RX

Dewax at 72°C for 30 seconds; Pretreatment in Bond Epitope Retrieval Solution (ER2 – EDTA pH9) for 20 minutes at 100°C; Peroxidase blocking for 5 minutes (room temperature), MSVA-613M 1:150 for 15 minutes (room temperature), Post primary (rabbit anti mouse) for 8 minutes (room temperature), Polymer (goat anti rabbit) for 8 minutes (room temperature), mixed DAB refine for 10 minutes (room temperature), hematoxylin for 5 minutes (room temperature).

These images reflect stainings by the protocol described above. It is of note that a comparable staining result can also be obtained by different protocols. In general, a longer pretreatment, a longer incubation time of the primary antibody, a higher antibody concentration, a higher temperature during incubation, and a longer incubation time of Post primary and or the Polymer result in stronger staining, potentially at the cost of more background staining. Modifications of the protocol with a strengthening effect on staining intensity in combination with changes of other parameters that result in lower staining intensity can result in a comparable result as shown above.

Roche – Ventana Discovery ULTRA

Pretreatment for 64 minutes at 100°C (pH 8,4); CM peroxidase blocking for 12 minutes (room temperature), MSVA-613M 1:150 for 20 minutes at 36°C, secondary antibody (anti-rabbit HQ) for 12 minutes at 36°C, anti-HQ HRP for 12 minutes at room temperature, DAB at room temperature, hematoxylin II at room temperature for 8 minutes, bluing reagent at room temperature for 4 minutes.

These images depict staining results obtained by the protocol described above. It is of note, that the Ventana machines generally require higher antibody concentrations than other commonly used autostainers because the antibodies are automatically diluted during the procedure. Various other protocols can result in an identical result as shown above. A longer pretreatment, a longer incubation time of the primary antibody, a higher antibody concentration, a higher temperature during incubation, and a longer incubation time of secondary antibody and or the anti-HQ HRP result in stronger staining, potentially at the cost of more background staining.

Impact of pH

The strongest KRT13 staining by MSVA-613M is obtained at a pH 9,0. However, pH 7,8 results in only a slight reduction of the staining intensity as compared to pH9. We thus consider pH7,8 as optimal for manual staining because of the better tissue preservation at pH7,8 than at pH 9,0.

• The diagnostic utility of KRT713 expression analysis should be investigated in a large cohort of tumors from different entities.
• The diagnostic utility of increased KRT13 expression in the skin is not known.
• The diagnostic utility of decreased KRT13 expression in non-keratinizing squamous epithelium of ectocervix, esophagus, oral cavity and lip is not known.
• The prognostic role of KRT13 expression in squamous cell carcinoma is unknown.

Utility of MSVA-613M is documented by a staining pattern that exactly matches the data summarized in the protein atlas. A strong positive KRT13 staining is almost exclusively seen in non-keratinizing squamous epithelial cells but not in the skin. In addition, all tissues known to not express KRT13 including those notorious for non-specific IHC background such as kidney and colonic mucosa are completely KRT13 negative.

Normal tissue gallery