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[The "classical" macrophage marker CD68 is strongly expressed in primary human fibroblasts]
L A Kunz-Schughart1, A Weber, M Rehli
1Institute of Pathology, University of Regensburg, Germany.
Insights
CD68 is not a reliable marker for macrophages. Fibroblasts and some tumor cells also express CD68 protein, challenging its use in diagnostics and research.
Area of Science:
- Immunology
- Cell Biology
- Histopathology
Context:
- CD68 is widely used as a specific marker for human monocytes and macrophages.
- Previous studies suggested potential cross-reactivity of anti-CD68 antibodies with non-myeloid cells.
- The reliability of CD68 as a macrophage-specific marker requires further investigation.
Purpose:
- To evaluate the specificity of CD68 expression in various cell types.
- To assess the reliability of monoclonal antibodies against CD68 for identifying macrophages.
- To compare CD68 expression in fibroblasts and carcinoma cells with that in monocytes and macrophages.
Summary:
- This study investigated CD68 protein and RNA expression in fibroblasts, carcinoma cell lines, monocytes, and macrophages using immunohistochemistry, flow cytometry, and RT-PCR.
- Fibroblasts from various tissues, including normal skin, breast, and tumor tissue, showed CD68 protein expression comparable to macrophages.
- RT-PCR confirmed CD68 mRNA expression in fibroblasts and some tumor cell types, indicating non-selective expression.
Impact:
- CD68 expression is not exclusive to macrophages, impacting its utility as a selective myeloid cell marker.
- Anti-CD68 antibodies should be used with caution for discriminating myeloid cells from fibroblasts in experimental and diagnostic settings.
- Findings necessitate re-evaluation of diagnostic protocols relying solely on CD68 for macrophage identification.
Aim:
Monoclonal antibodies against the human homologue of mouse macrosialin, CD68, are generally commercialized as markers for human monocytes and macrophages. Indeed, CD68 is considered as a selective marker for human myeloid cells, although several previous immunohistochemical studies indicate that some antibody clones also react with other hematopoietic and non-hematopoietic cell types. The aim of our study was to verify these observations and to evaluate the reliability of CD68 as a macrophage marker.
Methods:
We investigated protein and RNA expression of CD68 in various fibroblast types and carcinoma cell lines as compared to monocytes and macrophages using immunohistochemistry, flow cytometry, and specific RT-PCR. Different monoclonal antibody clones against CD68 were applied including KP-1 and EBM11.
Results:
As expected, the intensity of immunohistochemical and flow cytometric CD68 staining was dependent on both the antibody clone and the fixation procedure. However, fibroblasts isolated from normal skin, normal breast, breast tumor tissue, and osteoarthritis synovia clearly expressed CD68 protein at levels comparable to macrophages. The specificity of CD68 expression in fibroblasts was verified by RT-PCR which also showed some tumor cell types to express CD68 mRNA.
Conclusion:
Our findings clearly demonstrate that the expression of CD68 is not restricted to the macrophage lineage. This is highly relevant for experimental and diagnostic purposes, since anti CD68 antibodies cannot be accepted without reservations for the discrimination of myeloid cells and fibroblasts even in paraffin sections after formalin fixation.
