Expression of CD68 in Non‐Myeloid Cell Types

Abstract

CD68, the human homologue of macrosialin, is commonly regarded as a selective marker for human monocytes and macrophages. Its expression is thought to be regulated by a macrophage‐specific promoter. However, several immunohistochemical studies have indicated that CD68 antibodies also react with other haematopoietic and non‐haematopoietic cell types. We investigated the expression of CD68 in various primary cells and carcinoma cell lines using immunohistochemistry, flow cytometry, Western blot analysis and qRT‐PCR. Weak but significant immunoreactivity was detected in lymphocytes and several tumour cell lines whereas staining of primary fibroblasts and endothelial cells was comparable to macrophages. The intensity of CD68 staining in individual cell types depended on the antibody clone and the fixation technique. Anti‐CD68 mAb KP1 should be used with great caution for frozen tissue sections due to its reactivity with a wide variety of cell types. Also, care should be taken when distinguishing macrophages from fibroblasts/stromal cells in paraffin sections after formalin fixation since both cell types are stained highly positive for CD68. In accordance, mRNA expression of CD68 was not only detected in macrophages and monocytes but also in fibroblasts as well as endothelial cells and tumour cells, although with a varying intensity. Cloning of full length 5′‐sequences and determination of transcription start sites shows that macrophages and fibroblasts initiate transcription within the known promoter region; however, from different start sites, indicating alternative promoter architecture in myeloid versus non‐myeloid cells. We suggest that CD68 is not a selective macrophage marker but rather a lysosomal protein that is enriched in macrophages.

Leukocyte composition of human breast cancer

“…Although used successfully in multiple studies to relate TAM infiltration with clinically relevant outcomes, our results indicate that CD68 alone cannot accurately evaluate macrophage presence in human breast tissue given that multiple stromal cells express it and that a subset of these are CSF1R-and CD45-negative. We observed that the nonleukocytic CD68 + cells were predominantly located within tumor stroma and, thus, based on this localization and morphology, we speculate that CSF1R − CD68 + cells likely reflect tumor-associated fibroblasts or monocyte-derived fibrocytes in agreement with other reports (31,32,(49)(50)(51)(52). Our findings do not invalidate CD68 as a clinically relevant marker and, importantly, CSF1-response gene signatures have been identified in breast adenocarcinomas that are predictive of recurrence risk and metastasis (53,54).…”

Tumor‐associated macrophages correlate with response to epidermal growth factor receptor‐tyrosine kinase inhibitors in advanced non‐small cell lung cancer

“…32,33 Gottfried et al also reported that CD68 could be expressed on fibroblasts. 34 Counterstaining with cell type-specific markers in our study did reveal that very low ratio of CD68þ cells were fibroblasts or dendritic cells (Supporting Information Figures 2A and 2B). Nevertheless, the fibroblasts with their characteristic spindle cell shape were excluded from counting when calculating CD68þ TAMs.…”

CD68/macrosialin: not just a histochemical marker

“…Low expression of CD68 was found in several nonhematopoietic cell types including human umbilical cord mesenchymal stem cells, 27 fibroblasts, endothelial cells, and various tumor cell lines, 15 as well as intimal smooth muscle cells of human arteries. 28 Indeed, finding of CD68 in many non-myeloid cell types questions the mononuclear phagocyte lineage specificity of this surface marker even though CD68 expression is much stronger in myeloid cells.…”

“…14 However, cytochemical studies showed that non-myeloid cells can also express CD68 but to a lesser extent than myeloid cells. 15 In this review, we consider CD68 structure and expression in myeloid and non-myeloid cell types along with its role in normal and pathological conditions.…”