Detection of the CD56+/CD45- immunophenotype by flow cytometry in neuroendocrine malignancies

Abstract

Aims: Antibodies against CD56 are primarily used in flow cytometric studies to detect natural killer cells. However, they may be useful in the identification of neuroendocrine malignancies, especially if the cells do not express CD45, indicating a non-leucocyte origin.

Methods: A retrospective review was conducted on all solid tissue flow cytometric studies performed between January 1997 and September 2001, to identify all cases with a CD56+/CD45- immunophenotype.

Results: Twelve neuroendocrine malignancies (five metastatic small cell carcinomas, three Merkel cell carcinomas, two metastatic undifferentiated neuroendocrine carcinomas, one metastatic pancreatic neuroendocrine carcinoma, and one neuroblastoma) were identified.

Conclusions: CD56+/CD45- neuroendocrine malignancies are only rarely detected in the flow cytometric analysis of solid tissue samples. However, the recognition of this immunophenotype is important to avoid their misclassification as natural killer cell malignancies. Furthermore, flow cytometry assists in the rapid identification of such cases, so that appropriate immunohistochemical studies can be performed to facilitate their correct diagnosis.

Figure 1

Flow cytometric dot plots of a metastatic small cell carcinoma of unknown primary (case 10), demonstrating CD56 expression in the absence of CD45 expression. The lymphoid (red) and small cell carcinoma populations (green) were colour backgated from the log 90° side scatter versus anti-CD45–PECY5 (phycoerythrin-cyanin 5 tandem) plot (see (C) below). (A) Log 90° side scatter (SS) versus forward scatter (FS): the lymphoid population (red) overlaps the small cell carcinoma population (green). (B) FITC (fluorescein isothiocyanate) versus PE (phycoerythrin) isotype controls: demonstrating the presence of significant autofluorescence/non-specific staining (blue) in this sample. (C) Log 90° side scatter versus anti-CD45–PECY5: the lymphoid population (red) is clearly CD45+ whereas the small cell carcinoma population (green) is CD45−, although autofluorescence/non-specific staining results in apparent weak CD45 expression. (D) Anti-CD3–FITC versus anti-CD56–PE: the small cell carcinoma population (green) is CD56+/CD3− (autofluorescence/non-specific staining again results in apparent weak CD3 expression) and clearly distinct from the lymphocytes (red). (E) Anti-CD56–PE versus anti-CD45–PECY5: the small cell carcinoma population (green) is CD56+/CD45− (apparent weak CD45 expression as a result of autofluorescence/non-specific staining), whereas the lymphocytes (red) are CD45+.