In this study, 11 lung cancer cell lines and tissue of 89 NSCLC cases were immunohistologically examined for the expression of CD24 protein using a well-characterised monoclonal antibody. We found CD24 expression in 45% (five out of 11) of the cell lines: two adenocarcinoma cell lines (two out of six), two small cell lung cancer cell lines (two out of four) and one squamous cell cancer cell line (one out of one). This appears to match the data of Jackson et al
, who reported CD24 expression in SCLC cell lines and in some adenocarcinoma cell lines, although unfortunately no details were given for the NSCLC cell lines investigated (Jackson et al, 1992
). We found a higher incidence of CD24 expression (76% of cases) in NSCLC tissue. These different incidences of CD24 expression in cell lines and tissues might be explained in several ways. First, CD24-positive NSCLCs might be more difficult to culture as cell lines than CD24-negative tumours. Second, CD24 staining in tissue sections was often accentuated at cell–cell contacts, which are mostly missing in cell lines.
To our knowledge, this is the first study to examine the CD24 expression in a larger tumour sample of NSCLC. Methodologically, the usage of TMAs was very convenient to help processing a large set of cases. The validity of this approach was confirmed by comparison of the CD24 array staining results with matching conventional whole mount sections.
We found CD24 expression to be an independent predictor of shortened patient survival as evidenced by univariate and multivariate analyses in NSCLC. This raises the question of the biological function of CD24 in NSCLC. CD24 is expressed in a large variety of human tissues: physiologically, in developing or regenerating tissues (Figarella-Branger et al, 1993
; Shirasawa et al, 1993
; Poncet et al, 1996
; Cram et al, 1999
) and a few mature cell types such as, for example, granulocytes, keratinocytes (Redondo et al, 1998
) and renal tubules (Droz et al, 1990
). Pathologically, CD24 has been described in B-cell neoplasia (Pirucello and Lang, 1990
; Raife et al, 1994
; Lavabre-Bertrand et al, 1994
), renal cell carcinoma (Droz et al, 1990
), small cell lung cancer (Jackson et al, 1992
), nasopharyngeal carcinoma (Karran et al, 1995
), hepatocellular carcinoma (Huang and Hsu, 1995
), bladder carcinoma (Gromova et al, 1999
), glioma (Senner et al, 1999
), breast cancer (Fogel et al, 1999
; Liu and Vadgama, 2000
) and ovarian cancer (Welsh et al, 2001
; Kristiansen et al, 2002
). This ubiquitous expression obviously excludes a diagnostic use of CD24 as a specific marker for NSCLC or any other tumour type.
CD24 has been identified as a ligand to P-selectin (Sammar et al, 1994
), and it is conceivable that this function contributes to a more aggressive metastatic behaviour of CD24-positive tumour cells, as in vitro
evidence suggests (Aigner et al, 1995
; Friederichs et al, 2000
). P-selectin is a surface molecule expressed by activated endothelial cells and platelets (McEver, 1991
). It plays an important role in marginal adhesion and migration of cells under shear forces in the blood stream. Moreover, P-selectin deficiency has been linked to decreased rates of metastasis formation in mice, which underscores the relevance of this adhesion receptor in tumor progression (Kim et al, 1998
). Physiologically, its primary ligand is P-Selectin Glycoprotein Ligand-1 (PSGL-1), which is expressed, for example, by neutrophils. Possibly, CD24-expressing tumour cells can spread more easily because of their capacity to either form thrombi with activated platelets or to adhere to endothelia in the bloodstream as demonstrated for CD24-expressing breast cancer cells (Aigner et al, 1997
). This mechanism of metastasis does not only require expression of CD24 of the tumour, but also expression of P-selectin in platelets and endothelia of the vasculature of the target organs. Obviously, P-selectin expression on platelets and endothelia of lung cancer patients can hardly be studied comprehensively in vivo
. Importantly, P-selectin-mediated binding of SCLC to endothelial cells has already been shown in vitro
(Pottratz et al, 1996
) and the CD24/P-selectin pathway was found to initiate lung colonisation of the human lung adenocarcinoma cell line A125 in a mouse model (Friederichs et al, 2000
These observations are consistent with the view that CD24 is a cell surface protein that might be important for haematogenous metastasis formation, and we feel that this is the most likely explanation of the decreased survival times of strongly CD24- expressing tumours because most lung cancer patients die from systemic metastatic disease. In our study, clinicopathological parameters were only considered at the time of surgery. Thus, we did not find an association of CD24 with clinical stage since patients with haematogenous metastases are excluded from surgical therapy and are consequently not represented in our tumour sample. Also, the nonsignificant association of CD24 expression with nodal status is compatible with this interpretation, since lymphatic spread is a biologically different process, although it frequently precedes haematogenous dissemination.
This study establishes CD24 expression as an independent prognostic tumour marker in NSCLC. Our recent description of CD24 expression as an independent marker of shortened patient survival in epithelial ovarian cancer (Kristiansen et al, 2002
) further underscores the importance of CD24 in metastatic disease progression of human carcinomas. Prospectively, CD24 might aid the clinician in the selection of an appropriate therapy for individual patients. In this circumstance, it is of relevance to know that the intravenous administration of CD24-specific antibodies has been sucessfully used therapeutically to treat transplantation-associated B-cell proliferative syndrome (Fischer et al, 1991
; Garnier et al, 2002
). This new therapeutic option might deserve consideration in NSCLC patients as well.