MATERIALS AND METHODS

Immunohistochemistry for CD34 and αSMA was performed on serial sections taken from normal breast (n = 10), fibroadenoma (n = 10), infiltrating ductal carcinoma (n = 40), ductal carcinoma in situ (n = 55), and radial scar/complex sclerosing lesions (n = 20). A standard avidin biotin complex technique, without prior antigen retrieval, was used incorporating 10 minutes’ incubation in 60% H₂O₂ to block endogenous peroxidase. The primary antibodies were diluted in Dako antibody diluent (Qbend-10; Dako, Ely, Cambridgeshire, UK) at 1/100 for CD34 and 1/600 for αSMA. Staining of blood vessels was used as an internal positive control for CD34 and of normal myoepithelial cells for αSMA. For each case, serial sections were also stained with haematoxylin and eosin and in a proportion of cases serial sections were stained with anti-CD31 (Dako) as a further marker of endothelial cells.

Fibroblast reactivity for CD34 and αSMA was recorded as positive, negative, and—where there was focal loss—as reduced. Because many of the sections contained ducts and lobules showing a range of histopathological features, the relation between staining pattern and pathology was interpreted for each duct–lobular unit. Thus, in addition to 40 infiltrating ductal carcinomas (IDCs) and 10 fibroadenomas, 301 normal duct–lobular units were scored, 535 ducts with ductal carcinoma in situ (DCIS; 60 low grade, 77 intermediate, and 398 high grade), six cases showing microinvasion, 88 duct–lobular units exhibiting usual type epithelial hyperplasia (HUT), six with atypical ductal hyperplasia (ADH), 12 lobular carcinoma in situ (LCIS), 11 apocrine cysts, and 24 radial scars, four of which contained areas of reactive fibrosis following preoperative core biopsy (table 1).

The relation between the staining pattern and the different histological features was compared using the χ² test, and p < 0.05 was considered significant.

RESULTS

In the normal breast, both intralobular and extralobular fibroblasts exhibited strong uniform staining for CD34 and were αSMA negative. This pattern was maintained in the presence of other pathology, including adjacent carcinoma; thus, in all 301 normal duct–lobular units fibroblasts were CD34 positive and, with the exception of 23 cases, αSMA negative (fig 1A,B). Similarly, fibroblasts around glands showing HUT, LCIS, or apocrine cyst formation were CD34 positive/αSMA negative (fig 1C,D), with the exception of focal αSMA positivity around nine of 79 ducts with epithelial hyperplasia. Fibroadenomas showed a homogeneous pattern of staining, with fibroblasts being strongly CD34 positive and αSMA positive. In contrast, all IDCs and foci of microinvasion exhibited consistent loss of CD34 expression on fibroblasts, but acquisition of αSMA (fig 1E,F). Discrete staining for CD34 was evident on small calibre blood vessels within the stroma, confirmed by their expression of the vascular antigen CD31.

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Figure 1

Fibroblasts are CD34 positive around (A) normal breast ducts and lobules and (C) hyperplasia of usual type, but are negative for α smooth muscle actin (αSMA) (B, D). Small calibre blood vessels are also CD34 positive (arrowhead in A and B). In invasive carcinomas, peritumorous fibroblasts are uniformly (E) CD34 negative and (F) αSMA positive.

The most variable pattern of expression was seen on fibroblasts surrounding ducts with DCIS; however, the loss of CD34 expression was significantly more frequent in high nuclear grade compared with low or intermediate grade samples (p < 0.001). In high grade DCIS, fibroblasts were CD34 negative in 78% cases compared with 29 of 77 (33%) intermediate and 21 of 60 (35%) cases of low grade DCIS (table 1; fig 2). In some cases, partial loss of CD34 was evident around a duct. The incidence of reduced staining was again variable between the grades; however, only 12% of cases of high grade DCIS retained normal CD34 expression compared with 22 of 77 (29%) intermediate grade and 28 of 60 (47%) low grade cases. Staining of serial sections of 20 of these cases with CD31 demonstrated clearly the pattern of small blood vessel formation around the ducts, and these vessels were easily distinguishable from the fibroblast population. In most cases, loss of CD34 expression was accompanied by the acquisition of αSMA; however, this relation was not absolute, with gain of αSMA being more frequent than loss of CD34 (table 1), and only 27% of periductal fibroblasts in low grade DCIS were αSMA negative, compared with 47% retaining expression of CD34. Again, a mixed pattern of staining was evident in relation to ADH, with a proportion of CD34 negative fibroblasts in half the cases; however, in none of the six cases was there evidence of αSMA staining.

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Figure 2

In ducts displaying features of high grade DCIS, 78% were associated with (A, C) CD34 negative periductal fibroblasts and (B, D) the acquisition of α smooth muscle actin (αSMA). (C) Black arrows indicate retained CD34 on small blood vessels in contrast to the fibroblasts (white arrowhead). In low grade DCIS, (E) 47% of duct–lobular units retained CD34 on periductal fibroblasts with (F) 27% remaining αSMA negative; however, 35% of low grade DCIS ducts showed a similar expression pattern to high grade DCIS, with (G) loss of CD34 (arrows indicate CD34 positive blood vessels) and (H) gain of αSMA.

In the radial scars, most fibroblasts were CD34 negative/αSMA positive (fig 3), although some cases displayed CD34 positive/αSMA positive fibroblasts at the periphery of the lesion. The pattern was similar in those radial scars admixed with DCIS (n = 4) and those not associated with malignant change (n = 20). Interestingly, in four cases, areas of reactive fibrosis were evident from preoperative core biopsy, and in these sites the fibroblasts were uniformly CD34 negative/αSMA positive (fig 3).

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Figure 3

The predominant pattern in radial scars was (A, C) loss of CD34 expression associated with (B, C) acquisition of α smooth muscle actin (αSMA) expression. Strong αSMA expression was also evident around the glandular structures in relation to myoepithelial cells (arrow in D). In four radial scars, reactive fibrosis relating to previous core biopsy was present, and in these areas the spindle cells were uniformly (E) CD34 negative and (F) αSMA positive. Inserts in (E) and (F) are high power magnifications (×400) of the central area of fibrosis.

DISCUSSION

CD34 is a transmembrane glycoprotein that is thought to be involved in the modulation of cell adhesion and signal transduction,²⁸ and is expressed by mesenchymal cells at several sites, including the normal mammary stroma.^30,33 Loss of CD34 by mesenchymal cells has been described in several situations where there is malignant transformation of the mesenchymal population. Malignant phyllodes tumours of the breast exhibit lower levels of CD34 expression than benign phyllodes or fibroadenomas,^24,33,35 and CD34 is lost in sarcomas arising within CD34 positive dermatofibrosarcoma protruberans.³⁶ Loss of fibroblast CD34 has also been described in non-neoplastic fibroblasts around epithelial tumours, such as basal cell carcinoma³⁷ and colorectal carcinoma.³⁸

“Loss of CD34 may be related to invasive potential”

Our study has shown that fibroblast CD34 expression is consistently lost in invasive breast carcinomas that include microinvasion, and in a high proportion of cases of DCIS, particularly high grade lesions, which are thought to be more likely to progress to invasion.³⁹ Loss of expression is also seen in a proportion of ADH, but not around glandular structures showing LCIS. This is of interest because ADH and DCIS are regarded as premalignant lesions, whereas LCIS is considered to confer an increased risk of developing carcinoma, but the risk relates to the development of carcinoma in either breast, not to the site of the LCIS.⁴⁰ This raises the possibility that loss of CD34 may be related to invasive potential. The change in CD34 expression is very localised, with loss around a duct containing DCIS, but retained expression around adjacent normal breast glands, and this strongly implicates epithelial–mesenchymal interactions in the control of expression, as has been suggested previously in the case of phyllodes tumours.³³ What determines the loss of CD34 is of interest because not all DCIS cases show loss, and this may point to different functional states of the neoplastic epithelium, which are important in determining their invasive potential. However, whereas it has previously been suggested that the loss of CD34 is specific to malignancy,⁴¹ we show that this is not the case. Although most benign lesions, including HUT, retain CD34 expression, loss of CD34 was a consistent finding in the fibroblasts associated with areas of fibrosis following core biopsy and in radial scars; in this last case, whether they were associated with malignancy or not. Although on the surface this makes CD34 less valuable as a diagnostic marker of malignancy—particularly in sclerotic lesions—it raises interesting questions as to the state of differentiation of fibroblasts in different breast lesions, and the potential relevance of this in terms of fibroblast function. Some studies suggest that radial scars are associated with an increased risk of development of malignancy,⁴² and several other studies have shown similar alteration in the stroma of radial scars as seen in invasive carcinomas, such as increased hyaluronic acid⁴³ and increased expression of ED-A fibronectin and vascular endothelial growth factor.⁴⁴ The importance of loss of expression of CD34 on stromal cells in reactive fibrosis is unclear, although it could possibly be related to the terminal differentiation of these cells.

“It is essential that the changes in fibroblast phenotype associated with malignancy are carefully dissected, not only to validate their use as possible diagnostic markers, but also to establish their potential as therapeutic targets”

The importance of changes in fibroblast function in promoting tumour progression is increasingly recognised and such a tumour promoting effect is frequently attributed to the activation of fibroblasts to αSMA positive myofibroblasts. However, our study clearly demonstrates that acquisition of the myofibroblast phenotype is not indicative of malignancy in the breast, and it also demonstrates that not all myofibroblasts have the same phenotype, some being CD34 positive, as in fibroadenomas, and others being CD34 negative, as in invasive and many in situ lesions. The potential for the stromal response to tumours becoming a target for treatment is increasing, with clinical trials in place for fibroblast derived factors such as MMP⁴⁵ and Tenascin.⁴⁶ However, it is essential that the changes in fibroblast phenotype associated with malignancy are carefully dissected, not only to validate their use as possible diagnostic markers, but also to establish their potential as therapeutic targets.

The functional relevance of fibroblast CD34 expression and its loss is not clear, although it may represent a change from a multipotent mesenchymal cell to a committed cell type. However, the variable association between CD34 and αSMA expression demonstrates that not all myofibroblasts are the same, and that the loss of CD34 appears to be more closely related, although clearly not specific, to malignancy than the acquisition of αSMA. This suggests that αSMA may not be the most important marker of fibroblast function, and in vitro studies are now indicated to establish the role of CD34 in the breast fibroblast.