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To overcome the difficulty in obtaining quiescent monocytes , we studied the ability of promonocytic THP-1 cells to activate endothelial cells .
 endothelial cells, quiescent monocytes, promonocytic THP-1 cells
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: cell line, protein, RNA, DNA, cell type 
Text: {0} 
Answer:
 endothelial cells, quiescent monocytes, promonocytic THP-1 cells