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So called costimulatory signals , mediated by other cell surface interactions or soluble cytokines produced by antigen presenting cells , are also required for complete T cell activation .
 soluble cytokines, antigen presenting 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:
 soluble cytokines, antigen presenting cells