Tumor-associated fibroblasts recruit blood monocytes into tumor tissue.

Tumor-associated fibroblasts (TAF) and tumor-associated macrophages are the main stromal components in desmoplastic breast tumors. These host cell types were extensively studied individually with regard to tumor development and progression but little is known about their reciprocal interactions. To elucidate the role of TAF in the recruitment of monocytes (MO) we designed a 3D co-culture system of multicellular fibroblast spheroids of different origin, co-cultured with MO suspensions from healthy donors. Spheroids of tumor-derived but not of normal fibroblasts were extensively infiltrated by MO. A linear correlation between number of infiltrated MO and number of MO applied per spheroid was shown, indicating a distinct migratory MO subpopulation (approximately 15%) within the peripheral blood MO pool. Our data imply that MO migration into fibroblastic tumor areas may partially result from high expression of CCL2 (monocyte chemotactic protein-1), which was regulated by endogenous IL-6 as shown by neutralization experiments. The effect of CCL2 on MO migration was inhibited by CCL2 neutralizing antibody in tumor-derived fibroblast conditioned media in a Boyden chamber migration assay but not in spheroid culture. While this phenomenon needs further evaluation, our data clearly support the concept of fibroblasts as "sentinel cells" relevant for tumor progression.