Mesenchymal Contribution to Recruitment, Infiltration, and Positioning of Leukocytes in Human Melanoma Tissues

To understand factors that regulate leukocyte entry and positioning within human melanoma tissues, we performed a multiparametric quantitative analysis of two separated regions: the intratumoral area and the peritumoral stroma. Using two mesenchymal markers, fibroblast activation protein (FAP) and CD90, we identified three subsets of mesenchymal cells (MCs): (i) intratumoral FAP+CD90low/− MC, (ii) peritumoral FAP+CD90+ MC, and (iii) FAP−CD90+ perivascular MC. We characterized CD90+ MCs, which showed a stable CCL2-secretory phenotype when long-term expanded ex vivo, and heavily surrounded peritumoral Duffy antigen receptor for chemokine+ (DARC) postcapillary venules, supporting a role for these vessels in peritumoral inflammatory leukocyte recruitment. Conversely, the intratumoral area was variably invaded by FAP+CD90low/− MCs that colocalized with a distinct extracellular matrix (ECM) network. A positive correlation was observed between intratumoral stromal cell/ECM networks and leukocyte infiltration among tumor cells (TCs), as well as in a stroma-dependent xenograft tumor model. Adoptively transferred T lymphocytes preferentially infiltrated tumors composed of TC+MC, compared with TCs only. Altogether, our results suggest that a variety of MCs contribute to regulate different steps of leukocyte tumor infiltration, that is, CD90+ cells surrounding peritumoral vessels secrete CCL2 to recruit CCR2+ leukocytes at the tumor periphery, whereas intratumoral FAP+ cells organize a stromal scaffold that contact guide further invasion among densely packed tumor cells.