SURFACE PHENOTYPE AND FUNCTIONS OF TUMOR-INFILTRATING DENDRITIC CELLS- CD8 EXPRESSION BY A CELL SUBPOPULATION

Although the function and significance of tumor-infiltrating dendritic cells (TIDC) in the immune response to tumor have never been clearly demonstrated, their location suggests that they play a critical role i n the presentation of tumor antigen to specific T cells. We studied th e morphological and functional characteristics of interstitial dendrit ic cells (DC) located inside tumors obtained by injection of cancer ce lls into syngeneic rats. Single and double immunostaining of tumor sec tions revealed a dense network of cells which expressed class II major histocompatibility complex (MHC II) molecules. Cell morphology and su rface markers were characteristic of DC populations in other tissues. These DC were in close contact with tumor cells and increased in numbe r as the tumor grew larger. Unexpectedly, a subpopulation of morpholog ically characteristic TIDC expressed both CD8 and MHC II molecules. TI DC were purified from tumors by gradient centrifugation and immunobead s and characterized by morphology, ultrastructural study and surface m arkers studied by flow cytometry. TIDC were negative for the CD5 molec ule (a pan T cell marker), and were not labeled with 3.2.3 monoclonal antibody (mAb) (an NK cell marker) or with Ki-M2R mAb (a macrophage ma rker). A subpopulation of TIDC expressed the CD8 molecule, confirming the in situ results. TIDC expressed high levels of class I and class I I MHC molecules and the adhesion molecule ICAM-1. This expression is c ompatible with effective antigen presenting function. Purified TIDC tr iggered rapid and high levels of proliferation of tumor-immune T cells in vitro, demonstrating the potential of these cells to constitutivel y process and present tumor-associated antigens.