Simple SummaryTesticular germ cell cancer (TGCC) is characterized by an extensive immune cell infiltration, which generates a pro-inflammatory tumor microenvironment (TME). The aim of this study is to compare the interaction of tumor cells representative of two major TGCC subtypes with the immune system in an in vitro coculture model. We found that the non-seminomatous cell line NTERA-2 lacks immunostimulatory capacity and even inhibits T cell and monocyte activity, thus sharply contrasting the opposing properties of the seminomatous cell line TCam-2. We hypothesize that different immunological characteristics of tumor cell subtypes may explain some of the clinical characteristics of TGCC.AbstractTesticular germ cell cancer (TGCC) is subdivided into several subtypes. While seminomatous germ cell tumors (SGCT) are characterized by an intensive infiltration of immune cells which constitute a pro-inflammatory tumor micromilieu (TME), immune cells in non-seminomatous germ cell tumors (NSGCT) are differently composed and less abundant. Previously, we have shown that the seminomatous cell line TCam-2 promotes T cell and monocyte activation in a coculture model, resulting in mutual interactions between both cell types. Here we set out to compare this feature of TCam-2 cells with the non-seminomatous cell line NTERA-2. Peripheral blood T cells or monocytes cocultured with NTERA-2 cells failed to secrete relevant amounts of pro-inflammatory cytokines, and significantly downregulated the expression of genes encoding activation markers and effector molecules. In contrast, immune cells cocultured with TCam-2 cells produced IL-2, IL-6 and TNFα, and strongly upregulated the expression of multiple pro-inflammatory genes. Furthermore, the expression of genes involved in proliferation, stemness and subtype specification remained unaltered in NTERA-2 cells during coculture with T cells or monocytes, indicating the absence of mutual interactions. Collectively, our findings uncover fundamental differences between SGCT and NSGCT in their capability to generate a pro-inflammatory TME, which possibly impacts the clinical features and prognosis of both TGCC subtypes.