# Applications of Tumor Cells in an _In Vitro_ 3D Environment
## 7 Applications of 3D Models of Cancer Cells
### Immune Cells  
Given that the skin's immune component is the body's primary immune barrier and a fundamental source of virtually all skin diseases, considerable interest in incorporating it into 3D skin culture systems is highly anticipated [93; 111]. Undeniably, for a greater understanding of skin pathophysiology, the successful recreation of skin immunological functions is integral for mimicking native immune reactions, investigating key sensitization events, and modeling skin diseases [93; 94]. The skin's resident immune cells include dermal dendritic cells (DCs), Langerhans cells (LCs), macrophages, T cells, mast cells, and innate lymphoid cells (ILCs), and each skin layer possesses its own characteristic contingent of cells responsible for regulating immune responses [93; 94]. However, given the immense complexity of the skin as a biological barrier, comprised of diverse immune cell types involved in immunological responses, it is incredibly challenging to incorporate the entire immune component in hSEs. Thus, to date, most commercially available hSEs either lack the integrated immune component of the skin or focus on incorporating only a single immunological aspect of it [93; 94].  
To date, various approaches have been employed to integrate immune cells into 3D skin reconstruction models. The incorporation of LCs, which are antigen-presenting cells located in the _stratum spinosum_ of the epidermis, was the primary focus of the first skin immune-competent models that were developed [93; 111]. In 1997 and 2005, the [PERSON] group published two independent studies in which they reported a successful incorporation of cord-blood-derived LCs into a reconstructed human skin epidermis [112; 113]. However, given the lack of the dermal equivalent in their model as well as logistical limitations regarding the fresh cord blood supply and donor variation, the extensive application of the model was significantly restricted [111].  
With this in mind, [PERSON] et al. pushed ahead by incorporating MUTZ-3 cell-derived LCs, establishing the hSE VUmc model [103; 114]. MUTZ-3 cells are a human acute myeloid leukemia (AML) cell line that can be differentiated into LCs, known as MUTZ-LCs, through stimulation with the transforming growth factor \(\beta\) (TGF-\(\beta\)). This triggers MUTZ-3 cell pre-differentiation in MUTZ-LCs, indicated by the emergence of langerin, HLA-DR, and CD1a expression on the cell surface as well as the intracellular presence of Birbeck granules. Moreover, in response to stimulation by an allergen or irritant, the cells begin to resemble their native immune equivalents in the skin by expressing characteristic maturation markers, such as CCR7, CXCR4, and CD83, and acquiring the ability for CXCL12-induced migration [103; 111]. This model was then successfully employed as a standardized assay for irritant and sensitizer discrimination by investigating MUTZ-LCs' phenotypic plasticity and chemotactic migration into the dermis [111].  
Other 3D immune skin reconstruction models have also been developed, which contain melanoma cells, autologous T-cells, and fibroblasts that can be allogeneic [115]. This approach allows for the visualization and quantitation of T-cell-mediated tumor cell killing.