Engineering approaches for studying immune-tumor cell interactions and immunotherapy (1)
Cancer immunotherapy includes several methods to boost the immune system’s response to cancer such as cytokines, vaccines, oncolytic viruses, adoptive cell therapy, and immune checkpoints. The tumor microenvironment consists of many elements that can influence diffusion of oxygen and nutrients, as well hindering efficient drug delivery and immune surveillance. It is shown that the anti-tumoral or pro-tumoral behaviors of some immune cell types and the microenvironmental conditions that affect tumor development and dissemination, including processes such as tumor cell killing, immune cell recruitment, angiogenesis, lymphangiogenesis, and tumor cell metastasis.
Engineered 3D in vitro systems have the advantage of using human cells and offer a complexity similar to in vivo while retaining tight control over physical parameters and cell composition. Therefore, 3D in vitro systems can be used to recapitulate the physiological state of the cells within a body, allowing the study of both chemical and physical signaling between cells in the TME. The abundance of tumor infiltrating lymphocytes correlates with overall survival. Innate immune cells play important roles in the constant fight against pathogens and in wound healing. However, in cancer, they are usually hijacked or functionally compromised by tumor cells. Microfluidic devices allow us to recapitulate selected aspects of the TME, while decoupling the effects of chemical and physical conditions to observe interactions and signaling between tumor and immune cells. Figure 2G summarizes several of the mechanisms covered by these in vitro studies that allow us to combine the imaging and quantification of lymphocyte migration, activation, and cytotoxicity with therapeutics to gain a deeper understanding of how immune cells and cancer interact, ultimately leading to the design of more effective treatments.
Using microfluidic 3D cell culture, several reports have demonstrated that the interplay between tumor cells and the TME can affect immune cell migration. One group used a microfluidic device to co-culture human pancreatic adenocarcinoma (Panc1) and blood monocyte-derived macrophages in separate gel chambers. The TAMs and tumor organoids were embedded in collagen I, within either the same channel or an adjacent gel channel of the microfluidic device. The metastatic site was modeled in a microfluidic device with a human dermal microvascular endothelial cell monolayer-coated channel sandwiched between two collagen gel channels. A multilayered, open-top, suspended, microfluidic layers of 2D and 3D cell cultures allowed the assembly and separation of microfluidic chambers. One media channel coated with C166 mouse endothelial cells and loaded the other with Raw 264.7 macrophages and GL261 or CT-2A glioblastoma cells that were subsequently allowed to migrate into the gel. This enabled signaling between endothelial cells and macrophages via cytokines such as TGF-β and IL-1 that induced angiogenic sprouting toward the tumor cells and macrophages.
Activation of T cells and cytotoxicity
Three-dimensional systems for assessing T cell activation typically combine cancer cell spheroids and T cells, either in microwell plates or embedded in a hydrogel such as Matrigel or collagen. Three-dimensional, co-culture approaches to studying T cell activation in the context of cancer are valuable because of differences observed between activation in 2D versus 3D culture environments, with 3D culture more faithfully recapitulating in vivo cellular responses. In this model, which combined melanoma spheroids and primary T cells in microwell plates, the authors observed that most T cells remained around the periphery and did not invade the spheroids. These studies, in particular, highlight the need for in vitro experimental systems that are more representative of in vivo systems.
Patient-derived organotypic models contain cellular material and tissues from a single individual biopsy or surgical resection specimen. Organoids from patient tissue are typically treated in one of two ways, either using the air-liquid interface method or fully embedded in hydrogel. Studies in ex vivo murine organotypic models have demonstrated that they can recapitu- late in vivo drug sensitivities, lending credibility for performing similar studies with human organotypic models. Cytokine profiling of conditioned media in murine organotypic models was also able to identify robust features of response to immune checkpoint blockade (PD-1), suggesting that a similar approach could be used to assess response to checkpoint inhibition in patient-derived organo- typic models.
Three-dimensional, in vitro models of the tumor microenvironment must faithfully capture the reality of hu- man, in situ cancers to be relevant tools in cancer research. These complex in vitro systems such as micro- fluidic models possess many advantages, including the ability to observe and quantify cell motion through time, non-destructive sampling (such as cytokine analysis), as well as control over matrix stiffness and composition, interstitial flow, chemical gradients, and hypoxia. As our capability to mimic the immune microenvironment of cancer improves through the development of increasingly realistic and sophisticated 3D models, more researchers will begin to combine these approaches with other techniques such as single-cell sequencing or advanced microscopy methods, lending even greater clarity to our understanding of immune-tumor cell interactions and immunotherapy and paving the way for translation of these methods into the clinic.