Integrated cancer tissue engineering models for precision medicine

Abstract

Tumors are not merely cancerous cells that undergo mindless proliferation. Rather, they are highly organized and interconnected organ systems. Tumor cells reside in complex microenvironments in which they are subjected to a variety of physical and chemical stimuli that influence cell behavior and ultimately the progression and maintenance of the tumor. As cancer bioengineers, it is our responsibility to create physiologic models that enable accurate understanding of the multi-dimensional structure, organization, and complex relationships in diverse tumor microenvironments. Such models can greatly expedite clinical discovery and translation by closely replicating the physiological conditions while maintaining high tunability and control of extrinsic factors. In this review, we discuss the current models that target key aspects of the tumor microenvironment and their role in cancer progression. In order to address sources of experimental variation and model limitations, we also make recommendations for methods to improve overall physiologic reproducibility, experimental repeatability, and rigor within the field. Improvements can be made through an enhanced emphasis on mathematical modeling, standardized in vitro model characterization, transparent reporting of methodologies, and designing experiments with physiological metrics. Taken together these considerations will enhance the relevance of in vitro tumor models, biological understanding, and accelerate treatment exploration ultimately leading to improved clinical outcomes. Moreover, the development of robust, user-friendly models that integrate important stimuli will allow for the in-depth study of tumors as they undergo progression from non-transformed primary cells to metastatic disease and facilitate translation to a wide variety of biological and clinical studies.

Fig 1. Components of the ‘Cancer-Organ’ model.

To develop an accurate multi-dimensional understanding of the structure, organization, and complex relationships in cancers, we need to consider the following factors. Heterogeneous cancer cells reside in a complex tumor microenvironment, which consists of mechanical stimuli, non-malignant cell-cancer cell interactions, soluble signals, and extracellular matrix (ECM). The dimensionality of cell culture influences cancer cell motility and cellular interaction with the surrounding cells and ECM. Mechanical stimuli including shear, compressive, tensile, and viscoelastic forces, dynamically influence cancer cells as the tumor grows. Similarly, cellular interactions through direct contact with surrounding non-malignant cells and soluble signals alter communication and downstream signaling. Interactions between immune cells and cancerous cells are highly complex and can lead to immune evasion and support of tumor progression. All of these characteristics play an integral role in tumor progression and are critical to forming a complete picture of the ‘cancer-organ’ system.

Fig 2. Various engineering tools can help construct the complex picture of the ‘cancer-organ’ system.

Summarized here are the state-of-the-art cancer bioengineering models that we discuss in this review. Each model has inherent benefits and drawbacks that are discussed in more detail within the following sections. We have listed the components of the ‘cancer-organ’ system which can be probed with the specific model in the figure.

Fig 3. Various cell-cell interactions within the cancer-organ system.

Interactions of cancer and malignant cells with their surroundings help dictate their survival and phenotypes. Within the homeostatic non-transformed microenvironment, various cell-cell junctions are formed ensuring the proper polarization, orientation, and proliferation of the non-malignant cells. Cell-ECM interactions provide structure and mechanical stimuli to the cellular surroundings through points of adhesion. These native interactions are disrupted by the infiltrating cancer cells which interrupt cell-cell communications and displace healthy tissue. The cancer cells undergo the epithelial-mesenchymal transition in order to metastasize and do not experience the same proliferative inhibition provided by non-malignant cell-cell communication. Well-established communication between cancerous cells increases survival by avoiding anoikis and promoting chemoresistance. Finally, the surrounding ECM, which is stiffened by the presence of the expanding cancer mass, aids in additional cancer cell migration, and an altered mechanical environment will feed forward the progression of the disease.

Fig 4. The immune microenvironment of tumors contains cellular components from both the innate and adaptive immune systems, with functional immuno-modulation between all the different cell types.

Macrophages are typically the most abundant population of leukocytes within the TME, derived from both tissue-resident and circulating monocytic progenitors. The accumulation of tumor-associated macrophages is often correlated with the development of pathological phenotypes in cancer, which leads to the promotion of angiogenesis, metastasis, chemoresistance and functional suppression of adaptive immunity. The TME counterbalances activating natural killer (NK) cell signals with strong inhibitory signals to escape NK cell mediated immune surveillance and further reduce the phagocytic activity of NK cells. NK cells also exhibit functional anergic phenotypes with reduced phagocytosis and reduced amounts of cytoplasmic granules that contribute to tumor progression. Other granulocytes within the TME often recruited from circulating vasculature include neutrophils, basophils, eosinophils and mast cells. Tumors often experience reduced recruitment, but granulocytes are often re-programmed to a pro-tumor phenotype, promoting vascular normalization and stromal remodeling. Analysis of several solid tumors also indicate that they are infiltrated with T-cells and B-cells, recruited from circulating blood and lymphatic structures. The number of infiltrated T-cells offer significant prognostic value to cancers. However, the TME reprograms T-cells into an exhausted anergic state, leading to severe immune suppression, specifically of the Th and CTL (CD8+ cytotoxic T lymphocytes) phenotypes. Additionally, recruited naive T-cells are also converted to an insidious regulatory Treg phenotype, which contributes to suppressive immunomodulation. B-cells typically respond to tumor-derived antigens and elicit antibody responses through IgM secretion and direct stimulation of Th cells. Tumor-educated B-cells are immuno-suppressive, promote regulatory T-cells, and promote carcinogenesis. Myeloid derived suppressor cells are heterogeneous mixes of immature myeloid cells, found accumulated in lymphoid structures, blood, and the TME, and are heavily correlated with immune suppression. Myeloid derived suppressor cells are powerful inactivators of T-cells. Impaired myeloid differentiation also results in defective antigen presentation. Coupled with dysregulated T-cell priming by antigen presenters like dendritic cells, an overall immune suppressive landscape leads to tumor escape from immune surveillance.