Engineering novel biomaterials to treat autoimmune disease

Shadab Hassan

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Feb 25, 2017 · 14 min read

Concepts of Immunoengineering through the use of biomaterial adjuvants

Immunoengineering is a convergent field of science that aims to modulate the responses of the immune system using the various quantitative tools available for an engineer. The goal to bring about innovative immunotherapies that can be used to treat autoimmune disorders. This concept will rely on the relationship between antigen-presenting cells, notably the dendritic cells, with B and T cells in order to induce immunity against the target autoimmune disease. One of the key tools available to engineer such solutions are biomaterials, in the form of vaccine adjuvants, as it plays a critical role in controlling the immune cells. [1] When engineering a novel biomaterial for treatment, there are several physical and chemical variables to take into consideration to ensure that these biomaterials can activate the receptors efficiently and transport its antigens safely.

As mentioned, one of the most quantitative tools in the Immunoengineering toolbox are adjuvants. These are materials that cannot induce an adaptive immune response by itself, however can augment the response in conjunction with a specific antigen. Adjuvants made of biomaterials have been studied for its property of being able to modulate immune responses to a desired degree, making a potential therapeutic option to treat not only autoimmune disease, but other chronic diseases such as HIV as well. [1] [2]

Autoimmune Disease

The immune system is responsible for protecting the body against various pathogens. However, there are times when the immune system will attack healthy cells in the body by mistake. Such occurrences are referred to as autoimmune diseases. The specific triggers that causes the immune system to generate antibodies that attack these healthy cells are unknown. The early symptoms of an autoimmune disease are fatigue, muscle aches, inflammation, and low fever.

To be classified as autoimmune, the disease has to show tissue damage due to an adaptive immune response to autoantigens. These responses are intervened by autoreactive lymphocytes, and can also include pro-inflammatory cytokines, along with autoantibodies that target tissue and cause inflammation. As mentioned before, the specific triggers for many autoimmune diseases are not known. There are have been trends in genetic risk factors, more specifically, certain alleles of MHC class II molecules, however it is not a positive correlation. There is a noticeable trend in that autoimmune diseases such as Type 1 diabetes have a genetic factor. Demographics also seem to play a factor. Women, more specifically, African-American, Hispanic-American, and Native-American women, have a higher risk factor for some of these autoimmune diseases. [3] [4]

Throughout the 80+ types of autoimmune disease, there are two recognized categories of autoimmune disease: “organ-specific” and “systemic”. Organ-specific autoimmune diseases refer to when autoantigens from one or a select few organs are targeted by the immune system. A common example of this is Type I diabetes. Cytotoxic T-cells or autoantibodies formed by the immune system attack the insulin-producing pancreatic cells known as the islets of Langerhans, which can be seen in Figure 1. [5] Systemic diseases Systemic diseases affect a broad range of tissues. An example of this is systemic lupus erythematosus, which creates inflammation in multiple tissues due to the autoantigens it creates (such as chromatin) are prevalent throughout the body. In most cases, many of these diseases reach a chronic state because the autoantigens produced can never be extinguished from the body. [4] [5]

Treatment using biomaterials

The current method to treat autoimmune diseases is to use immunosuppressive drugs which weaken the immune system so that it does not attack itself. The downside to this is that it also weakens to system to fight off actual pathogens, making the user more susceptible to sickness. Immunomodulatory therapies which target the immune receptors or cytokines responsible for the disease are another option that allow for a more direct approach. However, this therapy cannot be used long term due to a rapid depletion of B and T cells with the receptors carrying health risks. In the short term, this therapy aims to disrupt the negative signaling pathway or by increasing the regulatory capacity of regulatory T-cells. [6] A different way to go about treating various autoimmune diseases is through the use biomaterials. It is in important tool for an engineer as it can exploit the immune systems biological and mechanical signals in order to control immune cells. Biomaterials can alter and retrain these immune cells towards their response to a specific phenotype of interest. The end goal would be to modify and control the innate and adaptive immune responses using these biomaterials as a new therapeutic option for treatment. This is making headway into a relatively new field known as Immunoengineering, which uses quantitative engineering tools to study the immune system at the molecular, cellular, and system level. This promising field hopes to alter the host-material relationship to our advantage. The main challenge will be engineering novel biomaterials to influence these immune cells in a controlled matter, as it requires an intricate design that takes into account of the physio-chemical properties of the selected biomaterial and the environment of the immune system. Figure 2 outlines how biomaterials can be used control the immune system using a specific antigen. Using this data, new or enhanced therapeutic options can be developed to alter and control immune responses. Through the shortcomings of traditional immunotherapy options, Immunoengineering is becoming a viable alternative for treatment. [1]

The approach that needs to be taken in order to treat autoimmune diseases is to exploit the immune systems biological and mechanical signals in order to control its immune cells. The various parameters for a biomaterial is critical in order to bring about these interactions and to optimize the response from the immune system. Autoimmune disease result from an unbalanced immune system. Dendritic cells are an essential part of this system as it initiates an immune response in response to pathogens, which then promotes the activation and differentiation of naïve T-cells. Dendritic cells control what subtype these T-cells differentiate into in order to best combat the invading pathogen. As such, dendritic cells also play a role in autoimmunity due to mistakes in self-tolerance, causing the creation of an immunosuppressive environment. It is critical that these mistakes are contained in order for the treatment to not backfire. Thus, the challenge for engineering biomaterials (in vaccine form or otherwise) is to ensure that it can diffuse across the tissue barriers to deliver its antigens to the dendritic cells. [4]

In the tissue microenvironment, biomaterial-based transporters such as polymeric nanoparticles, scaffolds, etc. can protect the antigen from the surrounding proteases and nucleases. In addition, it can send out both immune-stimulatory and immune-modulatory molecules (such as PAMPs) to the dendritic cells. These transporters are seen as the pathogen-mimicking particles that can safely deliver these antigens and PAMPs without inducing a negative response. It is imperative that the particles being delivered are taken up by the primary antigen presenting cells (APCs) and induce using PAMPs or other immune-stimulators. Because dendritic cells are phagocytic in nature, the biomaterial must be able to induce the phagocytic cells to target, activate, and move into the secondary lymphoid organs such as the spleen. Another method can be to design vaccines that can localize directly in the secondary lymphoid organs, specifically the lymph nodes. From there, it will present the specific antigens to residing dendritic cells. A practical example includes the use of artificial APCs composed of polymeric microparticles to present T-cell inducing ligands. [7] [8]

Biomaterial Adjuvants

Adjuvants have three main functional mechanisms: stabilizing the antigen to be presented, transporting this antigen, and inducing the innate immune response. It is important that the antigen remains stable long enough to communicate with the targeted immune cells as to create an immune response. Antigen transportation is important when the treatment plan involves dendritic cells. These cells possess pathogen recognition receptors on its surface that can recognize PAMPs. [9] Inducing the innate response is the most complicated of the three mechanisms. The immune response has two modules: an adaptive module, which reacts slowly to a pathogen, but is very prolific and can memorize its response, and an innate module, which acts much quicker, but cannot memorize the response. [10] A common side effect to using biomaterials is inflammation. Although the exact reasoning behind why this occurs is not known, researchers believe that this inflammation is of a similar response by the immune system during an innate response, such as that from an autoimmune disease. The hypothesis is that biomaterials are identified by the receptors of the innate immune cells. There is a possibility that the surface of biomaterials can imitate PAMPs, which communicate with pattern recognition receptors for innate immunity. [9]

Poly(lactide-co-glycolide) (PLGA) is a very popular choice of biomaterial as it has been shown to augment the efficiency of vaccines. [11] It is composed of two monomers: lactide and glycolide, created by the process of ring-opening polymerization. It has garnered much attention as a material which has controlled degradation rates and strong mechanical properties. [12] PLGA has been shown to upregulate CD40, 80, 86 along with MHC-II on dendritic cells. [13] Adjuvants made of Chitosan are also a viable choice. It is a porous derivative of chitin that has enhanced liquid absorbing capacity and cell interaction. It is biocompatible, biodegradable, nontoxic, and possesses antibacterial capabilities. [12] While there is a plethora of biomaterials to choose from, the choice of material is just the first step in the design process. From there, the various shape, size, and chemical components of the biomaterial need to be decided, and all play a factor in generating an immune response.

Biomaterial Design

The first parameter that needs to be considering when engineering a novel biomaterial is particle size. The size of both nano- and microparticles are an essential component in the design process as it effects how long it can retain in the system, and how well it can interact with the surface receptors of the dendritic cells, along with how well it can target the secondary lymphoid organs. Large particles (typically 200nm — 10µm) can be effectively phagocytosed by the dendritic cells, and then be able to be transported to the native lymphoid tissue and present its antigens to the T-cells. [8] Smaller particles (less than 100nm) have the ability to enter the lymphatic system directly and be transported to the resident dendritic cells in the secondary organs. [1] Particle size also plays a role in how well the antigen can influence the stimulation of toll-like receptors. These receptors are expressed in the immune system to identify PAMPs for pathogens to invade and infect host cells. [14] There have been prior studies of adding toll-like receptor ligands to vaccines to influence an innate immune response. It was shown that dendritic cells that have been activated with various toll-like receptor ligands will prompt different T-cell reactions. The goal of this is augment the effectiveness or control the immune response. Despite the potential use of biomaterials to control the toll-like receptor pathways, there has not been enough definitive studies for its use and efficacy. [15] Chen et al. pursued two parameters in material design: particle size and endosomal pH buffering, which could possibly link with TLR9-mediated innate immunity. The biomaterial composed of both micro and nano polystyrene particles which had an absorbed TLR9 agonist. The results showed that it was able to prompt a differentiated specific TLR9 cytokine reaction. The larger microparticles were shown to have a preference of secreting IL-6, whereas the nanoparticles were able to proliferate both IL-6 and IFN-a. [16] Further studies need to be performed that examine how the type of biomaterial and the antigen it is carrying affects the innate immune response. One of the challenges in inducing immunity is that the presented antigen going through the MHC-I pathway are handled within the cytosol. Along with endosomal pH buffering, endosomal escape is also something that needs to be considered. As PLGA has a good rate of degradation, which acidifies the endosome and causes a rupture. This is one of the likeliest reasons why PLGA has cross-presentation capabilities, meaning that it can present MHC-I molecules to cytotoxic T-cells. [17]

The second parameter to be considered is the shape of the particles. Agarwal et al. experimented with the shape and saw that bone marrow-derived dendritic cells take up large particles shaped as discs and rods more effectively than smaller-sized particles. [18] Petersen et al. provided data which showed that spherical polyanhydride nanoparticles induced a more significant activation of dendritic cells, compared to regular polyanhydride film by looking at the secretion of IL-12 and the upregulation of MHC-II. [19] Despite the promising trend, the exact relationship between the geometry of the particles and the response from the immune system is for the most part, not experimented. Various in vivo experiments with silicon particles showed a trend that discoid particles tend to concentrate in the lungs and heart, whereas cylindrical particles concentrated in the liver. The diameter of the particles also plays a role in the level of interaction between particles and the transport chain of specific organs. Spherical and cylindrical shaped particles tend to be easily taken up by cells, whereas discoidal-shaped cells had a lower rate of absorption. The activation of T-cells is a critical step in the immune response, and this is achieved by the organization and accumulation of ligands of the immunological synapse. [20] Steenblock et al. used spherical PLGA artificial antigen presenting cell systems to show proficient polyclonal and antigen-specific T-cell proliferation. [21] Babansee et al. also used PLGA by exposing thin films of it to dendritic cells. This in vitro experiment showed that the PLGA caused the cells to mature by upregulating MHC-II, CD-40, 80, and 86. [13] A possible hypothesis for the dendritic cells maturing is that the biomaterial serves as “danger signals” when used in conjunction with its antigen. The signal is created in response to an inflammatory reaction from the biomaterial. This signals the appropriate toll-like receptors for dendritic cell maturation. [9]

A third parameter which plays a pivotal role in modulation is the surface chemistry of the biomaterials, specifically its ionic charge and whether it is hydrophobic or hydrophilic. On a hydrophilic surface, the microparticles are absorbed into the cell. On the other hand, for the hydrophobic surface, microparticles are processed through a “zipper” system which entails the dendritic cells to transport its pseudopods once a connection has been made with the microparticle. Although the exact reason for the strong connection between the cell and the biomaterial’s surface, there are three possible explanations: the attraction between the negatively charged cell membrane to the positive charge of the microparticle, hydrophobic communications, or receptor-ligand connections. The zipper system has been shown to induced by hydrophilic surfaces that have been coated with positively charged polyamines such as polyethyleneimine, however the fate of the microparticles afterwards have been debated. [9] [22]

While the geometry and surface properties of the biomaterial have been closely examined for its apparent relationship in inducing an immune response, the structural properties must be considered as well. The porosity of and rate of degradation are important factors to consider when looking at dendritic cell migration. Large pore size and high degradation rates have been show to influence dendritic cells to move deeper in the presence of a chemokine released from a scaffold or hydrogel. In this area, naïve dendritic cells can be stored and be activated once an antigen has been presented and the danger signal causes it to return into the lymph nodes. [23]

Conclusion

The use of biomaterials to alter and control an immune response is a rapidly evolving field with potential to create therapeutic applications for autoimmune diseases. The use of vaccines as a container for multiple antigens, toll-like receptor ligands, chemokines, cytokines, etc. are being heavily investigated as a therapeutic treatment option. Through the concept of pathogen-mimicking, both micro- and nanoparticles have the ability to present antigens and PAMPs to dendritic cells, and augment antigen-specific responses from the immune system. The various parameters of these particles, such as size, geometry, surface and structural properties, along with choice of material, are all important aspects to consider when trying to optimize the innate immune response towards an antigen of interest. The use of synthetic hydrogels and scaffolds also supplements the response and can defend the particles from pathogens by increasing the rate of migration for the dendritic cells, and acting as a storage chamber until it is activated by a given antigen. By modulating the regulatory T-cell responses, there is much hope that this concept can be used to fight back against autoimmune diseases. Despite the amount of potential, the field of Immunoengineering is still in the infant stages, as much more understanding of how the engineering particles interact with immune cells is required.

References

1. Purwada, Alberto, Krishnendu Roy, and Ankur Singh. “Engineering Vaccines and Niches for Immune Modulation.” Acta Biomaterialia 10.4 (2014): 1728–740. Web.
2. Gupta, Rajesh K., and George R. Siber. “Adjuvants for Human Vaccines — current Status, Problems and Future Prospects.” Vaccine 13.14 (1995): 1263–276. Web.
3. “Autoimmune Diseases: What Are They? Who Gets Them?” WebMD. WebMD. Web. 19 Apr. 2016.
4. Murphy, Kenneth, Paul Travers, Mark Walport, and Charles Janeway. Janeway’s Immunobiology. New York: Garland Science, 2014. Print.
5. DiResta, G. Lecture 10: Endocrine System & Control Mechanisms — II. Anatomy, Physiology, and Biophysics II (April 2016)
6. Verbeke, Catia Stéphanie. “Antigen-specific Immune Modulation Using an Injectable Biomaterial.” (2014). Doctoral dissertation, Harvard University. Web.
7. Sunshine, Joel C., and Jordan J. Green. “Nanoengineering Approaches to the Design of Artificial Antigen-presenting Cells.” Nanomedicine 8.7 (2013): 1173–189. Web.
8. Jilek, S., H. Merkle, and E. Walter. “DNA-loaded Biodegradable Microparticles as Vaccine Delivery Systems and Their Interaction with Dendritic Cells.” Advanced Drug Delivery Reviews 57.3 (2005): 377–90. Web.
9. Jones, Kim S. “Biomaterials as Vaccine Adjuvants.” Biotechnol Progress Biotechnology Progress 24.4 (2008): 807–14. Web.
10. Matzinger, P. “The Danger Model: A Renewed Sense of Self.” Science 296.5566 (2002): 301–05. Web.
11. Eldridge, J.h., J.k. Staas, T.r. Tice, and R.m. Gilley. “Biodegradable Poly(dl-lactide-co-glycolide) Microspheres.” Research in Immunology 143.5 (1992): 557–63. Web.
12. Atala, Anthony, and James J. Yoo. “Polymers for Bioprinting.” Essentials of 3D Biofabrication and Translation: Print.
13. Babensee, Julia E., and Abhijit Paranjpe. “Differential Levels of Dendritic Cell Maturation on Different Biomaterials Used in Combination Products.” Journal of Biomedical Materials Research Part A J. Biomed. Mater. Res. 74A.4 (2005): 503–10. Web.
14. Kwissa, Marcin, Sudhir Pai Kasturi, and Bali Pulendran. “The Science of Adjuvants.” Expert Review of Vaccines 6.5 (2007): 673–84. Web.
15. Singh, Ankur, Pallab Pradhan, and Krishnendu Roy. “Immunobioengineering Approaches Towards Combinatorial Delivery of Immune-Modulators and Antigens.” Novel Immune Potentiators and Delivery Technologies for Next Generation Vaccines (2012): 161–81. Web.
16. Chen, Helen C., Bingbing Sun, Kenny K. Tran, and Hong Shen. “Effects of Particle Size on Toll-like Receptor 9-mediated Cytokine Profiles.” Biomaterials 32.6 (2011): 1731–737. Web.
17. Shen, Hong, Anne L. Ackerman, Virginia Cody, Alessandra Giodini, Ella R. Hinson, Peter Cresswell, Richard L. Edelson, W. Mark Saltzman, and Douglas J. Hanlon. “Enhanced and Prolonged Cross-presentation following Endosomal Escape of Exogenous Antigens Encapsulated in Biodegradable Nanoparticles.” Immunology 117.1 (2006): 78–88. Web.
18. Agarwal, R., V. Singh, P. Jurney, L. Shi, S. V. Sreenivasan, and K. Roy. “Mammalian Cells Preferentially Internalize Hydrogel Nanodiscs over Nanorods and Use Shape-specific Uptake Mechanisms.” Proceedings of the National Academy of Sciences 110.43 (2013): 17247–7252. Web.
19. Petersen, Latrisha K., Li Xue, Michael J. Wannemuehler, Krishna Rajan, and Balaji Narasimhan. “The Simultaneous Effect of Polymer Chemistry and Device Geometry on the in Vitro Activation of Murine Dendritic Cells.” Biomaterials 30.28 (2009): 5131–142. Web.
20. Decuzzi, P., B. Godin, T. Tanaka, S.-Y. Lee, C. Chiappini, X. Liu, and M. Ferrari. “Size and Shape Effects in the Biodistribution of Intravascularly Injected Particles.” Journal of Controlled Release 141.3 (2010): 320–27. Web.
21. Steenblock, Erin R., and Tarek M. Fahmy. “A Comprehensive Platform for Ex Vivo T-cell Expansion Based on Biodegradable Polymeric Artificial Antigen-presenting Cells.” Mol Ther Molecular Therapy 16.4 (2008): 765–72. Web.
22. Thiele, L. “Competitive Adsorption of Serum Proteins at Microparticles Affects Phagocytosis by Dendritic Cells.” Biomaterials 24.8 (2003): 1409–418. Web.
23. Singh, Ankur, Shalu Suri, and Krishnendu Roy. “In-situ Crosslinking Hydrogels for Combinatorial Delivery of Chemokines and SiRNA–DNA Carrying Microparticles to Dendritic Cells.” Biomaterials 30.28 (2009): 5187–200. Web.