[Image above] Graphic illustrating how ions released during bioactive glass dissolution affect dendritic cells, specialized cells that play a crucial role in the immune system. Credit: Katharina Schuhladen, Institute of Biomaterials, University of Erlangen-Nuremberg
In the May issue of the Bulletin, we looked at a few of the myriad applications of glass, including in energy and art. And tucked away at the end of the issue was one application of glass that may be less familiar to a general audience—bioactive glass.
Bioactive glass is a biomaterial, meaning it is engineered to interact with biological systems to support, enhance, or replace damaged tissue or a biological function. Since Larry Hench discovered the original 45S5 Bioglass formulation in 1969, “many new compositions and other types of [bioactive glasses] have been proposed for optimizing the body’s response,” a review article of bioactive glasses notes.
Unlike most metals, bioactive glasses are biocompatible, meaning they are not known to cause a toxic or injurious physiological or immune response in living tissue. As such, they are far superior to metals in some regards because they do not adversely affect the tissue they are supposed to help heal.
Although bioactive glasses are not expected to trigger harmful responses, they do still trigger responses—why else would they be so good at helping the body to heal? There still are a lot of unknowns surrounding how and what responses bioactive glasses trigger, however, so understanding these responses better helps us improve the healing abilities of bioactive glasses and avoid possible toxic responses.
One type of immune response that scientists do not clearly understand is how bioactive glasses affect the function of dendritic cells (DCs).
DCs are specialized immune cells that play a crucial role in initiating primary immune responses by processing and presenting antigens (a toxin or other foreign substance) to a type of white blood cell called a lymphocyte, which then triggers an immune response.
In other words, DCs alert lymphocytes to an intruder in the body, and then lymphocytes launch an attack. For a good visual depiction of how DCs initiate an immune response, check out the video below.
There are several different immune cells that alert the body to antigens, but DCs are considered the most powerful and influential. Thus, if scientists can learn how bioactive glasses affect DC functions, it could provide a means of control over a body’s immune response during healing.
In a new open-access study, researchers from the University of Erlangen-Nuremberg and University Hospital Erlangen in Germany investigated in vitro how different borate bioactive glasses affect mice DCs. In particular, the researchers specifically investigated how DC phenotype and function are impacted by bioactive glass ionic dissolution products (IDPs).
When a bioactive glass is placed in the human body, the physiological environment causes the glass to dissolve, triggering two main things to occur:
- Formation of an apatite layer on the tissue surface
- Release of biologically active ions (i.e., IDPs)
The particular IDPs depend on the glass composition, but much is known about the effects of certain ions. For example, calcium and silicon favor osteoblast differentiation, whereas boron stimulates fibroblast cells and angiogenesis. Boron glass in particular has proven exceptionally good at wound healing, as evidenced by collaborative work at Mo-Sci and the Missouri University of Science and Technology discussed in the May 2011 and May 2013 issues of the ACerS Bulletin.
As you can see, the IDP effects are significant and thus are an important place to begin investigation of how bioactive glasses affect DCs.
For the study, the researchers focused on bioactive glasses doped with copper and/or zinc.
“The main idea was to develop a bioactive glass suitable to be used in wound healing applications,” Aldo Boccaccini, ACerS Fellow and head of the Institute of Biomaterials at the University of Erlangen-Nuremberg, says in an email. “Therefore we chose zinc, which is known to be important in wound healing and to have some anti-inflammatory and bactericidal properties. In addition, we chose copper since it has been shown that copper is efficient against various bacteria and it provides additional angiogenic properties, which are two properties especially interesting in the treatment of chronic wounds.”
Unsurprisingly, the researchers also saw antibacterial properties of zinc and copper in their study. They note these effects can be explained by an increase in the pH value caused by dissolution of the bioactive glass, which makes the environment more alkaline and thus hostile to bacteria.
The more interesting results were what the researchers discovered about how IDPs directly interact with DCs.
In the case of zinc-doped bioactive glasses, the researchers discovered the IDPs provided several positive effects on DC function. In particular, zinc ions increased the number of certain DC surface markers, i.e., molecules involved in antigen presentation and activation of lymphocytes. As a result, the researchers saw an increased capacity of these DCs to stimulate T cells (a type of lymphocyte) to act against the antigens.
However, Boccaccini emphasizes that the same results were seen for bioactive glasses without doping as well. “It should be mentioned that no statistically significant difference between the cells being in contact with bioactive glass with and without zinc could be found (most probably due to the minor, not detectable amount of zinc being released),” he says.
In contrast, Boccaccini says the results for copper-doped bioactive glasses were “especially interesting” because “the immune reaction can be fine-tuned by changing the concentration (and therefore the release) of copper.”
Specifically, high concentrations of copper ions reduced the number of certain DC surface markers, thus reducing the capacity of DCs to stimulate T cell proliferation.
“This offers several interesting possibilities, such as actively inducing an immune reaction (as shown by the 1% B3-Cu bioactive glass) or to actively dampen an immune reaction (as shown by the 10% B3-Cu bioactive glass),” Boccaccini says.
In addition to the effects on DCs and T cells, the researchers also found some interesting results concerning effects on cytokines, a broad and loose category of small proteins important in cell signaling. While zinc-doped bioactive glasses increased secretion of certain pro-inflammatory cytokines, which are detrimental to health, copper-doped bioactive glasses reduced such cytokine levels.
While this study provided an intriguing first look at how bioactive glasses affect DCs and T cells, the results are difficult to apply to wound treating in humans because the studies were performed in mice cells. As such, Boccaccini says the researchers plan to repeat the experiments using human cells and also to further investigate the possible bioactive glass effects on B-lymphocytes as well as macrophages (another specialized immune cell that, like DCs, alert the body to antigens).
“Moreover, it will be also interesting to investigate whether our ion-doped bioactive glasses offer antiviral effects,” Boccaccini continues. “If those results are promising, we are also planning to test our bioactive glasses in a 3D in vitro wound healing model as well as in vivo.”
For continuing the research, Boccaccini says the researchers have established a collaboration with the Department of Immune Modulation of the University Hospital Erlangen.
The open-access paper, published in Biomaterials Science, is “Cu, Zn doped borate bioactive glasses: antibacterial efficacy and dose-dependent in vitro modulation of murine dendritic cells” (DOI: 10.1039/C9BM01691K).