A new RNA-based technique could enable vaccination directly through the skin without using needles. Using sugars, the technique can guide vaccines precisely to the skin’s immune cells and could eventually be used against viruses, bacteria, skin cancer and other diseases.
The skin is the body’s largest organ and contains a vast reservoir of immune cells that can be activated to fight disease.
These include Langerhans cells, located in the upper layers of the skin and acting as sentinels, keeping watch for anything foreign. When these sentinels detect something that does not belong, they present it to the rest of the immune system, which then launches a response.
The idea has therefore been to use the skin’s own sentinels as a kind of gateway to the immune system. The new study now shows that a small RNA-based structure can be built that works as a targeted delivery system to these cells. Using sugar molecules, the structure can make Langerhans cells take up proteins from cancer cells and present them to the rest of the immune system.
“Langerhans cells have receptors that can recognise the sugar molecule mannose. So what we do is link a surface protein from cancer cells to mannose sugar molecules to get it into these cells. This creates a powerful immune response against the very cancer cells that the foreign material comes from,” explains a researcher behind the study, Professor Jørgen Kjems from the Interdisciplinary Nanoscience Centre (iNANO) and the Department of Molecular Biology and Genetics at Aarhus University, Denmark.
The research has been published in the Journal of Controlled Release.
RNA scaffold holds sugar molecules at precisely the right distance
The structure is based on a nucleic acid–based Holliday Junction: a self-assembling scaffold comprising four short strands of RNA.
The scaffold is designed to hold the sugar molecules about four nanometres apart – precisely the distance that matches the receptors on the surface of Langerhans cells. It works a bit like a bunch of keys, in which several keys fit into their locks at the same time. This makes the structure much easier for the cells to recognise and take up.
The idea is to attach the proteins that you want the immune system to react against the scaffold carrying the sugar molecules.
When Langerhans cells take up these proteins or protein fragments, they are presented to the rest of the immune system, which mobilises an attack against cells carrying precisely these proteins on their surface.
The study shows that the technique can help to stimulate an immune response against a skin-cancer target, but according to Jørgen Kjems, the same principle could also be applied to other types of cancer or to external viruses and bacteria.
“What is special about the way we do it is that we use small RNA molecules to link everything together. It turns out that the mannose sugars are best recognised if they are positioned at a precise distance from one another, and we can create a small structure that presents them at the perfect distance. This gives better uptake by the Langerhans cells and therefore a stronger immune response,” he says.
Immune cells specifically took up RNA structures in human skin
But the crucial question was whether the system would also work in real human skin and not only as a theoretical construct. The researchers therefore isolated cells from human epidermis and exposed them to the RNA structures containing mannose.
The results showed that Langerhans cells took up the structures significantly better than structures without sugar and that both the number and the precise position of the sugar molecules improved uptake by the cells.
The next step was to apply the structures directly to living human skin using a Dermaroller, which creates tiny holes in the outermost layer of the skin, enabling the structures to reach the Langerhans cells.
Microscopic examinations confirmed that, here too, Langerhans cells specifically took up the RNA structures in the skin.
The researchers then investigated whether the system could not only find the right immune cells but also deliver a biological “target” for the immune system. To do this, they linked a peptide from melanoma cells to the RNA structure.
The peptide was attached to the RNA structure using a chemical trigger, which is cleaved inside the cell and releases the peptide where the immune system processes foreign proteins.
The experiments showed that the presentation of the peptide substantially improved when it was delivered via the targeted RNA structure rather than on its own.
“This means that much less of the peptide is needed to activate the immune system, because far more of it ends up in the right immune cells. In addition, you get a more effective response. This points to the possibility that, in the long term, we may be able to vaccinate against skin cancer simply by applying the solution to the skin,” says Jørgen Kjems.
The next step is testing the vaccine in mice
The next major step will now be to determine whether the technique can also provide real protection in a living animal. Experiments have therefore been launched in which mice are vaccinated by applying the RNA structures to the skin.
The aim is to investigate whether the effect also holds in a whole immune system – and not only in isolated skin cells.
The plan is also to test the structure against influenza or SARS-CoV-2 in established mouse models.
If the technique works, both sugars and target proteins could, in principle, be swapped out, so that the same RNA structure could serve as a kind of universal platform for many different types of immunotherapy.
“We are currently setting up experiments in which we vaccinate mice by applying the structure to the skin. This will hopefully demonstrate that we elicit a strong immune response. It is, of course, an important step that we have already demonstrated this in human skin, as it can often be difficult to translate results from animals to humans. In principle, the mannose sugars can also be replaced with other sugars targeting other immune cells elsewhere in the body, so the potential is huge,” he says.
