Austin discovery heralds vaccine development revolution

Immunologists at Melbourne's Austin Research Institute have made a major discovery about the functioning of the immune system, which could revolutionise vaccine development.

Assoc Prof Magda Plebanski's research team has discovered that, for dendritic cells -- the specialised sentinel cells that alert the immune system to infection and cancer -- size matters. In fact, it matters critically: in experiments in in vitro cell systems, and in animal models, the institute's team has shown that dendritic cells are uniquely adapted to detect and process virus-sized particles.

The magical figure is 40 nanometres -- dendritic cells react avidly to custom-designed synthetic vaccine particles in a very narrow size range around this figure, and induce powerful humoral (antibody-mediated) and cytotoxic (T-cell) arms of the immune system.

The Austin team's discovery is the basis of the DCtag adjuvant technology licensed to Melbourne vaccine developer Panvax, a subsidiary of biotechnology investment company Prima BioMed (ASX:PRR).

The team delayed publishing details of its discovery of what it is calling "size-mediated immunogenicity" until Panvax, its sponsor and commercial partner, could lock up the intellectual property relating to DCtag technology.

The discovery, reported in the journal Vaccinethis week (vol 23, p258), offers a simple explanation for the failure of peptide vaccines to realize their enormous therapeutic promise, and points to a simple solution.

The problem, according to Plebanski, is that dendritic cells simply fail to be triggered to stimulate strongly immune responses to peptide molecules when presented on their own because they are too small. Her team has shown the problem can be solved by mounting peptide and protein antigens on virus-sized polymer beads.

Moreover, they have shown they can fine-tune the immune system to mount a primarily antibody-mediated defence against infectious bacteria or parasites, or a cytotoxic T-cell response against viral infections and cancerous cells.

"Our main finding is that size matters to almost a ridiculous degree," Plebanski said. "What nobody had realised is that there has been a whole range of particulate vaccines out there: they use a variety of materials, that may be biodegradable or non-biodegradable.

"Nobody has systematically explored the basic nano-sizing mechanisms by which the body recognises that something is dangerous. We discovered that the immune system preferentially attacks antigen on particles of a very specific size.

"We compared a whole series of particles with tiny size variations, and were stunned at how narrow the optimum size range is. Within a narrow range around 40 nm, the immune response is magnified approximately tenfold; outside this peak, the response to smaller or larger particles falls away very rapidly."

Plebanski said it was no accident that the peak coincided with the size range for most viruses. "It makes a lot of sense that the immune system measures particle size," she said.

Plebanski's team covalently bonded a variety of short peptide sequences containing MHC Class I or II peptide or protein antigens to particles of inert materials like gold, polystyrene and silica. The only constant was that the particles were all around 40 nm in diameter, plus or minus 10 nm.

Irrespective of the core material, the antigen-coated particles generated "amazingly high" CD8 (cytotoxic) T-cell responses, and high antibody titres, confirming that particle size -- not composition -- is the critical determinant of the magnitude of the immune response.

"Basically, we were able to mimic the way the body responds to a natural infection," she said.

Plebanski said the nanobead-bonded antigens constitute a very clean, highly selective system for manipulating the immune response -- the nanobeads can carry peptide antigens selected to induce strong antibody- or T-cell-mediated immunity, or to bias the immune response towards antibody or cell-mediated immunity.

The advance will give immunologists the ability to apply rational-design principles to develop potent new vaccines to prevent -- or treat -- viral, bacterial and parasitic diseases, and cancer.

For more than a century, vaccine developers have been plagued by unpredictable or ineffectual responses from experimental vaccines -- including the new generation of peptide vaccines.

Using inert materials in the nanobeads may ensure that the immune response is directed solely at the conjugated antigens, not the beads themselves.

Most vaccines induce a strong antibody response against pathogenic microbes and viruses, but typically fail to elicit cell mediated (cytotoxic T-cell) immunity. Cell-mediated immunity is critical for eliminating infected cells harbouring actively replicating viruses, bacteria and parasites. Cancer vaccines must also induce a focused, cytotoxic T-cell response to eliminate cancerous cells.

Plebanski's team has also published a recent paper in the Journal of Immunology, describing the use of nanobead-conjugated whole proteins, rather than peptide fragments, in anti-cancer vaccines.

They have shown that the experimental vaccines rapidly and completely eliminate tumours in a mouse model of cancer. Plebanski said it had the potential to work in humans and is currently progressing development towards a human clinical vaccine trial: "The advantage of using whole proteins is that, in a heterogeneous population, you don't know which peptides are going to be recognised by different individuals, because of the diversity of MHC [major histocompatibility complex] restriction patterns."

By targeting the whole protein to the dendritic cells for recognition and processing, the individual's immune system can simply 'choose' which peptides it will target with antibody- and cell-mediated immunity. "We've now tried a whole range of proteins, including proteins from model tumours, proteins from RSV (respiratory syncytial virus, one of the agents of the common cold), and malaria," Plebanski said. The institute's team, in collaboration with a UK based biotechnology company, has also shown that the nanobead technology also induces a strong, dual-armed immune response to complex 'milkshakes' of proteins and other molecules from lysed tumour cells.

"In terms of potency, the immune response from DCtag technology is right up there with anything else in the research literature, including Freund's complete adjuvant," she said.

"It's very flexible and robust. Virtually all the antigens we have tested when put onto a nanobead of the right size generated a strong immune response. It's quite extraordinary."