How does systemic flagellin immunization induce mucosal IgA?

Vaccination has been pivotal to the improved control of infectious diseases seen in the last century. Despite this encouraging progress, infections remain responsible for around 20% of deaths worldwide, primarily in the very young and in the aged, but they also have a considerable impact on our livestock and thus our food chain. This is important, not just for the substantial economic impact it has on the Euro 2 trillion food production business, but because we acquire many deadly infections from our food.

Developing novel vaccines against infectious diseases is highly cost-effective and history demonstrates how effective they can be. Furthermore, as our options to treat bacterial infections using antibiotics diminish, because of increased resistance to these drugs and a paucity of new drugs coming through, vaccination looks increasing attractive as a way to prevent disease developing. Vaccines are so effective at preventing disease because they work by providing protection before the infection is established, i.e. when pathogen numbers are likely to be at their lowest.

In most cases, vaccines provide their protection through the induction of proteins, called antibodies, which are specific for a single part of the bacterial or viral pathogen. These proteins "stick" to the pathogen and prevent it from damaging us or flag up its presence to the cells of the immune system allowing it to be destroyed. Although antibodies are specific, they come in different flavours, so the type of antibody that vaccines induce in the blood that help us kill invading pathogens is called IgG. In contrast, the most common form of antibody at body surfaces, called mucosal sites, is IgA. So both IgG and IgA are highly efficient at protecting us but tend to predominate in different parts of the body. Because vaccines are often given subcutaneously or into the muscle they tend to only induce systemic IgG, but not mucosal IgA. This has prevented us from maximizing the potential for IgA to protect too. This work aims to address this issue.

What we have identified is a way to induce systemic IgG and mucosal IgA at the same time. This is important because it is difficult to achieve. To achieve this we have immunized mice subcutaneously with a key bacterial protein called flagellin. Normally, this protein helps bacteria move and adhere, but here we have used it as a Trojan Horse to induce strong IgG and IgA responses and so potentially exploit the potential of antibody to protect at both mucosal and systemic sites. Because in some parts of the immune system these IgG and IgA responses develop side by side we can examine them for their similarities and differences. This will allow us to generate new technologies to maximize how best to make an efficient vaccine to provide protection at multiple sites throughout the body.

Vaccination is a key strategy to control infectious disease in humans and economically important animals. Currently, most vaccine strategies rely on the induction of IgG, despite most pathogens entering the host via mucosal routes, where the predominant isotype is IgA. This is because most vaccines are administered systemically and this tends to result in limited IgA induction.

What we have identified is that the flagellin protein, FliC, from Salmonella Typhimurium, when given systemically (e.g. subcutaneously or intraperitoneally), induces IgG in the spleen but IgG and IgA in the mesenteric lymph node (MLN). This co-induction of IgG and IgA in the MLN is surprising since we do not see this in response to other proteins such as ovalbumin when given as a soluble or alum-precipitated protein. When we investigated the mechanism underlying this, we found that it was through the TLR5-mediated recruitment of CD103+CD11b+ dendritic cells (DC) from the lamina propria to the MLN.

Combined with our capacity to identify antigen-specific T and B cells we have a model where we can investigate how IgG and IgA responses can develop concurrently in the same lymph node. This will allow us to better understand the mechanisms through which switching is mediated and controlled and how best to exploit this to develop vaccines that induce systemic and mucosal responses.

One of the key ways this is likely to be regulated is through distinct populations of T follicular helper (Tfh) cells being induced. Using our model we wish to address the hypothesis that the MLN contains multiple populations of Tfh that are induced in parallel yet contribute differently to the regulation of IgA and IgG switching. We will also test the hypothesis that immunizing with sFliC-heterologous antigen conjugates induces IgA responses to the passenger antigen in the MLN and thus sFliC can act as a conjugate carrier for vaccine delivery.

Society
The importance of this work extends significantly beyond supporting academic research. By understanding how antibody responses develop we are helping understand how to improve vaccine development. This is vital. In our highly mobile society the risks from infectious diseases are increasing due to increased travel and altered global migration and extend also into the food-chain. This is because many of our infections are actively acquired from food (e.g. Salmonella) or livestock rearing acts as an incubator and enables pathogen diversity (e.g. influenza virus). In parallel, we have a decreasing efficacy of anti-microbial treatments due to resistance and only a modest number of anti-virals available. Vaccination is a cost-effective approach that can protect against infection at the extremes of age in those groups that are most susceptible. Furthermore, vaccination is an acceptable intervention to society at large, and as the media response to the recent measles outbreak demonstrates, is one that is diminishing in controversy. Indeed, the measles outbreak demonstrates the importance of vaccination programmes to protecting society and the consequences when there is insufficient vaccine coverage.

Industry

This work will have a marked impact upon industry. The vaccine market is estimated to be worth $52 billion by 2016 and is likely to increase as more vaccines are developed and are required due to the increasing challenge of anti-microbial resistance. Flagellin is a molecule of interest both as a vaccine component and as a carrier, as demonstrated in the "Benefits" section above. This is aided by its safe use in man its ability to induce immunity in the absence of exogenous adjuvant. Therefore, the benefits to industry from this work are two-fold. First, there is understanding the nature of the immune response to flagellin itself and second, there is the impact improved understanding of fate decisions in immunoglobulin switching will deliver.