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When the Immune System Forgets

Source: iStockphoto

Source: iStockphoto

By Hsei Di Law

A mutation is revealing the basis behind an immunodeficiency syndrome that stifles the antibody response to vaccination.

David Vetter’s life inspired the 1976 telemovie The Boy in the Plastic Bubble. Suffering from profound immunodeficiency, David was forced to live in a sterile plastic bubble for the entirety of his 12 years of life. The immense public attention he attracted brought this group of diseases under the spotlight in the late 1970s.

Immunodeficiency leaves one extremely vulnerable to infections due to missing elements of the immune system or their failure to function properly. A variant of immunodeficiency disorders known as DOCK8 syndrome occurs when both of the principal immune cell types, the B and T cells, are defective. Patients with this disease show the typical signs of combined B and T cell immunodeficiency: a range of health issues including repeated bacterial and viral skin infections, eczema, allergies and cancer. Like many immunodeficiencies, DOCK8 syndrome is a genetic disorder caused by a mutation in the gene known as “dedicator of cytokinesis 8”, or Dock8.

Our team at the John Curtin School of Medical Research is trying to understand this disease by studying a model of DOCK8 syndrome in mice. Because humans and mice are so similar at a genetic level, mice act as a model for what happens in humans, thus serving as a platform for researchers to look closely at the mechanisms of disease at the cellular and molecular levels. They also provide an opportunity for researchers to test out possible treatments.

As in humans, the DOCK8 mouse model carries a mutation in the Dock8 gene. Led by Dr Katrina Randall and Prof Chris Goodnow, we have carried out experiments in DOCK8 mutant mice to learn how the mutation leads to immune disease.

Akin to human diagnostic blood testing, we ran a full blood count in DOCK8 mutant mice and found that they had far fewer immune cells than normal mice. This paralleled the human condition, and is unsurprising in the face of symptoms of immune deficiency.

The DOCK8 mutation particularly affected the survival of a subset of cells known as CD8 T cells, which are important for protection against viral infections. DOCK8 mutant mice had only 50% of normal CD8 and CD4 T cell counts, and also exhibited a gross absence of another specialised subset, the marginal zone B cells.

Further investigations revealed more puzzling abnormalities in immune cell function. Interestingly, the DOCK8 mutation seems to affect long-term immune events while leaving early responses intact.

During an infection, the immune system responds by producing two waves of antibodies. The first and immediate wave of antibodies can be seen within 2 weeks. While these early antibodies keep things in check, another lot of antibodies mature, giving rise to a second wave of antibodies. While delayed, these mature antibodies are even more efficient than the first and live longer.

In order to investigate their antibody response, we injected a mixture of foreign antigens into DOCK8 mutant mice. This is not unlike vaccinating people to induce antibody production. When we measured antibody levels 2 weeks later we detected as many antibodies in DOCK8 mutant mice as normal mice, indicating that the first wave of the antibody response was intact.

However, when antibody levels were reassessed after 4 weeks, DOCK8 mutant mice had lost most of their antibodies while normal mice were able to sustain a sizable amount of mature antibodies. Therefore, despite normal initial antibody production, DOCK8 mutant mice failed to mount the second phase of the antibody response.

A second defect in DOCK8 mutant mice concerns the remarkable ability of our immune system to remember past infections. When an antigen makes its first contact, the immune system learns to recognise the invader. This information is stored in a special subset of immune cells called “memory cells”. These cells persist for the lifetime of the animal, so in each of us exists a record of past infections stored as long-term memory.

When the same invader attempts to make a second infection, memory cells are roused into action and trigger an immediate and efficient defence – a “recall response”. This snap reaction often means we avert any symptoms of sickness at all.

The DOCK8 mutation seems to particularly affect a type of memory cell known as the CD8 memory T cell. In order to investigate this, we isolated T cells from both normal mice and DOCK8 mutant mice. The T cells from both sources were then pooled and transferred into the blood systems of normal mice. These recipients were then infected with a modified flu virus that triggers T cell proliferation.

Seven days after infection we measured T cell proliferation, and found that DOCK8 mutant T cells expanded to a similar degree as normal T cells. This indicated that DOCK8 mutant T cells were normal at this early stage of the immune response.

To probe the late response, recipient mice were reinfected with the same flu virus at day 35. We anticipated that by this time, memory T cells would have formed and reinfection would provoke a recall response. Sure enough, normal memory T cells facilitated a boost of immunity but DOCK8 mutant memory T cells failed to mount a recall response. Again, these mutant cells appeared to behave normally at first, with defects appearing during later events.

When DOCK8 mutant T cells were looked at closely under a microscope we discovered what was really happening at a molecular level. One of the earliest events in T cell activation involves physical contact with another type of immune cell, the dendritic cell. During this process, the T cell clusters specific proteins to its surface to facilitate cell–cell interaction. However, DOCK8 mutant cells were unable to concentrate these proteins, suggesting that interaction with dendritic cells was compromised. This finding suggests that early interactive activities with dendritic cells may be important for the T cell to persist and function normally as a memory cell later on.

With the understanding that we have gained from mice, we are able to propose some explanations for symptoms seen in human patients.

First, patients seem to lack an antibody response to vaccines. Indeed our research shows that the DOCK8 mutation affects antibody maturation and maintenance at high levels.

The mice model also revealed that the DOCK8 mutation leads to an intrinsic defect in CD8 T cell survival, which explains the reduced number of immune cells in patients and consequent susceptibility to infections and cancer.

Additionally, the research shows that DOCK8 mutation leads to memory cell damage, which may lie at the root of repeated and recurring infections.

Already, these insights will help in the diagnosis and development of a treatment for those with DOCK8 immuno­­deficiency and other related immune diseases. Currently, patients are treated by bone marrow transplantation.

Beyond its use as a model for DOCK8 immuno­deficiency, this mouse strain also provides an opportunity to investigate the memory of our immune system. A loss of immune memory may help explain why some infections do not go away, such as cold sores and yeast infections. Additionally, working towards a greater understanding of antibody persistence and memory cell function may help inform vaccine research, which relies on inducing long-lasting immunity.

Hsei Di Law is a research technician at the John Curtin School of Medical Research, The Australian National University.