Research Overview

Immunology Division

The Division aims to understand the basic functioning of the immune system and the way it develops from stem cells in bone marrow. In collaboration with other laboratories, this basic information will be applied to improving vaccines and to avoiding autoimmune diseases. The Division has particular focus on three cells types: B lymphocytes that make antibodies; T lymphocytes that kill infected cells; and dendritic cells (DC) that collect foreign material, process it and present it to the lymphocytes to initiate immune responses. Research concerns these cells, their interactions and the molecular signals within the cells that govern their responses. With our identification of five DC subtypes, we are now exploring the immunological significance of this complexity. We have shown that these DC process antigens differently and have different effects on T lymphocytes. We have identified two novel DC surface molecules and discovered a new signalling pathway that stimulates DC into action. We have also examined the cellular and molecular factors governing B cell development. This information should lead to new ways of regulating immune responses.

Background

In the beginning, there is the bone marrow stem cell -matriarch of not one, but two great cellular dynasties: the lymphoid and myeloid families. While the Cancer and Haematology Division studies the myeloid branch, the Division of Immunology focuses on lymphoid cells, which are divided into two types: T cells and B cells. All these lymphocytes have their ultimate origin in bone marrow, and derive from the self-renewing population of primitive white blood cells, called stem cells. These are the same stem cells that also spawn oxygen-transporting red blood cells, blood clotting platelets and garbage-disposing macrophages.

Education of T cells

Three decades ago one of the Hall Institute's most famous scientists, Professor Jacques Miller (shown left in 1966), made one of immunology's most important discoveries. He showed that a then-mysterious gland called the thymus, located high in the chest, is essential for the immune response. This is because the thymus makes T lymphocytes or T cells (T means thymus) from the stem cells which migrate into the organ from bone marrow. The thymus could be regarded as the university of the immune system - it is here that the T cells learn to recognise foreign antigens and to ignore the myriad "self" antigens present in the body's own tissues.

Professor Ken Shortman and Dr Li Wu, winner of the Hall Institute's 1996 Burnet Prize, are tracing the genealogy of these T cells - the steps by which unspecialised stem cells progressively become, during their sojourn in the thymus, specialised T cells that are kill virus-infected cells or to produce the spectrum of soluble factors called cytokines that help coordinate the immune system's defensive response. The history of every individual T cell can be retraced, through a series of branching "decisions", all the way back to its multipotent stem-cell precursor. By back-tracking from the fully differentiated T cells, through progressively rarer and rarer cells, the researchers have been able to identify important branch points or control points in this process.

Using flow cytometry and cell-sorting, Dr Li Wu has been able to isolate the earliest precursor of T cells in the thymus - only one in 3000 thymus cells is of this type. Professor Shortman and colleagues now hope to identify the signals that direct the step-wise development of these early precursors into fully differentiated T cells.

Cytotoxic T cells, mistaken identity and the origins of autoimmune disease

Dr Bill Heath is investigating how cytotoxic ("hunter-killer") T cells direct the immune response. He is studying the origins of immunity to viruses and cancer and seeking clues to the origin of autoimmune diseases such as diabetes, rheumatoid arthritis or multiple sclerosis. How are cytotoxic T cells generated? How do they know to attack "foreign" cells, while ignoring normal body "self" cells? And why do they sometimes mistakenly attack "self"?

The Hall Institute researchers are modelling auto-immune disease with transgenic mouse strains that express a particular non-mouse protein called ovalbumin as if it was a normal "self" component. Some of the mice develop autoimmune diseases that replicate the features of the human disease. Others do not develop disease, even though they have the potentially lethal cytotoxic T cells. By tracing the control circuits that normally regulate cytotoxic T cells, the Institute's researchers hope to develop strategies to prevent autoimmune disorders.

The limited vision of T cells and their guidance by different types of dendritic cells

A T cell cannot respond to antigen directly. Instead it must participate in a special ritual involving an antigen-presenting cell - usually a dendritic cell. Dendritic cells take up external free antigens, process them and present fragments of the antigens in a special groove atop a MHC class II molecule anchored in its cell membrane. Dendritic cells also process internal antigens (self antigens or viral antigens expressed by virus-infected cells) and present these on the groove of MHC class I molecules. The T cell approaches the dendritic cell, checks out the displayed antigen on its MHC molecules, and if it fits the T cell antigen receptor, the T cell swings into action.

Dr. Bill Heath and colleagues have now identified a special route by which antigens from outside the antigen-presenting cell can enter the class I MHC pathway. This alternate route is important both in initiating and regulating T cell autoimmune responses. They are investigating the nature of the antigen-presenting cell responsible for this alternate pathway - they suspect it is a type of dendritic cell - in collaboration with Professor Shortman's laboratory.

Dendritic cells have long been considered as a type of myeloid cell, closely related to the phagocytic macrophage. Like macrophages and granulocytes, dendritic cells can be grown in culture from myeloid precursors using the myeloid hormone GM-CSF (granulocyte-macrophage colony-stimulating factor). So it was a great surprise when Professor Shortman and Dr Wu found dendritic cells being generated from their thymic lymphoid precursor population, via a process that does not require GM-CSF. There seems to be another lineage of dendritic cells, related to lymphocytes, rather than to myeloid cells. Both types of dendritic cell are able to display antigens and activate T cells, but the newly identified lymphoid-derived dendritic cells appear to have additional regulatory effects on T cells. They may "turn down the volume", preventing T cell responses from getting out of hand, and may be the key to switching off hostile T cells in autoimmune disease.

B cells and the search for the perfect antibody

While T cells develop in their own specialised organ, the thymus, antibody-producing B lymphocytes (B cells) develop from bone marrow, along with most other blood cells - but they complete their education and refine their skills in the specialised B cell follicles of the spleen and lymph nodes.

For every alien antigen, a B cell awaits, ready to produce an antibody to neutralise it. So said the Clonal Selection Hypothesis, formulated in the 1950s by one of WEHI's most famous former scientists, Professor Sir MacFarlane Burnet (left), Nobel Prize winner for Medicine and Physiology. At the time, Burnet's radical proposal remained unproven. Burnet's intellectual heir, and successor as Director, Professor Sir Gus Nossal (below, with Burnet), provided final proof.

Professor Nossal demonstrated that each B cell can make antibody specific for only one alien antigen, and that this functional constraint is predetermined. His crucial experiments disproved the so-called "instruction" theory, which proposed that antigens somehow moulded antibodies. Instead his studies substantiated the idea that antigen somehow "selected", out of all possible B cells, the unique cell whose antibody best fitted that specific the antigen.

The clonal selection theory suggests that B cell production involves two crucial steps. First, a precursor B-cell commits itself to a particular specificity, and is then activated when its owner contracts an infectious agent whose antigen matches the antibody of the B cell. Although this match often is not particularly precise the first time round, which might mean the immune system cannot rid the body of the infection, the fit can be - and is - improved. Dr David Tarlinton is studying how the select few B cells are chosen, become activated, improve their fit for antigen and start producing antibody.

The process is approximately as follows: the activated B cell begins to multiply, often with a little help from activated T cells. The B cell and its descendants begin to refine their fit through a trial-and-error process. The cells making the best-fitting antibodies are selected to participate in the next round of refinement; the rest are eliminated. This "survival of the fittest" process progressively improves the antibody design for that particular alien antigen. A few days after the onset of infection, only those B cells that make the best antibodies remain in the race; they proliferate and subdue the invader. Subsequently, some of the best- fitting B cells remain in the body as sentinels, guarding against any future infection by the same virus or bacteria. This cellular "memory" of past infections is the basis of most vaccinations, so by studying how system works, researchers may be able to develop improved vaccination procedures.

Lymphocyte signalling: inside and out

All stages of lymphocyte life history, both during lymphocyte development and after lymphocytes have been activated as part of the immune response, depend on external signals. Lymphocytes receive the signals in the form of soluble cytokines, or via contact with molecules on the surface of other cells. Lymphocytes respond to these signals via receptors on their surface; the activated receptor then initiates a signalling cascade inside the cell, which in turn activates genes that code for transcription factors. The transcription factors then bind to other genes, switching them on.

Dr Lynn Corcoran and her co-workers have discovered that antibody-producing B lymphocytes require a particular transcription factor called Oct-2 before they can perform many of their protective functions during the course of an infection. B cells lacking Oct-2 cannot switch on Oct-2-dependent genes, so they fail to divide and mature to antibody-producing cells when they encounter a foreign invader, or antigen. Dr Corcoran and her colleagues are trying to identify these Oct-2 target genes, hoping to define the critical molecules that a B cell requires to mount a strong, enduring immune response.

In Dr Steve Gerondakis' laboratory, researchers have been dissecting the complex interactions between the lymphocyte-specific NF B family of transcription factors. They have generated a knockout mouse lacking one member of this family, called c-Rel, which has notable defects in lymphocyte function. They are also analysing earlier steps in the signalling cascade, focusing on a family of phosphatase enzymes which act by dephosphorylating the activated form of another enzyme in the cascade, called MAP kinase. Again, their approach to dissecting the complex interactions involved has been to knock out a gene for just one member of the family.

The Hall Institute researchers have developed a knockout mouse lacking a gene called PAC 1, and are studying what happens to animals lacking this particular MAP kinase phosphatase. This painstaking step-by-step approach should finally unravel the complex sequential steps of signalling