Crystallisation of soluble FceRII and FcgRIII under microgravity conditions – Experiment SPC.

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Table of Contents

1. Introduction
2. Proposal
3. References

Introduction

Immunoglobulin receptor proteins (FcR) expressed on the surface of all immune competent cells mediate most humoral immune reactions.  These reactions ensure an efficient and broad immune response by triggering endocytosis of immune complexes, regulating antibody production by B-lymphocytes, as well as antibody-dependent cell-mediated cytotoxicity and release of inflammatory mediators that activate other immune cells.  At the same time FcRs mediate abnormally regulated immune reactions causing common conditions of allergies, asthma, infections, autoimmune disease and cancer, while also being prime targets of several virus infections [eg HIV (1), Ebola (2), Measles (3), Dengue (4)].  Being at the centre of both cellular and humoral immune responses the FcR is an ideal target for immunotherapy.

One such FcR, FceRII (CD23), was discovered in the late 1980’s and is now believed to be the central molecule in allergic responses, more important than the FceRI.  The functions of FceRII are diverse, including cellular adhesion, growth and differentiation of B and T lymphocytes, rescue from apoptosis, and release of cytotoxic mediators.  Furthermore, FceRII seems to play a pivotal role in regulating IgE production and hence being the primary cause of certain allergic conditions (5).  As is the case for many immunoglobulin receptors, soluble and membrane-bound forms are found.  However, the precise function(s) of soluble forms of FcR are unknown.  The objective of our research is to study both the protein structure and function of soluble immunoglobulin Fc receptors with a view to the rational design of immune therapies to treat allergies, autoimmune diseases, inflammatory and infectious reactions.  Since FceRII plays a major role in regulating IgE levels, the solution of its structure by X-ray crystallography would be of enormous benefit and use in designing immune therapies that could block the interaction of this receptor with IgE and other adhesion proteins such as CD11.  Ideally the best crystals would be those that include the FceRII in complex with its ligands, particularly IgE, however crystallization of FceRII alone has been impossible to achieve as yet.  This structure would be a necessary step towards more complex crystals and a better understanding of the diverse roles of this protein in human immunity.

A second receptor protein of interest is FcgRIII, an IgG receptor.  It is known that HIV infectivity is increased by the presence of certain antibodies in the human blood circulation, which raises questions about the usefulness of vaccinations against the virus.  The FcgRIII was shown to be responsible for this antibody-induced increase in HIV infectivity.  We have recently solved the structure of FcgRIII in complex with IgG at a resolution of 3.2 Å and sFcgRIII on its own at 2.5 Å (6).  Crystallisation of the FcgRIII under microgravity conditions could hopefully improve the quality of the crystals and allow higher resolution structures to be determined for the protein.  These structures would aid in our understanding of the structural and thermodynamic mechanisms of interaction between the FcgRIII and IgG, and ultimately to immune therapies that could block this interaction.  Such immune therapies would also be potentially capable of reducing the antibody-enhanced infectivity of HIV.