08 June 2002
The Lancet, Volume 359, Number 9322
CD44 is a simple name for a complicated molecule implicated in many inflammatory and neoplastic diseases. Almost daily, new results are published that strive to unravel how this cell-surface adhesion molecule works. And though to an outsider the possibility that all these details will ever tell a coherent story seems remote, researchers studying CD44 in inflammation are more positive. "In every model we look at", says Ellen Puré (Wistar Institute, Philadelphia, PA, USA), "CD44's mechanism of action is different. Eventually, though, we will discover the common parts to this and I believe that in the long term CD44 will be a very reasonable therapeutic target in inflammatory diseases."
Inflammatory responses to harmful stimuli are generally self-limiting. During the initial inflammation, inflammatory cells are recruited to the site of infection or damage. Here, both they and the local parenchymal cells release inflammatory mediators and the original insult is dealt with. The inflammatory response then usually resolves into a healing phase. But sometimes--in autoimmune diseases, for example--a chronic response can develop in which activated inflammatory cells accumulate in extravascular connective tissue and cause tissue destruction.
A model target for new anti-inflammatories?
CD44, which is involved at many stages of the inflammatory response, is expressed ubiquitously on leukocytes and parenchymal cells in an inactive form. During inflammation, its activity is upregulated so that it binds its major ligand--hyaluronan--more tightly. CD44 is actually a complex family of molecules, produced from one gene by splicing and post-translational modification. How the different forms are regulated and which forms are active in which situation is an aspect of CD44 under close scrutiny.
What is the evidence for CD44's involvement in inflammatory diseases? Mark Siegelman (UT Southwestern Medical Center, Dallas, TX, USA) studies juvenile chronic arthritis and systemic lupus erythematosus and reports a correlation between disease activity and the presence of T cells carrying activated CD44. "Our results suggest that waves of activated T cells come into the circulation every time there is an exacerbation of autoimmunity", he says. "However, we don't know if these T cells are the pathogenic cells and we need to find out more about them before we embark on any clinical trial aimed at blocking CD44 activity."
Ursula Günthert (University of Basel, Switzerland) also has data indicating that changes in CD44 occur in human diseases. She has found expression of specific CD44 splicing variants in inflamed brain areas in patients with multiple sclerosis but not in unaffected brain areas of patients or normal individuals. In addition, she has data on Crohn's disease indicating that the same variants are present only in inflamed gut tissue.
Most of the evidence for CD44's involvement in inflammatory diseases comes from animals. In one type of approach, says Siegelman, "people have thrown anti-CD44 antibodies at animal models of autoimmune diseases and in general they have inhibited disease development". The other type of animal work that is providing intriguing insights into the biology of CD44 uses genetically modified mice. "Rather surprisingly", says Paul Noble (Yale University School of Medicine, New Haven, CT, USA), "given that antibody blocking experiments indicated that CD44 was involved in lymphocyte homing and haematopoiesis, the CD44 null mouse survived just fine. It was only when these mice were challenged, that we really appreciated the effects of the lack of CD44." So, for example, Noble and Puré recently reported that, although wildtype mice recover from bleomycin-induced lung injury, CD44 null mice succumb to an unremitting inflammation in which there is impaired clearance of apoptotic neutrophils and hyaluronan fragments from the injury site (Science 2002; 296: 155-58). "My prediction was that these mice would be protected from lung injury", admits Noble. He and Puré explain their unexpected result by proposing that CD44 is pivotal in determining the normal progression from an inflammatory to a healing response. Thus, in their lung inflammation model, lack of CD44 prevented the transition to tissue healing and the animals succumbed to runaway inflammation.
In other situations, CD44 null mice survive better than their wildtype counterparts, presumably because the lack of CD44 tips physiological responses towards a favourable outcome. Thus, CD44 null mice crossbred with atherosclerosis-prone apo-E deficient mice have 75% less atherosclerosis than CD44 positive littermates. "Our results suggest that CD44 promotes atherosclerosis by both mediating inflammatory cell recruitment to atherosclerotic lesions and by altering smooth muscle cell function", says Puré.
Günthert, meanwhile, is studying mice null for specific variant forms of CD44. In a mouse colitis model, she found that knockout mice lacking exons v6 and v7 were largely unaffected whereas most wildtype mice died within 10 days from gut inflammation. "It is generally accepted that in chronic inflammation, something prevents activated lymphocytes undergoing apoptosis", she explains. "In our knockout mice there was a four-to-five fold increase in cell death and thus they only got a short-lived inflammation. We believe that upregulation of CD44 variants v6 and v7 normally blocks apoptosis." On the basis of these and other results in mouse models for multiple sclerosis and arthritis, Günthert is planning to develop v6 and v7 antibodies, which she believes may be of therapeutic use.
No-one studying CD44 is under any illusion about the challenges ahead.
"It's a frustrating and complicated molecule to work with", says Siegelman.
"And it is getting more and more complicated", admits Günthert wryly.
But, says Puré, "if we can understand how CD44 activity is regulated,
it really will be an excellent target for drug development, particularly
if, instead of trying to block CD44 at the cell surface, we concentrate
on developing small molecule inhibitors of CD44 activation".