By David N. Leff
Maybe it¿s because adult Americans are eating fewer fatty foods, kicking cigarettes, exercising more, taking pills to lower blood pressure and cholesterol levels and watching their weight.
¿Although there¿s been progress in treatment of atherosclerosis,¿ observed cell biologist/immunologist Ellen Puri, ¿especially prevention of acute cardiovascular events ¿ myocardial infarctions or stroke ¿ the bottom line is that cholesterol-lowering drugs have really had the impact of only about a 30 percent decrease in events over a five-year period. This suggests that there are other critical parameters that we need to identify.¿
Puri, a full professor at the Wistar Institute in Philadelphia, is senior author of a paper in the Journal of Clinical Investigation for October 2001. Its title: ¿The adhesion receptor CD44 promotes atherosclerosis by mediating inflammatory cell recruitment and vascular cell activation.¿
CD44 is an immune-system protein that perches on T cells, precursor B cells, monocytes and neutrophils. There it helps lymphocytes bind to endothelial cells that line the inner walls of arteries.
¿The normal function of the immune system,¿ Puri explained, ¿means the ability of immune cells to traffic around the blood and lymphoid organs, to play their role of immune surveillance ¿ looking for things, such as tumor cells and infections. That function seems unaffected if you inhibit CD44 or knock out its gene from a mouse.
¿One reason we chose to study CD44 in particular,¿ Puri told BioWorld Today, ¿is that about 15 years ago this molecule was thought to be important in homing of normal leukocytes that circulate in the blood to certain types of lymphoid organs in the normal immune system. And we showed that was not the case.¿
She continued: ¿Our idea is if you have a chronic disease that has to be treated for the long term, you don¿t want to interfere with some of those fundamental properties of the immune system, but you want to specifically interfere with inflammation. And we think that CD44 is a particularly good target in that regard.¿
Two KO Mice Score Against Atherosclerosis
The in vivo experiments she and her co-authors described in the journal enlisted two strains of knockout mice. One, lacking the gene encoding ApoE, was thereby prone to atherosclerosis. The other KO animal, deprived of CD44, led the authors to hypothesize that this deficit would resist atherogenesis.
¿We bred the ApoE KO mouse to the CD44 KO mouse,¿ she recounted, ¿fed them a normal Western¿ diet and kept them under normal living circumstances in Wistar¿s mouse colony. Then at 20 weeks of age, we sacrificed the animals, took out their aortas, and used several methods to measure the amount of atherosclerotic lesions, define the content of the lesions, and whether they¿d been altered. Other methods let us see if these data correlated or not with changes in their blood cholesterol levels.
¿An important part of the paper,¿ she went on, ¿points out that in fact it does not change cholesterol levels to lose CD44. The key result was the marked 75 percent reduction in the amount of lesions in these KO animals, compared to their wild-type littermates. That alone was gratifying, but what we need to do now is understand why.¿
Atherosclerosis sets in with appearance of cholesterol-laden macrophages (foam cells) in an artery¿s inner wall. Smooth muscle cells join this growing party, which goes on to form a plaque. Its expansion narrows the artery, constricting blood flow.
¿One of the really popular ideas at the moment,¿ Puri observed, ¿which I think is correct, is that even though we all have these atherosclerotic plaques, we¿re not all in danger of experiencing an acute event. That means you can have atherosclerotic lesions, and the blood just flows by; all is well. It doesn¿t cause an acute heart attack or stroke.
¿What causes the acute attack,¿ she continued, ¿is if there¿s a rupture of the lesion, and exposure to the circulation that leads to a blood clot that completely occludes the blood flow. The idea now is not just that there¿s plaque and how big it is ¿ but the molecular nature of that plaque. And the idea is that the more inflammatory the response going on in the vessel wall is, the more susceptible it is to rupture, whereas the more fibrotic it is ¿ meaning that rather than a lot of inflammatory cells, you get a lot of smooth muscle cells ¿ is much more resistant to rupture.
¿It¿s our idea that even if there¿s already plaque,¿ Puri pointed out, ¿we can shift the balance toward the more fibrotic response, and decrease the risk of thrombotic events or occlusions. We have lots of data now that suggest CD44 and its ligand can influence the balance between the more inflammatory response to the fibrotic. That is, CD44 tends to be pro-inflammatory, and its loss tends to be pro-fibrotic. If that¿s true, it makes CD44 a wonderful therapeutic target.
¿I don¿t think it will ever be a target on its own for one drug by itself,¿ Puri added, ¿but we think combination therapies, where we lower cholesterol and block some of these other molecular events that we know are contributing, will be used in combination with existing drugs to increase the benefits of treatment.¿
Human Trials In 10 To 15 Years
¿And as we develop inhibitors in the mouse model,¿ she pointed out, ¿we would then try it in the human in vitro system, and eventually, hopefully, go into patients. But that¿s obviously a long time down the road. But we¿re getting a much better understanding of translating things from mice to make them beneficial to humans. So I think now in terms of 10 or 15 years.
¿There¿s no doubt any more from our data and the literature that CD44 may be a very good target for chronic inflammatory diseases,¿ Puri pointed out. ¿Frankly, those are some of the best money makers in the business ¿ rheumatoid arthritis, atherosclerosis, asthma-associated airway inflammation. I believe that if a company was willing to collaborate, we have the assays for looking at the activation of CD44 and its ligand. If they had the libraries and the wherewithal for high-throughput screening, this is the time for us to collaborate.
¿And if we came up with CD44 inhibitors,¿ Puri concluded, ¿they would absolutely be patentable.¿