By David N. Leff
Asthma sufferers undergo myriad needle-prick skin tests to track down the countless environmental allergens that may be causing their symptoms. But there¿s a paradox underlying this assumption that asthma is an allergic malady, said pulmonologist and cell biologist Michael Holtzman, at Washington University in St. Louis.
He pointed out that ¿many asthmatic patients don¿t suffer from allergies, and most patients with allergies don¿t develop asthma. Even people in whom you could take an allergen and place it right in their airway,¿ Holtzman told BioWorld Today, ¿might get a local allergic response, but never develop asthma. So, obviously, something is missing from them.¿
That missing something is a whole alternative concept to the traditional view that asthma is entirely an allergic response. In the conventional scenario, an allergenic substance, such as dust or dander or mold, provokes the body¿s immune system to trigger inflammation of the constricted airways in an asthmatic attack.
¿One school of thought,¿ Holtzman said, ¿began with the idea that an allergic predisposition was associated with asthma. From that concept, they moved to identify the genes ¿ interleukins and immunoglobulin-E (IgE) ¿ that are responsive to allergens. That story has been pretty well shored up over the last couple of decades, with a lot of work in the mouse world suggesting that that sphere of the immune response ¿ the allergic responses ¿ is tied to the T helper lymphocyte type 2 (TH2).
¿There are two different T helper spheres,¿ Holtzman said. ¿On the one hand, there¿s this TH2 system; on the other, the TH1 system. And the allergy story is tied to the part of the immune system labeled TH2. Both T helper cell types help the other cells in the immune system take care of invaders. In the case of TH2, it¿s supposed to be more related to the kinds of stimuli that would lead to B-cell, humoral, immune responses, especially IgE antibody. It looks at antigens coming in, like parasites.
¿The TH1 system,¿ Holtzman said, ¿aims more at defense against viruses. And that system, people have thought to be down-regulated in asthma.¿
Reading From A Different Hymn Book
Reverting to the immune-response genes that trigger asthma, Holtzman pointed out that, ¿in the normal person, you get a viral infection in your airways or in your nose. The epithelial cells that line those tracts will respond to it by turning on a set of genes that act for viral defense. When a gene is activated, it¿s transcribed as messenger RNA. A lot of the genes that are important for immunity and inflammation are triggered to be activated by molecules known as cytokines. They exert their effects on these genes through an intermediate protein known as STAT. It stands for signal transducer and activator of transcription.¿ What STATs do is shuttle the cytokine¿s signal from the outside of the cell to its nucleus.
¿So, when cytokines bind to the airway epithelial cell, they activate STAT at the level of the cytokine receptor at the cell surface,¿ he said. ¿And then, once activated, STAT zips into the nucleus and binds to the target gene, which activates gene transcription. Tradition bases the role of cytokine on its level of production when triggered. But that¿s only part of the deal. Cytokines have to exert their effects as well as their output. And if you don¿t consider that part of the equation, you may be fooled by looking only at the level of production.¿
His final critique of current doctrine is that ¿the airway epithelial cell is left out of the story. The epithelial cell, as far as those people are concerned, is kind of an innocent bystander. They see it as an inert barrier against the environment. But those airway epithelia didn¿t take an active role in immunity. In asthma, they were more a victim of the immune system.
¿We didn¿t believe that,¿ Holtzman said. ¿They did take an active role, by virtue of their location at the ideal spot to defend us. We thought they would have a program capable of doing that, independent of the immune system. So, we went on a 20-year quest to prove that airway epithelial cells, and epithelial barrier cells in general ¿ in the skin, the gut ¿ are specially programmed with a set of immune response genes geared up to respond to bad things in the environment.
¿We focused on respiratory viruses because of our interest in asthma, and we knew those were triggers for the disease,¿ he said. ¿So, we identified a set of genes in epithelial cells that are especially networked for antiviral defense. We showed that they are activated ¿ without any need for the immune cells ¿ in response to viral infections.¿
From their cell and molecular biology studies, Holtzman and his colleagues knew that ¿STAT was a critical trigger of this gene network for antiviral defense.¿ So, using bronchoscopy and microscopy, they sampled the airway epithelia in 24 asthma-prone patients, plus cohorts of bronchitis-prone and normal individuals.
STAT Lurked In Asthma-Prone Epithelia Only
¿We brushed off some of the airway epithelial cells, and through the bronchoscope snipped a piece of airway tissue,¿ Holtzman said. ¿In both cases, we were able to assess STAT¿s activation state. If it was in the cytoplasm, we presumed it was inactive; if in the nucleus, active. And looking at those we saw it was activated in all cases in asthma, but not in the other two cohorts.¿
Holtzman is senior author of a paper in the May 1 issue of the twice-monthly Journal of Clinical Investigation. Its title: ¿Constitutive activation of an epithelial signal transducer and activator of transcription (STAT) pathway in asthma.¿
Looking toward therapeutic application of his findings, he said, ¿What we¿re proposing to do is develop engineered proteins that target STAT and down-regulate it to a normal level. It so happens we have a protein, the E1A oncoprotein, from the early region of adenovirus. So, when it infects the airways, E1A knocks out the immune response genes and stimulates the cell proliferation genes, so the virus goes on its merry way and makes us sick.
¿Therefore,¿ he said, ¿we engineered a mutant E1A that interfered with STAT. That allows us then to target STAT in the epithelial cell. We¿ve done it so far only in cells. Now, we¿ve engineered the whole thing in a gene therapy vector. So, we¿re ready to try it in a mouse model, and at some point probably somebody more commercially oriented than me will give it a try in a human trial.¿ n