Not-So-Mad Science: IL-13 vs. IL-4 In The Battle For Asthma!

(Previous asthma research-blogging here)

Marsha Wills-Karp, Jackie Luyimbazi, Xueying Xu, Brian Schofield, Tamlyn Y. Neben, Christopher L. Karp, Debra D. Donaldson (1998). Interleukin-13: Central Mediator of Allergic Asthma Science, 282, 2258-2261

Abstract:
The worldwide incidence, morbidity, and mortality of allergic asthma are increasing. The pathophysiological features of allergic asthma are thought to result from the aberrant expansion of CD4(+) T cells producing the type 2 cytokines interleukin-4 (IL-4) and IL-5, although a necessary role for these cytokines in allergic asthma has not been demonstrable. The type 2 cytokine IL-13, which shares a receptor component and signaling pathways with IL-4, was found to be necessary and sufficient for the expression of allergic asthma. IL-13 induces the pathophysiological features of asthma in a manner that is independent of immunoglobulin E and eosinophils. Thus, IL-13 is critical to allergen-induced asthma but operates through mechanisms other than those that are classically implicated in allergic responses.

There are many morbidly fascinating pathological changes associated with onset and clinical asthma. To wit, these include eosinophilia, mucus overproduction, mast cell/other inflammatory cell airway infiltration, and increased smooth muscle. There may also be scarring of the airways.

From what I currently understand about it, airway hypersensitivity generally happens after immune effector cells have infiltrated the underlying airway tissues. When these effector cells, which can include allergen-specific T-cells, mast cells, eosinophils, basophils, and even macrophages, are activated by an irritant (the allergen) they more or less cut loose and let wild with the localized inflammation. The localized inflammation, in turn, leads to more immune cell infiltration over time and concurrently the tissue itself undergoes histopathologically apparent changes, including thickening of the base layer of smooth muscle.

For example of immune effector cells getting activated, let's consider the most dramatic case: the mast cell. Mast cells are a type of white blood cell that expresses Fc receptors for IgE (IgE is the immunoglobulin most associated with allergic and anti-parasite responses) on it's surface. The Fc-bound IgE act as allergen-specific receptors that, when bound to their ligand, send a signal into the cell to degranulate. Mast cells store relatively massive amounts of inflammatory cytokines and peptides in large granules (e.g., histamines, prostaglandins, and leukotrienes) and they can, effectively, disgorge them all at once. This can lead to a very rapid spike in the systemic concentration of inflammatory effector molecules and subsequently extremely rapid onset of asthmatic symptoms. The same process is at work in acute food allergies.

Figure A: The mast cell is the one with the big lumpy nucleus in the center. The black dots surrounding it are granules packed with inflammatory molecules, just waiting to be released and wreak havoc. Those other 2 cells to the right are lymphocytes (according to the original caption on this TEM).

But what inflammatory molecules are required to invoke and/or sustain a hypersensitive airway response?

This paper examined the role of IL-13 in allergic asthma. According to Janeway's Immunobiology, IL-13 is involved in the differentiation of naive CD4+ T-cells into TH2 cells, which have been shown to be more involved in allergy than TH1 cells. IL-13 is also secreted by TH2 cells, apparently, and has been shown to have a direct effect on airway epithelial cells by which their proliferation in increased and differentiation into goblet cells (goblet cell metaplasia) is increased, which in turn leads to the increased mucus production seen in allergy and asthma. And when your organs are infected with multicellular parasites, IL-13 is there to help the organs make the changes they need to get rid of those parasites. And as if that weren't enough, IL-13 also increases smooth muscle contractility.

But IL-13 doesn't really do anything without the context of a TH2 immune response. Th2 cells are characterized by secretion of IL-4, and it should be noted that IL-4 and IL-13 share a subunit in their receptors.

Figure B: The left column has a normal lung biopsy (top) and a normal airway (bottom) from a Tbet+/+ mouse. The right column has the same measurements, but showing airway inflammation with lymphocyte and eosinophil infiltration (top) and remodeled airway with increased collagen (bottom) from a Tbet-/- mouse. The picture is blurry because I took it with my phone. It is from Janeway's Immunobiology, 7th ed., page 575. Tbet is a transcription factor that is necessary for the development of TH1 cells, so its abscence will invariably result in a TH2 inflammatory response (right column). Tbet is analogous to GATA3 in TH2 cells.

Allow me to explain T-cell differentiation really briefly:

1) Naive T-cells arrive in thymus.
2) Naive T-cells have to decide whether or not to be CD4+ or CD8+, which will result in being able to recognize MHCII or MHCI, respectively.
3) CD4+ T-cells get stimulated by DCs or stuff, and the resulting cytokine mileau determines whether they become TH1, TH2, Treg (also refered to as TH3), or TH17. They can also become memory T-cells of any of those variety later on in. Respectively, these cell types are characterized by secretion of IFNg, IL-4/IL-5, IL-10, and IL-17.
4) TH1, TH2, Treg, and TH17 all more or less have distinct biological roles, although the cytokine soup that gives rise to different types is messy (e.g., IL-2 just drives T-cell proliferation irrespective of subset) and often overlaps, and they'll even compete against each other (IL-12 drives TH1 proliferation but inhibits TH2 proliferation while IL-4 does the same for TH2 cells).

So, anyway, the group behind this paper found that while IL-4 is sufficient to initiate asthmatic events, IL-13 is required for the development of the airway hypersensitivity response (AHR). They used the standard ovalbumin (OVA) induced model of AHR and found that blocking IL-13 with an neutralizing fusion protein prevents the development of AHR. Apparently blocking IL-13 in mice who already have AHR results in their measures of AHR decreasing (specifically goblet cell metaplasia and mucus production). However, with all of this, blocking IL-13 had no effect whatsoever on net circulating IgE or eosinophilia.

These findings prove that IL-13 has a significant role in asthma. But they also imply that IL-13 does not play this role through classical allergy pathways, as IL-13 is found to be elevated in patients with both allergic and non-allergic asthma. This is further supported by the group's finding that daily intratracheally administration of IL-13 is sufficient to induce asthmatic pathology even in the abscence of antigen sensitization.

What I wonder about here is: how does it make biological sense for a molecule involved heavily in the production of allergen-specific TH2 cells to also operate completely independently of that cellular phenotype?

But what is important to human health is that this paper demonstrates that adminstration of IL-13 agonists or blockers may be of great therapeutic value to human asthmatics. This paper is 11 years old, and I don't currently know if anything has come of their findings, but still, it'd be cool if this really did have therapeutic value because, as my last post on asthma discussed (link up top), inhaled acute anti-inflammatories may only be getting to the pieces of lung that need it least (because they're the pieces that can still pump air, and if reacting tissue isn't pumping tidal volume, how can inhaled medicine get to it?). If this could be used daily as a preventative, I think it could greatly improve the quality of life for asthmatics everywhere.