Developmental Effects of Ambient Air Pollution

First off, props to Dr. Isis, who in the discussion of her Cairo haze post referenced the work of L. Calderón-Garcidueñas, which set me off into the literature. What I have initially found was sobering.

Air isn't something we often think about. We take it entirely for granted. We complain about how hot or humid it is, but we usually don't have to think about what in that air might be making us, or our children, sick. Those of us fortunate enough to live in Western, industrialized countries really are well-off in that we don't ever really have to pit our own health against the simple act of breathing. Other areas in the world aren't so fortunate.

Dr. Calderón-Garcidueñas' work focuses on the effects of airborne pollutants and particulate matter on the developing lungs of children in Mexico City, Mexico. But her work is actually relevant to anywhere that the air isn't shiny clean.

By radiograph, her team found that children in Mexico City with lifelong exposure to the polluted air had significantly increased rates of lung hyperinflation when compared to age-matched controls from a much less polluted area (Tuxpam, Ver [I'm guessing this is somewhere else in Mexico?]). Lung hyperinflation by itself in one person doesn't indicate a lot, it just means that there is more air in the small alveoli of the lung so it appears larger on a radiograph. This could mean asthma, emphysema, or even lung cancer, but could also just mean the patient was breathing hard. But when there is such a significant (p=0.0004) between groups it becomes indicative of pollution-induced lung dysfunction.

This is in addition to previous work demonstrating that children in this severly polluted area of Mexico City had altered nasal apparatus such that the mucociliary clearing/filter mechanism wasn't working so well as it should be. This is troubling because the loss of the nasal ciliary filter leads to an even greater dose of reactive gasses or particulate matters getting past it into the lower lungs, thus exacerbating the initial problem.

Referenced experiments in this paper showed even more going on. Particulate matter (PM) less than 5um in aerodynamic diameter was found to disproportionately wind up in the alveolar sacks of the lung while PM greater than 10um was found to locate to the proximal bronchioles. PM doesn't seem to affect lung epithelia directly, but it does activate alveolar macrophages to produce inflammatory cytokines (IL-6 and TNF) that utlimately will attract additional inflammatory infiltrate and further damage lung tissues. On the other hand, reactive gasses were found, in vitro and in animal models, to elicit secretion of IL-6, IL-8 (both inflammatory), and fibronectin (involved in repair if tissue damage) in lung epithelial cells.

Together, these 2 overlapping respsonses indicate the air pollution is driving a repetitive damage-repair cycle in lung tissues. Pollution damages lung tissue and activates the immune system to produce and inflammatory response. So the lung tissue tries to repair itself in an inflammed environment, which can lead to scar-like tissue regeneration. The altered tissue regeneration (as in the loss of the nasal cilia above) can lead to lung tissue more vulnerable to subsequent pollution injury. Over time, this kind of cycle leads to long-term tissue remodeling such as increased constrictive reactivity of existing smooth muscle (a hallmark of acute asthmatic responses), increased smooth muscle metaplasia (chronic asthma), eosinophilia (asthma and allergy reactivity), and even destruction of the walls dividing alveoli (a hallmark of emphysema). And what's saddest about this is that moving animal models of exposure to clean air did not completely reverse the exposure-related pathologies, which means that the lung damage will be a life-long legacy of any children that grew up in it.

So take a deep breath, and be thankful it's mostly clean.

Calderón-Garcidue˜nas, L. (2000). Respiratory tract pathology and cytokine imbalance in clinically healthy children chronically and sequentially exposed to air pollutants Medical Hypotheses, 55 (5), 373-378 DOI: 10.1054/mehy.2000.1070

(P.S. - Whomever came to this blog searching for "burning bronchioles so they won't constrict", please don't. I can think of few worse ways to die than by one literally drowning in necrotic, burnt lung tissue while gasping painfully for whatever air they can still get.)

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.