How does one go about developing an animal model for a human disease? Our ability to model the disease depends on our knowledge of the human phenotype that we are trying to model. In addition, what we call a “disease” may be more appropriately labeled as a “syndrome” because multiple overlapping or even distinct pathogenetic mechanisms may lead to different manifestations of a single disease. Thus, one should use animals to model specific disease phenotypes, not the whole syndrome.
Models are most likely to mimic true pathogenetic pathways when they are based on the known etiology or insult that causes the human disease. For example, with chronic obstructive pulmonary disease, we have an advantage in that we know that cigarette smoking causes it, and that exposure of mice to cigarette smoke leads to inflammation and airspace enlargement similar to that seen in humans (4). However, proper modeling is made difficult by the fact that long-term, low-dose exposure is required. Although this model is better suited for emphysema than for small airway disease for the anatomic reasons stated, modeling in infection could lead to a more pronounced airway phenotype. Now that we have identified specific mutations in protooncogenes (e.g., kras) (5) and growth factors (e.g., EGFR) in humans that lead to cancer, we are finding a remarkable ability to replicate the disease and response to therapy in the mouse (6). Obliterative bronchiolitis is a classic example of a disease for which we know the etiology—lung transplantation. Yet, the small size of the mouse precludes this surgical procedure.
Intratracheal bleomycin administration, a model for the fibroproliferative phase of acute respiratory distress syndrome and perhaps pulmonary fibrosis, is another complicated model (7). Working in its favor is the fact that bleomycin is a cause of human fibrosis; however, the acute inflammation–driven model that leads to fibrosis is different from the idiopathic pulmonary fibrosis that might have been initiated by inflammation but on diagnosis has switched to an inflammation-independent fibroproliferative process. This concept of acute inflammation being necessary to initiate but not necessarily sustain established disease is likely relevant to asthma models as well.