To understand how a specific immune response leads to chronic lung disease, it was critical to generate a representative experimental model that demonstrated a close linkage between acute viral infection and the subsequent development of chronic lung disease. To be faithful to the clinical condition, the model should also include the element of genetic susceptibility as well as virology, immunology, and pathology components that are similar to what occurs in humans. In that regard, epidemiological studies of humans indicate that respiratory syncytial virus (RSV) is the most common cause of serious respiratory illness in infancy and that this particular paramyxovirus is frequently associated with the later development of persistent asthma (Castro et al., 2008
; Sigurs et al., 2005
). Some experimental evidence suggests that a related paramyxovirus known as human metapneumovirus (hMPV) can cause chronic airway disease in mice, but mucus production was increased for only a short period of time after infection (Hamelin et al., 2006
). There is also a reported association between rhinovirus infection and childhood asthma (Jackson et al., 2008
), though in this instance, rhinoviruses appear to be associated with acute exacerbation of existing disease rather than acting as a primary cause of chronic disease. Thus, RSV appears to contain distinct ingredients to drive the chronic inflammatory lung disease that develops after the resolution of acute infection.
Based on the epidemiological findings from clinical studies, investigators have often used infection with RSV to develop a model of virus-induced chronic lung disease. In general, however, RSV (like most human pathogens) replicates poorly in the mouse lung unless the virus is first adapted using serial passage or other experimental approaches. To circumvent these issues, RSV is delivered at a high inoculums, but the resulting all-or-none pattern of illness often includes a severe alveolitis rather than the primary bronchiolitis that is typical of human disease (Graham et al., 1988
). To better capture this critical feature of human airway disease, we selected an infectious agent known as Sendai virus (SeV). This virus is a mouse parainfluenza-type I virus that is similar to the other paramyxoviruses (e.g., RSV, hMPV, and human parainfluenza virus) that more commonly infect humans. In contrast to infection with human pathogens such as RSV and hMPV, SeV replicates at high efficiency in the mouse lung and causes an acute virus-mediated inflammation of the small airways that is essentially indistinguishable from the RSV-mediated disease observed in humans (Shornick et al., 2008
; Tyner et al., 2005
; Walter et al., 2001
). In particular, the pattern of illness after SeV inoculation resembles the so-called “top-down” infection found in humans. In this case, an intermediate inoculum causes infection limited to the airway mucosa and inflammation that is largely restricted to peribronchial and bronchiolar tissues. Smaller inoculum will cause illness confined to the upper airways (i.e., sinusitis and bronchitis) whereas larger inoculums will cause disease that extends to the alveolar compartment (i.e., pneumonitis). Because chronic inflammatory disease is likely to be found in the small pulmonary airways, we suspect that a severe infection at this site is critical for the subsequent development of chronic lung disease (similar to the case in humans where severe RSV bronchiolitis is associated with chronic asthma).
Consistent with these principles, we found that the acute antiviral response to SeV infection is followed by a delayed but permanent switch to chronic airway disease in mice. This disease is characterized by overproduction of mucus [marked by mucous cell metaplasia (MCM)] and increased airway reactivity to inhaled methacholine (defined by AHR) (Patel et al., 2006
; Tyner et al., 2006
; Walter et al., 2002
). These disease traits of MCM and AHR are hallmark features of asthma and COPD in humans. These traits are also the primary causes of morbidity and mortality in these conditions, and therefore are primary targets for therapy. As developed in this review, the airway disease is first detected at 3 weeks after viral inoculation, reaches its maximal level at 7 weeks, and persists for at least a year later (Kim et al., 2008
; Patel et al., 2006
; Tyner et al., 2006
; Walter et al., 2002
). This time course is consistent with the one in humans, wherein chronic lung disease also lasts indefinitely after infection.
As noted above, another characteristic of chronic lung disease in humans that needs to be represented in the mouse model is genetic susceptibility. In that regard, we have observed that the development of chronic lung disease after SeV infection is manifested most vigorously in the C57BL/6J strain of inbred mice. By contrast, the BALBc/J strain of mice exhibits a very similar pattern of acute illness in the first week after viral inoculation, but fails to develop any significant acute disease at 3 weeks or chronic disease at 7 weeks after inoculation (Patel et al., 2006
). Other mouse strains (e.g., CV129, C3H/HeJ, or A/J) are so sensitive to SeV (and develop such severe alveolitis) that it is difficult to capture the top-down pattern of illness that is typical of severe RSV infection in humans. In both C57BL/6J and BALBc/J mice, the initial reverse transcriptase polymerase chain reaction (RT-PCR) analysis of virus in whole-lung homogenates indicated that SeV was completely cleared before the onset of airway disease on postinoculation week 3 (Patel et al., 2006
; Walter et al., 2002
). However, more sensitive PCR assays indicated that low levels of virus may persist for longer periods of time in each of these mouse strains (Kim et al., 2008
) and (E. Agapov and M. J. Holtzman, unpublished observations). The role for this remnant viral RNA in driving a chronic immune response still needs to be fully defined. This role as well as the one for host genetics will likely only be resolved after we define the type of immune response that causes the chronic disease found in this model. In that regard, the C57BL/6J strain provides a suitable genetic background for transgenic and knockout mice that could be used to define the immune mechanism for chronic inflammatory disease after viral infection.