**3. Pneumonia in the laboratory: animal models and mechanisms of infection**

#### **3.1. Animal models of pneumonia**

commonly affected by upper RTIs, such as sinusitis, tonsillitis, and otitis externa. On the other hand, men are at higher risk of developing otitis media, croup, and lower RTIs, including CAP [11]. Furthermore, these infections are more severe and show poorer outcomes and more complications in male than female individuals, leading to increased mortality, especially in CAP [21]. To date, the specific contributions of sex hormones or other factors, such as exposure to air pollution, socioeconomic, racial, and/or behavioral factors, obesity, and other comorbidities have only been explored in small studies [24, 53–56]. Several other factors including anatomic differences of the respiratory tract, behavioral, socioeconomic, and lifestyle factors have also been related with differences in incidence and severity of respiratory infections between genders [11, 41]. **Table 1** summarizes epidemiological data on associations of sex and environmental exposures on various lung health

**CAP pathogens Environmental associations/comorbidities**

*Anaerobes*, *Gram-negative bacilli* Aspiration, enteric chemical pneumonitis

*Coxiella burnetii* (Q fever) Contact with farm animals or parturient cats

**Table 3.** Common pathogens in community-acquired pneumonia and environmental associations.

*Coccidioides immitis* Travel to southwest USA

*Histoplasma capsulatum* Exposure to bats

*Anaerobes* Poor dental hygiene

Nursing home resident

Chronic obstructive lung disease

Recent influenza infection

Recent antibiotic therapy

Exposure to birds

Exposure to rabbits

Congestive heart failure, mixed infections

Structural disease of the lung: (bronchiectasis, cystic

Alcoholism

HIV infection

fibrosis)

*Streptococcus pneumoniae*, *Gram-negative bacilli*, *Anaerobes*, *Haemophilus influenzae*, *Staphylococcus aureus* (including *methicillin-resistant* forms), *Chlamydophila pneumoniae*,

*Streptococcus pneumoniae* (including drug-resistan*t S. pneumoniae*), *Anaerobes, Gram-negative bacilli*

*Streptococcus pneumoniae*, *Haemophilus influenzae*, *Salmonella*, *Cytomegalovirus*, *Cryptococcus*, *Pneumocystis jirovecii*, *Anaerobes*, *Mycobacterium tuberculosis*

*Streptococcus pneumoniae*, *Haemophilus influenzae*,

*Pseudomonas aeruginosa*, *Pseudomonas cepacia*,

*Streptococcus pneumoniae*, *Staphylococcus aureus*,

*Chlamydophila psittaci*, *Cryptococcus neoformans*,

Drug-resistant *pneumococcus*, *Pseudomonas aeruginosa*,

*Parainfluenza virus*, *Influenza virus A* and *B, Respiratory* 

*Mycobacterium tuberculosis*

8 Contemporary Topics of Pneumonia

*Moraxella catarrhalis*, *Legionella*

*Staphylococcus aureus*

*Haemophilus influenzae*

*Gram-negative bacilli*

*Histoplasma capsulatum*

*syncytial virus* (RSV)

outcomes including pneumonia in children and adults [10, 27, 57–72].

Wide-ranging research is required to understand the mechanisms underlying pulmonary diseases, such as pneumonia. Studies of human populations, *in vitro* experiments, and exploratory infections of species are needed to advance in the development of new treatments for this condition. Animal models have been widely used in the field and have often provided insight into the physiological processes associated with the disease.

A variety of species have been used as animal models of pneumonia. Even though some species, such as *Danio rerio* (zebrafish) and *Caenorhabditis elegans* (roundworm) do not use lungs to acquire oxygen, and do not have similar sexual characteristics when compared to humans, they can be useful models to provide valuable information about host-pathogen interactions in lung disease [73]. Researchers utilize zebrafish as an alternative vertebrate model to study the pathogen's ability to infect the host [74]. Because zebrafish embryos and 3-week old larvae look transparent, it is feasible to follow the evolution of lung infection in real time [74–76]. In addition, zebrafish have a developed adaptive immune system and a high rate of conserved gene orthologs in humans [77, 78]. On the other hand, *C. elegans* is used as a non-vertebrate model for studying lung bacterial agents. Interestingly, the immune system of *C. elegans* and humans has similar signaling cascades in response to infection [79]. Despite anatomical differences, it is possible to recognize pathogen-specific virulence factors in epithelial surfaces of *C. elegans*, making this model ideal for the study of host defense mechanisms. Likewise, insects, such as *Drosophila melanogaster* (fruit fly) are valuable models of infection for the analysis of bacterial pathogenesis and genetic contributions. Insects have an advanced antimicrobial defense mechanism and a complex and conserved immune system [80]. In addition, a large number of genes that encode for proteins in the immune system are found on the X chromosome, which promote a higher activation of toll and immune deficiency signaling in *D. melanogaster* females than males [81]. Together, all these species possess advantages, such as low cost of maintenance, short life span, small size, fast development, and rapid reproduction making them feasible models for the study of infectious diseases. However, most pneumonia studies performed in animals are conducted in mammals because of their anatomical, genetic, and morphological similarities with humans.

Larger mammalian species, such as rabbits, piglets, and primates are ideal for specialized experiments when physiological monitoring and therapies are evaluated [82]. Currently, primates are the only species able to assess primate-specific infectious agents, but due to ethical concerns, piglets are the most frequently used model to study ventilator-associated pneumonia (VAP). Even though large mammalian animals are phylogenetically close to the human species, the disadvantages associated with their use as models is that they are only useful for a limited number of studies, and they are expensive to house and feed, slow to breed, and genetically diverse. For this reason, infections in the lung have primarily been studied in small mammalian species, predominantly rodents. Rodents are small, inexpensive, and highly reproductive. Inbred strains are preferred to investigate genetically identical groups by facilitating the use of molecular approaches to understand the mechanisms of diseases. Since studies in mice have become popular in scientific research, the creation of new studies benefit from the extensive literature available regarding genetic engineering, immunological responses to pathogens and host defenses.

to the *C. albicans* resistance loci that modifies host responses in these mice. In addition, studies of the *Klra* natural killer (NK) receptors have demonstrated that *Klra15 is* expressed in 129/J mice, and *Klra12* is expressed in CBA/J and C3H/He mice. None of these, however, are

Understanding the Intersection of Environmental Pollution, Pneumonia, and Inflammation: Does...

http://dx.doi.org/10.5772/intechopen.69627

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Following infection, both human and mouse lungs produce immune mediators, such as cytokines, chemokines, and other components of the immune system. A regulator of IL-1β that is also highly expressed in mouse and human lungs after infection is prostaglandin E

Viruses can also lead to pneumonia. Influenza A and B viruses are the most common causes of pneumonia in adults, but other viruses can contribute to the disease development. The susceptibility of mice models to influenza viruses depends on the strain of virus used. The most commonly used strains in research are A/Puerto Rico/8/1934 (H1N1, PR8) or A/WSN/1933 (H1N1, HSN). Researchers also use several pandemic viruses, such as the 1918 H1N1 pandemic strain, highly pathogenic avian influenza (HPAI) viruses of the H5N1 subtype, certain H7 subtype viruses, a subset of low pathogenic avian influenza viruses, and the 2009 H1N1 pandemic strains. After viral infection in mice, several immunomodulatory mediators are released including IL-1β, IL-6, IL-8, MCP-1, MIP-1α/β, interferon-gamma inducible protein

(IP-10) and interferon-beta (IFN-β) in a somewhat strain-specific manner [98–100].

strains and molecular pathways in pneumonic mouse lungs may differ.

Animal research in viral pneumonia employs either BALB/c or C57BL/6 mice [68]. The majority of laboratory mice are vulnerable to disease and death after infection, whereas, wild mice are resistant to exposure. This is due to the lack of the antiviral factor Mx1 protein in inbred strains [72]. On the other hand, it is possible for researchers to adapt strains to mouse models. DBA/2J and A/J mice are more susceptible to diseases, even with viral isolates that were not adapted to mice, than the more frequently used BALB/C and C57BL/6 strains. Even though mouse-adapted strains are important to model seasonal H1N1 and H3N2 virus infections, certain influenza viruses cause disease in mice without prior adaptation [101]. Therefore, the interpretation of research outcomes in a particular strain may not be applicable in other

Typically, Th1 cells are important in the clearance of intracellular pathogens, whereas Th2 cells are associated with responses to parasites. C57BL/6 mice display a typical Th1-type bias to pathogens, whereas other strains, such as BALB/c, A/J, and DBA/2 mice, tend toward a Th2 response [102]. These variations may also be reflected in the M1 and M2 macrophage responses to antigen stimulation. In addition, the region *D7Mit341* to *D7Mit247* on mouse chromosome 7 has been reported to be a survival trait against illness associated with *S. pneumoniae*. Susceptibility to experimental pneumococcal infection is strain dependent. In this

peroxide leading to DNA damage and apoptosis in lung cells [50, 97].

), and its precursor enzyme cyclooxygenase-2 (COX2) [52]. Studies using depletion of alveolar macrophages have demonstrated that these contribute largely to the stimulation of pro-inflammatory cytokines, such as IL-6 and TNFα [48]. Moreover, interleukin-1β (IL-1β) is induced only by strains containing the cholesterol-dependent cytolysin, pneumolysin (PLY), a major virulence factor of pneumococci infection [51]. In addition, the levels of tolllike receptor 2 (TLR2) and toll-like receptor 4 (TLR4) increase after *S. pneumoniae* infection in the Crl:CD1 mouse strain [96]. Both BALB/c mice and human lungs liberate hydrogen

expressed in C57BL/6 mice [95].

(PGE<sup>2</sup>
