**3. Lung cells contribute to the systemic inflammatory response induced by lung inflammation**

The cells lining the airways are mainly epithelial cells but also include alveolar macrophages and both cell types are exposed to the external environment. They are the first responders in the lung when the lung is exposed to external factors such as cigarette smoke, air pollutants or infectious agents. These cells are critically important in the processing and neutralization of inhaled environmental contaminants which include airborne particulate matter (PM), cigarette smoke, bacteria and viruses, shown in Figure 2. Alveolar macrophages are one of the most potent producers of inflammatory mediators in the lung. It is known that human alveolar macrophages exposed to PM10 (EHC-93) [46] are able to phagocytose these particles *in vivo* [43] and *in vitro* [45] and produce, in a dose-dependent manner, an array of mediators such as IL-1β, IL-6 and TNF-α that are part of the innate immune response. To test the contribution of the mediators produced by alveolar macrophages to the systemic response, supernatants from alveolar macrophages, incubated *ex vivo* with urban PM, were instilled into the lungs of rabbits. The supernatants produced a systemic bone-marrow stimulation response similar to that produced by direct deposition of urban PM into the rabbit lung [42, 43]. Analysis of the supernatants showed that the proinflammatory mediators IL-1β and IL-6, the chemokine macrophage inflammatory protein (MIP)-1α and granulocyte macrophage colony-stimulating factor (GM-CSF) are elevated when macrophages are incubated with urban PM [45]. Studies showing a strong relationship between the quantity of particles phagocytosed by macrophages in lung tissue and the magnitude of the systemic response, after urban PM exposure (Figure 3), support the notion that the production of inflammatory mediators by alveolar macrophages is important and suggests that alveolar macrophages are significant contributors to the innate component of the systemic response following an inflammatory stimulus in the lung

Similar experiments using bronchial epithelial cells showed that, when exposed to urban PM, cells produce excess GM-CSF, IL-1β, IL-6, TNF-α, IL-8 and leukemia inhibitory factor (LIF) in a dose-dependent manner [47-49]. Some overlap was evident when comparing mediators produced by alveolar macrophages with those produced by bronchial epithelial cells after exposure to similar doses of urban PM, however, some distinct differences in the type and the magnitude of cytokine production was observed (Figure 4). The relative contributions of macrophages and epithelial cells in the production of mediators responsible for the systemic inflammatory response need to be determined. Alveolar macrophages are professional phagocytes and the magnitude of their cytokine production is significantly higher than bronchial epithelial cells, after the same level of exposure (Figure 4). These studies suggest that alveolar macrophages are key effector cells, responsible, at least, for generating the systemic inflammatory response associated with exposure to air pollution. However, although the macrophages are more potent producers of proinflammatory mediators expressed per cell basis, the airspace epithelial cells out-number the alveolar macrophages approximately ten Figure 2

times. Furthermore, the interaction between macrophages and epithelial cells has a synergistic effect on the production and release of mediators involved in the systemic inflammatory response [50], therefore alveolar macrophages and airspace epithelial cells both play central roles in the activation of the innate immune response and the production of inflammatory

Nature and Consequences of the Systemic Inflammatory Response Induced by Lung Inflammation

http://dx.doi.org/10.5772/57392

85

**Figure 4.** Cytokines produced by human AMs and bronchial epithelial cells (HBECs) when exposed to 100 µg/mL of PM10 (EHC-93] for 24 h. Differences between two groups were compared by Mann–Whitney U test. Alveolar macro‐ phages produced significantly more IL-6, IL-1β and GM-CSF than bronchial epithelial cells when exposed to the same

The roles of other lung cells such as connective tissue cells (fibroblast, smooth muscle cells), immune cells (lymphocytes and dendritic cells) and vascular cells (endothelium) in the systemic response to lung inflammation are less clear. Several studies have documented increased levels of endothelial specific markers (soluble P, E and L-selectin, intercellular adhesion molecule [ICAM]-1, vascular cell adhesion molecule [VCAM]-1 and endothelin-1) present in the circulation during lung inflammation [51-53] but whether these mediators come directly from the lung or are released secondary to the initial circulating proinflammatory mediators such as IL-1β and TNF-α, is unclear. Mediators released from connective tissue cells and immune cells of the adaptive immune responses tend to be more localized in cellular niches

**4. Mediators of the systemic inflammatory response induced by lung**

Lung inflammation has been associated with an array of different circulating cellular or noncellular mediators that may differ significantly depending on the type and the character of the

mediators involved in the systemic response to lung inflammation.

amount of PM10 [162].

**inflammation**

with less of a systemic consequence.

inflammatory response in the lung.

**Figure 2.** Photomicrographs of ambient particles phagocytosed by alveolar macrophages (A and D) and bronchial epi‐ thelial cells (B and C). **A and B:** Ambient particles (EHC-93] in alveolar macrophages (A) and both type I and type II epithelial cells (B) in rabbits exposed to 5 mg EHC-93 twice a week for 4 wks. **C:** Particles in primary cultures of human bronchial epithelial cells exposed to EHC-93 [100 µg/ml) for 24h. **D:** Particles in alveolar macrophages exposed to EHC-93 [100 µg/ml) for 24 h. The bar represents 10 µm [162]. Figure 3

**Figure 3.** Relationship between the fraction of alveolar macrophages (AMs) that phagocytosed PM10 particles and the transit time of PMNs though the bone marrow. Rabbits were exposed to 5 mg PM10 (EHC-93) twice a week for 4 weeks, and AMs with particles in their cytoplasm were enumerated using quantitative histological methods. Dividing PMNs in the marrow were labeled with 5-bromo-2-deoxyuridine and the transit time of PMNs through the bone mar‐ row was measured. Faster transit times of PMNs through the marrow were associated with an increased percentage of AMs with phagocytosed particles (*R*2 = 0.46, p < 0.05) [162].

times. Furthermore, the interaction between macrophages and epithelial cells has a synergistic effect on the production and release of mediators involved in the systemic inflammatory response [50], therefore alveolar macrophages and airspace epithelial cells both play central roles in the activation of the innate immune response and the production of inflammatory mediators involved in the systemic response to lung inflammation.

**PM10 particles**

R2 = 0.463 p < 0.05

**(%)**

**C PM D <sup>10</sup> particles**

EHC-93 [100 µg/ml) for 24 h. The bar represents 10 µm [162].

of AMs with phagocytosed particles (*R*2 = 0.46, p < 0.05) [162].

Transit time of neutrophilsthrough

Figure 3

the bone marrow(hrs)

**Figure 2.** Photomicrographs of ambient particles phagocytosed by alveolar macrophages (A and D) and bronchial epi‐ thelial cells (B and C). **A and B:** Ambient particles (EHC-93] in alveolar macrophages (A) and both type I and type II epithelial cells (B) in rabbits exposed to 5 mg EHC-93 twice a week for 4 wks. **C:** Particles in primary cultures of human bronchial epithelial cells exposed to EHC-93 [100 µg/ml) for 24h. **D:** Particles in alveolar macrophages exposed to

**6 8 10 12 14 16 18 20 22 24**

% PM10 positive alveolar macrophage

**% PM10 positive Alveolar Macrophage**

**Figure 3.** Relationship between the fraction of alveolar macrophages (AMs) that phagocytosed PM10 particles and the transit time of PMNs though the bone marrow. Rabbits were exposed to 5 mg PM10 (EHC-93) twice a week for 4 weeks, and AMs with particles in their cytoplasm were enumerated using quantitative histological methods. Dividing PMNs in the marrow were labeled with 5-bromo-2-deoxyuridine and the transit time of PMNs through the bone mar‐ row was measured. Faster transit times of PMNs through the marrow were associated with an increased percentage

Figure 2

84 Lung Inflammation

**Figure 4.** Cytokines produced by human AMs and bronchial epithelial cells (HBECs) when exposed to 100 µg/mL of PM10 (EHC-93] for 24 h. Differences between two groups were compared by Mann–Whitney U test. Alveolar macro‐ phages produced significantly more IL-6, IL-1β and GM-CSF than bronchial epithelial cells when exposed to the same amount of PM10 [162].

The roles of other lung cells such as connective tissue cells (fibroblast, smooth muscle cells), immune cells (lymphocytes and dendritic cells) and vascular cells (endothelium) in the systemic response to lung inflammation are less clear. Several studies have documented increased levels of endothelial specific markers (soluble P, E and L-selectin, intercellular adhesion molecule [ICAM]-1, vascular cell adhesion molecule [VCAM]-1 and endothelin-1) present in the circulation during lung inflammation [51-53] but whether these mediators come directly from the lung or are released secondary to the initial circulating proinflammatory mediators such as IL-1β and TNF-α, is unclear. Mediators released from connective tissue cells and immune cells of the adaptive immune responses tend to be more localized in cellular niches with less of a systemic consequence.
