**9. T cells and Alveolar macrophages**

T cell mediated immunity is important in the lung in order to protect the host from inhaled pathogens and environmental antigens [89-91]. Furthermore, most of the environmental antigens encountered by the lung are innocuous. Hence it is crucial for the resident immune cells to distinguish such inert antigens from those derived from pathogens and respond appropriately. The immunoregulatory mechanisms employed by the lung include the components for surfactant lining [92], products secreted by alveolar type II epithelial cells as well as alveolar macrophages which altogether actively suppress T cell proliferation. More‐ over, AM induces anergy (immune unresponsiveness) in T cells which is reversible upon its removal from culture [91]. T cells isolated from lungs of rodents express low levels of IL2 receptor (IL2R/CD25) [91]. The Immunoregulators associated with AM include prostaglan‐ dins, leukotrienes, IL1 and nitric oxide [89]. AMs also downregulate local pulmonary immune responses against intratracheally administered T cell dependent antigens [93].

AMs from rat, mouse and human differ markedly in its potency to inhibit mitogen-induced T cell proliferation. However, T cells stimulated in the presence of AMs display a similar phenotype in all species examined, i.e. CD3 downregulation, upregulation of IL2R, increased IL2 production and inability to respond to IL2 [94]. Thus, AMs appear to allow T cell activation and effector functions, while selectively inhibiting T cell proliferation. The suppressive activity of AMs is restricted to the final step in the activation process, (i.e. proliferation). Through CD3 downregulation, IL2R expression and IL2 secretion appear to proceed normally [94].

T cells from humans produce less IL2 than autologous blood-derived T cells [95]. FoxP3 is a master regulator of regulatory T cells (Treg). FoxP3 expression is regarded as definitive marker for cells with regulatory function in mice and humans [96, 97]. AMs obtained from BAL of mice or humans enhance FoxP3 expression in CD4+FoxP3-T cells in vitro. Activation of naïve T cells in the presence of either AMs or AM-conditioned media prevents T cell proliferation. This effect can be reversed by inhibiting binding of retinoic acid to its receptor or by blocking TGFβ1 signaling [80]. AMs induce intracellular FoxP3 expression in CD4+T cells. It promotes IL10 while suppressing IFNγ production by the same T cell in vitro [80]. AMs induce an immune unresponsiveness in T cells and also affects its proliferation.

#### **10. Role of macrophages in chronic respiratory diseases**

Macrophages have the ability to elicit an immune response and resolve the inflammatory processes. The macrophages of the respiratory system are involved in the pathogenesis of different respiratory diseases such as COPD, asthma and pulmonary fibrosis. Different phenotypes of macrophages are involved in these respiratory diseases which play an impor‐ tant role in either inflammation and / or resolution process.

## **11. Chronic obstructive pulmonary disease (COPD)**

YM1 are also expressed in higher levels and are used as phenotypic markers for M2 polarized macrophages. CCL13, CCL14, CCL17, CCL18 and CCL24 [87] are specifically induced in M2a macrophages [64, 88], whereas M2b macrophages rather characterized by expression of high levels of CCL20, CXCL1, CXCL2 and CXCL3; whereas M2c macrophages express high levels

T cell mediated immunity is important in the lung in order to protect the host from inhaled pathogens and environmental antigens [89-91]. Furthermore, most of the environmental antigens encountered by the lung are innocuous. Hence it is crucial for the resident immune cells to distinguish such inert antigens from those derived from pathogens and respond appropriately. The immunoregulatory mechanisms employed by the lung include the components for surfactant lining [92], products secreted by alveolar type II epithelial cells as well as alveolar macrophages which altogether actively suppress T cell proliferation. More‐ over, AM induces anergy (immune unresponsiveness) in T cells which is reversible upon its removal from culture [91]. T cells isolated from lungs of rodents express low levels of IL2 receptor (IL2R/CD25) [91]. The Immunoregulators associated with AM include prostaglan‐ dins, leukotrienes, IL1 and nitric oxide [89]. AMs also downregulate local pulmonary immune

AMs from rat, mouse and human differ markedly in its potency to inhibit mitogen-induced T cell proliferation. However, T cells stimulated in the presence of AMs display a similar phenotype in all species examined, i.e. CD3 downregulation, upregulation of IL2R, increased IL2 production and inability to respond to IL2 [94]. Thus, AMs appear to allow T cell activation and effector functions, while selectively inhibiting T cell proliferation. The suppressive activity of AMs is restricted to the final step in the activation process, (i.e. proliferation). Through CD3

T cells from humans produce less IL2 than autologous blood-derived T cells [95]. FoxP3 is a master regulator of regulatory T cells (Treg). FoxP3 expression is regarded as definitive marker for cells with regulatory function in mice and humans [96, 97]. AMs obtained from BAL of mice or humans enhance FoxP3 expression in CD4+FoxP3-T cells in vitro. Activation of naïve T cells in the presence of either AMs or AM-conditioned media prevents T cell proliferation. This effect can be reversed by inhibiting binding of retinoic acid to its receptor or by blocking TGFβ1 signaling [80]. AMs induce intracellular FoxP3 expression in CD4+T cells. It promotes IL10 while suppressing IFNγ production by the same T cell in vitro [80]. AMs induce an

Macrophages have the ability to elicit an immune response and resolve the inflammatory processes. The macrophages of the respiratory system are involved in the pathogenesis of

downregulation, IL2R expression and IL2 secretion appear to proceed normally [94].

immune unresponsiveness in T cells and also affects its proliferation.

**10. Role of macrophages in chronic respiratory diseases**

responses against intratracheally administered T cell dependent antigens [93].

of CXCL13, CCL16 and CCL18 [64].

38 Lung Inflammation

**9. T cells and Alveolar macrophages**

COPD is a global epidemic, mainly caused by cigarette smoke exposure (smokers disease) and high particulate air pollution (as associated to in-house cooking) with an alarming increase in its mortality rate [98]. COPD is characterized by an inflammatory airway obstruction and loss of alveolar tissue thereby causing reduced respiratory surface area (emphysema). Macrophag‐ es are elevated and accumulate in small airways, bronchioles and alveoli during COPD irrespective of the disease severity. Macrophages monitor and respond to their microenvir‐ onment that can define tissue remodelling and possibly control other inflammatory events. AMs in the lung are an important source of both proteinases and antiproteinases. They secrete a series MMPs (1,9 and 12) [10, 99-101] and tissue inhibitor of metalloproteinases (TIMPs). In addition, they also secrete lysosomal cysteine proteinases [Cathepsin K, L, S (CTSK, CTSL and CTSS respectively)] and their inhibitor cystatin C (CST3) [10, 102]. An imbalance between proteinase and antiproteinase is considered to be an important event in the pathogenesis of COPD [103].

Macrophage derived MMPs as well as cathepsins are elastinolytic [10, 102, 104] and are important in airway inflammation and development of emphysema [9, 10, 101].Elastinolysis, an essential event of emphysema [105] results in the destruction of lung tissues during COPD [106, 107]. AMs have also been shown to release neutrophil elastase in vitro [102, 108]. There is a positive association between macrophage numbers in the alveolar walls and the presence of mild to moderate emphysema as well as the degree of small airways disease in patients with COPD [109, 110]. Dysregulated expression of macrophage MMPs either directly or indirectly by cigarette smoke exposure can lead to lung parenchyma destruction, characteristic of emphysema.

The role of different subsets of AMs in the pathogensis of COPD is yet to be fully ascertained. Increased expression of iNos in AMs is found in patients with COPD [111-113]. Smoke exposure enhances the release of pro-inflammatory cytokines such as IL1β, IL6, IL8 and TNFα [114-118] in the lungs which are markers of M1 macrophage polarization. There is also contradictory transcriptome based evidence that M2 polarized alveolar macrophage may contribute to COPD pathogenesis [119]. Further, COPD exacerbation, characterized by severe shortness of breath, is a common occurrence, which is usually caused due to an infection or exposure to environmental pollutants. Impaired phagocytosis, a characteristic feature of M1 polarized macrophages is also considered to be an important cause for increased COPD severity [120]. Analysis of BAL fluid of COPD patients suggests that smoking cessation partly changes the macrophage polarization from a pro-inflammatory M1 towards an anti-inflam‐ matory M2 macrophage phenotype [121]. M2 polarized alveolar macrophage have been shown to produce MMP12 which plays an important role in cigarette smoke induced emphysema [122-124]. It could be considered that COPD pathogenesis is largely contributed by dysfunction of macrophages rather than a single subset of AMs.

Sun and coworkers [136] reported that increased numbers of AAMs induced by over expres‐ sion of IL10 results in induction of lung fibrosis in mice. Accordingly increased expression of CD206 (another marker of AAM) and IL4 is observed in patients with IPF and systemic sclerosis [137, 138]. Secretion of IL4 and IL13 by T cells is required for fibroblast migration and proliferation and their subsequent differentiation into myofibroblasts. IFNγ, in contrast, attenuates the fibrotic response and induces collagen degradation [139]. Paired immunoglo‐ bulin like receptor beta (PIRB) contributes to the pathogenesis of pulmonary fibrosis via the negative regulation of macrophage effector function and fibrogenic mediator expression. PIRB negatively regulates IL4-induced macrophage activation. Various studies have also demon‐ strated the role of AAM production in pulmonary fibrosis [140]. Relmα, a hallmark M2 macrophage marker is upregulated in pulmonary fibrosis which is controlled by IL4/ IL13-and

The Th2 cytokines IL4 and IL13, like TGFβ1, directly stimulate collagen synthesis in mouse and human fibroblasts [142]. They also promote the development of the classic myofibroblast phenotype in human lung fibroblasts [143]. Macrophages accumulate in areas of fibrotic injury but their role remains incompletely understood. A study using silica-induced model of lung fibrosis found that the IL4Rα-dependent differentiation of AAM is critical for the induction

an enzyme especially expressed by M2 macrophages is also overexpressed in patients with IPF, especially in a progressing stage suggesting that this enzyme plays a role in the patho‐

Polarization of macrophages to the M1 phenotype attenuates pulmonary fibrosis [65]. Fibrosis may be independent of monocyte and lung macrophage activity during the inflammation phase of bleomycin injury, a frequently used animal model for IPF. The depletion of lung macrophages during the inflammatory phase of bleomycin injury has no effect on the early as well as peak stage of lung fibrosis. However, during the progressive phase, lung macrophage depletion reduces the degree of pulmonary fibrosis. Depletion of lung macrophages during the resolution phase of bleomycin induced lung fibrosis slowed down the process [147]. Macrophages may promote resolution during the reversible phase of bleomycin induced pulmonary fibrosis [148]. Overexpression of MMP9 by AMs has the capability to attenuate the fibrosis induced by bleomycin [149]. ExMacs are recruited to the lung after noninfectious injury by bleomycin and are the major source of macrophages derived CXCL10 [46]. ExMacs are

, MHCIIin Gr-1 int and are separated from resident AMs by high expression of both

CD11b and CX3CR1. Everson and colleagues [147] separated AMs into 18 density defined subpopulations. Bleomycin altered the proportions of these subpopulations and enhanced the production of TNFα production in these specific subpopulations. Bleomycin-treated *Pirb(-/-)* mice displayed an increased expression of collagen and IL4 associated profibrogenic markers Relmα, MMP12, TIMP1, and osteopontin, which were localized to AMs. Thus, macrophages may have a role in resolution during the reversible phase of bleomycin induced pulmonary

Th2 response required to trigger fibrosis [144]. Chitotriosidase,

Macrophage Polarization in Lung Biology and Diseases

http://dx.doi.org/10.5772/57567

41

STAT6-dependent pathways [141].

and maintenance of the CD4+

CD11c+

fibrosis [148].

genesis of diffuse lung disease-associated fibrosis [145, 146].
