**6. Age-associated dysfunction of alveolar macrophages**

As discussed in Section 2, recent epidemiological data indicate that older adults are vulnerable to acute LRTIs that are attributable to either bacteria or viruses, and the globally increasing life expectancy further reinforces this fact. Phagocytosis by alveolar macrophages is responsible for the frontline defense against inhaled bacteria and fungi (Section 3), and the pro-inflammatory responses assist the defense by promoting phagocytosis (Section 4). During viral infection, efferocytosis of alveolar macrophages is indispensable to prevent uncontrolled pneumonia caused by DAMPs that leak from damaged and dead cells (Section 5). Since alveolar macrophages are characterized by advanced phagocytosis and efferocytosis, the decline in their activity is likely associated with the age-dependent exacerbation of acute LRTIs (**Figure 1**). In this section, we discussed the past and recent progress in the findings regarding age-related dysfunction of alveolar macrophages.

### **6.1 Age-associated decline in alveolar macrophage phagocytosis**

A previous study demonstrated that macrophages accounted for approximately 95% of the bronchoalveolar lavage fluid cells in both young and aged mice [91]. The absolute numbers of alveolar macrophages were also similar, but they indicated an age-related decrease when adjusted for lung weight, as discussed later (subsection 6.5). The percentage of alveolar macrophages capable of phagocytosing latex beads was approximately 80% and 60% in young and aged mice, respectively, and the difference was statistically significant. Like bacteria, phagocytosis against non-opsonized latex beads is mediated by MSR1 and CD36 [92]. Thus, these results suggest that aging is associated with reduced expression of scavenger receptors and/or an impaired ability to transduce engulfment signals, leading to an age-dependent decline in alveolar macrophage phagocytosis (**Figure 1A**). This finding is supported by recent evidence from in vivo studies. The phagocytic capacity of each alveolar macrophage for intranasally instilled latex beads was lower in aged mice than in young mice [93]. In this study, aged mice also exhibited decreased cell surface expression levels of MSR1, but not of CD36 and CD206, in alveolar macrophages (**Figure 1A**).

#### **Figure 1.**

*Intracellular events involved in age-associated dysfunction of phagocytosis (A), pro-inflammatory responses (B), and efferocytosis (C) in alveolar macrophages. (A) Age-associated decline in phagocytosis is mediated by reduced expression levels of MSR1 and Rac1. (B) Age-associated decline in pro-inflammatory responses is due to elevated expression levels of A20, which inactivates TRAF6, an upstream signaling protein of NF-κB, to suppress Streptococcus pneumoniae-stimulated signaling activation of TLRs. (C) Age-associated decline in efferocytosis is possible to be dependent on reduced expression levels of Rac1, which transmits engulfment signal associated with the TAM receptors Axl and MerTK.*

## *Physiological Role of Alveolar Macrophage in Acute Lower Respiratory Tract Infection… DOI: http://dx.doi.org/10.5772/intechopen.110509*

Moreover, alveolar macrophages in aged mice exhibited reduced phagocytosis after intratracheal injection of *E. coli*, which could be attributed to the reduced constitutive expression levels of Rac1 and resultant attenuated F-actin polymerization and filopodia formation (**Figure 1A**) [55]. No studies on human alveolar macrophages have been reported; however, unlike animal studies wherein the laboratory environment is maintained, identifying only the pure effects of aging in humans without other confounding factors is challenging. This is because smoking habits [94–96], chronic alcohol abuse [95, 97], and exposure to air pollutants [95] have been found to adversely influence alveolar macrophage phagocytosis.

### **6.2 Age-associated decline in alveolar macrophage pro-inflammatory responses**

Studies indicate that increased susceptibility to pneumococcal infection in elderly people is associated with a compromised initial response to TLR signaling in alveolar macrophages (**Figure 1B**). For instance, alveolar macrophages from aged mice exhibit suppressed responsiveness to in vitro LPS stimulation [98]. Notably, aged mice exhibited reduced survival, impaired bacterial clearance, and attenuated prompt pro-inflammatory cytokine production after intratracheal challenge with *S. pneumoniae*, which was accompanied by attenuated *S. pneumoniae*- or its cell wallstimulated phosphorylation of NF-κB p65 subunit, p38 MAPK, and JNK, in alveolar macrophages (**Figure 1B**) [99]. Further result was presented as a possible mechanism. In aged mice, the expression of A20 is specifically elevated in alveolar macrophages, which reduces *S. pneumoniae* exposure-induced IL-6 production (**Figure 1B**) [100]. A20 is known to inactivate TRAF6 in the cytosol, resulting in defects in its common downstream NF-κB, p38 MAPK, and JNK signaling cascades [101]. Thus, during pneumococcal infection, TLR9 signaling-mediated upregulation of alveolar macrophage phagocytosis can also be impaired in aged mice or humans (subsection 4.3.3) (**Figure 1B**). Notably, in an in vitro *Mycobacterium tuberculosis* infection model, compared with alveolar macrophages from young mice, those from aged mice constitutively expressed similar levels of TLR2, TLR4, and TLR9. They were able to produce equivalent levels of IL-12 and TNF-α in response to infection, while the contribution of TLR2 signaling to pro-inflammatory cytokine production was distinctly reduced in aged mice [102]. This suggests that phenotypes associated with age-dependent deterioration of TLR signaling differ according to the type of bacteria and possibly the composition of their virulence factors.

## **6.3 Age-associated decline in alveolar macrophage efferocytosis**

Aged mice indicated significant deterioration in survival rate and clinical score after intranasal instillation with influenza A virus, which also caused increased inflammation, accumulation of apoptotic cells in the alveoli, and impaired ability to bind to and engulf apoptotic neutrophils in alveolar macrophages [93]. In this study, alveolar macrophages from aged mice retained normal Axl expression levels but had markedly reduced levels of MSR1, as discussed above (Section 6.1). Further, MSR1 suppresses excessive inflammation by mediating the internalization of DAMPs by macrophages in a mouse model of ischemic stroke brain injury [103]. In addition, MSR1 participates in Tyro 3 signaling in macrophages to mediate efferocytosis in a mouse model of acute aortic dissection [104]. However, since alveolar macrophages express Axl or MerTK, but not Tyro 3 (Section 5.2), whether the age-associated decline in efferocytosis is caused by defects in the MSR1-Tyro 3 signaling axis is

unclear. Engulfment of apoptotic cells via TAM receptors requires Rac1 activation (Section 5.3), and Rac1 expression is depleted in alveolar macrophages from aged mice (Section 6.1), implying that reduced Rac1 expression is involved in the ageassociated decline in efferocytosis (**Figure 1C**). In summary, the decreased processing capacity for DAMPs due to suppressed MSR1 expression and decreased efferocytosis activity due to suppressed Rac1 expression in alveolar macrophages can be involved in the exacerbation of viral infection.
