In the elderly pneumococcal pneumonia is characterized by its rapid onset, severity, and high case-fatality rate [4
]. Consistent with reports describing increased levels of activated NFkB in aged tissues [19
], elevated levels of pro-inflammatory cytokines in serum and BALF of healthy seniors [22
], and positive regulation of pIgR and PAFr by nuclear localized NFkB [11
], we determined that aged mice had increased levels of pIgR and PAFr in their lungs, moreover were more susceptible to infection with S. pneumoniae
. In agreement with findings by Yende et al., which have shown that elevated levels of TNFα and IL-6 are alone sufficient to increase the risk for CAP [36
], we determined that mice infused with TNFα were more susceptible to S. pneumoniae
than those that received saline. Our observation of increased pIgR and PAFr protein in these mice provides a molecular mechanism that helps to explain how aging and pre-infection inflammation, such as that which is frequently observed in individuals with underlying morbidities (e.g. cardiovascular disease), increases susceptibility to CAP.
Because of the inflammatory nature of pneumococcal disease, we hypothesized that during pneumonia elevated levels of pIgR and PAFr would be present in the lungs of aged mice. Unexpectedly, we observed the opposite, and determined that despite considerably greater bacterial burden, aged mice had lower levels of pIgR and PAFr in the lungs and less NFkB activation. Importantly, young mice infused with TNFα had normal pIgR and PAFr expression/production as well as NFkB activation during infection (data not shown) suggesting that this muted response was not the result of prolonged exposure to pro-inflammatory cytokines. TLRs-1, 2 and 4 detect pneumococcal PAMPs and initiate a cell signaling cascade that activates NFkB. Further experiments determined that aged mice had significantly reduced protein levels of TLRs 1, 2, and 4 in their lungs and produced less TNFα and IL-6 when challenged with purified pneumococcal components. These observations help to explain the reduced levels of activated NFkB observed in infected aged mice. It is plausible that diminished TLR levels would lead to reduced cell-signaling, activation of NFkB and subsequent cytokine production. Reduced cytokine production would further diminish autocrine and paracrine NFkB activation through the TNFα and IL-6 receptor pathways.
Multiple age-related immune defects have been reported that increase susceptibility to pneumonia including reduced mucociliary clearance of air-borne particles, diminished macrophage function and T-cell activation, and decreased antibody avidity [37
]. While ADTD has already been described for peritoneal macrophages from aged C57/Bl6 mice, as well as peripheral monocytes from healthy elderly humans [40
], this is the first report to indicate that age-dependent TLR dysfunction occurs in the lungs and that it may contribute towards susceptibility to pneumonia. Interestingly, elderly with pneumonia often have disease presentation that would be considered atypical for mature adults. One or more of the 3 classic symptoms for pneumonia (cough, fever, dyspnea) are absent in >50% of the elderly with pneumonia and ~10% show no signs of infection with exception to confusion or delirium [43
]. TLR dysfunction is a possible explanation for why these individuals fail to show overt symptoms of infection such as fever. Importantly, age-related defects in TLR-signaling may be occurring simultaneously with diminished TLR protein levels. Studies have shown a 50% reduction in the expression of protein kinases (e.g. p38 and JNK) and their phosphorylation in peritoneal macrophages isolated from aged mice versus young controls [41
At this time it is unclear which cells in the lungs experience ADTD. While published data has shown that macrophages experience ADTD [40
], the possibility of epithelial cell TLR-dysfunction is supported by the fact that we observed these age-dependent changes using whole lung extracts which are composed primarily of non-lymphoid cells. Which lung cells experience ADTD is an important question that remains to be answered. Another important consideration is that changes in pIgR and TLR protein levels were not mirrored by changes in the corresponding mRNAs. However, multiple studies have shown that the amount of mRNA detected does not always correlate with the observed protein levels [44
]. Possible explanations include regulation of protein production by post-translational modification and age-dependent differences in protein degradation.
Multiple respiratory tract pathogens including Haemophilus influenzae, N eisseria meningitidis
, and Pseudomonas aeroginosa
express ChoP on their surface and use it to bind PAFr [46
]. Increased lung expression of PAFr as a result of aging/chronic inflammation would facilitate the ability of these bacteria to cause respiratory disease. Conflicting reports exist regarding the ability of S. pneumoniae
CbpA to attach to mouse pIgR [49
], however it is undisputed that CbpA binds to human pIgR. Increased levels of pIgR in the lungs of the elderly would also most likely increase their susceptibility to pneumonia. Finally, age-dependent TLR-1, 2, and 4 defects would impair the ability of aged animals to detect a variety of microorganisms and delay activation of the immune response, again increasing susceptibility to pneumonia. It remains undetermined if other TLRs are also negatively affected by age in the lungs.
At first glance AAI and TLR dysfunction appear to be paradoxical. However, AAI occurs in uninfected aged animals; a very different physiological condition than during infection. In , we propose a model that suggests AAI primes the lungs for S. pneumoniae attachment and facilitates the establishment of a lower respiratory tract infection. Subsequently, ADTD inhibits detection of the bacteria and results in a delayed/muted immune response. Our finding that AAI primes the lungs for infection through pIgR and PAFr, combined with the finding that aged mice experience lung TLR dysfunction suggests that in addition to the already described age-associated immune defects, conditions are favorable for the development of S. pneumoniae infection. Because these events occur before the innate immune system is fully engaged, these studies suggest that an infection can become well established before an adequate immune response is initiated.
Illustration explaining how age-associated inflammation and Toll-like receptor dysfunction increases the susceptibility of the elderly to pneumococcal pneumonia