By Joanna Lyford, Senior medwireNews Reporter
The composition of inflammatory lung infiltrate in people with pneumonia caused by Streptococcus pneumoniae differs by age, an in-situ study reveals.
Specifically, neutrophilic granulocytes are more abundant in older versus younger patients whereas the reverse is true for alveolar macrophages and M1 macrophages.
The findings offer new insights into the innate immune response to pneumococcal infection, say the authors, and suggest potential avenues for therapeutic intervention.
Alexandar Tzankov (University Hospital, Basel, Switzerland) and team obtained samples of inflammatory lung infiltrate from 22 patients with microbiologically proven pneumococcal pneumonia. This included five young patients aged under 30 years (mean 8.4 years), eight middle-aged patients aged between 30 and 70 years (mean 55.9 years) and nine elderly patients older than 70 years (mean 86.6 years).
Writing in Pathobiology, the authors say that macrophages and granulocytes typically predominated in the infiltrate whereas lymphocytes were much less abundant. The distribution of B lymphocytes (detected by CD20 antibodies) and T lymphocytes (detected by CD3 antibodies) did not differ among the three age groups.
However, the proportion of neutrophilic granulocytes detected by CD15 staining was significantly higher in elderly than young patients, at 94.5% versus 75.0%.
Meanwhile, macrophages detected by CD14 antibodies were significantly more prevalent in young than in elderly patients, at 30.0% versus 10.0%. The same was true for M1 macrophages, detected by HLA-DR antibodies, at 52.0% in young versus 11.4% in elderly patients.
Tzankov and co-authors remark that neutrophils in elderly people typically show a reduced defence capacity, leading to increased susceptibility to bacterial infections, despite an increase in absolute numbers of granulocytes compared with younger patients.
Macrophages in the elderly similarly display a reduced phagocytic capacity and impaired oxidative burst, which again hampers the ability to fight infection. “Impaired monocyte/macrophage function results in failure to eliminate pathogens and prolongs their survival in cells (e.g. tuberculosis). This is accompanied by chronic activation of the innate immune system, leading to persistent inflammation”, they write.
Noting that this is the first study to characterise the inflammatory infiltrate of pneumococcal pneumonia in situ, the researchers conclude: “[R]estoring macrophage function and decreasing neutrophilic influx in the elderly might be crucial for reducing the burden of pneumonia in this age group.”
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