Streptococcus pneumoniae strain-dependent lung inflammatory responses in a murine model of pneumococcal pneumonia

The inherent properties of an invading bacterium may influence the cytokine profile that is ultimately produced. We determined the alterations in proinflammatory (TNF-alpha, IL-1, and IL-6) and anti-inflammatory cytokine (IL-10) expressions in lung tissues within the first 48 h after infection in mice with pneumonia induced by direct intratracheal inoculation of five different pneumococcal strains.

Experimental murine model of Streptococcus pneumoniae pneumonia.

Female BALB/cby mice aged 8-10 weeks.

Five S. pneumoniae clinical isolates were used in this study. The strains included two serotype 3 strains (P4241 and P30606), two serotype 6 strains (P26772 and P23477), and one serotype 19 strain (P15986). The trachea of anesthetized animals was cannulated via the mouth with a blunt needle, and 50 micro l bacterial suspension of two different inocula (their respective 100% lethal inoculum and the same 10(5) CFU/mouse inoculum of S. pneumoniae strains) were instillated. At predetermined times after pneumococcal infection, i.e., time 0 (preinfection) and 2, 4, 6, 12, 24, and 48 h postinfection in experimental groups, lung tissues were sampled from groups of three mice to quantify lung pro- and anti-inflammatory mediators. The experiments were repeated at least three times.

Pneumonia induced by five different pneumococcal isolates resulted in pronounced differences in the local pro- and anti-inflammatory profiles. For example, with a 100% lethal inoculum of S. pneumoniae, the extent and timing of TNF-alpha expression varied greatly among strains, ranging from 2,643 to 10,022 pg/g and from 4 to 48 h, respectively. Moreover, TNF-alpha productions within 48 h postinfection measured by the 48 h area under the curve were differed significantly, ranging from 59,700 to 275,825. These different profiles were not serotype dependent. Comparable results were obtained when IL-1, IL-6, and IL-10 expressions in lung tissues were studied.

These findings confirm that the production of the pro- and anti-inflammatory mediators are critically dependent not only upon the different species of bacteria used to establish the experimental infection but also upon the different strains of a specific bacterial species used, i.e., S. pneumoniae in this study. These substantially different host responses were not serotype dependent. Moreover, the profile of lung pro-and anti-inflammatory cytokines within 48 h postinfection, at least in this pneumonia model, was not related to outcome of animals.