Role of Pneumolysin’s Complement-Activating Activity during Pneumococcal Bacteremia in Cirrhotic Rats

Abstract

We investigated the role of pneumolysin’s complement-activating activity during Streptococcus pneumoniae bacteremia in a hypocomplementemic, cirrhotic host. Isogenic mutant pneumococcal strains, in which pneumolysin was expressed from a plasmid, were used. These strains included H+C+, expressing wild-type pneumolysin with both cytolytic and complement-activating activity; PLY−, carrying the plasmid without the pneumolysin gene; and, H+C−, expressing pneumolysin with cytolytic activity only. In control rats, intravenous infection with 2.0 × 107 CFU of H+C+ per ml of blood resulted in a decrease in bacteremia of 3.5 log units by 18 h postinfection and 55% mortality. By contrast, cirrhotic rats infected similarly with the H+C+ strain demonstrated a 0.2-log-unit increase in bacteremia by 18 h postinfection and 100% mortality. Both control and cirrhotic rats cleared the PLY− strain more effectively from their bloodstreams by 18 h postinfection (6.2 and 5.6 log unit decreases, respectively). Infection with the PLY− strain also resulted in low mortality (0 and 14%, respectively) for control and cirrhotic rats. When infected with the H+C− strain (without complement-activating activity), both groups cleared the organism from their bloodstreams nearly as well as they did the PLY− strain. Furthermore, the mortality rate for control and cirrhotic rats was identical after infection with the H+C− strain. These studies suggest that pneumolysin production contributes to decreased pneumococcal clearance from the bloodstream and higher mortality in both control and cirrhotic rats. However, pneumolysin’s complement-activating activity may uniquely enhance pneumococcal virulence in the hypocomplementemic, cirrhotic host.

Cirrhosis-induced defects in innate pulmonary defenses against Streptococcus pneumoniae

“…Finally, reduced pulmonary complement levels would make the cirrhotic host particularly vulnerable to complement depletion by the complement-activating activity of pneumolysin. This would be consistent with previous work from our laboratory using an intravenous pneumococcal infection model, in which the complement-activating activity of pneumolysin was especially detrimental for cirrhotic rats [ 34 ]. Unlike control animals, the cirrhotic animals could not adequately replenish serum complement that was consumed by pneumolysin during the bacteremic infection [ 46 ].…”

“…WU2 is also a clinical pneumococcal strain that produces a type 3 capsule but, unlike ATCC 6303 belongs to PspA clade 2. WU2 strains are lethal to our rats when given intravenously [ 34 ] but not when infected transtracheally. The Ply+ and Ply- mutants were created from a spontaneously occurring rough (nonencapsulated) WU2 isolate (WU2R) as described previously [ 53 , 54 ].…”

“…Our laboratory has previously shown that, like their human counterparts, cirrhotic rats with ascites are hypocomplementemic [ 34 ] and are more susceptible to pneumococcal infection. They have increased mortality compared to control animals following both pulmonary and intravenous pneumococcal challenges with type 3 S. pneumoniae [ 13 , 34 ]. In addition, we have shown that the complement-activating activity of pneumolysin is particularly detrimental to cirrhotic rats following an intravenous challenge [ 34 ].…”

“…They have increased mortality compared to control animals following both pulmonary and intravenous pneumococcal challenges with type 3 S. pneumoniae [ 13 , 34 ]. In addition, we have shown that the complement-activating activity of pneumolysin is particularly detrimental to cirrhotic rats following an intravenous challenge [ 34 ]. We therefore hypothesized that the severely cirrhotic host has increased mortality from pneumococcal pneumonia due to compromised innate defenses, and pneumolysin may also be especially detrimental to the cirrhotic rats during a respiratory tract infection.…”

Role of Streptococcus pneumoniae Proteins in Evasion of Complement-Mediated Immunity

“…Similarly, complementation of a PLY negative mutant with a recombinant PLY presenting intact complement activation capacity but no hemolytic activity was able to restore the bacterial virulence, indicating an independent contribution of the complement activation motif of PLY for pneumococcal virulence in this infection model ( Rubins et al, 1995 ). However, in models of pneumococcal bacteremia, a significant role for the complement activity of PLY was only observed in cirrhotic rats (which produce reduced amounts of complement), with no effect in animals exhibiting normal complement production ( Alcantara et al, 1999 ). This result suggests that the contribution of the complement activation motif of PLY is important in host niches or systems where a limited amount of complement is available ( Propst-Graham et al, 2007 ; Marriott et al, 2008 ).…”