Streptococcus pneumoniae (the neumococcus), a Gram-positive bacterium, accounts for approximately 40% of all cases of community-acquired pneumonia (CAP) and is a leading cause of bacteremia and sepsis. Individuals hospitalized for invasive pneumococcal disease (IPD) are at an increased risk for sudden death as a result of adverse cardiac events, in particular new or worsened congestive heart failure. Thus, some form of cardiac damage seems to occur during these infectious episodes. Herein we describe the novel observation of cardiac lesions formed within the myocardium of septic mice with IPD. Lesion formation was positively correlated with bacterial burden in the blood as well as serum levels of cardiac troponin, a clinical marker for cardiac damage. Lesion formation was also concomitant with changes in electrophysiology, which indicated a progressive loss of cardiac contractility. Lesions increased in severity during the infection, and had a marked absence of infiltrated immune cells, which stands in stark contrast to abscesses typically seen formed by other Gram-positive bacteria. Importantly, cardiac lesion formation were both bacterial and mouse strain independent. Notably, pneumococci could be visualized within the lesions, and were confined to the heart following both intratracheal and intravenous challenge. These cardiac lesions may explain the high incidence of adverse cardiac events in humans with severe CAP. To examine the mechanism(s) responsible for cardiomyocyte death and to explore the potential for protection by immunization against pneumolysin we propose the following Specific Aims:
Aim 1 : Determine the impact of pneumococcal cell wall and pneumolysin on cardiomyocyte death and lesion formation. We have observed the presence of TUNEL positive cells, pneumolysin, and IL-1? present within cardiac lesions suggesting that inflammasome-dependent apoptosis may be occurring. To determine the mechanism(s) of cardiomyocyte death we will examine lesion formation in knockout (KO) and caspase inhibitor treated mice that are deficient in key regulators of established apoptosis pathways. Complementary in vitro studies using HL-1 cardiomyocytes will examine extrinsic and intrinsic apoptosis using caspase inhibitors. Additional studies will determine if pneumococcal cell wall and the toxin pneumolysin act independently or synergistically on cell death.
Aim 2 : Test if neutralization of pneumolysin activity with antibody protects mice against lesion formation. Pneumolysin is detected within cardiac lesions and is presumably damaging to cardiomyocytes. Since antibodies against pneumolysin are neutralizing and confer significant protection against S. pneumoniae, we hypothesize that immunization against pneumolysin should prevent cardiac lesion formation following bacterial challenge.
This proposal will examine the molecular basis for cardiomyocyte death in our novel observation of cardiac lesion formation by S. pneumoniae. It will also investigate the possibility for cardiac protection by neutralization of pneumolysin by immunization. Findings identified by this study may indicate potential targets for therapeutic intervention.