Abstract
Klebsiella pneumoniae is a leading cause of nosocomial and community-acquired pneumonia. Infections from
this pathogen have become more common and severe due to the emergence of antimicrobial resistant strains.
Neutrophils play a critical role in pulmonary bacterial clearance but an excessive pulmonary recruitment of
these cells leads to collateral lung damage, acute lung injury, and worse patient outcomes. The mechanisms
underlying an excessive neutrophil recruitment in bacterial pneumonia patients are poorly understood. We
have previously reported a critical role for leptin and its receptor (LepR) in neutrophil recruitment,
phagocytosis, bacterial killing and eicosanoids synthesis during bacterial pneumonia. A common LepR
polymorphism, (Q223R LepR) has been found in approximately 50% of the general population with no effects
on BMI. Homozygous expression of Arginine (R) at the 223 position (RR LepR) as compared with the ancestral
allele A, encoding for Glutamine Q, QQ or the heterozygous QR, of the extracellular domain of the LepR is
associated with worse outcomes in intra-abdominal E.histolytica, Clostridium difficile infections (CDI) in
humans and mice and in patients with non-appendicular secondary peritonitis. Poor outcomes in CDIs were
associated with tissue and blood neutrophilia in humans and mice and enhanced leukocyte CXCR2 and Cd11b
expression, and elevated cytokines (IL-1¿, IL-6, IFN-¿, MIF and TNF-a) in RR LepR mice. Excessive neutrophil
recruitment in RR LepR mice in CDI could be ameliorated by pretreating mice with a neutralizing antibody
against IL-1ß, MIF or by blocking CXCR2.The impact of Q223R LepR mutation in host defense against
bacterial pneumonia is unknown. It is critically important to determine the impact of host genetic factors that
regulate neutrophil recruitment during bacterial infections since they might provide novel insights into
mechanisms of disease. The long-term goal is to determine the consequences of common leptin mutations in
host defense against pulmonary infections. The overall objective of this proposal is to determine the impact of
the Q223R LepR polymorphism on the host response against Klebsiella pneumoniae in mice and in bacterial
pneumonia patients with the RR LepR mutation. Achieving these objectives will determine if the RR LepR
mutation has a negative impact on mechanisms of pulmonary host defense in mice, if this mutation is a risk
factor for more severe illness from bacterial pneumonia patients, and provide preliminary data for a subsequent
grant proposal. The central hypothesis is that the RR LepR mutation impairs host defense against Klebsiella
pneumonia by inducing an excessive pulmonary inflammatory response characterized by elevated blood and
lung neutrophils, higher lung injury, and greater mortality compared with QQ LepR mice. In patients with the
RR LepR mutation will have a more severe presentation of bacterial pneumonia and worse outcomes
compared with patients with the QQ or QR LepR genotype. The results of these experiments lend themselves
to translational medicine with the LepR signaling pathways as targets for therapy.