A novel role of TLR9 in peritoneal B cell in regulating host defense during polymicrobial sepsis

Li Xu, University of Pittsburgh School of Medicine;Union Hospital,Tongji Medical College,Hua zhong University of Science and Technology; Meihong Deng, University of Pittsburgh school of Medine; Rosemary Hoffman, University of Pittsburgh School of Medicine; Patricia Loughran, University of Pittsburgh School of Medicine; Timothy Billiar, University of Pittsburgh

TLR9 is known to play important roles in sepsis.TLR9 deficient (TLR9^-/-)mice are resistant to polymicrobial sepsis.

However, the mechanisms and biological pathways by which TLR9 regulates host defense are yet to be identified.

To investigate this, mice were subjected to a clinical relevant polymicrobial sepsis model, cecal ligation and puncture(CLP), for 18 hours.

As expected, global deletion of TLR9 increased survival and decreased bacterial load in peritoneal as well as systemic inflammation after CLP. The number of peritoneal PMNs in TLR9^-/- mice was significantly higher than in WT mice after CLP (WT CLP 9.26±1.03*10⁶ ;TLR9 KO CLP 16.2±5.08*10⁶), suggesting that TLR9 may regulate host defense via PMN recruitment. However, specifically knockout of TLR9 in myeloid cells was not phenocopy of global TLR9^-/- after CLP; suggesting that TLR9 on myeloid cell did not play critical roles in host defense. Interestingly, the number of peritoneal B cells assessed by flow cytometry was significantly higher in TLR9^-/- mice than their control at baseline as well as after CLP (Number of peritoneal B cells: WT Control 4.85±1.46*105; TLR9 KO Control 33.8±5.44*105;WT CLP 1.39±1.12*105;TLR9 KO CLP 6.42±3.07*105).Importantly, the circulating level of natural antibody IgM, which is important for opsonization of bacteria for PMN phagocytosis, was significantly increased in TLR9^-/- mice than their control at baseline and after CLP.

Our data suggest an unrecognized role of TLR9 in B cells in regulation of host defense via modulation of IgM secretion from B cell.