Mechanisms of Immuno-Modulation With The TLR4 Agonist Monophosphoryl Lipid A in the Treatment of Sepsis
Monophosphoryl lipid A (MPLA) is a Toll-like receptor 4 agonist that has been approved for use as a vaccine adjuvant in humans. In these studies, we evaluated the effect of MPLA treatment on the innate immune response to systemic infection in mice with emphasis on bacterial clearance, leukocyte recruitment and function, as well as potential signaling pathways involved in TLR4 immmuno-modulation. Mice received either intravenous or intraperitoneal treatment with MPLA prior to induction of bacterial peritonitis by cecal ligation and puncture (CLP), intraperitoneal or intravenous infection with Pseudomonas aeruginosa, or burn wound infection. In each model, MPLA treated mice showed significantly improved survival compared to vehicle-treated controls. MPLA treatment prevented infection-associated hypothermia, and enhanced bacterial clearance despite causing attenuation of pro-inflammatory cytokine production. When MPLA was administered after a CLP core temperature was not preserved, but bacterial clearance was still enhanced. Concentrations of pro-inflammatory cytokines such as TNFα, IL-1β, and IL-6, that are regulated through the MyD88-dependent signaling pathway, were markedly lower in the plasma of MPLA-treated mice whereas plasma concentrations of Trif-dependent gene products such as G-CSF, MCP-1 and RANTES were largely unaffected. Treatment with MPLA increased the numbers of granulocytes, undifferentiated myeloid cells, and macrophages at infection sites. MPLA treatment also increased the percentage and total numbers of myeloid cells mediating phagocytosis of bacteria. Depletion of monocytes did not eliminate the enhanced bacterial clearance induced by MPLA, but depletion of Granulocytes did. MPLA treatment also increased the recruitment of immature myeloid cells expressing myeloid derived suppressor cell markers. While TLR4 deficient mice were un-affected by pretreatment with MPLA, Trif Deficient mice were still tolerized by MPLA treatment prior to CLP. Cytokine analysis further supports the theory that MPLA exerts its effects primarily through the MyD88 independent pathway. In conclusion, these studies show that MPLA treatment significantly augments the innate immune response to bacterial infection by enhancing bacterial clearance despite attenuation of pro-inflammatory cytokine production. The enhanced bacterial clearance is mediated, in part, by increased numbers of myeloid cells with effective phagocytic functions at sites of infection and this is dependent upon activation of TLR4.