Innate immunity to intracellular bacterial infections

Intracellular bacterial pathogens have developed mechanisms to enter and invade cells, to survive the immune response and to replicate inside the host. We studied the innate mechanisms that have evolved in the host to battle intracellular bacterial pathogens, such as the obligate intracellular Chlamydia pneumoniae and the facultative intracellular Listeria monocytogenes, which invade the respiratory and the gastrointestinal tracts in humans.

Infection of murine bone marrow-derived macrophages (BMM) with C. pneumoniae induces IFN-α/β-dependent IFN-γ secretion leading to the control of intracellular bacterial growth. We studied the molecular details of chlamydial-induced IFN-α and IFN-γ expression in BMM. We demonstrated that TLR4, but not TLR2, TLR6 or TLR9, is essential for the control of C. pneumoniae infection. We found that TLR4-MyD88-IRAK4-dependent signaling is necessary for IFN-α and IFN-γ mRNA expression, and protection against infection of BMM with C. pneumoniae. In C. pneumoniae-infected BMM, IFN-α/β- dependent STAT1 was necessary for increased IFN-γ mRNA accumulation and bacterial growth control. Enhancement of IFN-γ mRNA levels and control of C. pneumoniae infection also required NF-κB activation. We showed that NF-κB activation is TRAF6-dependent, but independent of TLR4-MyD88-IFN-α/β signaling in intracellular bacterial infection. In C. pneumoniae-infected IRF3-/- BMM, IFN-α and IFN-γ mRNA levels and bacterial levels were not altered compared to the WT. However, IFN-β-/- BMM showed higher loads of C. pneumoniae and no expression of IFN-α and IFN-γ mRNA in comparison to the WT BMM. In conclusion, we demonstrated that TLR4-MyD88-IFN-α/β-STAT1-dependent signaling, as well as TLR4-MyD88-independent but TRAF6-dependent NF-κB activation play a role in IFN-γ expression and protection against C. pneumoniae infection in BMM.

We then studied the protective role of STAT1 in mice infected intranasally with C. pneumoniae. STAT1 mediated an IFN-α/βR- and IFN-γR-dependent protection against C. pneumoniae infection in vivo. STAT1 phosphorylation was detected after chlamydial infection in IFN-α/βR-/- and IFN-γR-/- mice, but not in IFN-α/βR-/-/IFN-γR-/- mice. T cells released IFN-γ and conferred protection against C. pneumoniae in a STAT1-independent fashion. STAT1 mediated microbicidal mechanisms of non-hematopoietic cells, leading to control of intracellular infection in vivo. Thus, STAT1 mediates a cooperative effect of IFN- α/β and IFN-γ on non-hematopoietic cells, resulting in protection against C. pneumoniae in pulmonary infection.

We next addressed the role of NOD1 in growth control of L. monocytogenes. NOD1 conferred protection to intraperitoneal and subcutaneous infection of L. monocytogenes, and controlled the dissemination of L. monocytogenes into the brain. NOD1 was not involved in the generation of adaptive immune responses or the recruitment of inflammatory cells. Non- hematopoietic cells accounted for the NOD1-mediated resistance to L. monocytogenes. Furthermore, L. monocytogenes-infected NOD1-/- BMM, fibroblasts and astrocytes showed increased bacterial load, and IFN-γ-induced inhibition of bacterial growth was dampened in NOD1-/- BMM. Surprisingly, a number of important inflammatory cytokines, chemokines, growth factors and metalloproteases were increased in NOD1-/- compared to WT fibroblasts as determined by microarray analysis. In conclusion, NOD1 confers non-hematopoietic cell- mediated resistance to infection with L. monocytogenes in vivo. It plays a role in the control of infection in BMM, fibroblasts and astrocytes, and is required for IFN-γ-mediated L. monocytogenes growth control in BMM.

List of scientific papers

I. Rothfuchs AG, Trumstedt C, Wigzell H, Rottenberg ME (2004). Intracellular bacterial infection-induced IFN-gamma is critically but not solely dependent on Toll-like receptor 4-myeloid differentiation factor 88-IFN-alpha beta-STAT1 signaling. J Immunol. 172(10): 6345-53.
https://pubmed.ncbi.nlm.nih.gov/15128825

II. Trumstedt C, Eriksson E, Lundberg AM, Yang TB, Yan ZQ, Wigzell H, Rottenberg ME (2007). Role of IRAK4 and IRF3 in the control of intracellular infection with Chlamydia pneumoniae. J Leukoc Biol. 81(6): 1591-8. Epub 2007 Mar 14
https://pubmed.ncbi.nlm.nih.gov/17360955

III. Rothfuchs AG, Trumstedt C, Mattei F, Schiavoni G, Hidmark A, Wigzell H, Rottenberg ME (2006). STAT1 regulates IFN-alpha beta- and IFN-gamma-dependent control of infection with Chlamydia pneumoniae by nonhemopoietic cells. J Immunol. 176(11): 6982-90.
https://pubmed.ncbi.nlm.nih.gov/16709859

IV. Mosa A, Trumstedt C, Eriksson E, Soehnlein O, Janik K, Klos A, Dittrich-Breiholz, Kracht M, Hidmark Å, Wigzell H, Rottenberg M E (2008). Nonhematopoietic cells control the outcome of infection with Listeria monocytogenes in a NOD1-dependent manner. [Submitted]