Role of Toll-Like Receptor 13 in Innate Immune Recognition of Group B Streptococci

Abstract

c Murine Toll-like receptor 13 (TLR13), an endosomal receptor that is not present in humans, is activated by an unmethylated motif present in the large ribosomal subunit of bacterial RNA (23S rRNA). Little is known, however, of the impact of TLR13 on antibacterial host defenses. Here we examined the role of this receptor in the context of infection induced by the model pathogen group B streptococcus (GBS). To this end, we used bacterial strains masked from TLR13 recognition by virtue of constitutive expression of the ErmC methyltransferase, which results in dimethylation of the 23S rRNA motif at a critical adenine residue. We found that TLR13-mediated rRNA recognition was required for optimal induction of tumor necrosis factor alpha and nitrous oxide in dendritic cell and macrophage cultures stimulated with heat-killed bacteria or purified bacterial RNA. However, TLR13-dependent recognition was redundant when live bacteria were used as a stimulus. Moreover, masking bacterial rRNA from TLR13 recognition did not increase the ability of GBS to avoid host defenses and replicate in vivo. In contrast, increased susceptibility to infection was observed under conditions in which signaling by all endosomal TLRs was abolished, i.e., in mice with a loss-of-function mutation in the chaperone protein UNC93B1. Our data lend support to the conclusion that TLR13 participates in GBS recognition, although blockade of the function of this receptor can be compensated for by other endosomal TLRs. Lack of selective pressure by bacterial infections might explain the evolutionary loss of TLR13 in humans. However, further studies using different bacterial species are needed to prove this hypothesis.

Streptococci Engage TLR13 on Myeloid Cells in a Site-Specific Fashion

“…We found that TLR13 2/2 BMDMs were strongly impaired in the cytokine response to GBS (Fig. 1B), which was in accordance with data on GBS strains masked from TLR13 recognition (14). Moreover, UNC-93B and TLR13 appeared to serve as nonredundant signaling components in the same pathway.…”

“…However, little is known about the contribution of TLR13 to the inflammatory program in MFs infected with streptococci or any other Gram-positive bacteria. This is largely due to the fact that the data on streptococci interacting with cells from TLR13-knockout mice are mostly limited to the response to isolated bacterial RNA, whereas data from in vitro or in vivo infection are sparse (14,15).…”

Bacterial RNA: An Underestimated Stimulus for Innate Immune Responses

“…However, in line with the observations of Deshmukh et al (27), immune cell activation by live GBS was greatly diminished in Unc93B1 3d, but not in TLR7/8/9 triple-deficient, cells. Similarly, in an in vivo infection model, Unc93B1 mutation, but not erm-mediated masking of TLR13 recognition, increased lethality and bacterial burdens (64). Collectively, these data indicate that TLR13-dependent recognition of bacterial RNA plays a redundant role in the recognition of live S. agalactiae, likely due to functional compensation by other endosomal Unc93B1-dependent TLRs.…”

“…Two important points to consider are the release of RNA from bacteria and, thus, the mechanisms of how to get access to any innate immune receptor and second, a point that is often overlooked, the concentration necessary to trigger an immune response. From the studies using purified bacterial RNA, it seems obvious that bacterial RNA, in general, can activate innate immune cells, because Gram-positive and Gram-negative RNA preparations induced cytokine secretion (28,30,31,36,38,39,64). Yet the contribution of RNA to whole bacteria recognition has been less well analyzed.…”

“…Dependency on bacterial internalization, phagosomal acidification, and Unc93B1 (27, 68) indicated a predominant role for the nucleic acid-sensing pathways in the recognition of S. agalactiae by an unidentified MyD88-dependent receptor. In light of the subsequent identification of TLR13 as a receptor for bacterial RNA in murine immune cells (39), Signorino et al (64) investigated the immune responses induced by an erm methylase-overexpressing S. agalactiae strain in which TLR13 recognition is camouflaged. Remarkably, TLR13-mediated detection of 23S rRNA was required for in vitro cytokine induction in response to heat-killed bacteria, whereas no significant effect was observed when live bacteria were used as stimulus.…”