TIR-domain-containing protein C of uropathogenic E. coli CFT073 as a modulator of innate immune checkpoints

Urinary tract infections are one of the most common community-acquired infections worldwide, affecting approximately 150 million people each year. Uropathogenic E. coli are responsible for the vast majority of UTIs. In order to infect the lower and upper urinary tract, they express a wide variety of virulence factors. One of these virulence factors is TcpC, a Toll/interleukin-1 receptor domain-containing protein produced by various E. coli strains of the phylogenetic group B2, including CFT073. Studies have shown that TcpC is able to inhibit TNFα and IL-1β release of mouse macrophages during an infection with E. coli CFT073. It is suggested that TcpC is able to inhibit cytokine release of these cells by binding to specific proteins of the TLR4 signaling cascade and the NLRP3 inflammasome. I now report that TcpC is able to stimulate cytokine release of immune and bladder epithelial cells during infection. Human monocytes and human bladder epithelial cells release higher amounts of proinflammatory cytokines after infection with TcpC-producing CFT073 strains compared to a TcpC knockout. Differentiation of monocytes to macrophages abrogates this TcpC-dependent effect. Infection of T24/83ΔTLR4 cells suggests that exclusively TLR4 is responsible for a proinflammatory reaction. Furthermore, infection of T24/83ΔMyD88 bladder epithelial cells suggests that the TcpC-induced stimulation of proinflammatory cytokines is MyD88-independent. THP-1 cells treated with conditioned medium in which TcpC was overexpressed at different levels showed that TcpC inhibited cytokine release after stimulation with LPS plus ATP at low levels of induction. Deletion of the TIR-domain of TcpC leads to a loss of the inhibitory capabilities, showing that it is crucial for the function of the protein in this context. Thus, during an infection of monocytes with CFT073, the proinflammatory response is increased compared to the TcpC knockout strain, whereas treatment with culture supernatants containing TcpC inhibits the proinflammatory response of LPS plus ATP stimulated monocytes. In summary, I conclude that the TcpC-induced inhibition versus stimulation of release of proinflammatory cytokines may depend on the direct contact between CFT073 and eukaryotic cells. Since TcpC is able to bind to TLR4 and MyD88, I think it affects myddosome formation after stimulation of cells with LPS, which may be further influenced by the direct contact of the bacterium with the immune cells.