Transduction of Functionally Contrasting Signals by Two Mycobacterial PPE Proteins Downstream of TLR2 Receptors

Abstract

As pathogen-associated molecular pattern sensors, the TLRs can detect diverse ligands to elicit either proinflammatory or anti-inflammatory responses, but the mechanism that dictates such contrasting immune responses is not well understood. In this work, we demonstrate that proline–proline–glutamic acid (PPE)17 protein of Mycobacterium tuberculosis induces TLR1/2 heterodimerization to elicit proinflammatory-type response, whereas PPE18-induced homodimerization of TLR2 triggers anti-inflammatory type responses. Ligation of TLR1/2 caused an increased recruitment of IL-1R–associated kinase (IRAK)1, MyD88, and protein kinase C (PKC)ε to the downstream TLR-signaling complex that translocated PKCε into the nucleus in an IRAK1-dependent manner. PKCε-mediated phosphorylation allowed the nuclear IRAK3 to be exported to the cytoplasm, leading to increased activation of ERK1/2, stabilization of MAPK phosphatase 1 (MKP-1), and induction of TNF-α with concomitant downregulation of p38MAPK. Silencing of TLR1 inhibited PPE17-triggered cytoplasmic export of IRAK3 as well as TNF-α induction, suggesting an important role of TLR1/2 heterodimer in regulating proinflammatory responses via the IRAK3-signaling pathway. In contrast, PPE18-mediated homodimerization of TLR2 caused poorer cytoplasmic export of nuclear IRAK3 and MKP-1 stabilization, resulting in increased p38MAPK activation. Our study hints to a novel mechanism that implicates PKCε–IRAK3–MKP-1 signaling in the regulation of MAPK activity and inflammatory cascades downstream of TLR2 in tuberculosis.

At Least Two Genes Encode Many Variants of Irak3 in Rainbow Trout, but Neither the Full-Length Factor Nor Its Variants Interfere Directly With the TLR-Mediated Stimulation of Inflammation

“…Previous investigations revealed that IRAK3 is equally distributed in the cytoplasm and nucleus of human cells and shifts toward the cytoplasm upon immune stimulation (45, 46). We wanted to see, whether this also applies to the ortholog from trout and whether the Irak3 variants might alter this pathogen-stimulated spatial redistribution of the factor.…”

The N-terminal domain of Mycobacterium tuberculosis PPE17 (Rv1168c) protein plays a dominant role in inducing antibody responses in active TB patients

“…This indicates that though the N-terminal regions of PPE proteins share a conserved sequence, they may be structurally different. Earlier we demonstrated that N-terminal fragments of PPE17 and PPE18 proteins induce contrasting signaling cascades in macrophages due to their different conformations [30,35,42,43]. Thus, it is interesting to speculate that though the N-terminal region of about 180 amino acid residues of PPE proteins are conserved, the subtle variation in the sequence and length in the C-terminal region could change the three-dimensional epitopes of PPE proteins resulting in more unique immunoreactivity to N-PPE17.…”

Innate immune receptors in development of Mycobacterium tuberculosis infection

“…M. tuberculosis и компоненты ее клеточной стенки распознаются TLR1, TLR2, TLR4, TLR7 и TLR9 [45,49,79,103], что приводит к MyD88-зависимой активации антибактериальных эффекторных путей и продукции NO-оксидазы, а также провоспалительных цитокинов, таких как TNFα, IL-12, хемокинов [73]. Такие гликолипиды M. tuberculosis, как липо арабиноманнан, липоманнан и фосфатидилинозитол маннозид, а также белок пролин-пролин-глутаминовой кислоты (PPE)-17 подают сигнал через гетеродимеры TLR2-TLR1 [41,105]. При этом показано, что миколовые кислоты могут ингибировать TLR2 в альвеолоцитах 1 и 2 типа и альвеолярных макрофагах [93].…”