Cellular trafficking of lipoteichoic acid and Toll-like receptor 2 in relation to signaling; role of CD14 and CD36

Abstract

Lipoteichoic acid (LTA) is a central inducer of inflammatory responses caused by Gram-positive bacteria, such as Staphylococcus aureus, via activation of TLR2. Localization of TLR2 in relation to its coreceptors may be important for function. This study explores the signaling, uptake, and trafficking pattern of LTA in relation to expression of TLR2 and its coreceptors CD36 and CD14 in human monocytes. We found TLR2 expressed in early endosomes, late endosomes/lysosomes, and in Rab-11-positive compartments but not in the Golgi apparatus or endoplasmic reticulum (ER). Rapid internalization of fluorescently labeled LTA was observed in human monocytes, colocalizing with markers for early and late endosomes, lysosomes, ER, and Golgi network. Blocking CD14 and CD36 with antibodies inhibited LTA binding and LTA-induced TNF release from monocytes, emphasizing an important role for both molecules as coreceptors for TLR2. Importantly, blocking CD36 did not affect TNF release induced by N-palmitoyl-S-[2,3-bis(palmitoyloxy)-(2R,S)-propyl]-(R)-cysteinyl-seryl-(lysyl)3-lysine or LPS. Expression of CD14 markedly enhanced LTA binding to the plasma membrane and also enhanced NF-kappaB activation. LTA internalization, but not NF-kappaB activation, was inhibited in Dynamin-I K44A dominant-negative transfectants, suggesting that LTA is internalized by receptor-mediated endocytosis but that internalization is not required for signaling. In fact, immobilizing LTA and thereby inhibiting internalization resulted in enhanced TNF release from monocytes. Our results suggest that LTA signaling preferentially occurs at the plasma membrane, is independent of internalization, and is facilitated by CD36 and CD14 as coreceptors for TLR2.

Divergent roles of Toll‐like receptor 2 in response to lipoteichoic acid and Staphylococcus aureus in vivo

“…To determine whether these surface molecules are important for cytokine response to LTA by primary microvascular endothelial cells, HDMEC were pre-incubated with blocking antibodies to CD14 and CD36 (n 5 3). FA6-152 is a mAb that recognizes amino acids 155-185, the immunodominant domain, of CD36, and has been shown to inhibit LTA binding and cytokine production by human monocytes [23]. We found that the IL-6 response was reduced by 60% by anti-CD14, but anti-CD36 had no effect (Fig.…”

“…In addition to further increases in TLR2, CD14 and CXCL10 transcripts (6.171.4-, 14.874.8-and 3.971.5-fold, respectively) compared to treatment with IFN-g alone, there was significant upregulation of a number of inflammatory genes including the chemokines CCL2 and IL-8 (6.372.1-and 28.2719.3-fold, respectively), cytokines To further define the interaction between LTA and TLR2 on primary human endothelial cells, the role of co-receptors was examined. The requirement of CD36 and CD14 for murine macrophages and human monocytes in response to LTA in vitro is well documented [3,8,22,23]. The scavenger molecule CD36, in particular, is deemed to be critical in vivo as CD36-deficient mice suffer significantly higher mortality from a systemic S. aureus infection compared to WT mice [3,22].…”

Phagosomal signaling byBorrelia burgdorferiin human monocytes involves Toll-like receptor (TLR) 2 and TLR8 cooperativity and TLR8-mediated induction of IFN-β

“…The same group demonstrated that TLR2 and MyD88 colocalize to the phagosomal vacuole of RAW macrophages infected with a live vaccine Ft strain (14). TLR2 also can become available for signaling from within endosomal vacuoles in human monocytes stimulated with internalized staphylococcal lipoteichoic acid (56). O'Connell et al (57) demonstrated that intracellular TLR2 was responsible for the initiation of signal transduction events by Chlamydia trachomatis.…”

“…Several studies have now demonstrated that TLR2 is recruited to the phagosome, where it becomes available for signaling (14,(55)(56)(57). Structurally, this model is not at all unanticipated, given that the phagosomal membrane is initially derived from the plasma membrane (58,59).…”

Recombinant human Hsp70 protects against lipoteichoic acid-induced inflammation manifestations at the cellular and organismal levels

“…TLR4 and TLR2, in cooperation with the CD14 receptor, recognize Gram-negative or Gram-positive pathogens, respectively. Lipoteichoic acid, which represents the major component of the Gram-positive bacterial cell wall, stimulates mammalian phagocytes by activating the TLR2, CD14, and CD36 receptors (Lotz et al 2004;Nilsen et al 2008). LPS activates phagocytes that explore components of the TLR4/CD14/MD2 complex as well as a few other proteins and receptors (Leon et al 2008).…”