Platelet-derived soluble TREM-Like Transcrip-1 (TLT-1) induces human and mouse Dendritic Cell activation through a TLR-4-dependent mechanism. (136.32)

Triggering Receptor Expressed on Myeloid Cells-Like Transcrip-1 (TLT-1) is a member of the multifunctional TREM family of receptors that also include TREM-1, 2, 3, and TLT-2. TLT-1 is expressed on megakaryocytes and platelets at low levels, but upon activation with thrombin, its membrane expression is up-regulated and shed, yielding a soluble form (sTLT-1). Here we describe for the first time the ability of human sTLT-1 to activate human monocytes and antigen-presenting cells. Treatment of myeloid Dendritic Cells (DCs) with sTLT-1 resulted in an increase of co-stimulatory molecules, induction of pro-inflammatory cytokines production, and their ability to prime lymphocyte proliferation. sTLT-1 failed to activate murine Bone Marrow-derived Dendritic Cells from TLR4-deficient mice, and blocking antibodies against TLR4 reduced sTLT-1-induced cytokine production by human monocytes and DCs, indicating that TLR4 is involved in sTLT-1 signaling. Therefore, human sTLT-1 has the ability to endow APCs with the capacity to initiate adaptive immune responses, and may therefore be a potential target of anti-inflammatory therapies.

TREM-like transcript-1 protects against inflammation-associated hemorrhage by facilitating platelet aggregation in mice and humans

Triggering receptor expressed on myeloid cells-like (TREM-like) transcript-1 (TLT-1), a type 1 single Ig domain orphan receptor specific to platelet and megakaryocyte alpha-granules, relocates to the platelet surface upon platelet stimulation. We found here that patients diagnosed with sepsis, in contrast to healthy individuals, had substantial levels of soluble TLT-1 (sTLT-1) in their plasma that correlated with the presence of disseminated intravascular coagulation. sTLT-1 bound to fibrinogen and augmented platelet aggregation in vitro. Furthermore, the cytoplasmic domain of TLT-1 could also bind ezrin/radixin/moesin family proteins, suggesting its ability to link fibrinogen to the platelet cytoskeleton. Accordingly, platelets of Treml1-/- mice failed to aggregate efficiently, extending tail-bleeding times. Lipopolysaccharide-treated Treml1-/- mice developed higher plasma levels of TNF and D-dimers than wild-type mice and were more likely to succumb during challenge. Finally, Treml1-/- mice were predisposed to hemorrhage associated with localized inflammatory lesions. Taken together, our findings suggest that TLT-1 plays a protective role during inflammation by dampening the inflammatory response and facilitating platelet aggregation at sites of vascular injury. Therefore, therapeutic modulation of TLT-1-mediated effects may provide clinical benefit to patients with hypercoagulatory conditions, including those associated with inflammation.