TLT-1 (TREM-Like transcript-1) Protects against Hemorrhage Associated with Inflammation by Facilitating Platelet Aggregation.

TREM-like transcript-1 (TLT-1) is a novel, platelet-specific, membrane protein that is stored in a-granules and brought to the platelet surface upon activation. Little is known about its function. Here we build upon our recent findings that a soluble fragment of TLT-1 (sTLT) is released upon platelet activation and can be detected in the serum, but not the plasma, of healthy individuals. Evaluation of patients admitted to the University Hospital, Ramon Ruiz Arnau, in Bayamon Puerto Rico with the diagnosis of sepsis for the presence of plasma sTLT demonstrated significant levels of sTLT in septic individuals as compared to controls, indicating that TLT-1 plays a role in the early stages of sepsis. To directly assess this possibility we generated TLT-1 knockout mice and characterized their hemostatic function. Compared to wild type mice, TLT-1^−/− platelets displayed decreased aggregation when treated with thrombin, collagen, ADP, or U46619 in vitro. The reduced platelet aggregation of TLT-1^−/− mice translates to a 30% increase in bleeding time in tail snip assays compared to controls, supporting TLT-1’s role in vascular homeostasis. To evaluate TLT-1’s role in sepsis we challenged mice with lipopolysaccharide (LPS). LPS challenged TLT-1 null mice die faster and have a significantly lower rate of survival than wild type mice. We used the localized Shwartzman reaction to model the pathology associated with sepsis in a controlled lesion. Although neutrophil infiltration, thrombus formation, and the formation of microclots was not significantly higher in TLT-1^−/− mice, the hemorrhage associated with Shwartzman lesions of these mice was nearly double and lesion size was greater than three times that of wild type mice. Moreover, we demonstrate a recombinant sTLT augments platelet aggregation in vitro suggesting a mechanistic explanation for the increased hemorrhage in TLT-1^−/− mice. Thus these studies reveal an important role for TLT-1 in the maintenance of vascular hemostasis by facilitating platelet aggregation through incorporation of sTLT into the forming clot. This work suggests that therapeutic intervention using tools such as anti-TLT-1 single chain antibodies that inhibit platelet aggregation may provide a novel approach to the control of the thrombotic response.