A TREM family member, TLT-1, is found exclusively in the α-granules of megakaryocytes and platelets

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Abstract
The triggering receptors expressed on myeloid cells (TREMs) have drawn considerable attention due to their ability to activate multiple cell types within the innate immune system, including neutrophils, monocyte/macrophages, and dendritic cells, via their association with DAP12. TLT-1 (TREM-like transcript-1) lies within the TREM gene cluster and contains the characteristic single V-set immunoglobulin (Ig) domain of the family, but its longer cytoplasmic tail is composed of both a proline-rich region and an immune receptor tyrosine-based inhibitory motif, the latter known to be used for interactions with protein tyrosine phosphatases. Here we report that TLT-1 is expressed exclusively in platelets and megakaryocytes (MKs) and that TLT-1 expression is up-regulated dramatically upon platelet activation. Consistent with this observation, confocal microscopy demonstrates that TLT-1 is prepackaged, along with CD62P, into both MK and platelet α-granules. Differences in thrombin-induced redistribution of CD62P and TLT-1 indicate that TLT-1 is not simply cargo of α-granules but may instead regulate granule construction or dispersal. Together these data show that that TLT-1 does not function to inhibit members of the TREM family but instead may play a role in maintaining vascular hemostasis and regulating coagulation and inflammation at sites of injury.