MERTK as negative regulator of human T cell activation.

The aim of this study was to test the hypothesis whether MERTK, which is up-regulated in human DCs treated with immunosuppressive agents, is directly involved in modulating T cell activation. MERTK is a member of the TAM family and contributes to regulating innate immune response to ACs by inhibiting DC activation in animal models. However, whether MERTK interacts directly with T cells has not been addressed. Here, we show that MERTK is highly expressed on dex-induced human tol-DCs and participates in their tolerogenic effect. Neutralization of MERTK in allogenic MLR, as well as autologous DC-T cell cultures, leads to increased T cell proliferation and IFN-γ production. Additionally, we identify a previously unrecognized noncell-autonomous regulatory function of MERTK expressed on DCs. Mer-Fc protein, used to mimic MERTK on DCs, suppresses naïve and antigen-specific memory T cell activation. This mechanism is mediated by the neutralization of the MERTK ligand PROS1. We find that MERTK and PROS1 are expressed in human T cells upon TCR activation and drive an autocrine proproliferative mechanism. Collectively, these results suggest that MERTK on DCs controls T cell activation and expansion through the competition for PROS1 interaction with MERTK in the T cells. In conclusion, this report identified MERTK as a potent suppressor of T cell response.

TAM receptor function in the retinal pigment epithelium.

The TAM receptor tyrosine kinase Mer is expressed by cells of the retinal pigment epithelium (RPE), and genetic studies have demonstrated that Mer is essential for RPE function. RPE cells that lack Mer exhibit a severely compromised ability to phagocytose the distal ends of photoreceptor (PR) outer segments, which leads to the complete postnatal degeneration of photoreceptors and to blindness. Although in vitro experiments have implicated Gas6 as the critical TAM ligand for this process, we find that Gas6 mutant mice have a histologically intact retina with no photoreceptor degeneration. We further find that, in addition to Mer, RPE cells also express another TAM receptor--Tyro 3--and that both of these receptors are instead activated independently by the Gas6-related ligand Protein S. This protein is also expressed by RPE cells. Finally, we demonstrate that loss of Mer function is accompanied by a substantial down-regulation in Tyro 3 as well. These observations indicate that both Mer and Tyro 3 act in mouse RPE cells and suggest that their biologically relevant ligand in these cells is Protein S.