Summary The T-cell immunoglobulin domain and mucin domain (TIM) family, including TIM-1, TIM-2, TIM-3 and TIM-4, is a relatively newly described group of molecules with a conserved structure and important immunological functions, including T cell activation, induction of T-cell apoptosis and T-cell tolerance, and the clearance of apoptotic cells. TIM-1 costimulates T-cell activation and enhances cytokine production. In humans, TIM-1 also serves as a susceptibility gene for allergy and asthma. TIM-3, expressed on T cells and dendritic cells, regulates T-cell apoptosis and immune tolerance. In contrast, TIM-4, which is expressed primarily on antigen-presenting cells and which is a receptor for phosphatidylserine, regulates T-cell activation and tolerance, in part by mediating the uptake and engulfment of apoptotic cells. The TIM molecules thus have surprisingly broad activities affecting multiple aspects of immunology.

TIM-4, a member of the TIM family expressed on antigen-presenting cells, binds to phosphatidylserine exposed on the surface of apoptotic bodies. However, the significance of this interaction in vivo remains unknown because other receptors have been implicated in the clearance of apoptotic bodies and could compensate for the TIM-4 deficiency in vivo. In this study, we describe the generation of TIM-4-deficient mice and address whether TIM-4 serves a unique function in vivo. We show that TIM-4 −/− peritoneal macrophages and B-1 cells do not efficiently engulf apoptotic bodies in vitro, or clear apoptotic bodies in vivo. TIM-4-deficient mice have hyperactive T and B cells, elevated levels of serum Ig, and develop antibodies to double-stranded DNA. Taken together, we show that TIM-4 is critical for the clearance of apoptotic bodies in vivo, and that lack of TIM-4 results in aberrant persistence of apoptotic bodies leading to dysregulated lymphocyte activation and signs of systemic autoimmunity.phosphatidylserine receptor | phagocytosis | autoantibody | apoptosis T he T cell, Ig, mucin domain (TIM) family includes four expressed Tim genes (Tim-1 through Tim-4) and four predicted genes (Tim-5 through Tim-8) on mouse chromosome 11B1.1, whereas a syntenic region on human chromosome 5q33.2 contains three TIM genes (TIM-1, TIM-3, and TIM-4) (1-3). The syntenic genetic intervals containing the TIM loci in both mice and man have been linked to susceptibility to autoimmune and allergic disorders (2-4).TIM-4, the only TIM protein not expressed on T cells, is found on antigen-presenting cells (APC), including dendritic cells and macrophages (5, 6). First described as a ligand for TIM-1 (7), which is expressed on activated T cells, we showed that TIM-4 costimulates T cell activation by promoting cell survival (5). We now know that TIM-4 exerts a bimodal effect on T cell immunity by inhibiting naïve T cells during the initiation phase of an immune response, and later enhancing T cell responses during the elicitation phase of T cell immunity (6). It is unclear, though, whether activating or inhibitory functions of TIM-4 predominate in vivo. With the generation of TIM-4-deficient mice, we are able to directly address the in vivo role of TIM-4.Two independent groups found that TIM-4 binds to phosphatidylserine (PtdSer) and that blocking TIM-4 with anti-TIM-4 mAb inhibits the engulfment of apoptotic bodies (AB) by TIM-4-expressing macrophages (8,9). Although these studies clearly showed that TIM-4 binds to AB through PtdSer, the physiologic role of TIM-4 in vivo has not been addressed owing to lack of TIM-4-deficient mice. It remains unknown whether TIM-4 is necessary for clearing AB in vivo or whether other receptors that bind AB, such as BAI1 (10) and stabilin-2 (11), compensate for TIM-4 deficiency.Clearance of AB is a critical mechanism for maintaining normal tissue homeostasis in multicellular organisms. AB that are not rapidly cleared progress to secondary necrosis, during which autoand neoantigens are exposed in an inflammato...

TIM (T cell, Ig, mucin) proteins can regulate T cell immune responses. Tim-4 mRNA is not expressed in T cells, but exclusively in APCs. Tim-4 is a ligand for Tim-1 and Tim-4.Ig fusion protein was shown to either inhibit or expand T cells. However, the molecular basis for such opposite effects was not defined. By generating mAbs, we show that expression of Tim-4 protein is restricted to CD11c+ and CD11b+ cells and is up-regulated upon activation. We show that Tim-4 specifically phosphorylates Tim-1 and induces T cell expansion by enhancing cell division and reducing apoptosis. Tim-4 also induces the phosphorylation of signaling molecules LAT, Akt, and ERK1/2 in T cells. Tim-4, expressed on APCs, is a costimulatory molecule that promotes T cell expansion and survival by cross-linking Tim-1 on T cells.