T and B cell hyperactivity and autoimmunity associated with niche-specific defects in apoptotic body clearance in TIM-4-deficient mice

Abstract

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...

Molecular mechanism for differential recognition of membrane phosphatidylserine by the immune regulatory receptor Tim4

“…purposes of phagocytic removal (15,16,(20)(21)(22). The crystal structure of Tim4 in complex with a single-PS head group has provided invaluable insight into how Tim4 specifically recognizes a single-PS molecule (23).…”

Blocking monoclonal antibodies of TIM proteins as orchestrators of anti-tumor immune response

“…Another obstacle to the application of anti-TIMs mAbs is the possibility of inducing adverse toxicity, most commonly reversible autoimmunity or systemic inflammatory reactions. 52,61 In conclusion, TIM molecules play critical roles in the regulation of tumor immunosurveillance and anti-tumor immunity, and anti-TIMs mAbs can be utilized as potent tools to manipulate the functions of various immune cells, and thus orchestrate immune responses against tumors. The molecules have the potential to greatly impact the therapeutic effects of cancer regimens.…”

“…For example, TIM-4 serves as a critical phagocytosis receptor in peritoneal macrophages, since the deficiency of TIM-4 results in a significant decrease in the ability of peritoneal macrophages to engulf apoptotic cells. 61 However, splenic and tumor-associated macrophages were still able to engulf apoptotic cells even in the absence of TIM-4, suggesting the existence of other phagocytosis receptors expressed in these subsets of APCs. 61,62 Recent studies have identified a novel role of TIM-4 in the regulation of tumor immunity and antitumor immune responses.…”

“…61 However, splenic and tumor-associated macrophages were still able to engulf apoptotic cells even in the absence of TIM-4, suggesting the existence of other phagocytosis receptors expressed in these subsets of APCs. 61,62 Recent studies have identified a novel role of TIM-4 in the regulation of tumor immunity and antitumor immune responses. 62 TIM-4 plays an important role in controlling tumor-specific responses through regulation of processing and presentation of antigens derived from phagocytized apoptotic tumor cells.…”

The impact of the TIM gene family on tumor immunity and immunosuppression

“…55 (d) TIM-4 expression is largely restricted to APCs, and it serves as a phosphatidylserine receptor that regulates the engulfment of apoptotic cells; it interacts with TIM-1 on T cells to regulate the differentiation of Th1 and Th17 cells. 34,57,59 APC, antigen-presenting cell; DAMP, damage-associated molecular pattern; DC, dendritic cell; NK, natural killer; Sema4A, Semaphorin 4A; TIM, T-cell immunoglobulin mucin. acids into DC endosomes, which blocks innate immune signaling pathways upstream of PRR-sensing systems.…”