Ligation of the transmembrane protein T cell Ig and mucin domain (Tim)-1 can costimulate T cell activation. Agonistic Abs to Tim-1 are also capable of inducing T cell activation without additional stimuli. However, little is known about the biochemical mechanisms underlying T cell stimulation or costimulation through Tim-1. We show that a tyrosine in Tim-1 becomes phosphorylated in a lck-dependent manner, whereupon it can directly recruit p85 adaptor subunits of PI3K. This results in PI3K activation, which is required for Tim-1 function. We also provide genetic evidence that p85 expression is required for optimal Tim-1 function. Thus, we describe a pathway from Tim-1 tyrosine phosphorylation to the PI3K signaling pathway, which appears to be a major effector of Tim-1-mediated T cell activation.
Proteins of the T cell Ig and mucin domain (TIM)3 family function to modulate T cell activation and effector function. This family consists of four Tim proteins in the mouse and three TIM proteins in the human (1, 2, 3). The TIM proteins are cell surface, Ig superfamily receptors that may also be secreted and which share the following domain structure: an extracellular Ig-like domain, followed by a polymorphic mucin-like domain, a transmembrane domain, and a cytoplasmic tail of variable length (1, 2, 3).
Of these family members, TIM-1 has been implicated in the immune regulation of asthma and atopic diseases (3, 4). More recent studies have shown that ligation of TIM-1 with an agonistic Ab or with a ligand (TIM-4) can costimulate proliferation of T cells as well as cytokine production, both in vivo and in vitro, and abrogate tolerance in a Th2 model of respiratory tolerance, collectively suggesting that it usually functions as a positive immune regulator (5, 6). Our own prior work also showed that murine Tim-1 can transmit a costimulatory signal for activation and cytokine production of T cells (7). Finally, a recent report demonstrated that human TIM-1 can physically associate with the TCR-CD3 complex to up-regulate T cell activation signals (8). Nevertheless, depending on the affinity and targeted epitope of the cross-linking Ab, and also possibly the timing of its application, opposing conclusions have been reached regarding whether TIM-1 positively or negatively regulates T cell activation and disease outcomes (9, 10, 11, 12). Thus, a greater understanding of the biochemical signals mediated by TIM-1 is required to mechanistically explain its role in immune activation.
The TCR/CD3 complex and costimulatory molecules interact with key intracellular signaling proteins that determine which downstream effector pathways are triggered. Of these, the PI3K have been frequently implicated in costimulatory signaling (13). These enzymes regulate diverse biological processes, including cell growth, proliferation, cytoskeletal organization, apoptosis, and vesicular trafficking (14). Among the various classes of PI3K, class IA proteins are linked to tyrosine kinase-dependent signaling downstream of lymphocyte Ag receptors, costimulatory receptors, and cytokine receptors. These proteins generally exist as heterodimers of a regulatory subunit (p85) and a catalytic subunit (p110). Class I PI3K function to phosphorylate inositol rings at the 3′ position to generate phosphatidylinositol-3,4-bisphosphate, and phosphatidylinositol-3,4,5-trisphosphate (14). These lipids are stationed at the inner leaflet of the plasma membrane, where their major function appears to be to recruit signaling molecules with pleckstrin homology domains. Additionally, the p85 adaptor subunit contains several other interesting domains, including SH2, SH3, and proline rich domains, and a region with homology to Rho family GTPase-activating proteins. These domains facilitate protein-protein interactions that may be able to mediate signal transduction events independently of p110 (15).
To date, there is little known about the molecular basis for signal transduction downstream of TIM-1. We previously showed that the cytoplasmic tail of Tim-1 could be inducibly tyrosine phosphorylated, and that Tim-1 function in human and murine T cell lines requires a conserved tyrosine in the cytoplasmic tail (7). In this present study, we show that Tim-1 tyrosine phosphorylation and activation of downstream transcription factors require the src family kinase Lck, which is also critical for TCR-mediated signal transduction. Following phosphorylation, Tim-1 recruits the PI3K adaptors p85α and p85β. Finally, we show that Tim-1 can enhance activation of primary murine T cells in a manner that requires p85 expression and PI3K activity.
Materials and Methods
Cell lines and mice
In addition to parental Jurkat T cells, the lck-deficient variant JCaM.1 and reconstituted JCaM1/Lck were also used (16, 17). A fast-growing variant of the D10 T cell clone was obtained from M. Krummel (University of California, San Francisco, CA) and was described previously (7, 18). p85β knockout (k.o.) and p85α/β double k.o. mice have been described (19). C57BL/6 mice were purchased from Jackson ImmunoResearch Laboratories. Jurkat, D10, and primary T cells were cultured as previously described (7).
Abs and reagents
Clonotypic Ab to the Jurkat TCR (C305) was from A. Weiss (University of California, San Francisco). Anti-human CD28, anti-mouse (CD3), anti-mCD28, and PE anti-mCD25 were from Caltag Laboratories. Anti-PI3K p85 (rabbit anti-serum) and anti-phospho-tyrosine (4G10) were from Upstate Biotechnology/Chemicon International. Biotin-anti-mCD28 (37.51), biotin-anti-mCD3ε (145-2C11) and FITC-conjugated anti-CD69 were from BD Biosciences. Biotin-anti-mCD4 (L3T4) was from eBioscience. Streptavidin was from Zymed Laboratories. Rabbit anti-hamster IgG and allophycocyanin-conjugated anti-rat IgG were from Jackson ImmunoResearch Laboratories. Akti 1/2, PMA, PP2, and LY294002 were from EMD Chemicals. Anti-flag (M2) and β-actin mAbs and ionomycin were from Sigma-Aldrich. Rat anti-mouse Tim-1 (3B3) was described previously (5). Anti-phospho Akt (S473) rabbit mAb was from BioSource International/Invitrogen.
Transient transfections and luciferase assay
T cell lines were transfected by electroporation using a gene pulser (Bio-Rad), at a setting of 250–260V and 960 μF, in cuvettes containing 1–2 × 107 cells with 0.4 ml antibiotic-free media. The amount and type of DNA plasmids transfected for an experiment are as described in the respective figure legends. Following transfection, cells were cultured in 10 ml complete medium for 16–20 h and then stimulated for 6 h in 96 U-bottom well plates. Reporter assays were performed as described previously (20). Luciferase activity was determined with an Orion luminometer (Zylux).
SH2 domain arrays
A 13 amino acid peptide based on the Tim-1 cytoplasmic tail was synthesized, including Y276 and six amino acids on either side (Peptide Synthesis facility, Biotechnology Center, Molecular Medicine Institute, University of Pittsburgh). The peptide was biotinylated on the N terminus and the tyrosine was phosphorylated. This was then subjected to HPLC; the end product obtained was greater than 90% pure, as determined by mass spectrometry. The purified peptide was incubated with the SH2 domain array, which was processed as per the manufacturer’s instructions (Panomics). Processed arrays were imaged on a Kodak Image Station 2000R, with accumulation to 2000 gray levels. When used as a competitive inhibitor, phenylphosphate was added both after blocking and during peptide incubation, at 100 mM. Quantitation was performed by averaging the intensity values for the duplicate p85α and p85β spots and dividing by the average of all positive control spots. These values were then compared for experiments performed with or without phenylphosphate to derive a percent inhibition.
TIM-1 tyrosine phosphorylation and endogenous p85 association
Jurkat, or Lck mutants JCaM1.6 or JCam1-Lck cells (2 × 107), were transfected with flag-tagged Tim-1 or flag tagged mutant Tim-1 constructs, stimulated with pervanadate (1/100) (21), and/or coupled with PP2 (10 μM). Nonidet P-40 lysates were immunoprecipitated with M2-conjugated agarose beads (Sigma-Aldrich) and separated by SDS-PAGE followed by Western blotting onto a polyvinylidene difluoride membrane (20). Blots were probed with M2 and HRP-conjugated anti-mouse IgG (Pierce) or with anti-p85 pan Ab and HRP-conjugated Protein A (Pierce). They were developed with Super Signal Pico ECL substrate (Pierce) and digitally imaged on a Kodak Image Station 2000R, with accumulation set to stop at 2000 gray levels. For detection of tyrosine phosphorylation, blots were then stripped and re-probed with 4G10.
Analysis of T cell activation markers
CD4+ T cells were purified from spleens of C57BL/6 mice or from p85 β k.o. or p85 α/β double k.o. mice by negative selection (Miltenyi Biotec). T cells were stimulated in 24-well plates coated with rabbit anti-hamster secondary Ab (10 μg/ml overnight at 4°C) followed by primary Abs (2–3 h at 37°C) of hamster anti-mouse CD3 (0.02–1.0 μg/ml) and anti-mouse CD28 (0.5 μg/ml). T cells were harvested at days 1 and 3 poststimulation and examined for CD69 and CD25 surface expression, respectively, by flow cytometry on a BD LSR II. Tim-1 expression was determined with 3B3 mAb, followed by anti-rat IgG Ab.
Purified splenic CD4+ T cells from p85β or p85α/β k.o. mice were stimulated as indicated at one million per well in a 24-well plate. Supernatants were assayed for murine IL-2 using an OptEIA kit from BD Biosciences, according to the manufacturer’s instructions.
Regulation of Tim-1 tyrosine phosphorylation
We showed previously that Tim-1 can be inducibly phosphorylated on tyrosine and that tyrosine 276 in the cytoplasmic tail is required for Tim-1-mediated costimulation of NFAT/AP-1-dependent transcription in T cells (7). This tyrosine is conserved in context between different species (1) and is contained in a motif favorable for recognition by src-family tyrosine kinases, as indicated by the charged residues glutamic acid and aspartic acid upstream (22). To ascertain whether src kinases are indeed required for Tim-1 phosphorylation, we examined tyrosine phosphorylation in the presence of PP2, a potent inhibitor of src kinases (23). Jurkat cells were transfected with epitope-tagged Tim-1, and stimulated with pervanadate, in the presence or absence of PP2, followed by immunoprecipitation (IP) of Tim-1 and anti-phosphotyrosine Western blotting. As shown in Fig. 1,A, tyrosine phosphorylation of Tim-1 was induced by treatment with pervanadate. Inclusion of PP2, along with stimulation by pervanadate, led to a significant decrease in the phosphorylation intensity, which was similar to basal levels. This indicates a requirement of src kinases for Tim-1 tyrosine phosphorylation, even under conditions where potent tyrosine kinase activation is induced, i.e., by pervanadate. Because Lck is a prominent src family tyrosine kinase essential for TCR-mediated T cell activation, we used a Lck-deficient mutant variant of Jurkat cells, JCaM1 (17), to probe the role of Lck in Tim-1 phosphorylation and function. The ability of Tim-1 to become tyrosine phosphorylated in JCaM1 cells was severely compromised (Fig. 1,B), even after treatment with the potent stimulus pervanadate. In contrast, JCaM1 cells that have been stably reconstituted with Lck (JCaM1-Lck) showed a recovery of inducible Tim-1 phosphorylation (Fig. 1,B) similar to what was observed in wild-type cells (Fig. 1,A). To establish the functional relevance of lck-dependent Tim-1 tyrosine phosphorylation, we examined Tim-1-mediated NFAT/AP-1 activation in JCaM1 cells. Consistent with our previously published data with parental Jurkats (7), Tim-1 expression in Lck-reconstituted JCaM1 cells resulted in enhanced transcription of an NFAT/AP-1 luciferase reporter (Fig. 1 C), both basally as well as in conjunction with stimulation through the TCR. In contrast, Lck-deficient JCaM1 cells displayed little, if any, reporter activity either with or without stimulation, despite the fact that Flag-Tim-1 expression was equivalent on both cell lines (data not shown). These data suggest that Tim-1 enhances T cell activation, at least in part, by coupling to phosphotyrosine-dependent signaling pathways.
Interaction of Tim-1 with p85 subunits of PI3K
One mechanism by which phosphotyrosine-dependent signaling complexes are formed is through phosphotyrosine-SH2 domain interactions. To identify SH2 domain-containing signaling proteins that might specifically bind to phosphotyrosine 276 of the Tim-1 tail, a solid phase screen was performed. The TranSignal SH2 domain array from Panomics contains 38 SH2 domains of various signaling proteins, folded in their native conformation and immobilized in duplicate on a membrane. A 13 amino acid peptide was synthesized (Fig. 2,A), which contained the tyrosine 276 of Tim-1 tail and 6 amino acids on either side. The peptide was tyrosine phosphorylated and biotinylated and incubated with the SH2 domain array membrane (for key see Fig. 2,B). As shown in Fig. 2,C, two prominent sets of spots were obtained, which correspond to the N-terminal SH2 domain of p85 α and p85 β. Other spots that were not consistent and of a much lower signal intensity corresponded to SH2 domains from Ras GTPase-activating protein (RAS-GAP) and the tyrosine kinase Fyn. We next performed the assay in the presence of a competitive inhibitor—phenylphosphate—to confirm that the binding observed was dependent on phosphotyrosine. As shown in Fig. 2 D, a much weaker signal was obtained when the SH2 array was incubated with phosphorylated Tim-1 peptide in the presence of phenylphosphate. By comparing the p85 signal with the positive control spots along the right and bottom edges, we determined that binding of the peptide to the p85α and p85β N-terminal SH2 domains was inhibited by 99 and 95%, respectively.
To validate the Tim-1-p85 association with full-length proteins, the interaction was also established in T cells by conventional IP. Flag-Tim-1-transfected Jurkat cells were stimulated with pervanadate, followed by anti-Flag IP and Western blotting with a pan-p85 Ab. Although there was a reproducibly strong interaction of Tim-1 with p85 under conditions of pervanadate stimulation, a weaker and less consistent association was also detected under resting conditions (Fig. 3,A). Consistent with the functional importance of tyrosine 276 in the Tim-1 cytoplasmic tail (7), there was a loss of inducible p85 binding to mutant forms of Tim-1 either lacking the cytoplasmic tail or with Y276 mutated to phenylalanine (Fig. 3,B). In agreement with data discussed above, this interaction was also significantly diminished in cells treated with PP2 (Fig. 3,C). A major target for PI3K-derived lipids is the serine/threonine kinase Akt, activation of which is absolutely dependent upon PI3K. We therefore examined whether endogenous Tim-1 ligation results in activation of Akt, as assayed by its phosphorylation at serine 473. As shown in Fig. 3 D, we observed inducible Akt activation, in both primary T cell blasts and the Th2 T cell clone D10, after stimulation (of untransfected cells) with Abs to CD3 and CD28 or Tim-1. Akt activation by either CD3/CD28 or Tim-1 was completely inhibited by pretreatment with the PI3K inhibitor LY294002 (data not shown). These results demonstrate that Tim-1 can interact with p85 and activate PI3K signaling, when expressed in T cells, in a phosphotyrosine-dependent manner.
Role of p85 and PI3K in Tim-1 function
p85 proteins function as regulatory subunits of class IA PI3K and, as such, control localization and activation of the associated catalytic p110 proteins. To determine whether the catalytic property of PI3K is required for Tim-1-mediated costimulation of downstream NFAT/AP-1 dependent transcription, D10 cells (which possess a normally regulated PI3K pathway) (18) were cotransfected with an NFAT/AP-1 luciferase reporter, along with either empty vector or Tim-1. These cells were then treated with a range of stimuli, in the presence or absence of LY294002, an inhibitor of PI3K activity (24). As shown in Fig. 4,A, stimulation through CD3 and CD28 induced an enhanced activation of the reporter in Tim-1-transfected cells, compared with cells transfected with empty vector. However, in the presence of LY294002, the effect mediated by Tim-1 was markedly reduced. These data suggest that one mechanism by which Tim-1 costimulates T cell activation is through the catalytic function of PI3K. Therefore, we examined the role of a known downstream target of PI3K—the serine/threonine kinase Akt/protein kinase B (25), which, as shown above, is activated by Tim-1. A similar reporter assay was performed as in Fig. 4,A, this time with the addition of the Akt inhibitor Akti 1/2 (26). As shown in Fig. 4,B, Tim-1-mediated activation of NFAT/AP-1 was effectively inhibited by Akti 1/2. We attempted to more definitively determine the role of p85 in Tim-1 function by short interfering RNA-mediated knock-down of p85 in D10 T cells. We were able to achieve at best ∼60% decrease of the protein (Fig. 4,C), likely due in part to the necessity to eliminate expression of both p85α and p85β; this resulted in a partial inhibition of Tim-1 function, as determined with the NFAT/AP-1 reporter assay (Fig. 4,D). Taken together, the data in Fig. 4 indicate that Tim-1 exerts an effect on NFAT/AP-1 at least in part through the PI3K/Akt pathway.
One of the earliest activation markers expressed by T cells is the cell surface glycoprotein CD69, which appears within several hours following TCR cross-linking and persists for ∼3 days (27, 28). To assess the effect of Tim-1 on CD69 up-regulation, purified CD4+ T cells from C57BL/6 mice were activated with a range of concentrations of plate-bound anti-CD3, together with a fixed amount of anti-CD28, in the presence or absence of Tim-1 Ab. Ligation of Tim-1 substantially enhanced the up-regulation of CD69 expression (Fig. 5,A), with the greatest effect observed at lower concentrations of anti-CD3. An increase in CD69 expression was also observed on primary T cells treated with anti-Tim-1 alone. In light of data discussed above, we were interested in determining whether PI3K catalytic activity is required for Tim-1-mediated CD69 up-regulation. As shown in Fig. 5 A, LY294002 significantly inhibited CD69 up-regulation induced by Tim-1 ligation, either alone or in conjunction with anti-CD3/CD28.
Another inducible cell surface marker essential for T cell commitment to effector function is the high affinity IL-2 receptor α-chain (CD25). To evaluate whether Tim-1 selectively enhanced expression of the early activation Ag CD69, or whether this phenomenon was a more general consequence of its effects on T cell activation, we also examined CD25 up-regulation (Fig. 5,B). As observed with CD69, Tim-1 cross-linking costimulated CD25 surface expression and this effect was more evident with lower concentrations of anti-CD3. Again, as with CD69, Tim-1 ligation alone led to some up-regulation of CD25 expression. Treatment with LY294002 significantly inhibited Tim-1-mediated CD25 up-regulation, both by itself and in conjunction with CD3/CD28 signals. Thus, data presented in Fig. 5 suggest that Tim-1 induces expression of the activation markers CD25 and CD69 in a PI3K-dependent manner.
To determine more conclusively the relevance of the p85 interaction for endogenous Tim-1 function, a mouse model of p85 deficiency mice was used (19). These mice were generated by crossing Pik3r1f mice, which have floxed alleles of Pik3r1 (encoding p85α, p50α, and p55α), with Pik3r2n mice, which are deficient in the germline for Pik3r2 (encoding p85β). The resulting mice were then bred to Lck-Cre transgenic mice, yielding mice with T cells specifically lacking expression of PI3K regulatory subunits from the double negative stage of thymocyte development forward. T cells from p85β null mice develop normally and possess no detectable defects in PI3K signaling or any compensatory changes in PI3K isoform expression, so these mice served as controls (19, 29). p85 double k.o. mice also possess T cells with no gross developmental problems but have selective defects in TCR-dependent signaling and cytokine production (19).
We first assessed expression of Tim-1 on T cells from p85-deficient mice. Importantly, expression of Tim-1 was not impaired on p85α/β k.o. T cells, with expression in the resting (low levels) and activated (higher levels) state indistinguishable from p85β or wild-type mice (Fig. 6,A). To confirm and extend the inhibitor studies performed with T cells from C57BL/6 mice, we examined costimulatory Tim-1 effects on surface expression of CD69 and CD25 in CD4+ T cells from these p85 mutant mice. Representative flow cytometry plots for results obtained with CD69 are shown in Fig. 6, B and C. Thus, while stimulation with anti-CD3/CD28 alone was relatively unaffected by the complete lack of p85 in this experiment (compare C with B), the ability of Tim-1 to costimulate CD69 up-regulation was severely impaired.
Because we and others have observed both stimulatory and costimulatory effects of Tim-1 ligation, we wanted to determine whether both or either of these activities would be affected by p85 deficiency. We therefore examined the effects of Tim-1 Ab 3B3 on up-regulation of CD69 (Fig. 7,A, C, and E) and CD25 (Fig. 7,B, D, and F), either in the presence of CD3/CD28 engagement (Fig. 7, A–D) or without any additional stimuli (Fig. 7, E and F). As can clearly be seen from these data, the absence of p85 expression, in the p85α/β double k.o. T cells, had a profound effect on the ability of Tim-1 to induce increases in CD69 and CD25, either by itself or in conjunction with CD3/CD28 signaling. Previous studies with p85-deficient mice had demonstrated that the proliferation defect was largely corrected with the addition of exogenous IL-2 (19). Although activation markers were assessed relatively early in the course of T cell activation, we wanted to ensure that proliferation differences did not underlie the effects we had observed. Thus, as expected, addition of IL-2 to the cultures had no effect on the results (data not shown).
Finally, we examined cytokine production by the p85 mutant T cells and found that Tim-1 induction of IL-2 secretion was impaired in the p85 double k.o. cells, compared with T cells lacking p85β alone (Fig. 8). In contrast to the more potent stimulatory effects of Tim-1 Ab on CD69 and CD25 induction, Tim-1 crosslinking by itself had a small effect on IL-2 secretion (note difference in scales between A and B), which was partially impaired in the p85 α/β-deficient T cells (Fig. 8,A). However, in the presence of sub-optimal anti-CD3/CD28 stimulation, Tim-1 ligation significantly enhanced the production of IL-2, an effect that is severely impaired in the p85 α/β-deficient T cells (Fig. 8 B).
Recent studies have demonstrated that the transmembrane protein Tim-1 can influence the course of an immune response through direct effects on T cell activation. However, the biochemical signaling pathways underlying these effects of Tim-1 ligation are not understood. Using a combination of biochemical, pharmacological, and genetic approaches, we delineate a signaling pathway, starting with Src kinase-mediated phosphorylation of Tim-1, to recruitment and activation of PI3K and Akt, which are required for induction of NFAT/AP-1-dependent transcription and up-regulation of surface activation markers and cytokines that are important for T cell activation and function.
The data we present here reveal a requirement for the Src family kinase Lck for Tim-1 tyrosine phosphorylation and subsequent NFAT/AP-1-dependent transcription. Our current model is that Lck first phosphorylates the cytoplasmic tail of Tim-1, which allows subsequent recruitment of signaling proteins to Y276, a residue that is required for Tim-1 function in T cell lines (7). It should be noted that our data are also consistent with a possible role for the related tyrosine kinase Fyn in the regulation of Tim-1 function. Thus, the JCaM.1 cell line, while lacking expression of Lck, also contains little, if any, Fyn protein (30). In addition, the kinase inhibitor PP2, although relatively specific for Src family kinases, cannot distinguish between possible functions for Lck or Fyn. Finally, in some SH2 domain array experiments, we noted weak binding of the Tim-1 phospho-Y276 peptide to the SH2 domain of Fyn. Further investigation is therefore warranted to address whether Fyn contributes to Tim-1 function.
Using an unbiased approach, we find that phosphorylation of Y276 leads to recruitment of the p85 subunits of PI3K, and we further confirm this interaction with endogenous p85 in T cells expressing Tim-1. Interestingly, interaction with p85 is a common feature shared by other T cell accessory molecules, such as CD28, ICOS, and CTLA-4. However, in contrast to these molecules, which preferentially bind the C-terminal SH2 domain of p85 (31, 32, 33, 34, 35), Tim-1 binds the amino-terminal SH2 domain, as revealed by the SH2 domain array. Another difference is that p85 binds to the CD28 family members via a consensus YXXM motif, whereas it binds to Tim-1 at a noncanonical sequence (YIVE). Although the amino-terminal p85 SH2 domain has a 10-fold lower avidity for the YXXM motif than does the C-terminal SH2 domain (31), work from Songyang and colleagues (36) suggests that YMXM is the preferred target for both domains. We have attempted to coimmunoprecipitate an isolated N-terminal p85α SH2 domain construct with Tim-1, but have been unable to do so (unpublished data), possibly due to the low affinity of the interaction in the absence of other sequences. Further investigation will be required to more precisely define how the Tim-1-p85 complex forms and what other proteins may be involved. Nonetheless, our ability to coimmunoprecipitate p85 with Tim-1 in a phosphotyrosine-dependent manner (Fig. 3) and the observed interaction between the purified N-SH2 and peptide (again, phosphotyrosine-dependent; Fig. 2) suggest that this is a direct interaction.
Consistent with its recruitment of p85, Tim-1 requires PI3K and Akt activity to enhance T cell activation, because LY294002 and Akti 1/2 inhibit Tim-1 function. We also used a recently described genetic model, in which there is a complete lack of expression of the p85 regulatory subunits of PI3K selectively in T cells (19). This is accompanied by a significant reduction in levels of the catalytic p110 subunit isoforms (which are normally stabilized by p85 binding), resulting in specific abrogation of class IA PI3K activity in T cells. Consistent with previous data (19), CD25 and CD69 up-regulation triggered by TCR stimulation was only mildly affected in the p85β and double k.o. T cells. However, the ability of Tim-1 to costimulate CD25 and CD69 is severely impaired in the double k.o. T cells, when compared with p85β-deficient T cells. Again, consistent with previous findings (19), p85β-deficient T cells behave identically to wild-type T cells with regard to CD25 and CD69 induction (data not shown).
T cell activation induced by Tim-1 alone was observed in our earlier studies performed in Jurkat T cells and is also in agreement with other studies where hyperproliferation of T cells was observed in mice treated with soluble mTim-1 molecules, as well as activation of primary human cells treated with anti-Tim-1 Ab (6, 7, 8). A recent study has shown that Tim-1 can associate with the TCR-CD3 complex in the basal state, an interaction that is increased by Tim-1 or TCR/CD3 ligation (8). Tim-1 association with the TCR/CD3 complex could lead to aggregation of both, which may facilitate Tim-1 phosphorylation and activation of downstream signaling pathways through both TCR-dependent and -independent pathways. Indeed, we show that increasing amounts of agonistic anti-Tim-1 alone are accompanied by increased surface expression of CD69 and CD25. Although there is a significant dampening of this response in T cells from p85 double k.o. mice, higher concentrations of anti-Tim-1 are eventually sufficient to induce CD69 and CD25 expression. Though these T cells completely lack p85 regulatory subunits, proximal TCR-induced signaling seems relatively unaffected, because ZAP-70 phosphorylation is intact (19). Given that Tim-1 may also induce ZAP-70 phosphorylation through its interaction with the TCR (8), the residual effects of Tim-1 observed in p85-deficient T cells may be TCR-dependent.
Data presented here support the model that Tim-1 functions as a positive costimulatory molecule. However, anti-Tim-1 Abs targeting different epitopes of the Tim-1 extracellular domain have been shown to initiate either positive or negative effects on T cell activation (9, 10, 12). Thus, two anti-Tim-1 Abs, RMT1-10 and 3B3, differing by 17-fold in binding affinity to the same or closely related Tim-1 epitope in the IgV domain, exert opposite effects on T cell function (9). Although Tim-1 ligation by RMT1-10 apparently inhibits T cell activation, crosslinking with the higher affinity Ab 3B3 potentiates T cell activation. Recent structural studies also predict different ligand binding modes for the TIM family members (37, 38), consistent with direct and indirect evidence for multiple ligands for these proteins (2, 6, 7, 39, 40, 41). Thus, it is possible that more than one ligand might interact with Tim-1 concurrently on different faces of the molecule. Furthermore, homotypic interactions between individual Tim-1 molecules are mediated through their Ig domains, interactions that may be affected by glycosylation of the polymorphic mucin domain (38). The ultimate outcome of Tim-1 engagement may therefore depend upon the nature of the ligand(s) with which it interacts, as well as the timing and stoichiometry of such interactions. Differences in functional outcomes after Tim-1 ligation may result from divergent effects of Tim-1 ligation on T cell signaling pathways under such circumstances.
Although our study indicates that PI3K plays an important role in mediating Tim-1 signaling, this does not exclude the possibility that additional molecules can also bind to Tim-1 and transmit signals to other downstream pathways. Based on data presented here, we suggest that ligation of Tim-1, when coupled with TCR stimulation, enhances downstream signaling pathways that induce T cell activation, at least in part through PI3K and its downstream effector Akt. Future studies will focus on the mechanisms by which Tim-1-mediated PI3K activation affects T cell activation.
We are grateful to A. Weiss for providing C305 Ab. We also thank V. Kuchroo for sharing data before publication.
The authors have no financial conflict of interest.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
This work was supported by grants from the National Institutes of Health (AI67544 to L.P.K., AI50831 to D.A.F., P01 AI054456 and HL069507 to R.H.D., and T32 AI60573 to J.S.O.).
Abbreviations used in this paper: TIM, T cell Ig and mucin domain; k.o., knockout; IP, immunoprecipitation; stim, stimulated; unstim, unstimulated; m, mouse.