Comitogenic effect of solid-phase immobilized anti-1F7 on human CD4 T cell activation via CD3 and CD2 pathways

We have recently developed a mAb, anti-1F7, which defines a family of structures found to include the molecule recognized by anti-Ta1 (CD26). In this paper, we demonstrated that binding of 1F7 by solid-phase immobilized anti-1F7 mAb but not anti-Ta1 mAb has a comitogenic effect by inducing proliferation of human CD4+ T lymphocytes in conjunction with submitogenic doses of anti-CD3 or anti-CD2. The proliferative response induced via the CD3-1F7 or CD2-1F7 pathways is associated with the IL-2 autocrine pathway, including IL-2 production. IL-2R expression and anti-IL-2R (Tac) inhibition. Furthermore, solid-phase immobilization of anti-1F7 but not anti-Ta1 acts in conjunction with submitogenic doses of PMA to mediate a comitogenic effect in the absence of anti-CD3 or anti-CD2, leading to CD4+ T cell proliferation. PMA treatment, in the meantime, leads to enhanced expression of 1F7 on the T cell surface. Despite its functional association with both pathways of activation, however, the 1F7 structure is not comodulated with the CD3/TCR complex nor the CD2 molecule. These findings thus suggest that the CD26 Ag is involved in CD3 and CD2-induced human CD4+ T cell activation.     

Cell surface modulation of CD26 by anti-1F7 monoclonal antibody. Analysis of surface expression and human T cell activation

In this paper, we examined in detail the ability of anti-1F7 to modulate 1F7 (CD26) surface expression as well as analyzed the functional relationship between the surface expression of CD3, CD2, and CD26 and human T cell activation. We showed that anti-1F7-induced modulation is an energy-dependent process that occurs via capping and internalization of the Ag-antibody complex. Although the recovery rate for Ag reexpression of 1F7 following optimal modulation is relatively delayed, reexpression of 1F7 is greatly accelerated following phorbol ester treatment. Most importantly, we demonstrated that modulation of the CD26 Ag leads to an enhancement in the proliferative activity of modulated human T cells treated with anti-CD3 or anti-CD2, which is preceded by an enhancement in Ca2+ mobilization. CD26 modulation also led to an increase in anti-CD3- or anti-CD2-mediated T cell clone proliferation. Finally, whereas modulation of the CD26 Ag has an effect on CD3- or CD2-induced T cell activation, modulation of the CD3/TCR complex inhibits the proliferative response of T cells incubated with anti-CD3 plus anti-1F7 or anti-CD2 plus anti-1F7. However, modulation of the CD2 structure does not affect anti-CD3- plus anti-1F7-induced human T cell activation. The above results thus provide additional evidence that the CD26 Ag plays an integral role in the regulation of human T cell activation.     

Responsiveness of fetal and adult CD4-, CD8- thymocytes to T cell activation

Day-14 fetal CD4-, CD8- thymocytes showed a greater proliferative response to PMA + IL-4 than did adult double-negative thymocytes. In contrast, adult double-negative thymocytes were more responsive to PMA + IL-1 + IL-2 or to IL-1 + IL-2 alone. The adult double-negative thymocytes showed significantly greater proliferation than fetal thymocytes after stimulation via anti-CD3 or anti-Thy-1 in the presence or absence of interleukins (IL-1 + IL-2 or IL-4). Adult CD4-, CD8- thymocytes also exhibited greater calcium mobilization following anti-CD3 stimulation IL-2-dependent activation with anti-Thy-1 or IL-1 + IL-2 in the absence of PMA resulted in marked expansion of CD 3+, F23.1+, CD4-, CD8- thymocytes, a population absent in fetal thymocytes but constituting 4% of pre-cultured CD4-, CD8- adult thymocytes. IL-4 + PMA failed to expand this CD 3+ population. It is hypothesized that before expression of functional TCR, T cell development may be more dependent on activation pathways not using IL-2; after TCR expression, IL-2-dependent pathways, including Thy-1-mediated stimulation, become functional.     

Regulation of CD4 and CD8 surface expression on human thymocyte subpopulations by triggering through CD2 and the CD3-T cell receptor

Human thymocytes bearing the CD4 and/or CD8 antigens can be fractionated into cells with an immature and more mature phenotype based on their quantitative expression of the CD3 Ag (J. Immunol. 138:3108; J. Immunol. 139:1065). We show that the expression of CD4 and CD8 on thymocyte subpopulations with low CD3 (CD3L) and high CD3 (CD3H) is regulated by activation through the CD2 molecule and perturbation of the CD3-T cell receptor complex (CD3-Ti). Similar to its previously reported effects on peripheral T cells, PMA was able to induce the down-regulation of surface CD4, but not CD8, on thymocyte subpopulations. PMA could induce CD4 and CD8 phosphorylation in both CD3L and CD3H fractions. These results suggest that if changes in phosphorylation represent the mechanism by which CD4 and CD8 are able to transmit signals, this mechanism is operative in both CD3L and CD3H subpopulations. Treatment with anti-T11(2) and anti-T11(3) antibodies (CD2 activation pathway) resulted in partial down-regulation of CD4 but not CD8 surface expression on both CD3L and CD3H thymocytes. Similar treatment had no detectable effect on peripheral T cells. The down-regulation of surface CD4 induced by activation via CD2 could be inhibited by treatment of thymocytes with anti-CD3 antibodies. Treatment of thymocytes with anti-CD3 alone or following CD2 activation induced the selective down-regulation of surface CD8 within 15 minutes. These results suggest that CD2 and CD3-Ti triggering may regulate CD4 and CD8 surface expression on thymocytes. Furthermore, these results suggest that "cross-talk" between the CD2 and CD3-Ti pathway of activation may involve CD4 and CD8 molecules.     

CD7 augments T cell proliferation via the interleukin-2 autocrine pathway.

The role of CD7, a T cell differentiation antigen, in T cell function is not known at present; this study evaluates the effect of anti-CD7 mAb in PMBC cultures activated with suboptimal concentrations of lectins, antigens, and anti-CD3 mAb. We found that the inclusion of anti-CD7 resulted in increased IL-2 production and IL-2R-alpha expression in these cultures. H-7, a protein kinase C (PKC) inhibitor, and genistein, a protein tyrosine kinase (PTK) inhibitor, significantly suppressed the proliferation of T cells in comitogenic assays. This suggested that the comitogenic effect mediated by CD7 molecule involved both the PKC and the PTK pathways of T cell activation. These drugs appeared to affect the CD7-mediated effects by inhibiting the IL-2 autocrine pathway, especially the up-regulation of IL-2R-alpha since inhibition was not relieved with exogenous rIL-2. Taken together, our results suggest that CD7 augments T cell function by up-regulating IL-2R-alpha expression and IL-2 production via multiple pathways of protein phosphorylation.     

Purified lymphocyte function-associated antigen-3 (LFA-3) activates human thymocytes via the CD2 pathway

Defining the cellular and molecular mechanisms of interaction of developing thymocytes with nonlymphoid cells of the thymic microenvironment is critical for understanding normal thymus function. We have previously shown that the CD2/LFA-3 adhesion pathway is important in the interaction of thymocytes with a variety of LFA-3+ nonlymphoid thymic microenvironment cell types. Moreover, T cell activation via the CD2 (alternative, Ag independent) pathway is considered an important mechanism for intrathymic T cell proliferation. To study the relevance of CD2/LFA-3 interactions to human thymocyte activation, we have used purified LFA-3 Ag in several in vitro assays of thymocyte proliferation. Whereas LFA-3 Ag alone did not induce thymocyte proliferation, LFA-3 Ag in combination with the anti-CD2 antibody, CD2.1, and rIL-2 induced marked thymocyte proliferation. Additionally, the anti-CD28 antibody, Kolt2, could substitute for rIL-2, resulting in thymocyte activation induced by LFA-3 Ag in combination with antibodies CD2.1 and Kolt2. In both triggering systems, LFA-3 induced thymocyte activation was dependent upon the concentration of LFA-3 Ag. LFA-3 Ag-dependent thymocyte activation was directed primarily toward CD1-, mature thymocytes. Finally, intact SRBC that express the sheep homolog of LFA-3, T11TS, in combination with antibody CD2.1 and rIL-2 could also induce thymocyte activation. These data suggest that interaction of LFA-3 molecules with thymocyte CD2 molecules may provide a component of the stimulus for normal intrathymic thymocyte activation leading to thymocyte proliferation.     

CD5 costimulation up-regulates the signaling to extracellular signal-regulated kinase activation in CD4+CD8+ thymocytes and supports their differentiation to the CD4 lineage

CD5 positively costimulates TCR-stimulated mature T cells, whereas this molecule has been suggested to negatively regulate the activation of TCR-triggered thymocytes. We investigated the effect of CD5 costimulation on the differentiation of CD4+CD8+ thymocytes. Coligation of thymocytes with anti-CD3 and anti-CD5 induced enhanced tyrosine phosphorylation of LAT (linker for activation of T cells) and phospholipase C-gamma (PLC-gamma) compared with ligation with anti-CD3 alone. Despite increased phosphorylation of PLC-gamma, this treatment down-regulated Ca2+ influx. In contrast, the phosphorylation of LAT and enhanced association with Grb2 led to activation of extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase. When CD3 and CD5 on CD4+CD8+ thymocytes in culture were coligated, they lost CD8, down-regulated CD4 expression, and induced CD69 expression, yielding a CD4+(dull)CD8-CD69+ population. An ERK inhibitor, PD98059, inhibited the generation of this population. The reduction of generation of CD4+CD8- cells resulted from decreased survival of these differentiating thymocytes. Consistent with this, PD98059 inhibited the anti-CD3/CD5-mediated Bcl-2 induction. These results indicate that CD5 down-regulates a branch of TCR signaling, whereas this molecule functions to support the differentiation of CD4+CD8+ thymocytes by up-regulating another branch of TCR signaling that leads to ERK activation.     

Interaction of CD2 with its ligand, LFA-3, in human T cell proliferation

Recently, it has been demonstrated that lymphocyte function-associated Ag (LFA-3) is a natural ligand for CD2 and that this receptor-ligand interaction functions in cell-cell adhesion. In this report, we demonstrate that LFA-3 plays a role in T cell activation. L cells were transfected with human genomic DNA and sorted for expression of LFA-3. We demonstrate that LFA-3+ L cells, together with anti-CD3 mAb or with suboptimal doses of PHA, stimulate proliferation of human peripheral blood T cells. Furthermore, thymocyte proliferation was induced by LFA-3+ L cells and suboptimal doses of PHA. Proliferation was inhibited by mAb directed against either CD2 or LFA-3. Stimulation of thymocytes by the combination of PHA and LFA-3+ L cells resulted in the increased expression of the IL-2R, as well as of the surface Ag 4F2, transferrin receptor, and HLA-DR. These data support the conclusion that LFA-3 plays a role in CD2-dependent T cell activation. LFA-3 is widely distributed and is expressed on all APC and target cells. Thus, the ability of the CD2/LFA-3 interaction to costimulate with an anti-CD3 mAb suggests that the CD2/LFA-3 interaction may be involved not only in an Ag-independent alternate pathway of T cell activation but also in Ag-specific T cell activation.     

Maturation versus death of developing double-positive thymocytes reflects competing effects on Bcl-2 expression and can be regulated by the intensity of CD28 costimulation

Immature double-positive (DP) thymocytes mature into CD4(+)CD8(-) cells in response to coengagement of TCR with any of a variety of cell surface "coinducer" receptors, including CD2. In contrast, DP thymocytes are signaled to undergo apoptosis by coengagement of TCR with CD28 costimulatory receptors, but the molecular basis for DP thymocyte apoptosis by TCR plus CD28 coengagement is not known. In the present study, we report that TCR plus CD28 coengagement does not invariably induce DP thymocyte apoptosis but, depending on the intensity of CD28 costimulation, can induce DP thymocyte maturation. We demonstrate that distinct but interacting signal transduction pathways mediate DP thymocyte maturation signals and DP thymocyte apoptotic signals. Specifically, DP maturation signals are transduced by the extracellular signal-related kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway and up-regulate expression of the antiapoptotic protein Bcl-2. In contrast, the apoptotic response stimulated by CD28 costimulatory signals is mediated by ERK/MAPK-independent pathways. Importantly, when TCR-activated thymocytes are simultaneously coengaged by both CD28 and CD2 receptors, CD28 signals can inhibit ERK/MAPK-dependent Bcl-2 protein up-regulation. Thus, there is cross-talk between the signal transduction pathways that transduce apoptotic and maturation responses, enabling CD28-initiated signal transduction pathways to both stimulate DP thymocyte apoptosis and also negatively regulate maturation responses initiated by TCR plus CD2 coengagement.     

LFA-1在胸腺细胞活化信号传导中的功能研究

在ConA和固相抗CD_3单抗刺激系统中,应用抗LFA-1/ICAM-1单抗,研究其在胸腺细胞活化中的功能作用,结果证明,培养初期加入可溶性抗LFA-1可完全阻断ConA活化胸腺细胞增殖,对固相抗CD3单抗诱导的胸腺细胞活化也表现出相同的抑制效应,但对ConA刺激24h后的胸腺细胞应答以及IL-1 IL-2诱导的胸腺细胞增殖无影响。在可溶性抗LFA-1单抗的存在下,ConA诱导胸腺细胞合成IL-2和IL-6的能力显著下降,IL-2R的表达降低。此外,当用固相抗LFA-1和固相抗CD3或用二抗交联LFA-1和CD3刺激胸腺细胞时,抗LFA-1则具有明显地促增殖应答效应,单纯固相抗LFA-1刺激或交联LFA-1均无诱导活化作用,研究结果表明,LFA-1是未成熟胸腺细胞活化的重要辅助分子之一,它可参与TCR/CD3途径介导的早期活化信号的传导,并为胸腺细胞表达IL-2R 和产生IL-2可能提供复合刺激信号。     

"CD3low" human thymocyte populations can readily be triggered via the CD2 and/or CD28 activation pathways whereas the CD3 pathway remains nonfunctional

We have investigated the role of the CD2 and the CD28 Ag-independent pathways of activation on CD3low thymocytes. We previously showed that anti-CD28 mAb synergized with anti-CD2 mAb directed against epitopes T11.1 and T11.2, in the activation of purified resting T cells or unseparated thymocytes. Proliferation induced via CD2 plus CD28 was mediated via an IL-2-dependent pathway and was not affected by prior modulation of the CD3-TCR complex. Here, we show that a subset of CD3low thymocytes, although unresponsive to CD3 activation, can be activated to proliferate through the CD2 or the CD28 pathways, in the presence of exogenous IL-2. The mitogenic combination of mAb to CD2 and CD28 induces a proliferation of thymocytes which, in absence of exogenous lymphokines, is restricted to the more mature intrathymic subpopulation, CD1a-. However, CD3low thymocytes can also be triggered through the CD2 plus CD28 activation pathways but require at least addition of exogenous IL-2 to proliferate. This study demonstrates that a fraction of immature CD3low thymocytes possesses functional CD2 and CD28 surface molecules at a time when CD3 is not yet functional.     

Transgenic expression of RasGRP1 induces the maturation of double-negative thymocytes and enhances the production of CD8 single-positive thymocytes

RasGRP1 is a guanine nucleotide exchange factor for Ras that is required for the efficient production of both CD4 and CD8 single-positive thymocytes. We found that RasGRP1 expression is rapidly up-regulated in double-negative thymocytes following pre-TCR ligation. Transgenic overexpression of RasGRP1 compensated for deficient pre-TCR signaling in vivo, enabling recombinase-activating gene 2(-/-) double-negative thymocytes to mature to the double-positive stage. RasGRP1 transgenic mice had a 4-fold increase in CD8 single-positive thymocytes, most of which had atypically low levels of CD3. The RasGRP1 transgene lowered the threshold of TCR signaling needed to initiate proliferation of single-positive thymocytes, with this effect being particularly evident among CD8 single-positive cells. In 3-day cultures, TCR stimulation via anti-CD3 caused a 10-fold increase in the ratio of CD8 to CD4 thymocytes among RasGRP1 transgenic vs nontransgenic thymocytes. These results demonstrate that in addition to driving the double-negative to double-positive transition, increased expression of RasGRP1 selectively increases CD8 single-positive thymocyte numbers and enhances their responsiveness to TCR signaling.     

Human T-cell leukemia virus type I-induced proliferation of human thymocytes requires the presence of a comitogen

HTLV-I has recently been shown to be a direct activator of resting human peripheral T cells. In order to determine the susceptibility of T-cell precursors to HTLV-I mitogenic activity we have exposed human thymic T cells to uv-inactivated HTLV-I. Unlike mature T cells, thymocytes were not directly susceptible to HTLV-I-induced activation although agglutination of cells did occur after exposure to HTLV-I alone. However, in the presence of another stimulus, phyto-hemagglutinin or anti-CD3 monoclonal antibodies and accessory cells, thymocytes proliferated when exposed to HTLV-I. Concanavalin A did not induce HTLV-I comitogenic activity. HTLV-I-induced thymocyte proliferation was enhanced by autologous or heterologous accessory cells. This proliferation was shown to be mediated by the interleukin-2/interleukin-2 receptor pathway. Simultaneous stimulation by HTLV-I and nonmitogenic doses of phytohemagglutinin were required both for the production of interleukin-2 and for the expression of the interleukin-2 receptor. These data demonstrated functional differences between peripheral T cells and thymocytes.     

Activation of CD3/TCR negative human thymocytes via CD28 molecule.

We have observed that the CD28 molecule was present on the cell surface of a large fraction of resting CD3- thymocytes (40 to 100%). Interestingly, the majority (greater than 90%) of surface CD3-CD28-cells reacted in the cytoplasm with anti-CD28 (CK248, 9.3) and anti-CD3 epsilon chain mAbs (Leu4, OKT3). Along this line, we found that CD28 surface expression could be induced within 18 hr on CD3-CD28- thymocytes using very low doses of phorbol-13-myristate-12-acetate (PMA). This event was accompanied by the appearance of CD25 and CD69 activation antigens but not of CD3/TCR complex. These results were further confirmed by immunoprecipitation studies. It is noteworthy that the T-cell activation pathway initiated via the CD28 molecule is functional in resting CD3- thymocytes in the presence of PMA and/or IL2. Finally, stimulation of CD3- immature thymocytes via CD28 gave rise to a large fraction (about one-third) of CD3-CD8+ cells.     

CD3Ti+ human thymocyte-derived clones displaying a differential response to activation via CD3Ti and CD2

Previous studies indicated that, unlike peripheral T-cells, freshly isolated thymocytes show little or no proliferation to activation signals via either the antigen/MHC receptor complex (CD3Ti) or the CD2 structure, unless exogenous IL-2 or phorbol esters are added. To investigate these differences in more detail, we have studied the response of clonal populations of mature thymocyte subsets as well as peripheral T-cell clones to activation via either CD3Ti or CD2. Here we report the characterization of three clones belonging to different subsets of mature thymocytes: CD3+ CD4+ (Ti alpha/beta), CD3+ CD8+ (Ti alpha/beta), and CD3+ CD4- CD8- (Ti gamma/delta). All three clones could be induced to proliferate to insolubilized anti-CD3 mAb. In contrast, activating anti-CD2 mAbs, which induced proliferation in all peripheral T-cell clones tested, did not induce an appreciable proliferation of the thymocyte clones. The latter required additional signals provided by the phorbol ester PMA. However, anti-CD2 mAbs were able to induce early activation events such as phosphoinositide turnover and [Ca2+]i increase to an extent similar to the ones elicited by anti-CD3 mAb. These results further support previous findings suggesting that mature thymocytes are not functionally identical to peripheral T-cells.     

The Functional Role of B7 Molecules on the Induction of Thymocyte Activation and Apoptosis

To evaluate the role of B7 on thymocyte activation and apoptosis, we took advantage of TCR transgenic mice in which the majority of thymocytes express a uniform TCR that is specific for ovalbumin. We also prepared Chinese hamster ovary (CHO) cells expressing B7 and appropriate class II molecules. We found that the apoptosis of double-positive thymocytes by TCR-mediated signaling, which presumably represents negative selection, requires a costimulatory signal provided by B7-1 or B7-2. The requirement of B7-1 costimulation for the apoptosis of thymocytes does not change in either low or high antigenic peptide loading. We also demonstrated that two signals through TCR and CD28 augmented the proliferation of thymocytes, and the requirement of CD28-mediated signal by B7-1 or B7-2 for thymocyte proliferation became less evident when high doses of antigenic peptide were loaded, indicating that the intensity of TCR-mediated signal determines the requirement of B7-mediated second signal for thymocyte proliferation.     

Expression and tyrosine phosphorylation of the T cell receptor zeta-subunit in human thymocytes.

Recent evidence suggests that the zeta-subunit of the TCR complex plays a critical role in transducing signals initiated by the Ag receptor heterodimer. Because thymic maturation involves specific interactions between the TCR complex and thymic stromal cells, the zeta-subunit has been postulated to also play a role in this process. To assess the potential for zeta to contribute to thymocyte maturation, we have used an anti-zeta mAb (TIA-2) to quantitate its expression in mature (CD3bright) and immature (CD3dim and CD3-) populations of human thymocytes. Using both flow cytometric and immunoblotting analysis, we found that the relative expression of TCR-zeta varied directly with the surface expression of CD3. Importantly, TCR-zeta was detected in the majority of CD3- thymocytes, indicating that its expression precedes the surface appearance of CD3:TCR. In thymocytes, TCR-zeta was found to be constitutively phosphorylated on tyrosine residues. The relative expression of phospho-zeta varied directly with the maturational stage of the thymocyte, with the mature (CD3bright), single positive cells accounting for most of the phospho-zeta found in the human thymus. The expression of phospho-zeta could be significantly increased by activating thymocytes with mAb reactive with either CD3 or CD2. These results suggest that TCR-zeta is functionally linked to the major thymocyte activation receptors.     

Evidence of synergy between Thy-1 and CD3/TCR complex in signal delivery to murine thymocytes for cell death.

The potential role of Thy-1 in CD3/TCR complex-mediated signal delivery to murine thymocytes was studied. Ag-mimicking cross-linked anti-CD3 mAb stimulated suspension of thymocytes from adult (6 to 8 wk old) mice for a brisk free cytoplasmic calcium ion ([Ca2+]i) rise, low level of inositol phosphate production, and marginal increase in tyrosine-specific phosphorylation of 110/120-kDa and 40-kDa cellular proteins. Weak but sustained [Ca2+]i rise, low inositol phosphate production, and weak protein tyrosine phosphorylation were also induced by the cross-linked anti-Thy-1 mAb that mimicked the putative natural ligand. The signal delivered via either of these two pathways was however insufficient for definitively promoting cell death and DNA fragmentation in the adult thymocytes. Here we demonstrated that anti-Thy-1 mAb synergized with anti-CD3 mAb for inducing a long-lasting prominent [Ca2+]i rise, definite inositol 1,4,5-triphosphate and inositol 1,3,4,5-tetrakiphosphate production, and extensive tyrosine-specific phosphorylation of 110/120-, 92-, 75-, and 40-kDa proteins, which resulted in marked promotion of cell death and DNA fragmentation in the adult thymocytes. This unique anti-Thy-1 antibody activity was confirmed to be directed to glycosylphosphatidylinositol-anchored Thy-1, and was distinguished from the known anti-L3T4 activity that augmented the CD3-mediated signal transduction in a different manner. The synergistic actions of anti-CD3 and anti-Thy-1 mAb obligatorily required the cross-linking of the two mAb together. The anti-CD3 and anti-Thy-1 mAb cross-linked together acted on immature thymocytes from newborn (less than 24 h after birth) mice for rather more extensive promotion of protein tyrosine phosphorylation and cell death. In addition, they affected peripheral T lymphocytes for accelerating protein tyrosine phosphorylation but not cell death. These results suggest a novel function of glycosylphosphatidylinositol-anchored Thy-1 as a possible unique intrathymic intensifier of the CD3/TCR complex-delivered signal for negative thymocyte selection.     

Loss of c-Cbl RING finger function results in high-intensity TCR signaling and thymic deletion

Signaling from the T-cell receptor (TCR) in thymocytes is negatively regulated by the RING finger-type ubiquitin ligase c-Cbl. To further investigate this regulation, we generated mice with a loss-of-function mutation in the c-Cbl RING finger domain. These mice exhibit complete thymic deletion by young adulthood, which is not caused by a developmental block, lack of progenitors or peripheral T-cell activation. Rather, this phenotype correlates with greatly increased expression of the CD5 and CD69 activation markers and increased sensitivity to anti-CD3-induced cell death. Thymic loss contrasts the normal fate of the c-Cbl-/- thymus, even though thymocytes from both mutant mice show equivalent enhancement in proximal TCR signaling, Erk activation and calcium mobilization. Remarkably, only the RING finger mutant thymocytes show prominent TCR-directed activation of Akt. We show that the mutant c-Cbl protein itself is essential for activating this pathway by recruiting the p85 regulatory subunit of PI 3-kinase. This study provides a unique model for analyzing high-intensity TCR signals that cause thymocyte deletion and highlights multiple roles of c-Cbl in regulating this process.