Prolonged TCR/CD28 Engagement Drives IL-2-Independent T Cell Clonal Expansion through Signaling Mediated by the Mammalian Target of Rapamycin

Abstract

Proliferation of Ag-specific T cells is central to the development of protective immunity. The concomitant stimulation of the TCR and CD28 programs resting T cells to IL-2-driven clonal expansion. We report that a prolonged occupancy of the TCR and CD28 bypasses the need for autocrine IL-2 secretion and sustains IL-2-independent lymphocyte proliferation. In contrast, a short engagement of the TCR and CD28 only drives the expansion of cells capable of IL-2 production. TCR/CD28- and IL-2-driven proliferation revealed a different requirement for PI3K and for the mammalian target of rapamycin (mTOR). Thus, both PI3K and mTOR activities were needed for T cells to proliferate to TCR/CD28-initiated stimuli and for optimal cyclin E expression. In contrast, either PI3K or mTOR were sufficient for IL-2-driven cell proliferation as they independently mediated cyclin E induction. Interestingly, rapamycin delayed cell cycle entry of IL-2-sufficient T cells, but did not prevent their expansion. Together, our findings indicate that the TCR, CD28, and IL-2 independently control T cell proliferation via distinct signaling pathways involving PI3K and mTOR. These data suggest that Ag persistence and the availability of costimulatory signals and of autocrine and paracrine growth factors individually shape T lymphocyte expansion in vivo.

Abrogating Cbl-b in effector CD8+ T cells improves the efficacy of adoptive therapy of leukemia in mice

“…Consistent with previous studies (35,36), IL-2 blockade inhibited some but not all antigen-induced proliferation of naive wild-type TCR gag cells ( Figure 3A). In contrast, IL-2 blockade did not affect the proliferation of naive TCR gag Cblb -/-cells ( Figure 3A) or cell survival, as we observed significant expansion of Cblb -/-effector cells (data not shown).…”

“…Proliferation of resting T lymphocytes depends upon the engagement of TCR and costimulatory pathways, such as CD28. In naive T cells, initiation of T cell cycling can occur in the absence of IL-2 (33, 35, 53) but appears to require prolonged TCR/CD28 engagement and the downstream activation of PI3K/PKB, Akt, and mammalian target of rapamycin (36). Cbl-b antagonizes TCR-and CD28-dependent proximal signaling of PI3K/PKB, Akt, and NF-κB (18)(19)(20)54), and the enhanced proliferation of Cblb -/-T cells has been shown to be dependent on PI3K (19).…”

Conditional Knockout Mice Reveal an Essential Role of Protein Phosphatase 4 in Thymocyte Development and Pre-T-Cell Receptor Signaling

“…Moreover, PP4 associates with, dephosphorylates, and inhibits histone deacetylase 3 (29), which was originally cloned from phytohemagglutinin-activated T cells and whose mRNA is upregulated in peripheral blood mononuclear cells cultured with anti-CD3 antibody (3). PP4 also associates with its regulatory subunit ␣4 to regulate mammalian target-of-rapamycin (mTOR) signaling, which has been shown to be required for anti-CD3-and anti-CD28-induced, IL-2-independent T-cell proliferation and is also important for T-cell survival (2,17). Thus, the interplay between PP4 and its different regulatory subunits may switch the activity mode of PP4 toward different targets, leading to various PP4-mediated signaling pathways.…”

Proliferating CD4+ T Cells Undergo Immediate Growth Arrest upon Cessation of TCR Signaling In Vivo

“…Under such conditions, continued TCR ligation can, in fact, interfere with this autocrine proliferative pathway (12). Our own investigations of CD4 ϩ T cell proliferation have extended this work, but further indicate that sustained TCR and CD28 signaling can be both necessary and sufficient to maintain an optimal rate of cell division when access to IL-2 in the in vitro cultivation system is limited (13)(14)(15). IL-2 has not been observed to play a major role in Ag-dependent naive CD4 ϩ T cell clonal expansion in vivo (16).…”