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Introduction mTOR (mammalian target of rapamycin) is a ubiquitously expressed serine/threonine kinase that affects a number of key cell functions, including protein synthesis and proliferation. 1,2 mTOR associates with either protein called raptor or another named rictor to form the mTORC1 and mTORC2 complexes, respectively. The signaling pathways activated by mTORC1 have thus far been much better characterized. 1-3 Accordingly, mTORC1 acts by activating p70S6 kinase 1 (p70S6K1) and inhibiting the 4E binding protein 1 (4E-BP1). In turn, p70S6K1 phosphorylates an S6 protein of the 40S ribosomal subunit (S6rp) at several sites, including serines 235 and 236. The exact mechanisms of mTOR activation are still under investigation, but at least 2 separate, distinct signals are required. Whereas the first is provided by the cell membrane receptors for growth factors, such as insulin and insulin-like growth factor (IGF), the second is generated by nutrients. [4][5][6][7] The IGF and other receptors activate cell-signaling pathways PI3K/Akt 8-10 and ERK/ MEK. 11-13 Both these signaling pathways have been implicated in mTORC1 activation through an indirect mechanism by suppressing the activity of the tuberous sclerosis complex proteins TSC1 and TSC2 which, in turn, inhibit activity of mTORC1 through inactivating the G protein Rheb. mTORC1 can be functionally inactivated by inhibitors from the rapamycin family. Rather than occupying the enzymatic kinase/ATP-binding domain, rapamycin-type compounds block the binding of mTORC1 to the FKBP12 protein and, consequently, inhibit activity of the complex. Rapamycin and its derivatives are highly potent and specific for mTORC1. They are currently used clinically as immunosuppressive drugs and are evaluated as therapeutic agents in various types of cancer. 1,2 From the several cytokines such as IL-2, IL-7, IL-15, and IL-21 that signal through the receptors that share the common ␥ chain (␥c), IL-2 is by far the best characterized. It signals through the receptor that, in addition to the ␥c, contains the second signaling chain ␤ and, in the case of high-affinity receptor, also the IL-2-specific, signal nontransducing ␣ chain. In normal immune cells, IL-2 has been shown to activate Jak/STAT signaling, PI3K/Akt and MEK/ERK signaling pathways. 14 In addition to the ␥c, the receptor for IL-7 contains a second IL-7-specific signaltransducing ␣ chain. IL-7 has been implicated in promoting maturation and survival of T lymphocytes. [15][16][17][18] In turn, IL-15 shares with IL-2 both receptor signaling chains, ␥ and ␤, and, similar to IL-2, also has the IL-15-specific, nontransducing ␣ chain. Consequently, IL-2 and IL-15 share a number of properties, including stimulation of the T-, natural killer (NK)-, and B-cell proliferation and functional maturation, but certain features unique to each of them have also been described. 14,19 IL-21 also displays a spectrum of effects on the immune cells, 20 with its ability to increase cytotoxicity of both NK 21 and CD8 ϩ T 22 cells being the best define...

We examined activation of the mTOR signaling pathway in situ in the primary, normal reactive and patient-derived post-transplant lymphoproliferative disorder (PTLD) tissue samples. We accomplished this analysis by immunohistochemistry on formalin-fixed, paraffin-embedded specimens using a set of highly specific antibodies that permitted us to determine phosphorylation status of the key serines in the mTOR target proteins. Our results demonstrate that the mTOR signaling pathway is activated in reactive tissue in a highly distinct fashion with positive, typically enlarged cells being present primarily in the germinal center and, to a lesser degree, in interfollicular areas with mantle zone being conspicuously negative. We could demonstrate mTOR activation in the lesional cells in the entire spectrum of PTLD subtypes, regardless of their Epstein-Barr virus genome expression status. These data demonstrate the ubiquitous activation of the mTOR signaling pathway in PTLD and indicate that mTOR inhibitors may be effective in treatment and, notably, prevention of PTLDs given their immunosuppressive properties. Furthermore, our results define potential biomarkers of the therapeutic response. Because the constitutive mTOR activation has also been identified in cells isolated from other hematologic malignancies, the ability to examine the in vivo mTOR signaling may have implications reaching beyond the PTLD field.

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Using immunohistochemistry with antibodies against the phosphoserine residues in both S6rp and 4E binding protein 1, we identified the activation of the mammalian target of rapamycin (mTORC)1 pathway in 29 cases of AIDS-related lymphoma. These cases represented a diverse spectrum of histological types of nonHodgkin lymphoma (24 cases) and classic Hodgkin lymphoma (five cases). mTORC1 was also activated in the hyperplastic but not involuted follicles of HIV-associated lymphadenopathy in eight cases, supporting the notion that mTORC1 activation is a common feature of transformed lymphocytes irrespective of either their reactive or malignant phenotype. We also found that in B-cell lines that represent diffuse large B-cell lymphoma, Burkitt lymphoma, Epstein-Barr virus-infected lymphocytes, and human herpesvirus 8-positive primary effusion lymphoma, inhibitors of Syk, MEK, and, seemingly , phosphoinositide 3 kinases suppressed mTORC1 activation , in particular when these inhibitors were used in combination. These findings indicate that AIDS-related lymphoma and other histologically similar types of lymphomas that are derived from transformed B lymphocytes may display clinical responses to inhibitors that directly target mTORC1 or , possibly , upstream activators of the mTORC1 pathway.