Effects of tolerance induction on early cell cycle progression by Th1 clones

p21(Cip1) up-regulated during histone deacetylase inhibitor-induced CD4(+) T-cell anergy selectively associates with mitogen-activated protein kinases

Histone deacetylase inhibitor n-butyrate induced proliferative unresponsiveness in antigen-stimulated murine CD4(+) T cells. T cells anergized by n-butyrate demonstrated reduced interleukin-2 (IL-2) secretion and decreased activating protein 1 (AP-1) activity upon restimulation. Mechanistic studies determined that the cyclin-dependent kinase (cdk) inhibitor p21(Cip1) was up-regulated in the anergic CD4(+) T cells. p21(Cip1) is known to inhibit the cell cycle through its interaction with cdk, proliferating cell nuclear antigen (PCNA) or c-Jun N-terminal kinase (JNK). p21(Cip1) did not preferentially associate with PCNA or cdk in anergic T helper type 1 (Th1) cells. Instead, among the three interaction partners, p21(Cip1) was found to interact with phospho-JNK and phospho-c-jun selectively in the anergic CD4(+) T cells. The activity of c-jun and downstream transcription factor AP-1 were suppressed in the anergic Th1 cells. In contrast, p21(Cip1) and the two phospho-proteins were never detected concurrently in the control CD4(+) T cells. The n-butyrate-induced p21(Cip1)-mediated inhibition of JNK and c-jun represents a novel potential mechanism by which proliferative unresponsiveness was maintained in CD4(+) T cells.

HIV-1 actively replicates in naive CD4(+) T cells residing within human lymphoid tissues

Although HIV-1 gene expression is detected in naive, resting T cells in vivo, such cells are resistant to productive infection in vitro. However, we found that the endogenous microenvironment of human lymphoid tissues supports de novo infection and depletion of this population. Cell cycle analysis and DNA labeling experiments established that these cells were definitively quiescent and thus infected de novo. Quantitation of the "burst size" within naive cells further demonstrated that these cells were productively infected and contributed to the local viral burden. These findings demonstrate that lymphoid tissues support active HIV-1 replication in resting, naive T cells. Moreover, these cells are not solely reservoirs of latent virus but are permissive hosts for viral replication that likely targets them for elimination.

CTLA-4 regulates induction of anergy in vivo

The requirement for CTLA-4 during the induction of peripheral T cell tolerance in vivo was investigated using naive TCR transgenic T cells lacking CTLA-4. CTLA-4(-/-) T cells are resistant to tolerance induction, as demonstrated by their proliferative responses, IL-2 production, and progression into the cell cycle. Following exposure to a tolerogenic stimulus in vivo and restimulation in vitro, wild-type T cells are blocked at the late G1 to S restriction point of the cell cycle. In contrast, CTLA-4(-/-) T cells enter into the S phase of the cell cycle, as shown by downregulation of p27(kip1), elevated cdk2 kinase activity, and Rb hyperphosphorylation. Thus, CTLA-4 has an essential role in determining the outcome of T cell encounter with a tolerogenic stimulus.

Induction of T cell anergy in the absence of CTLA-4/B7 interaction

Immunologic tolerance in T lymphocytes is maintained through both thymic and peripheral contributions. One peripheral tolerance mechanism is the induction of T cell anergy, a form of nonresponsiveness resulting from incomplete T cell activation, such as stimulation through the TCR in the absence of costimulation. Recent reports have suggested that engagement of the inhibitory receptor CTLA-4 by its B7 ligand is critical for the initiation of anergy. We tested the importance of CTLA-4 in anergy induction in primary T cells with an in vitro anergy system. Using both CTLA-4/B7-blocking agents and CTLA-4-deficient T cells, we found that T cell anergy can be established in the absence of CTLA-4 expression and/or function. Even in the absence of CTLA-4 signal transduction, T cells activated solely through TCR ligation lose the ability to proliferate as a result of autocrine IL-2 production upon subsequent receptor engagement. Thus, CTLA-4 signaling is not required for the development of T cell anergy.

Transduction of human progenitor hematopoietic stem cells by human immunodeficiency virus type 1-based vectors is cell cycle dependent

Human immunodeficiency virus (HIV) type 1 vectors are highly efficient in their ability to transduce human progenitor hematopoietic stem cells (PHSC). Although mitosis was not required for transduction of these cells, transduction rates were much greater once cells had been cultured in the presence of cytokines. Transduction rates, however, rarely exceeded 70%. We demonstrate here that there is a distinct subpopulation that is more easily transduced by HIV vectors. These cells were distinguished by a disproportionate population in the S/G2/M phases of the cell cycle. By sorting them prior to transduction, we found that those cells in either the G1 or S/G2/M fraction were more readily transduced than G0 cells. Maintaining the cells in G0 by omitting cytokines from the medium reduced transduction rates by up to 10-fold. Addition of cytokines to the medium immediately after transduction did not improve the transduction efficiency as measured by expression of the transgene. Analysis of replication intermediates indicated that the block to transduction of G0 cells operated near the time of initiation of reverse transcription. These results suggest that although lentivirus vectors can transduce nondividing PHSC, transduction efficiency is severalfold greater once the cells exit G0 and enter G1. Further characterization of these more transducible cells and identification of the cellular factors responsible may enhance transduction while maintaining the pluripotentiality of the PHSC.

Progression to the G1b phase of the cell cycle is required for completion of human immunodeficiency virus type 1 reverse transcription in T cells

Successful infection by human immunodeficiency virus type 1 (HIV-1) requires the activation of target cells. Infection of quiescent peripheral CD4 lymphocytes by HIV-1 results in incomplete, labile, reverse transcripts. In the present study, we isolated highly purified quiescent T cells and utilized the CD3/CD28 activation pathways as well as cell cycle inhibitors to further define the role of costimulation and cell cycle progression in HIV-1 reverse transcription. Activation with alphaCD3 alone resulted in cell cycle progression into only G1a and incomplete HIV-1 reverse transcription. Costimulation through the CD28 receptor and transition into G1b was required to efficiently complete the reverse transcription process. These findings have relevance to immune activation in vivo, since lymphocytes rendered anergic by a single activation signal would be nonpermissive for productive infection with HIV-1. Importantly, these data also suggest that HIV vector-based genetic transduction strategies might be successful only in target cells that transition into the G1b phase of the cell cycle.

Ability of tolerized Th1 and Th2 clones to stimulate B cell activation and cell cycle progression

Tolerant and nontolerant murine Th1 and Th2 clones, specific for human gamma-globulin (HGG), were compared for their ability to promote cell cycle entry and progression by B cells in vitro. When stimulated with HGG, nontolerant Th1 and Th2 clones induced similar increases in B cell membrane MHC class II levels--a phenomenon associated with early B cell activation. Nontolerant Th1 and Th2 clones also induced B cell DNA synthesis, an event associated with subsequent G1 phase traversal, although Th2 cells were more efficient than Th1 cells in stimulating this activity. Exposure of Th clones to tolerogen in the form of HGG-pulsed chemically fixed APC inhibited the ability of Th1 clones, but not Th2 clones to promote polyclonal B cell DNA synthesis in HGG-stimulated secondary cultures. However, Th1 clones exposed to tolerogen did not lose their ability to increase the expression of MHC class II molecules on B cells in these cultures. These results indicate that tolerance induction does not inhibit the ability of Th1 clones promote B cell cycle progression. In contrast, exposure of Th2 cells to tolerogen does not inhibit significantly the ability of these cells to stimulate B cell cycle entry or progression.