T cell signaling mechanisms that regulate HIV-1 infection

TLR2 activation enhances HIV nuclear import and infection through T cell activation-independent and -dependent pathways

TLR2 activation plays a crucial role in Neisseria gonorrheae-mediated enhancement of HIV infection of resting CD4(+) T cells. We examined signaling pathways involved in the HIV enhancing effect of TLR2. TLR2 but not IL-2 signals promoted HIV nuclear import; however, both signals were required for the maximal enhancing effect. Although TLR2 signaling could not activate T cells, it increased IL-2-induced T cell activation. Cyclosporin A and IkBα inhibitor blocked TLR2-mediated enhancement of HIV infection/nuclear import. PI3K inhibitor blocked HIV infection/nuclear import and T cell activation and exerted a moderate inhibitory effect on cell cycle progression in CD4(+) T cells activated by TLR2/IL-2. Blockade of p38 signaling suppressed TLR2-mediated enhancement of HIV nuclear import/infection. However, the p38 inhibitor did not have a significant effect on T cell activation or TCR/CD3-mediated enhancement of HIV infection/nuclear import. The cell cycle arresting reagent aphidicolin blocked TLR2- and TCR/CD3-induced HIV infection/nuclear import. Finally, cyclosporin A and IκBα and PI3K inhibitors but not the p38 inhibitor blocked TLR2-mediated IκBα phosphorylation. Our results suggest that TLR2 activation enhances HIV infection/nuclear import in resting CD4(+) T cells through both T cell activation-dependent and -independent mechanisms.

The biology of FoxP3: a key player in immune suppression during infections, autoimmune diseases and cancer

The Transcription factor FoxP3 belongs to the forkhead/winged-helix family of transcriptional regulators and shares general structural features with other FoxP family members. FoxP3 functions as a master of transcription for the development of regulatory T-cells (Treg cells) both in humans and in mice. Natural genetic mutations ofFoxP3 that disrupt its function in humans result in an autoimmune syndrome called Immune Polyendocrinopathy, Enteropathy, X-linked (IPEX) and in mice, its deletion causes the Scurfy phenotype, with similar pathology. The finding that FoxP3 is required for the development and function of Tregs has led to an explosion of research in determining its regulation and function in the immune system. Understanding the biological properties of FoxP3 has a wide range of implications for immune tolerance, autoimmune disorders, inflammation and immune response to infectious diseases and cancer.

Human immunodeficiency virus persistence and production in T-cell development

Human immunodeficiency virus type 1 (HIV-1) replication depends on CD4 and coreceptor expression as well as host factors associated with the activation state of the cell. To determine the impact of the activation stage of thymocytes on the HIV-1 life cycle, we investigated R5 and X4 HIV-1 entry, reverse transcription, and expression in discrete thymocyte subsets at different stages of T-cell development. Early after infection, preferential entry and replication of R5 HIV-1 were predominantly detected in mature CD3(+/hi) CD27(+) thymocytes. Thus, R5 HIV-1 targets the stage of development where thymocytes acquire functional responsiveness, which has important implications for HIV pathogenesis. In contrast, X4 HIV-1 expression and replication were primarily found in immature CD3(-/+/low) CD27(-) CD69(-) thymocytes. HIV-1 proviral burden and virus expression in thymocyte subsets correlated with the expression of the highest levels of the respective coreceptor. R5 and X4 HIV-1 entered and completed reverse transcription in all subsets tested, indicating that the activation state of thymocytes and coreceptor expression are sufficient to support full reverse transcription throughout development. Although R5 HIV-1 is expressed mainly in mature CD3(+/hi) CD27(+) thymocytes, 5.3% of HIV-1-infected immature thymocytes express R5 HIV-1, indicating that potentially latent viral DNA can be established early in T-cell development.

HIV infection of naturally occurring and genetically reprogrammed human regulatory T-cells

A T-cell subset, defined as CD4(+)CD25(hi) (regulatory T-cells [Treg cells]), was recently shown to suppress T-cell activation. We demonstrate that human Treg cells isolated from healthy donors express the HIV-coreceptor CCR5 and are highly susceptible to HIV infection and replication. Because Treg cells are present in very few numbers and are difficult to expand in vitro, we genetically modified conventional human T-cells to generate Treg cells in vitro by ectopic expression of FoxP3, a transcription factor associated with reprogramming T-cells into a Treg subset. Overexpression of FoxP3 in naïve human CD4(+) T-cells recapitulated the hyporesponsiveness and suppressive function of naturally occurring Treg cells. However, FoxP3 was less efficient in reprogramming memory T-cell subset into regulatory cells. In addition, FoxP3-transduced T-cells also became more susceptible to HIV infection. Remarkably, a portion of HIV-positive individuals with a low percentage of CD4(+) and higher levels of activated T-cells have greatly reduced levels of FoxP3(+)CD4(+)CD25(hi) T-cells, suggesting disruption of the Treg cells during HIV infection. Targeting and disruption of the T-cell regulatory system by HIV may contribute to hyperactivation of conventional T-cells, a characteristic of HIV disease progression. Moreover, the ability to reprogram human T-cells into Treg cells in vitro will greatly aid in decoding their mechanism of suppression, their enhanced susceptibility to HIV infection, and the unique markers expressed by this subset.