Normal T cell homeostasis: the conversion of naive cells into memory-phenotype cells

CD8 T cell quiescence revisited

Naïve T cells are typically considered to be in a default state of quiescence, whereas memory T cells undergo basal proliferation and quickly exhibit effector responses when stimulated. Over the past few years, however, a more complex picture has emerged, with evidence that naïve T cell quiescence is actively enforced, and that heterogeneity among naïve T cells influences their capacity to escape quiescence in response to homeostatic cues. Furthermore, the active state of memory T cells may also be instructed, requiring contact with dendritic cells to avoid reversion to quiescence. Here, we discuss these new findings and propose that there is much more flexibility in the quiescent state of naïve and memory T cells than previously thought.

FOXO3 regulates CD8 T cell memory by T cell-intrinsic mechanisms

CD8 T cell responses have three phases: expansion, contraction, and memory. Dynamic alterations in proliferation and apoptotic rates control CD8 T cell numbers at each phase, which in turn dictate the magnitude of CD8 T cell memory. Identification of signaling pathways that control CD8 T cell memory is incomplete. The PI3K/Akt signaling pathway controls cell growth in many cell types by modulating the activity of FOXO transcription factors. But the role of FOXOs in regulating CD8 T cell memory remains unknown. We show that phosphorylation of Akt, FOXO and mTOR in CD8 T cells occurs in a dynamic fashion in vivo during an acute viral infection. To elucidate the potentially dynamic role for FOXO3 in regulating homeostasis of activated CD8 T cells in lymphoid and non-lymphoid organs, we infected global and T cell-specific FOXO3-deficient mice with Lymphocytic Choriomeningitis Virus (LCMV). We found that FOXO3 deficiency induced a marked increase in the expansion of effector CD8 T cells, preferentially in the spleen, by T cell-intrinsic mechanisms. Mechanistically, the enhanced accumulation of proliferating CD8 T cells in FOXO3-deficient mice was not attributed to an augmented rate of cell division, but instead was linked to a reduction in cellular apoptosis. These data suggested that FOXO3 might inhibit accumulation of growth factor-deprived proliferating CD8 T cells by reducing their viability. By virtue of greater accumulation of memory precursor effector cells during expansion, the numbers of memory CD8 T cells were strikingly increased in the spleens of both global and T cell-specific FOXO3-deficient mice. The augmented CD8 T cell memory was durable, and FOXO3 deficiency did not perturb any of the qualitative attributes of memory T cells. In summary, we have identified FOXO3 as a critical regulator of CD8 T cell memory, and therapeutic modulation of FOXO3 might enhance vaccine-induced protective immunity against intracellular pathogens.

CD8 T cells are vital for controlling infections with viruses, intracellular bacteria and protozoa. Induction of T and B cell memory is the basis of vaccinations and cellular immunity to intracellular pathogens depends upon the number and quality of memory CD8 T cells. Understanding the mechanisms that control various facets of CD8 T cell memory is of fundamental importance for development of effective vaccines. In this study, we have identified the transcription factor FOXO3 as a crucial regulator of the magnitude of CD8 T cell memory. During a T cell response, FOXO3 limits the number of memory CD8 T cells by inhibiting the accumulation of memory precursor effector cells that give raise to long-lived CD8 T cells. Loss of FOXO3 activity in T cells led to a durable increase in the number of memory CD8 T cells, and the functional quality of FOXO3-deficient memory CD8 T cells was unaffected by FOXO3 deficiency. Thus, our studies suggest that targeting FOXO3 activity may be a fruitful strategy to augment vaccine-induced CD8 T cell memory and protective immunity.

Defective CD8 T cell responses in aged mice are due to quantitative and qualitative changes in virus-specific precursors

Aging is associated with suboptimal CD8 T cell responses to viral infections. It is not clear whether these poor responses are due to environmental influences or quantitative and qualitative changes in the pool of responding CD8 T cells. Our studies demonstrated several deleterious age-related changes in the pool of Ag-specific CD8 T cells that respond to infection. The majority of CD8 T cells from uninfected aged mice was CD44(Hi) and had increased expression of inhibitory receptors including PD1, LAG3, 2B4, and CD160. These aged CD44(Hi) CD8 T cells were transcriptionally similar to exhausted CD8 T cells found during chronic infections. In addition, the number of virus-specific precursors in aged mice prior to infection was decreased up to 10-fold, and many of these Ag-specific precursors had high expression of CD44 and PD1. Finally, TCR transgenic studies demonstrated that the CD44(Hi) Ag-specific CD8 T cells from unimmunized aged and young mice were qualitatively inferior compared with CD44(Lo) CD8 T cells from aged or young donors. Thus, a decrease in precursor frequency as well as qualitative changes of CD8 T cells during aging are directly related to impaired immunity.

Immune reconstitution inflammatory syndrome: the trouble with immunity when you had none

Some individuals who are infected with HIV rapidly deteriorate shortly after starting antiretroviral therapy, despite effective viral suppression. This reaction, referred to as immune reconstitution inflammatory syndrome (IRIS), is characterized by tissue-destructive inflammation and arises as CD4(+) T cells re-emerge. It has been proposed that IRIS is caused by a dysregulation of the expanding population of CD4(+) T cells specific for a co-infecting opportunistic pathogen. Here, we argue that IRIS instead results from hyper-responsiveness of the innate immune system to T cell help, a mechanism that may be shared by the many manifestations of IRIS that occur following the reversal of other types of immunosuppression in pathogen-infected hosts.

Heme oxygenase-1 (HO-1) is constitutively up-regulated in top alpinists

Alpinists who challenge Mt. Everest need adaptation to hypoxia before the attack of Mt. Everest. Although this adaptation is important for the success of climbing Mt. Everest, the molecular mechanism on the adaptation to hypoxia is not well understood. In order to clarify this mechanism, we investigated hypoxia-induced gene expressions specific for top alpinists using microarray analyses. We report here that heme oxygenase-1 (HO-1) is significantly higher in the blood of top alpinist compared with non-alpinists. Although HO-1 expression of non-alpinists is also up-regulated in response to hypoxia, HO-1 level of the top alpinists are constitutively higher than that of non-alpinists. Serial examinations of HO-1 in one top alpinist revealed that the higher expression of HO-1 is maintained in high-level several months after the attack of top mountains. Taken together with the biochemical function of HO-1 that catalyzes heme into CO and bilirubin, HO-1 expression may improve the circulation and compensate with oxidative tissue damages induced by hypoxia. These data also suggest that peripheral blood has the memory on hypoxia independent of antigens by maintaining the high-level of HO-1 expression in top alpinists, which merits the rapid adaptation to hypoxia for 8000m climbing.

Protein kinase C η is required for T cell activation and homeostatic proliferation

Protein kinase C η (PKCη) is abundant in T cells and is recruited to the immunological synapse that is formed between a T cell and an antigen-presenting cell; however, its function in T cells is unknown. We showed that PKCη was required for the activation of mature CD8+ T cells through the T cell receptor. Compared with wild-type T cells, PKCη-/- T cells showed poor proliferation in response to antigen stimulation, a trait shared with T cells deficient in PKCθ, which is the most abundant PKC isoform in T cells and was thought to be the only PKC isoform with a specific role in T cell activation. In contrast, only PKCη-deficient T cells showed defective homeostatic proliferation, which requires self-antigen recognition. PKCη was dispensable for thymocyte development; however, thymocytes from mice doubly deficient in PKCη and PKCθ exhibited poor development, indicating some redundancy between the PKC isoforms. Deficiency in PKCη or PKCθ had opposing effects on the relative numbers of CD4+ and CD8+ T cells. PKCη-/- mice had a higher ratio of CD4+ to CD8+ T cells compared to that of wild-type mice, whereas PKCθ-/- mice had a lower ratio. Mice deficient in both isoforms exhibited normal cell ratios. Together, these data suggest that PKCη shares some redundant roles with PKCθ in T cell biology and also performs nonredundant functions that are required for T cell homeostasis and activation.

Origin and plasticity of MHC I-associated self peptides

Endogenous peptides presented by MHC I molecules represent the essence of self for CD8 T lymphocytes. These MHC I peptides (MIPs) regulate all key events that occur during the lifetime of CD8 T cells. CD8 T cells are selected on self-MIPs, sustained by self-MIPs, and activated in the presence of self-MIPs. Recently, large-scale mass spectrometry studies have revealed that the self-MIP repertoire is more complex and plastic than previously anticipated. The composition of the self-MIP repertoire varies from one cell type to another and can be perturbed by cell-intrinsic and -extrinsic factors including dysregulation of cellular metabolism and infection. The complexity and plasticity of the self-MIP repertoire represent a major challenge for the maintenance of self tolerance and can have pervasive effects on the global functioning of the immune system.

Understand memory, design better vaccines

Naive lymphocytes have a finite lifespan and are continually replaced by input from generative organs. In contrast, memory cells or their progeny can last a lifetime. The expanded populations of memory cells and their more widespread distribution provide protection against recurrent infection.