Memory: the incomplete unhappening of differentiation

Tim-3 Is Not Required for Establishment of CD8+ T Cell Memory to Lymphocytic Choriomeningitis Virus

Tim-3 is a transmembrane protein that is best known for being highly expressed on terminally exhausted CD8+ T cells associated with chronic infection and tumors, although its expression is not limited to those settings. Tim-3 is also expressed by CD8+ T cells during acute infection and by multiple other immune cell types, including CD4+ Th1 and regulatory T cells, dendritic cells, and mast cells. In this study, we investigated the role of Tim-3 signaling on CD8+ T cell memory using a Tim-3 conditional knockout mouse model and mice lacking the signaling portion of the Tim-3 cytoplasmic domain. Together, our results indicate that Tim-3 has at most a modest effect on the formation and function of CD8+ memory T cells.

p27(Kip1) negatively regulates the magnitude and persistence of CD4 T cell memory

Much is known about the differentiation of naive T cells into distinct lineages of effector cells, but the molecular mechanisms underlying the generation and maintenance of CD4 T cell memory are poorly characterized. Our studies ascribe a novel role for the cell cycle regulator p27(Kip1) as a prominent negative regulator of the establishment and long-term maintenance of Th1 CD4 T cell memory. We demonstrate that p27(Kip1) might restrict the differentiation and survival of memory precursors by increasing the T-bet/Bcl-6 ratio in effector CD4 T cells. By promoting apoptosis and contraction of effector CD4 T cells by mechanisms that are at least in part T cell intrinsic, p27(Kip1) markedly limits the abundance of memory CD4 T cells. Furthermore, we causally link p27(Kip1)-dependent apoptosis to the decay of CD4 T cell memory, possibly by repressing the expression of γ-chain receptors and the downstream effector of the Wnt/β-catenin signaling pathway, Tcf-1. We extend these findings by showing that the antagonistic effects of p27(Kip1) on CD4 T cell memory require its cyclin-dependent kinase-binding domain. Collectively, these findings provide key insights into the mechanisms underlying the governance of peripheral CD4 T cell homeostasis and identify p27(Kip1) as a target to enhance vaccine-induced CD4 T cell memory.

Immune markers and correlates of protection for vaccine induced immune responses

Vaccines have been a major innovation in the history of mankind and still have the potential to address the challenges posed by chronic intracellular infections including tuberculosis, HIV and malaria which are leading causes of high morbidity and mortality across the world. Markers of an appropriate humoral response currently remain the best validated correlates of protective immunity after vaccination. Despite advancements in the field of immunology over the past few decades currently there are, however, no sufficiently validated immune correlates of vaccine induced protection against chronic infections in neither human nor veterinary medicine. Technological and conceptual advancements within cell-mediated immunology have led to a number of new immunological read-outs with the potential to emerge as correlates of vaccine induced protection. For T(H)1 type responses, antigen-specific production of interferon-gamma (IFN-γ) has been promoted as a quantitative marker of protective cell-mediated immune responses over the past couple of decades. More recently, however, evidence from several infections has pointed towards the quality of the immune response, measured through increased levels of antigen-specific polyfunctional T cells capable of producing a triad of relevant cytokines, as a better correlate of sustained protective immunity against this type of infections. Also the possibilities to measure antigen-specific cytotoxic T cells (CTL) during infection or in response to vaccination, through recombinant major histocompatibility complex (MHC) class I tetramers loaded with relevant peptides, has opened a new vista to include CTL responses in the evaluation of protective immune responses. Here, we review different immune markers and new candidates for correlates of a protective vaccine induced immune response against chronic infections and how successful they have been in defining the protective immunity in human and veterinary medicine.

Th17 cells are long lived and retain a stem cell-like molecular signature

Th17 cells have been described as short lived, but this view is at odds with their capacity to trigger protracted damage to normal and transformed tissues. We report that Th17 cells, despite displaying low expression of CD27 and other phenotypic markers of terminal differentiation, efficiently eradicated tumors and caused autoimmunity, were long lived, and maintained a core molecular signature resembling early memory CD8(+) cells with stem cell-like properties. In addition, we found that Th17 cells had high expression of Tcf7, a direct target of the Wnt and β-catenin signaling axis, and accumulated β-catenin, a feature observed in stem cells. In vivo, Th17 cells gave rise to Th1-like effector cell progeny and also self-renewed and persisted as IL-17A-secreting cells. Multipotency was required for Th17 cell-mediated tumor eradication because effector cells deficient in IFN-γ or IL-17A had impaired activity. Thus, Th17 cells are not always short lived and are a less-differentiated subset capable of superior persistence and functionality.