Cutting edge: naive T cells masquerading as memory cells

This study shows that naive CD8 T cells can acquire characteristics of memory T cells in the absence of stimulation with specific Ag simply by the process of homeostatic proliferation under lymphopenic conditions. This Ag-independent T cell differentiation pathway did not result in up-regulation of early activation markers (CD69, CD25, CD71), but expression of several memory markers (CD44, CD122, Ly6C) increased progressively with successive divisions. These markers were then stably expressed, and these cells also became more responsive functionally to specific Ag. Thus, all "memory" phenotype T cells in an individual may not be true Ag-experienced cells and may include naive cells masquerading as memory cells. These findings are specially relevant in cases of disease or treatment-induced lymphopenia such as in HIV-infected individuals or transplant recipients. In addition, this study may have implications for autoimmunity because homeostatic proliferation of naive T cells requires interaction with self peptide plus MHC molecules.     

Cutting edge: murine cytomegalovirus induces a polyfunctional CD4 T cell response

CD4 T lymphocytes regulate the adaptive immune response to most viruses, both by providing help to CD8 T cells and B cells as well as through direct antiviral activity. Currently, no mouse cytomegalovirus (MCMV)-specific CD4 T cell responses are known. In this study, we identify and characterize 15 I-A(b)-restricted CD4 T cell responses specific for MCMV epitopes. CD4 T cells accumulate to high levels in the spleen and lungs during acute infection and produce multiple cytokines (IFN-gamma, TNF, IL-2, IL-10, and IL-17). Interestingly, IL-17 and IFN-gamma production within epitope-specific cells was found to be mutually exclusive. CD4 T cells recognizing a peptide derived from m09 were only detectable at later times of infection and displayed a unique cytokine production profile. In total, this study reveals that the MCMV-specific CD4 T cell response is complex and functionally diverse, highlighting its important role in controlling this persistent pathogen.     

Cutting edge: memory CD8 T cell maturation occurs independently of CD8alphaalpha

As memory CD8 T cells form during acute viral infection, several changes in gene expression and function occur, but little is known about the control of this process. It was reported previously that the homodimer CD8alphaalpha was involved in generating IL-7Ralphahigh memory CD8 T cell precursors, and consequently, protective memory CD8 T cells did not form in animals significantly impaired in CD8alphaalpha expression (E8(I)-/- mice). However, the precise contribution of CD8alphaalpha to sustained IL-7Ralpha expression and other memory CD8 T cell-associated changes has not been investigated. We found that IL-7Ralpha expression and generation of memory CD8 T cells that protect against secondary viral infection was considerably normal in E8(I)-/- animals. Interestingly, virus-specific CD4 T cell responses were elevated, and the relative surface levels of CD8alphabeta in activated T cells were reduced in E8(I)-/- mice compared with wild-type animals. Our results indicate that memory CD8 T cell development can occur independently of CD8alphaalpha.     

Cutting edge: antigen-independent CD8 T cell proliferation

Recent analyses of CD8 T cell responses to Listeria monocytogenes infection demonstrate that the duration of in vivo T cell proliferation is not determined by the amount or duration of Ag presentation. However, the extent to which T lymphocytes are capable of proliferating in the absence of Ag is unknown. Herein we demonstrate that CD8 T lymphocytes undergo up to eight rounds of proliferation in the absence of Ag following transient, 2.5-h in vitro antigenic stimulation. Ag-independent expansion of CD8 T cells is driven by IL-2 and is further augmented by IL-7 or IL-15. These experiments clearly demonstrate that CD8 T cells undergo prolonged proliferation following transient Ag exposure and support the notion that in vivo CD8 T cell expansion following infection can be uncoupled from Ag presentation.     

Cutting edge: Enhanced IL-2 signaling can convert self-specific T cell response from tolerance to autoimmunity

Naive and memory T cells show differences in their response to antigenic stimulation. We examined whether this difference extended to the peripheral deletion of T cells reactive to self-Ag or, alternatively, the induction of autoimmunity. Our results show that although both populations where susceptible to deletion, memory T cells, but not naive T cells, also gave rise to autoimmunity after in vivo presentation of skin-derived self-Ags. The same migratory dendritic cells presented self-Ag to both naive and memory T cell populations, but only the latter had significant levels of the effector molecule granzyme B. Memory T cells also expressed increased levels of the high affinity IL-2 receptor chain after self-Ag recognition. Provision of IL-2 signaling using a stimulatory complex of anti-IL-2 Ab and IL-2 drove the otherwise tolerant naive T cells toward an autoimmune response. Therefore, enhanced IL-2 signaling can act as a major selector between tolerance and autoimmunity.     

Cutting edge: gut microenvironment promotes differentiation of a unique memory CD8 T cell population

Whether tissue microenvironment influences memory CD8 T cell differentiation is unclear. We demonstrate that virus-specific intraepithelial lymphocytes in gut resemble neither central nor effector memory CD8 T cells isolated from spleen or blood. This unique phenotype arises in situ within the gut, suggesting that anatomic location plays an inductive role in the memory differentiation program. In support of this hypothesis, memory CD8 T cells changed phenotype upon change in location. After transfer and in vivo restimulation, gut or spleen memory cells proliferated, disseminated into spleen and gut, and adopted the memory T cell phenotype characteristic of their new environment. Our data suggests that anatomic location directly impacts the memory T cell differentiation program.     

Cutting edge: protective response to pulmonary injury requires gamma delta T lymphocytes

Gamma delta intraepithelial lymphocytes are thought to coordinate responses to pathogens that penetrate the epithelial barrier. To directly test this, mice were inoculated with Nocardia asteroides. At doses that were nonlethal for control mice, gamma delta-deficient mice became severely ill and died within 14 days. Histologic examination of these lungs demonstrated the presence of severe tissue damage and unimpeded bacterial growth in the gamma delta-deficient mice compared with neutrophilic lesions and clearance of the organism in control mice. Interestingly, ozone exposure that targets a comparable lung region also resulted in diffuse epithelial necrosis associated with a similar lack of neutrophil recruitment in gamma delta-deficient mice. These data demonstrate that gamma delta intraepithelial lymphocytes can protect the host from pathogenic and nonpathogenic insults by targeting the inflammatory response to epithelial necrosis.     

Cutting edge: CD8+ T cell priming in the absence of NK cells leads to enhanced memory responses

It is uncertain whether NK cells modulate T cell memory differentiation. By using a genetic model that allows the selective depletion of NK cells, we show in this study that NK cells shape CD8(+) T cell fate by killing recently activated CD8(+) T cells in an NKG2D- and perforin-dependent manner. In the absence of NK cells, the differentiation of CD8(+) T cells is strongly biased toward a central memory T cell phenotype. Although, on a per-cell basis, memory CD8(+) T cells generated in the presence or the absence of NK cells have similar functional features and recall capabilities, NK cell deletion resulted in a significantly higher number of memory Ag-specific CD8(+) T cells, leading to more effective control of tumors carrying model Ags. The enhanced memory responses induced by the transient deletion of NK cells may provide a rational basis for the design of new vaccination strategies.     

Cutting edge: rapid in vivo killing by memory CD8 T cells

In this study, we examined the cytotoxic activity of effector and memory CD8 T cells in vivo. At the peak of the CTL response following an acute lymphocytic choriomeningitis virus infection, effector CD8 T cells exhibited extremely rapid killing and started to eliminate adoptively transferred target cells within 15 min by a perforin-dependent mechanism. Although resting memory CD8 T cells are poorly cytolytic by in vitro (51)Cr release assays, there was rapid elimination (within 1-4 h) of target cells after transfer into immune mice, and both CD62L(high) and CD62L(low) memory CD8 T cells were able to kill rapidly in vivo. Strikingly, when directly compared on a per cell basis, memory CD8 T cells were only slightly slower than effector cells in eliminating target cells. These data indicate that virus specific memory CD8 T cells can rapidly acquire cytotoxic function upon re-exposure to Ag and are much more efficient killers in vivo than previously appreciated.     

Cutting edge: long-term B cell memory in humans after smallpox vaccination

Memory B cells are a central component of humoral immunity, and yet little is known about their longevity in humans. Immune memory after smallpox vaccination (DryVax) is a valuable benchmark for understanding the longevity of B cell memory in the absence of re-exposure to Ag. In this study, we demonstrate that smallpox vaccine-specific memory B cells last for >50 years in immunized individuals. Virus-specific memory B cells initially declined postimmunization, but then reached a plateau approximately 10-fold lower than peak and were stably maintained for >50 years after vaccination at a frequency of approximately 0.1% of total circulating IgG(+) B cells. These persisting memory B cells were functional and able to mount a robust anamnestic Ab response upon revaccination. Additionally, virus-specific CD4(+) T cells were detected decades after vaccination. These data show that immunological memory to DryVax vaccine is long-lived and may contribute to protection against smallpox.     

Asymmetric cell division safeguards memory CD8 T cell development

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Cutting edge: a single MHC class Ia is sufficient for CD8 memory T cell differentiation

Recent studies have suggested a role for MHC class Ib molecules in providing signals for memory T cell differentiation during the early phases of acute infection. To test this hypothesis, we assessed the development of effector and memory CD8 T cells in transgenic mice expressing a single chain H-2D(d)/beta2-microglobulin (beta2M) fusion protein on a beta2M-deficient background. These mice thus express a single MHC class Ia in the absence of all other beta2M-dependent class Ia and Ib molecules. Following infection with a recombinant vaccinia virus expressing a known D(d)-restricted epitope from HIV-1 gp160, the development of effector and memory cells CD8 T cells was comparable to control mice. Furthermore, these memory cells responded rapidly and robustly to antigenic restimulation. Therefore, we conclude that full CD8 memory differentiation requires only a single MHC class Ia chain, ruling out a requirement for MHC class Ib molecules in this process.     

Asymmetric cell division during T cell development controls downstream fate

During mammalian T cell development, the requirement for expansion of many individual T cell clones, rather than merely expansion of the entire T cell population, suggests a possible role for asymmetric cell division (ACD). We show that ACD of developing T cells controls cell fate through differential inheritance of cell fate determinants Numb and α-Adaptin. ACD occurs specifically during the β-selection stage of T cell development, and subsequent divisions are predominantly symmetric. ACD is controlled by interaction with stromal cells and chemokine receptor signaling and uses a conserved network of polarity regulators. The disruption of polarity by deletion of the polarity regulator, Scribble, or the altered inheritance of fate determinants impacts subsequent fate decisions to influence the numbers of DN4 cells arising after the β-selection checkpoint. These findings indicate that ACD enables the thymic microenvironment to orchestrate fate decisions related to differentiation and self-renewal.     

Cutting edge: contributions of apoptosis and anergy to systemic T cell tolerance

Multiple pathways can induce and maintain peripheral T cell tolerance. The goal of this study was to define the contributions of apoptosis and anergy to the maintenance of self-tolerance to a systemic Ag. Upon transfer into mice expressing OVA systemically, OVA-specific DO11 CD4+ T cells are activated transiently, cease responding, and die. Bim is the essential apoptosis-inducing trigger and apoptosis proceeds despite increased expression of Bcl-2 and Bcl-x. However, preventing apoptosis by eliminating Bim does not restore proliferation or cytokine production by DO11 cells. While Foxp3 is transiently induced, anergy is not associated with the stable development of regulatory T cells. Thus, apoptosis is dispensable for tolerance to a systemic self-Ag and cell-intrinsic anergy is sufficient to tolerize T cells.     

Asymmetric cell division

Asymmetric cell division is a conserved mechanism for partitioning information during mitosis. Over the past several years, significant progress has been made in our understanding of how cells establish polarity during asymmetric cell division and how determinants, in the form of localized proteins and mRNAs, are segregated. In particular, genetic studies in Drosophila and Caenorhabditis elegans have linked cell polarity, G protein signaling and regulation of the cytoskeleton to coordination of mitotic spindle orientation and localization of determinants. Also, several new studies have furthered our understanding of how asymmetrically localized cell fate determinants, such as the Numb, a negative regulator Notch signaling, functions in biasing cell fates in the developing nervous system in Drosophila. In vertebrates, analysis of dividing neural progenitor cells by in vivo imaging has raised questions about the role of asymmetric cell divisions during neurogenesis.     

Cutting edge: Hierarchy of maturity of murine memory B cell subsets

The paucity of murine memory B cell markers has been a significant impediment to the study of memory. The most commonly used marker is IgG, which is neither sensitive nor specific, because activated nonmemory cells can be IgG(+), and memory cells can be IgM(+). In this article, we show that, together, PD-L2 (CD273), CD80, and CD73 define at least five phenotypic subsets of murine memory B cells. These subsets are generated from naive cells bearing a single BCR in response to a single T-dependent Ag. This diversity is independent of class switch, because IgG(1)- and IgM-bearing memory cells are found within each compartment. Memory subsets defined by PD-L2, CD80, and CD73 are biologically distinct from one another, because they differ in ontogeny and selection. Together, these distinctions suggest that there is a spectrum of memory B cells and progressive acquisition from more naive-like to more memory-like properties.     

Cutting edge: local recall responses by memory T cells newly recruited to peripheral nonlymphoid tissues

Infection results in the formation of a circulating effector memory T cell population able to enter peripheral tissues either in the steady state or in response to localized infection. As a consequence, recall is thought to result from a phased response first involving those T cells already at the site of infection followed by the infiltration of memory cells from the wider circulation. We have recently reported that tissue-resident T cells can undergo stimulation and proliferation in response to local infection. In this study, we examine the proliferation of memory T cells newly recruited from the circulation. Our results show that although recruitment of circulating memory cells is nonspecific in nature, there is preferential proliferation of specific T cells within infected tissues. Thus, expansion represents a means of local Ag-specific enrichment of T cells recruited from a circulating memory pool of mixed specificities.     

Cutting edge: diminished T cell TLR expression and function modulates the immune response in human filarial infection

Patent lymphatic filariasis is characterized by profound Ag-specific T cell hyporesponsiveness with impaired IFN-gamma and IL-2 production. Because T cells have been shown to express a number of TLR and to respond to TLR ligands, we hypothesized that diminished T cell TLR function could partially account for the T cell hyporesponsiveness in filariasis. T cells expressed TLR1, TLR2, TLR4, and TLR9, and the baseline expression of TLR1, TLR2, and TLR4, but not TLR9 was significantly lower in T cells of the filarial-infected individuals compared with the uninfected individuals (both endemic and nonendemic). TLR function was significantly diminished in the T cells of filarial-infected individuals based on decreased T cell activation/cytokine production in response to TLR ligands. Thus, diminished expression and function of T cell TLR is a novel mechanism underlying T cell immune tolerance in lymphatic filariasis.