Development, trafficking, and function of memory T-cell subsets

Shaping and reshaping CD8+ T-cell memory

The ability to develop and sustain populations of memory T cells after infection or immunization is a hallmark of the adaptive immune response and a basis for protective vaccination against infectious disease. Technical advances that allow direct ex vivo identification and characterization of antigen-specific CD8+ T cells at various stages of the response to infection or vaccination in mouse models have fuelled efforts to characterize the factors that control memory CD8+ T-cell generation. Here, we dissect the input signals that shape the characteristics of the memory CD8+ T-cell response and discuss how manipulation of these signals has the potential to reshape CD8+ T-cell memory and improve the efficacy of vaccination.

Transcription factor KLF2 regulates the migration of naive T cells by restricting chemokine receptor expression patterns

The migration patterns of naive and activated T cells are associated with the expression of distinct sets of chemokine receptors, but the molecular basis for this regulation is unknown. Here we identify Krupple-like factor 2 (KLF2) as a key transcriptional factor needed to prevent naive T cells from expressing inflammatory chemokine receptors and acquiring the migration patterns of activated T cells. Lineage-specific deletion of KLF2 resulted in fewer naive T cells in the blood and secondary lymphoid organs, whereas it expanded naive T cell numbers in nonlymphoid tissues; these effects were associated with altered expression of inflammatory chemokine receptors on naive T cells. KLF2 repressed the expression of several chemokine receptors, including CCR3 and CCR5. We thus conclude that KLF2 maintains proper T cell migration patterns by linking T cell movement and transcriptional regulation of chemokine receptor expression patterns.

SOCS1 downregulation in dendritic cells promotes memory T-cell responses

SOCS1-1 is crucial for control of immune cell cytokine expression, including those cytokines known to enable memory T-cell formation and homeostasis. In this study, we found that immunization with SOCS1-downregulated bone marrow-derived dendritic cells generated increased antigen-specific CD8(+) T memory cells and antigen-specific responses, as measured by ELISPOT, CTL assays, serum ELISAs, and T-cell proliferation assays. Bone marrow-derived dendritic cells in which SOCS1 was downregulated expressed increased levels of surface IL-15Ra and thymic leukemia (TL) antigen, both of which support memory cell development. This work supports a crucial role for SOCS1 in regulation of dendritic cell-directed generation of memory T-cell responses.

The alpha1beta1 integrin and TNF receptor II protect airway CD8+ effector T cells from apoptosis during influenza infection

Primary viral infections of the lung induce potent effector CD8 T cell responses. To function in the influenza-infected airways, CD8 T cells must be able to resist cell death. The majority of the CD8 T cells in the airways and lung parenchyma expressed CD49a, the alpha-chain of the type IV collagen receptor VLA-1, and these cells were highly activated, producing both IFN-gamma and TNF-alpha. In the airways, where type IV collagen is abundant, but not the spleen, the CD49a(+) CD8 cells had reduced proportions of annexin V and caspase 8, and >80% expressed the TNF-alpha receptor II, while Fas, TNFR-I, and CD27 expression were similar to CD49a(-) cells. Furthermore, the CD49a(+), but not CD49a(-), CD8 T cells from the airways were resistant to active induction of apoptosis in the presence of type IV collagen and TNF-alpha in vitro. We propose that TNFR-II and the VLA-1 synergize to protect effector CD8 T cells in the infected airways from apoptosis during the acute infection.

Assessing ageing of individual T lymphocytes: mission impossible?

Effector T lymphocytes are the progeny of a limited number of antigen-specific precursor cells and it has been estimated that clonotypic human T cells may expand million fold on their way reaching high cell numbers that are sufficient for immune protection. Moreover, memory T cell responses are characterized by repetitive expansion of antigen-specific T cell clonotypes, and limitations in the proliferative capacity could lead to immune senescence. Because telomeres progressively shorten as a function of cell division, telomere length is a powerful indicator of the replicative in vivo history of human T lymphocytes. In this review, we summarize observations made over the last decade on telomere length dynamics of well-defined T cell populations derived from healthy donors and patients with infectious disease or cancer. We focus on T cell differentiation, T cell ageing, and natural and vaccine induced immune responses. We also discuss the scientific evidence for in vivo replicative senescence of antigen-specific T cells, and evaluate the available methods for measuring telomere lengths and telomerase activity, and their potential and limitations to increase our understanding of T cell physiology.

Method for assessing the similarity between subsets of the T cell receptor repertoire

The CD8+ T cell response is important in the control of many viral and other infections. There have been many studies aimed at better understanding the influence of T cell receptor diversity on immune responses and the evolution of the T cell receptor repertoire over time and through the various stages of immune responses to infection. In recent years, there has been an increase in both the number of studies using T cell receptor data and the volume of T cell receptor data generated per study. Appropriate analytical tools are required to analyse this data. We present a robust approach to assessing the similarity between samples of the T cell receptor repertoire, which we demonstrate on published data of subsets of the influenza A virus D(b)NP366(-) and D(b)PA224(-)specific CD8+ T cell responses in mice sorted on the expression of CD62L, which is a marker distinguishing central and effector memory cells.

Manipulating the rate of memory CD8+ T cell generation after acute infection

Infection with Listeria monocytogenes elicits expansion in numbers of Ag-specific CD8+ T cells, which then undergo programmed contraction. The remaining cells undergo further phenotypic and functional changes with time, eventually attaining the qualities of memory CD8+ T cells. In this study, we show that L. monocytogenes-specific CD8+ T cell populations primed in antibiotic-pretreated mice undergo brief effector phase, but rapidly develop phenotypic (CD127(high), CD43(low)) and functional (granzyme B(low), IL-2-producing) characteristics of memory CD8+ T cells. These early memory CD8+ T cells were capable of substantial secondary expansion in response to booster challenge at day 7 postinfection, resulting in significantly elevated numbers of secondary effector and memory CD8+ T cells and enhanced protective immunity compared with control-infected mice. Although early expansion in numbers is similar after L. monocytogenes infection of antibiotic-pretreated and control mice, the absence of sustained proliferation coupled with decreased killer cell lectin-like receptor G-1 up-regulation on responding CD8+ T cells may explain the rapid effector to memory CD8+ T cell transition. In addition, antibiotic treatment 2 days post-L. monocytogenes challenge accelerated the generation of CD8+ T cells with memory phenotype and function, and this accelerated memory generation was reversed in the presence of CpG-induced inflammation. Together, these data show that the rate at which Ag-specific CD8+ T cell populations acquire memory characteristics after infection is not fixed, but rather can be manipulated by limiting inflammation that will in turn modulate the timing and extent to which CD8+ T cells proliferate and up-regulate killer cell lectin-like receptor G-1 expression.

Cutting edge: migration to nonlymphoid tissues results in functional conversion of central to effector memory CD8 T cells

Memory CD8 T cells, essential for defense against intracellular pathogens, are heterogeneous with respect to phenotype and function. Constitutively lytic effector memory cells primarily reside in nonlymphoid tissues, whereas secondary lymphoid tissues contain functionally quiescent central memory cells. However, the mechanism by which functionally distinct memory populations are maintained is unknown. In this study, we show that resting CD8 memory cells modified their functional abilities upon entry into nonlymphoid tissues, as exemplified by the induction of granzyme B and lytic activity. Contemporaneously, the costimulator CD27 was down-regulated. These findings hold important implications for memory cell lineage development and tissue-specific immunity.

Comparative prime-boost vaccinations using Semliki Forest virus, adenovirus, and ALVAC vectors demonstrate differences in the generation of a protective central memory CTL response against the P815 tumor

Tumor-specific Ags are potential target molecules in the therapeutic treatment of cancer. One way to elicit potent immune responses against these Ags is to use recombinant viruses, which activate both the innate and the adaptive arms of the immune system. In this study, we have compared Semliki Forest virus (SFV), adenovirus, and ALVAC (poxvirus) vectors for their capacity to induce CD8(+) T cell responses against the P1A tumor Ag and to elicit protection against subsequent challenge injection of P1A-expressing P815 tumor cells in DBA/2 mice. Both homologous and heterologous prime-boost regimens were studied. In most cases, both higher CD8(+) T cell responses and better tumor protections were observed in mice immunized with heterologous prime-boost regimens, suggesting that the combination of different viral vectors is beneficial for the induction of an effective immune response. However, homologous immunization with SFV provided potent tumor protection despite a rather moderate primary CD8(+) T cell response as compared with mice immunized with recombinant adenovirus. SFV-immunized mice showed a rapid and more extensive expansion of P1A-specific CD8(+) T cells in the tumor-draining lymph node after tumor challenge and had a higher frequency of CD62L(+) P1A-specific T cells in the blood, spleen, and lymph nodes as compared with adenoimmunized mice. Our results indicate that not only the magnitude but in particular the quality of the CD8(+) T cell response correlates with tumor protection.

Memory generation and maintenance of CD8+ T cell function during viral persistence

During infection with viruses that establish latency, the immune system needs to maintain lifelong control of the infectious agent in the presence of persistent Ag. By using a gamma-herpesvirus (gammaHV) infection model, we demonstrate that a small number of virus-specific central-memory CD8+ T cells develop early during infection, and that virus-specific CD8+T cells maintain functional and protective capacities during chronic infection despite low-level Ag persistence. During the primary immune response, we show generation of CD8+ memory T cell precursors expressing lymphoid homing molecules (CCR7, L-selectin) and homeostatic cytokine receptors (IL-7alpha, IL-2/IL-15beta). During long-term persistent infection, central-memory cells constitute 20-50% of the virus-specific CD8+ T cell population and maintain the expression of L-selectin, CCR7, and IL-7R molecules. Functional analyses demonstrate that during viral persistence: 1) CD8+ T cells maintain TCR affinity for peptide/MHC complexes, 2) the functional avidity of CD8+ T cells measured as the capacity to produce IFN-gamma is preserved intact, and 3) virus-specific CD8+ T cells have in vivo killing capacity. Next, we demonstrate that at 8 mo post-virus inoculation, long-term CD8+ T cells are capable of mediating a protective recall response against the establishment of gammaHV68 splenic latency. These observations provide evidence that functional CD8+ memory T cells can be generated and maintained during low-load gammaHV68 persistence.

Methods for comparing the diversity of samples of the T cell receptor repertoire

Analysis of T cell receptor (TCR) data has become a crucial element in many studies aimed at better understanding the evolution of the T cell repertoire and the role of TCR diversity in immune responses. In this paper we focus on comparing the diversity between samples of the TCR repertoire. We discuss some of the limitations and potential problems inherent in some of the more popular approaches to comparing samples of the TCR repertoire and we suggest alternate methods that both avoid these problems and enrich the analysis of TCR data. Examples from published studies of the CD8(+) T cell responses to the influenza A virus D(b)NP(366) and D(b)PA(224) epitopes in mice are used to demonstrate the implementation of these methods. One example involves a comparison between the central and effector memory T cell subsets, defined on the basis of CD62L expression, and the other examines changes in the TCR repertoire over time.

The relevance of cytokines for development of protective immunity and rational design of vaccines

Cytokines are key regulators of the immune system that shape innate and adaptive immune responses. An adequate balance of the cytokine environment is critical to achieve protective immunity and to avoid immunopathology. Present knowledge allows a deeper understanding of the cytokine network and their sometimes conflicting roles in the development of immune responses, as well as their relevance in the establishment and maintenance of immunological memory. New insights have been gained into the role of different T cell subsets for protection against infection or tumor growth. The incorporation of cytokines as molecular adjuvants in vaccines has been attempted to strengthen vaccine-induced immune responses, and as a rational approach to modulate cytokine milieu in vivo and tailor host immunity for specific situations. These approaches have been tried in experimental models and veterinary species, and a few of them have entered into clinical trials. However, manipulating the cytokine network to modulate immune responses is not a simple task, because cytokine functions are complex and the final effects on the immune response will depend on timing and length of exposure, cell(s) targeted and other cytokines present in the same microenvironment. Here, we will review our present understanding on the role of cytokines in the development of effector and memory T cell responses. Also the potential use of cytokines as molecular adjuvant for vaccines against infectious diseases and cancer will be revised.

Persistent antigen presentation after acute vesicular stomatitis virus infection

Long-term antigen expression is believed to play an important role in modulation of T-cell responses to chronic virus infections. However, recent studies suggest that immune responses may occur late after apparently acute infections. We have now analyzed the CD8 T-cell response to vesicular stomatitis virus (VSV), which is thought to cause to an infection characterized by rapid virus clearance by innate and adaptive immune system components. Unexpectedly, virus-encoded antigen was detectable more than 6 weeks after intranasal VSV infection in both draining and nondraining lymph nodes by adoptively transferred CD8 T cells. Infection with Listeria monocytogenes expressing the same antigen did not result in prolonged antigen presentation. Weeks after VSV infection, discrete T-cell clustering with dendritic cells within the lymph node was observed after transfer of antigen-specific CD8 T cells. Moreover, memory CD8 T cells as defined by phenotype and function were generated from naïve CD8 T cells entering the response late after infection. These findings suggested that protracted antigen presentation after an apparently acute virus infection may contribute to an ongoing antiviral immune response.