CD8 T cell responses to influenza virus infection in aged mice

Influenza is one of the most common infectious diseases afflicting humans, particularly the elderly. The murine model has been widely employed for investigation of immunity to influenza virus infection. In this paper, we review the recent advances in understanding the diminished CD8 T cell immune response to influenza virus infection in aged mice. Possible mechanisms of impaired CD8 T cell responses with aging are addressed, including: (1) the role of dendritic cells (DCs); (2) the effect of age-associated changes in the T cell repertoire; and (3) the interactions with CD4 T cells, including T regulatory (Treg) cells and CD4 T helper cells. The aged murine model of the CD8 T cell response to influenza virus is helping to elucidate the mechanisms of immunosenescence which can lead to therapeutic improvements in the primary CD8 T cell response to new infections, as well as the development of new strategies for immunization to prevent influenza in the elderly.     

Expression of the DX5 antigen on CD8+ T cells is associated with activation and subsequent cell death or memory during influenza virus infection

The antigen recognized by the DX5 antibody (DX5 antigen) is expressed on all murine NK cells. In the present study we found that a proportion of CD8+ T cells (approximately 5%) also express the DX5 antigen in uninfected mice, and that numbers of CD8+ T cells expressing DX5 are significantly higher in the lungs of influenza virus-infected mice representing up to 50% of all CD8+ T cells on day 10 post infection. The expression of the DX5 antigen on CD8+ T cells was associated with a memory phenotype in uninfected C57BL/6 mice and with an activation phenotype during influenza virus infection. Interestingly, when lymphocytes were isolated from lungs of influenza virus-infected mice on day 10 post infection and adoptively transferred into recombination activating gene-1 (RAG1)-deficient mice, CD8+DX5+ cells could not be recovered from the recipient mice 2 days later. Moreover, CD8+DX5+ cells were not detected when lung cells were removed from day 10 influenza virus-infected mice and cultured in vitro for 2 days. However, CD8+DX5+ cells could be detected when apoptosis inhibitors were added to these cultures, suggesting that the CD8+DX5+ cells underwent apoptosis during cell culture. Furthermore, almost all DX5 expressing CD8+ cells from lungs of mice on day 10 post influenza virus infection stained positively with Annexin-V. Taken together, the data suggest that CD8+ T cells expressing DX5 are associated with an activation/memory phenotype and are biased towards apoptosis.     

In situ imaging reveals different responses by naïve and memory CD8 T cells to late antigen presentation by lymph node DC after influenza virus infection

Pulmonary influenza infection causes prolonged lymph node hypertrophy while processed viral antigens continue to be presented to virus‐specific CD8 T cells. We show that naïve, but not central/memory, nucleoprotein (NP)‐specific CD8 T cells recognized antigen‐bearing CD11b⁺ DC in the draining lymph nodes more than 30 days after infection. After these late transfers, the naïve CD8 T cells underwent an abortive proliferative response in the mediastinal lymph node (MLN), where large clusters of partially activated cells remained in the paracortex until at least a week after transfer. A majority of the endogenous NP‐specific CD8 T cells that were in the MLN between 30 and 50 days after infection also showed signs of a continuing response to antigen stimulation. A high frequency of endogenous NP‐specific CD8 T cells in the MLN indicates that late antigen presentation may help shape the epitope dominance hierarchy during reinfection.     

CD8 alpha alpha homodimer expression and role in CD8 T cell memory generation during influenza virus A infection in mice

While the precise function of CD8alphaalpha homodimer expression on peripheral T cells is uncertain, recent evidence indicates that it facilitates survival and differentiation of lymphocytic choriomeningitis virus (LCMV)-specific memory CD8alphabeta T cell precursors in vivo. Here, we show that the CD8alphaalpha homodimer is also transiently up-regulated on influenza A virus-specific CD8alphabeta T cells after infection in vivo, temporally correlating with increased levels of the memory T cell development- and survival-related molecules IL-7Ralpha and Bcl-2, respectively. Unlike with LCMV, however, deletion of the CD8alphaalpha enhancer I does not abrogate CD8alphaalpha homodimer expression or manifest a significant impact on the generation of virus-specific, functional effector and central memory T cells in influenza A virus infection. These results demonstrate that the role of CD8alphaalpha in the generation of antiviral CD8 T cell memory is complex, presumably because various viral stimuli differentially regulate CD8alphaalpha expression. Further studies are needed to define those ligands that induce CD8alphaalpha on T cells during acute viral infections, and the general relevance of this process to memory T cell formation.     

CD4 T cells inhibit the CD8 T cell response during low-dose virus infection

CD4 T cells are not thought to play a significant role in generating an effective primary CD8 T cell response to most viral infections. We have challenged this view by demonstrating that antigen-specific CD4 T cells can indeed suppress the proliferation of antigen-specific naive CD8 T cells in response to low doses of vesicular stomatitis virus. This finding is in contrast to the established observations that at high antigen loads CD4 T cells play little role in generating CD8 T cell responses, and that in non-infectious model systems CD4 T cells actually help the CD8 T cell response. Our results suggest that at low infectious doses, CD4 T cells play a much larger role in controlling infections than previously appreciated.     

CD4 T cell responses to influenza infection

Immune responses to viral infections involve a complex orchestration between innate signals and adaptive responses of specific T and B cells. Anti-viral CD4 cells can direct CD8 responses by secreting a Type 1 panel of cytokines including IFN-gamma, IL-2 and TNF-alpha and can drive B cell production of IgG2a to neutralize infective viral particles. This review will focus specifically on the role of CD4 cells in the immune response to influenza, an acute, localized respiratory viral infection. We suggest that CD4 cells act as direct effectors in protection against influenza, may contribute to immunopathology and generate functionally distinct memory subsets.     

Acute Trypanosoma cruzi infection differentially affects CD3 and Thy-1 mediated T cell activation.

Monoclonal antibodies directed against the Thy-1 molecule or the CD3 complex were used to analyze the activation of T cells from mice acutely infected with Trypanosoma cruzi. When stimulated with G7, a mitogenic anti-Thy-1 monoclonal antibody, spleen cells from infected mice showed a markedly reduced or absent response that could not be restored by varying the culture time or the antibody concentration. However, cells from acutely infected animals proliferated to 145-2C11, an anti-CD3 monoclonal antibody. Flow cytometric analysis showed that the impaired response to G7 could not be attributed to a lack of expression of Thy-1 or CD3. Indeed, G7 seemed to deliver a positive signal to the cells since the proliferative response was completely restored by the addition of PMA. Moreover, purified T cells from infected mice responded to G7 in the presence of accessory cells from uninfected animals. These results suggest that a defective co-stimulatory cell function could be involved in the immunosuppression. In addition, our data present evidence against a generalized T cell anergy in the acute phase of the disease, since CD3-mediated activation was normal.     

Viral neuraminidase treatment of dendritic cells enhances antigen-specific CD8(+) T cell proliferation, but does not account for the CD4(+) T cell independence of the CD8(+) T cell response during influenza virus infection

In vitro studies demonstrate that the increased alloreactive T cell response to dendritic cells (DC) that are treated with either live or inactivated influenza virus A/PR/8/34 is due to viral neuraminidase (NA) activity. Since virus-specific cytotoxic T lymphocytes (CTL) play an important role in immunity to heterologous influenza strains, we compared the activation of CD8(+) T cells by untreated and NA-treated DC. Increased CTL activity was induced by NA-treated DC both in vitro and in vivo. Since the generation of CTL in response to influenza virus infection does not require prior "activation" of DC by CD4(+) T cells (as is the case for many antigens), we asked whether NA activity contributed to this unconditional CD8(+) T cell response. This was not the case. Future studies will determine the factors that are responsible for the CD4(+) T-cell-independent influenza virus-specific CTL response.     

Particle shape dependence of CD8+ T cell activation by artificial antigen presenting cells

Previous work developing particle-based acellular, artificial antigen presenting cells (aAPCs) has focused exclusively on spherical platforms. To explore the role of shape, we generated ellipsoidal PLGA microparticles with varying aspect ratios (ARs) and synthesized aAPCs from them. The ellipsoidal biomimetic aAPCs with high-AR showed significantly enhanced in vitro and in vivo activity above spherical aAPCs with particle volume and antigen content held constant. Confocal imaging indicates that CD8+ T cells preferentially migrate to and are activated by interaction with the long axis of the aAPC. Importantly, enhanced activity of high-AR aAPCs was seen in a mouse melanoma model, with high-AR aAPCs improving melanoma survival compared to non-cognate aAPCs (p = 0.004) and cognate spherical aAPCs (p = 0.05). These findings indicate that particle geometry is a critical design criterion in the generation of aAPCs, and may offer insight into the essential role of geometry in the interaction between CD8+ T cells and biological APCs.     

The roles of CD4 and CD8 in T cell activation

CD4 and CD8 T cell surface molecules play a role in T cell recognition and activation by binding to their respective class II and class I major histocompatibility complex (MHC) ligands on an antigen presenting cell (APC). Though CD4 and CD8 are capable of binding to MHC molecules in the absence of the T cell receptor (TCR), increasing evidence suggests that they may primarily function by complexing with the TCR to form a 'co-receptor' for recognition of antigen-bound MHC. Using gene transfer studies we have demonstrated that CD4 and CD8 can augment antigen-induced IL-2 production through different mechanisms dependent on whether or not they can bind MHC independently of the TCR or complexed with the TCR. Under circumstances where CD4 and CD8 can bind to the same MHC ligand as the TCR, they potentiate antigen-induced IL-2 production maximally by a mechanism in large part dependent on their cytoplasmic tails. Enhancement of antigen-induced IL-2 production can also occur under circumstances where CD4 and CD8 bind on MHC ligand distinct from that recognized by the TCR. In this instance, the magnitude of this enhancement is not as great and appears (at least for CD8) to be independent of the cytoplasmic tail and the associated p56lck. The dependence of co-receptor function on the cytoplasmic tail of CD4 or CD8 may reflect the activity of the associated intracellular tyrosine kinase p56lck.(ABSTRACT TRUNCATED AT 250 WORDS)     

Decreased CD4 and increased CD8 counts with T cell activation is associated with chronic helminth infection

We have previously reported the presence of marked immune dysregulation with a dominant Th2 profile, in a population of Ethiopian immigrants (ETH) in Israel heavily infected with helminths. In order to characterize better this immune dysregulation we studied by flow cytometry the expression of several activation markers on peripheral T cell populations, and lymphocyte apoptosis, in blood samples obtained from 63 ‘new’ ETH (recently arrived), 18 ‘old’ ETH (> 5 years since immigration) and 34 non-Ethiopian Israelis. The main findings in the ‘new’ ETH group in comparison with the non-Ethiopian controls were: (i) decreased CD4 and increased CD8 lymphocyte counts; (ii) elevated levels of activated T cells (CD3, CD4 and CD8) expressing HLA-DR; (iii) decreased levels of ‘naive’ CD4⁺ cells (CD45RA⁺), with increased levels of ‘memory’ CD4⁺ cells (CD45RO⁺); (iv) decreased numbers of CD28⁺ CD8⁺ lymphocytes; (v) marked increase in lymphocyte apoptosis. These T cell alterations and activation profile remained unchanged in 10 ‘new’ ETH in whom the helminth infections persisted for 6–11 months. In contrast, in 18 ‘old’ ETH, without helminth infections, the T cell activation profile was within the normal range. These findings suggest that chronic helminth infections may have a profound effect on the immune system of the host that disappears after eradication of these infections and adjustment to the new environment. It should therefore be taken into consideration for every immunomodulation therapy and especially in vaccine design and trials, in regions endemic for helminth infections.     

Characterization of CD8+ T cell repertoire diversity and persistence in the influenza A virus model of localized, transient infection

Influenza virus infection of C57BL/6 mice provides a well-characterized model for the study of acute CD8(+) T cell responses and for the analysis of memory in the absence of antigen persistence. The advent of tetramer reagents and intracellular cytokine staining, coupled with techniques such as single cell RT-PCR and influenza reverse genetics, has enabled the detailed molecular dissection of different epitope-specific primary, memory and secondary immune CD8(+) T cell responses. The approach offers novel insights into the factors determining the selection of immune repertoires, and their functional consequences for CD8(+) T cell-mediated immunity.     

Oscillating CD8(+) T cell effector functions after antigen recognition in the liver

When hepatitis B virus (HBV)-specific CD8(+) cytotoxic T lymphocytes (CTLs) are adoptively transferred into HBV transgenic mice, they enter the liver, recognize antigen, secrete interferon gamma (IFNgamma), inhibit viral replication, and kill their target cells, causing hepatitis. In the current study, we examined the impact of antigen recognition on the evolution of the activation phenotype, antiviral effector functions, expansion and contraction kinetics, and compartmentalization of the transferred CTLs. The results reveal that noncytolytic and cytolytic effector functions and expansion-contraction kinetics of the CTLs are regulated asynchronously and in an oscillatory manner as a consequence of antigen recognition in the liver and in association with PD-1 upregulation. We suggest that such oscillations maximize viral clearance and minimize tissue injury during HBV infection and that poor coordination of these events could lead to viral persistence and chronic liver disease.     

Decreased production of CD8 (T8) antigen after immunotherapy

The working mechanism(s) of immunotherapy still remains ill defined. As T cells bearing CD8 antigen possess suppressor/cytotoxic function, this study was conducted to examine the effect of immunotherapy on the production of CD8 antigen. Peripheral blood mononuclear cells (MNC) were obtained from 21 newly diagnosed and 23 hyposensitized (>1 year) asthmatic children and 13 agematched normal children. MNC were stimulated with crude mite extract (Dermatophagoides farinae) for 7 days and with phytohemagglutinin and concanavalin A for 3 days. The CD8 antigen and interleukin-2 receptor (IL-2R) in plasmas and culture supernatants were measured by CELLFREE T8 and IL-2R test kits (T Cell Sciences, USA). The results showed the following. (1) Plasma CD8 antigen was markedly increased in new patients compared to normals (536.7 ± 212.3 vs 222.5 ± 104.0 units/ml;P<0.001) and decreased to normal after immunotherapy (275.7 ± 98.5 units/ml). (2) When stimulated with mite allergen, MNC from both new and hyposensitized patients produced a much greater amount of CD8 antigen compared to those from normals. However, after immunotherapy MNC tended to produce less CD8 antigen, although not to a significant degree. (3) No difference in CD8 antigen production was seen among three groups when lymphocytes were stimulated with mitogens. (4) Production of CD8 antigen paralleled that of IL-2R. Thus, CD8 production was specifically decreased after immunotherapy and this fact reflects a hyposensitized state of T cells after long-term, repeated injection of allergens.     

CD4(+) and CD8(+) T cell death during human immunodeficiency virus infection in vitro.

We have evaluated the death of CD4(+) and CD8(+) T cells during in vitro human immunodeficiency virus (HIV) infection of peripheral blood mononuclear cells (PBMC) and tonsilar tissue. Acute infections with several X4 and R5 HIV isolates induced a decrease in cell viability that was higher in infections with X4 viruses and correlated with an increased rate of CD4(+) T-cell death. In CD4(+) T cells, the primary X4 isolate AOM induced higher levels of death than the laboratory X4 isolates IIIB and NL4-3 or the R5 isolates BaL and MDM. An effect on CD8(+) T-cell viability was exclusively observed in infections by X4 viruses, including the NL4-3 strain, in both PBMC and tonsilar tissue. This effect was dependent on the env gene of the infecting isolate and required productive HIV replication in CD4(+) but not in CD8(+) T cells. Our results suggest that X4 and R5 HIV isolates depleted CD4(+) T cells to a different extent and that CD8(+) T-cell viability may also be affected by mechanisms other than those acting in CD4(+) T cells.     

CD8 T cell dysfunction during chronic viral infection

Clearance of primary infection often leads to the development of highly functional memory T cells capable of rapid and long-lasting protective immunity. By contrast, chronic infections can result in T cell dysfunction and poor pathogen control. In this review, we will discuss recent work that highlights two main types of T cell dysfunction during chronic infection: exhaustion of effector functions and altered memory T cell development.     

The evolution of antigen-specific CD8+ T cell responses after natural primary infection of humans with Epstein-Barr virus

Epstein-Barr virus (EBV) is a persistent, gamma-herpes virus that infects 90% of the human population. Primary infection, particularly if it is delayed until adolescence or beyond, may cause acute infectious mononucleosis and persistent infection may be associated with the development of several malignancies. CD8(+) T cells play a critical role in controlling both the primary and persistent phases of infection. This review summarises work that has been done characterising the primary immune responses to EBV. It goes on to describe the down regulation of the primary immune response and to discuss some of the factors that may be involved in determining the death or survival of populations of antigen-specific CD8(+) T cells. Finally it describes features of the populations of memory cells that mediate the long-term control of EBV in healthy seropositive individuals. The studies show differences in the responses to epitopes from lytic cycle versus latent proteins and highlight the complexity of naturally occurring, in vivo, immune responses. A clear understanding of the means by which CD8(+) T cells control EBV is important if we are to successfully develop vaccines and other forms of immunotherapy for the virus and its related malignancies.     

CD8+ T 细胞活化与分化的分子机制

CD8+ T 细胞识别由MHC玉分子递呈的抗原肽,由于大多数有核细胞都表达MHCⅠ分子,因此CD8+ T 细胞在清除被病毒、胞内菌、寄生虫等感染的细胞或突变的肿瘤细胞中发挥着重要作用。识别病原微生物等抗原后,CD8+ T 细胞活化并分化形成多种类型的效应和记忆细胞,不仅能及时清除被感染的细胞,也能形成长期保护。各亚群CD8+ T 细胞的表面分子、功能和定位不同,细胞存活的时间、再次感染时的增殖能力和效应功能也有所差别。本文主要讨论CD8+ T 细胞如何受到多条信号通路和转录因子调控,活化和分化成不同类型的效应和记忆细胞,并对临床应用T 细胞抵御肿瘤和病原微生物的进展作一简单概述。     

Role of co-stimulation in CD8+ T cell activation

The two-signal model states that activation of naive T cells requires a signal 1 stimulus through the TCR and a co-stimulatory signal 2. By contrast, signal 1 alone is sufficient for pre-activated T cells. Recently, however, it has been shown that under certain conditions T cells can bypass the requirement for co-stimulation. For example, CD28- deficient mice, when immunized with lymphocytic choriomeningitis virus, mount a vigorous cytotoxic T lymphocyte response and clear the virus. As a continuous effort to unravel the mechanisms of T cell activation, we previously reported activation of hybridoma T cells by recombinant single-chain MHC molecules in the absence of antigen-presenting cells. In such reconstitution experiments, since the signals delivered to the T cells are well controlled, the contribution of any known or unknown signals can be ruled out. In the present study, we analyzed the requirements for activation of naive T cells by using splenocytes from TCR transgenic mice as a source of responding cells. We observed that naive CD8+ T cells are fully activated by signal 1 alone, but that co- stimulation lowers their activation threshold. Previously activated T cells are fully responsive, even when the first stimulation was performed in the absence of co-stimulation. They display a low activation threshold and are insensitive to co-stimulation. The physiological relevance of this finding and its consequences for immunotherapy as well as for our understanding of self-tolerance are discussed.      

Dynamics of blood-borne CD8 memory T cell migration in vivo

Memory T cells are distributed throughout the body following infection, but the migratory dynamics of the memory pool in vivo is unknown. The ability of circulating microbe-specific memory T cells to populate lymphoid and nonlymphoid tissues was examined using adoptive transfer and parabiosis systems. While migration of memory CD8 T cells to lymph nodes and peritoneal cavity required G(i)-coupled receptor signaling, migration to the spleen, bone marrow, lung, and liver was independent of this pathway. Following parabiosis, memory T cells rapidly equilibrated into the lymphoid tissues, lung, and liver of each parabiont, implying most memory cells were not obligately tissue resident. Equilibration of memory cell populations was delayed in the brain, peritoneal cavity, and intestinal lamina propria, indicating controlled gating for entry into these tissues. In addition, memory cell migration to the lamina propria required beta7 integrins. Thus, the blood-borne T cell pool serves to maintain the homeostasis of tissue-based memory populations.