Search

Summary: Immunization with vaccinia virus (VACV), the virus comprising the smallpox vaccine, induces memory CD8+ T cells that protect from subsequent infections with smallpox in humans or the related ectromelia virus (ECTV) in mice. Memory CD8+ T cells largely mediate these effects by expanding into secondary effectors that secrete the antiviral cytokine interferon-γ (IFN-γ) and induce cytolysis via releasing factors such as perforin, which permeabilizes target cells. We show that protection from ECTV infection after VACV immunization depends on the initial memory cell frequency and ability of expanded secondary effectors to kill infected targets in a perforin-dependent manner. Although IFN-γ is essential for antiviral protection, it can be produced by either secondary effectors or concomitant primary effector CD8+ T cells recruited to the response. Thus, during lethal virus challenge, memory CD8+ T cells are required for cytolytic killing of infected cells, but primary effectors can play important roles by producing IFN-γ. [Copyright &y& Elsevier]

The antigens recognized by individual CD8+ T cells are small peptides bound to major histocompatibility complex (MHC) class I molecules. The CD8+ T cell response to a virus is restricted to several peptides, and the magnitudes of the effector as well as memory phases of the response to the individual peptides are generally hierarchical. The peptide eliciting a stronger response is called immunodominant (ID), and those with smaller-magnitude responses are termed subdominant (SD). The relative importance of ID and SD determinants in protective immunity remains to be fully elucidated. We previously showed that multispecific memory CD8+ T cells can protect susceptible mice from mousepox, an acute lethal viral disease. It remained unknown, however, whether CD8+ T cells specific for single ID or SD peptides could be protective. Here, we demonstrate that immunization with dendritic cells pulsed with ID and some but not all SD peptides induces memory CD8+ T cells that are fully capable of protecting susceptible mice from mousepox. Additionally, while natural killer (NK) cells are essential for the natural resistance of nonimmune C57BL/6 (B6) to mousepox, we show that memory CD8+ T cells of single specificity also protect B6 mice depleted of NK cells. This suggests it is feasible to produce effective antiviral CD8+ T cell vaccines using single CD8+ T cell determinants and that NK cells are no longer essential when memory CD8+ T cells are present. [ABSTRACT FROM AUTHOR]

The extent to which direct- and cross-presentation (DP and CP) contribute to the priming of CD8+ T cell (TCD8+) responses to viruses is unclear mainly because of the difficulty in separating the two processes. Hence, while CP in the absence of DP has been clearly demonstrated, induction of an anti-viral TCD8+ response that excludes CP has never been purposely shown. Using vaccinia virus (VACV), which has been used as the vaccine to rid the world of smallpox and is proposed as a vector for many other vaccines, we show that DP is the main mechanism for the priming of an anti-viral TCD8+ response. These findings provide important insights to our understanding of how one of the most effective anti-viral vaccines induces immunity and should contribute to the development of novel vaccines. [ABSTRACT FROM AUTHOR]

The two major antiviral effector mechanisms of CD8+ T cells are thought to be perforin (Prf)-mediated cell lysis and gamma interferon (IFN-?)-mediated induction of an antiviral state. By affecting the expression of proteins involved in antigen presentation, IFN-? is also thought to shape the magnitude and specificity of the CD8+ T cell response. Here we studied the roles of Prf and IFN-? in shaping the effector and memory CD8+ T cell responses to vaccinia virus (VACV). IFN-? deficiency resulted in increased numbers of anti-VACV effector and memory CD8+ T cells, which were partly dependent on increased virus loads. On the other hand, Prf-deficient mice showed an increase in the number of VACV-specific CD8+ T cells only in the memory phase. Treatment of the mice with the antiviral drug cidofovir reduced the numbers of effector and memory cells closer to wild-type levels in IFN-?-deficient mice and reduced the numbers of memory CD8+ T cells to wild-type levels in Prf-deficient mice. These data suggest that virus loads are the main reason for the increased strength of the CD8 response in IFN-?- and Prf-deficient mice. Neither Prf deficiency nor IFN-? deficiency had an effect on the immunodominance hierarchy of five Kb-restricted CD8+ T cell determinants either during acute infection or after recovery. Thus, our work shows that CD8+ T cell immunodominance during VACV infection is not affected by the effects of IFN-? on the antigen presentation machinery. [ABSTRACT FROM AUTHOR]

Numerous attempts to produce antiviral vaccines by harnessing memory CD8 T cells have failed. A barrier to progress is that we do not know what makes an Ag a viable target of protective CD8 T cell memory. We found that in mice susceptible to lethal mousepox (the mouse homolog of human smallpox), a dendritic cell vaccine that induced memory CD8 T cells fully protected mice when the infecting virus produced Ag in large quantities and with rapid kinetics. Protection did not occur when the Agwas produced in low amounts, evenwith rapid kinetics, and protection was only partial when the Ag was produced in large quantities but with slow kinetics. Hence, the amount and timing of Ag expression appear to be key determinants of memory CD8 T cell antiviral protective immunity. These findings may have important implications for vaccine design. [ABSTRACT FROM AUTHOR]

Unlabelled: It has been shown in various infection models that CD4(+) T cell help (TH) is necessary for the conditioning, maintenance, and/or recall responses of memory CD8(+) T cells (CD8M). Yet, in the case of vaccinia virus (VACV), which constitutes the vaccine used to eradicate smallpox and is a candidate vector for other infectious diseases, the issue is controversial because different groups have shown either T(H) dependence or independence of CD8M conditioning, maintenance, and/or recall response. In agreement with some of these groups, we show that T(H) plays a role in, but is not essential for, the maintenance, proliferation, and effector differentiation of polyclonal memory CD8(+) T cells after infection with wild-type VACV strain Western Reserve. More important, we show that unhelped and helped anti-VACV memory CD8(+) T cells are similarly efficient at protecting susceptible mice from lethal mousepox, the mouse equivalent of human smallpox. Thus, T(H) is not essential for the conditioning and maintenance of memory CD8(+) T cells capable of mounting a recall response strong enough to protect from a lethal natural pathogen. Our results may partly explain why the VACV vaccine is so effective., Importance: We used vaccinia virus (VACV)--a gold standard vaccine--as the immunogen and ectromelia virus (ECTV) as the pathogen to demonstrate that the conditioning and maintenance of anti-VACV memory CD8(+) T cells and their ability to protect against an orthopoxvirus (OPV) infection in its natural host can develop in the absence of CD4(+) T cell help. Our results provide important insight to our basic knowledge of the immune system. Further, because VACV is used as a vaccine in humans, our results may help us understand how this vaccine induces protective immunity in this species. In addition, this work may partly explain why VACV is so effective as a vaccine., (Copyright © 2015, American Society for Microbiology. All Rights Reserved.)

Immunization with vaccinia virus (VACV), the virus comprising the smallpox vaccine, induces memory CD8(+) T cells that protect from subsequent infections with smallpox in humans or the related ectromelia virus (ECTV) in mice. Memory CD8(+) T cells largely mediate these effects by expanding into secondary effectors that secrete the antiviral cytokine interferon-γ (IFN-γ) and induce cytolysis via releasing factors such as perforin, which permeabilizes target cells. We show that protection from ECTV infection after VACV immunization depends on the initial memory cell frequency and ability of expanded secondary effectors to kill infected targets in a perforin-dependent manner. Although IFN-γ is essential for antiviral protection, it can be produced by either secondary effectors or concomitant primary effector CD8(+) T cells recruited to the response. Thus, during lethal virus challenge, memory CD8(+) T cells are required for cytolytic killing of infected cells, but primary effectors can play important roles by producing IFN-γ., (Copyright © 2013 Elsevier Inc. All rights reserved.)

The antigens recognized by individual CD8(+) T cells are small peptides bound to major histocompatibility complex (MHC) class I molecules. The CD8(+) T cell response to a virus is restricted to several peptides, and the magnitudes of the effector as well as memory phases of the response to the individual peptides are generally hierarchical. The peptide eliciting a stronger response is called immunodominant (ID), and those with smaller-magnitude responses are termed subdominant (SD). The relative importance of ID and SD determinants in protective immunity remains to be fully elucidated. We previously showed that multispecific memory CD8(+) T cells can protect susceptible mice from mousepox, an acute lethal viral disease. It remained unknown, however, whether CD8(+) T cells specific for single ID or SD peptides could be protective. Here, we demonstrate that immunization with dendritic cells pulsed with ID and some but not all SD peptides induces memory CD8(+) T cells that are fully capable of protecting susceptible mice from mousepox. Additionally, while natural killer (NK) cells are essential for the natural resistance of nonimmune C57BL/6 (B6) to mousepox, we show that memory CD8(+) T cells of single specificity also protect B6 mice depleted of NK cells. This suggests it is feasible to produce effective antiviral CD8(+) T cell vaccines using single CD8(+) T cell determinants and that NK cells are no longer essential when memory CD8(+) T cells are present.