Expansion of Protective CD8⁺T-Cell Responses Driven by Recombinant Cytomegaloviruses

Abstract

CD8؉ T cells are critical for the control of many persistent viral infections, such as human immunodeficiency virus, hepatitis C virus, Epstein-Barr virus, and cytomegalovirus (CMV). In most infections, large CD8؉ -T-cell populations are induced early but then contract and are maintained thereafter at lower levels. In contrast, CD8؉ T cells specific for murine CMV (MCMV) have been shown to gradually accumulate after resolution of primary infection. This unique behavior is restricted to certain epitopes, including an immunodominant epitope derived from the immediate-early 1 (IE1) gene product. To explore the mechanism behind this further, we measured CD8؉ -T-cell-mediated immunity induced by recombinant MCMV-expressing epitopes derived from influenza A virus or lymphocytic choriomeningitis virus placed under the control of an IE promoter. We observed that virus-specific CD8؉ -T-cell populations were induced and that these expanded gradually over time. Importantly, these CD8 ؉ T cells provided long-term protection against challenge without boosting. These results demonstrate a unique pattern of accumulating T cells, which provide long-lasting immune protection, that is independent of the initial immunodominance of the epitope and indicates the potential of T-cell-inducing vaccines based on persistent vectors. CD8ϩ T lymphocytes play a critical role in the control of persistent infections such as human immunodeficiency virus, hepatitis C virus, Epstein-Barr virus, and cytomegalovirus (CMV). Defining the nature of CD8 ϩ T cells mediating immunological protection and the rules governing the maintenance of these cells is crucial not only for our understanding of the pathogenesis of such infections but also for the design of T-cell-based vaccines. It is clear that memory T-cell populations may persist in the absence of antigen or further triggering of their T-cell receptors (1,41,49). However, it is also clear that the biology of the inducing pathogen can strongly influence the magnitude and the quality of the resulting T-cell populations. Recent studies with major histocompatibility complex (MHC) class I peptide tetrameric complexes (tetramers) have revealed that human CD8 ϩ -T-cell populations specific for different pathogens may show considerable heterogeneity in phenotype (3). In murine models it has been shown that the recirculation pattern, the activation status, and ultimately the protective capacity of memory T cells may also be influenced by the nature of the primary infection (5,10,36,60,63). We examined in detail the unique T-cell responses induced by murine CMV (MCMV) infection and explored the protective capacity of CD8 ϩ T cells generated in response to these pathogens.MCMV has been used for decades as an animal model for analyzing the biology and immunology of human herpesvirus infections in general and human CMV (HCMV) in particular. Similar to its human counterpart, MCMV is not eliminated by the host, and persistent, lifelong, and usually latent infection is established (40). Primary infection and main...

The (gradual) rise of memory inflation

“…However, epitopes in the C57BL/6 model (ie, restricted by H‐2b as opposed to H‐2d) were not yet described. Using a recombinant virus expressing well‐described epitopes from influenza nucleoprotein (NP) and LCMV glycoprotein (GP) expressed at the C terminus of the immediate early 2 (IE2) molecule (which is redundant for full viral replication), C57BL/6 mice were infected and immune responses tracked over time 2. In fact, the initial results were rather disappointing, since acutely very little was seen.…”

“…The first point is that it depends on the viral context, since epitopes which show inflation in the adenovirus or MCMV models do not induce inflation if presented in other recombinant settings such as vaccinia viruses—in other words it is not an inherent property of the peptide (for example related to some aspect of cross‐reactivity) 16, 22. Within a given model where inflation can occur, the answer does not appear to be related to initial immunodominance, since even very subdominant responses can inflate at later time‐points 2, 3, 4, 71. Similarly, it is not dependent on the affinity of the epitope itself for MHC, since responses to the same epitope can be converted from a non‐inflationary to an inflationary type by modulating the epitope context 2, 3, 71…”

“…For the rhesus macaque‐based cytomegaloviruses, there are a series of additional modifications regarding immune evasion and, interestingly, cellular targeting which can have quite profound impacts on the specificities observed, some of which are unconventional (eg, HLA‐E restricted) and can have potentially potent antiviral properties 86, 87, 88. Overall—even in settings where more conventional CD8 + T‐cell responses are induced—there is evidence that these responses can provide protection against viruses and cancers 2, 43. Thus, understanding what the rules are that govern the induction and maintenance of these populations is very relevant to vaccines and so the models proposed have translational relevance.…”

“…It is a striking phenomenon that was made quite evident by the use of MHC‐peptide tetramers to track responses in the infected animals over time 2, 3, 4. The term is now in reasonably common use5, 6 and the actual features of the responses induced which can be called “inflationary” are quite robustly reproducible between different laboratories using different virus stocks, mouse lines, and infection protocols.…”

The Context of Gene Expression Defines the Immunodominance Hierarchy of Cytomegalovirus Antigens

“…The cloning of CMV as bacterial artificial chromosomes (BAC) in Escherichia coli (6) allowed quick and efficient targeted mutagenesis of the CMV genome, as well as the generation of CMVs expressing defined heterologous target Ags (7). Initial results showed that these recombinants were able to induce T cell responses to peptides derived from other viruses, such as influenza A or lymphocitic choriomeningitis virus (7), and that provided immune protection against recombinant poxviruses expressing the target Ag.…”