Bi-specific MHC heterodimers for characterization of cross-reactive T cells

Heterologous Immunity and Persistent Murine Cytomegalovirus Infection

One's history of infections can affect the immune response to unrelated pathogens and influence disease outcome through the process of heterologous immunity. This can occur after acute viral infections, such as infections with lymphocytic choriomeningitis virus (LCMV) and vaccinia virus, where the pathogens are cleared, but it becomes a more complex issue in the context of persistent infections. In this study, murine cytomegalovirus (MCMV) was used as a persistent infection model to study heterologous immunity with LCMV. If mice were previously immune to LCMV and then infected with MCMV (LCMV+MCMV), they had more severe immunopathology, enhanced viral burden in multiple organs, and suppression of MCMV-specific T cell memory inflation. MCMV infection initially reduced the numbers of LCMV-specific memory T cells, but continued MCMV persistence did not further erode memory T cells specific to LCMV. When MCMV infection was given first (MCMV+LCMV), the magnitude of the acute T cell response to LCMV declined with age though this age-dependent decline was not dependent on MCMV. However, some of these MCMV persistently infected mice with acute LCMV infection (7 of 36) developed a robust immunodominant CD8 T cell response apparently cross-reactive between a newly defined putative MCMV epitope sequence, M57_727-734, and the normally subdominant LCMV epitope L_2062-2069, indicating a profound private specificity effect in heterologous immunity between these two viruses. These results further illustrate how a history of an acute or a persistent virus infection can substantially influence the immune responses and immune pathology associated with acute or persistent infections with an unrelated virus.

IMPORTANCE

This study extends our understanding of heterologous immunity in the context of persistent viral infection. The phenomenon has been studied mostly with viruses such as LCMV that are cleared, but the situation can be more complex with a persistent virus such as MCMV. We found that the history of LCMV infection intensifies MCMV immunopathology, enhances MCMV burden in multiple organs, and suppresses MCMV-specific T cell memory inflation. In the reverse infection sequence, we show that some of the long-term MCMV-immune mice mount a robust CD8 T cell cross-reactive response between a newly defined putative MCMV epitope sequence and a normally subdominant LCMV epitope. These results further illustrate how a history of infection can substantially influence the immune responses and immune pathology associated with infections with an unrelated virus.

Vaccination and heterologous immunity: educating the immune system

This review discusses three inter-related topics: (1) the immaturity of the neonatal and infant immune response; (2) heterologous immunity, where prior infection history with unrelated pathogens alters disease outcome resulting in either enhanced protective immunity or increased immunopathology to new infections, and (3) epidemiological human vaccine studies that demonstrate vaccines can have beneficial or detrimental effects on subsequent unrelated infections. The results from the epidemiological and heterologous immunity studies suggest that the immune system has tremendous plasticity and that each new infection or vaccine that an individual is exposed to during a lifetime will potentially alter the dynamics of their immune system. It also suggests that each new infection or vaccine that an infant receives is not only perturbing the immune system but is educating the immune system and laying down the foundation for all subsequent responses. This leads to the question, is there an optimum way to educate the immune system? Should this be taken into consideration in our vaccination protocols?

Generation of cellular immune memory and B-cell immunity is impaired by natural killer cells

The goal of most vaccines is the induction of long-lived memory T and B cells capable of protecting the host from infection by cytotoxic mechanisms, cytokines and high-affinity antibodies. However, efforts to develop vaccines against major human pathogens like HIV and HCV have not been successful, thereby highlighting the need for novel approaches to circumvent immunoregulatory mechanisms that limit induction of protective immunity. Here we show that mouse natural killer (NK) cells inhibit generation of long-lived virus-specific memory T- and B-cells as well as virus-specific antibody production after acute infection. Mechanistically, NK cells suppressed CD4 T cells and follicular helper T cells (T_FH) in a perforin-dependent manner during the first few days of infection, resulting in a weaker germinal center (GC) response and diminished immune memory. We anticipate that innovative strategies to relieve NK cell-mediated suppression of immunity should facilitate development of efficacious new vaccines targeting difficult-to-prevent infections.

Relapsing-remitting central nervous system autoimmunity mediated by GFAP-specific CD8 T cells

Multiple sclerosis (MS) is an inflammatory disease of the CNS that causes the demyelination of nerve cells and destroys oligodendrocytes, neurons, and axons. Historically, MS has been thought to be a CD4 T cell-mediated autoimmune disease of CNS white matter. However, recent studies identified CD8 T cell infiltrates and gray matter lesions in MS patients. These findings suggest that CD8 T cells and CNS Ags other than myelin proteins may be involved during the MS disease process. In this article, we show that CD8 T cells reactive to glial fibrillary acidic protein (GFAP), a protein expressed in astrocytes, can avoid tolerance mechanisms and, depending upon the T cell-triggering event, drive unique aspects of inflammatory CNS autoimmunity. In GFAP-specific CD8 TCR-transgenic (BG1) mice, tissue resident memory-like CD8 T cells spontaneously infiltrate the gray matter and white matter of the CNS, resulting in a relapsing-remitting CNS autoimmunity. The frequency, severity, and remissions from spontaneous disease are controlled by the presence of polyclonal B cells. In contrast, a viral trigger induces GFAP-specific CD8 T effector cells to exclusively target the meninges and vascular/perivascular space of the gray and white matter of the brain, causing a rapid, acute CNS disease. These findings demonstrate that the type of CD8 T cell-triggering event can determine the presentation of distinct CNS autoimmune disease pathologies.

Disparate epitopes mediating protective heterologous immunity to unrelated viruses share peptide-MHC structural features recognized by cross-reactive T cells

Closely related peptide epitopes can be recognized by the same T cells and contribute to the immune response against pathogens encoding those epitopes, but sometimes cross-reactive epitopes share little homology. The degree of structural homology required for such disparate ligands to be recognized by cross-reactive TCRs remains unclear. In this study, we examined the mechanistic basis for cross-reactive T cell responses between epitopes from unrelated and pathogenic viruses, lymphocytic choriomeningitis virus (LCMV) and vaccinia virus. Our results show that the LCMV cross-reactive T cell response toward vaccinia virus is dominated by a shared asparagine residue, together with other shared structural elements conserved in the crystal structures of K(b)-VV-A11R and K(b)-LCMV-gp34. Based on analysis of the crystal structures and the specificity determinants for the cross-reactive T cell response, we were able to manipulate the degree of cross-reactivity of the T cell response, and to predict and generate a LCMV cross-reactive response toward a variant of a null OVA-derived peptide. These results indicate that protective heterologous immune responses can occur for disparate epitopes from unrelated viruses.

CD8+ T cell-derived IFN-γ prevents infection by a second heterologous virus

Persistent viral infection is often associated with dysfunctional immune responses against unrelated pathogens. Lymphocytic choriomeningitis virus (LCMV) can establish acute or chronic infections in mice and is widely used as a model for persistent virus infections in humans. Mice infected with LCMV develop a transient defect in Ag-specific immunity against heterologous viral infection. Although it has been proposed that LCMV infection induces an immunosuppressed state within the host, our data show that infected mice successfully clear vaccinia virus through a mechanism that involves CD8(+) T cell-derived IFN-γ. This observation demonstrates that chronic LCMV infection does not impair protective immunity against heterologous viral challenge. Rather, a natural sterilizing immunity is induced following a primary infection that prevents a secondary infection. Our findings suggest a need to re-evaluate current thoughts about the immune suppression that might occur during a persistent infection.

Simplified sequence-specific oligonucleotide-based polymerase chain reaction protocol to characterize human major histocompatibility complex A*02 and A*24 specificities

Major histocompatibility complex (MHC) multimer technology is used in studies of high scientific and clinical interest for the identification, analysis, purification, and adoptive transfer of virus-specific T cells. MHC peptide multimers are usually specific for MHC A*02 or A*24 specificities because both specificities exhibit a high worldwide frequency. However, commercially available typing methods perform complete typing instead of browsing for these prevailing specificities. In this study we demonstrate an easy and accessible polymerase chain reaction-based method to accurately identify A*02 and A*24 samples in a cost-effective way.

Harnessing the power of genomics and immunoinformatics to produce improved vaccines

The role of cellular immunity as a mediator of protection against disease is gaining recognition, particularly with regard to the many pathogens for which we presently lack effective vaccines. As a result, there is an ever-increasing need to understand the T-cell populations induced by vaccination and, therefore, T-cell epitopes responsible for triggering their activation. Although the characterization and harnessing of cellular immunity for vaccine development is an active area of research interest, the field still needs to rigorously define T-cell epitope specificities, above all, on a genomic level. New immunoinformatic epitope mapping tools now make it possible to identify pathogen epitopes and perform comparisons against human and microbial genomic data sets. Such information will help to determine whether adaptive immune responses elicited by a vaccine are both pathogen-specific and protective, but not crossreactive against host or host-associated sequences that could jeopardize self-tolerance and/or human microbiome-host homeostasis. Here, we discuss advances in genomics and vaccine design and their relevance to the development of safer, more effective vaccines.