Amelia R. Hofstetter

Diverse roles of non-diverse molecules: MHC class Ib molecules in host defense and control of autoimmunity

While the prime function of classical MHC class I molecules (MHC-I) is to present peptide antigens to pathogen-specific cytotoxic T cells, non-classical MHC-I antigens perform a diverse array of functions in both innate and adaptive immunity. In this review we summarize recent evidence that non classical MHC-I molecules are not only recognized by pathogen-specific T cells but that they also serve as immunoregulatory molecules by stimulating a number of distinct non-conventional T cell subsets.

Heterogeneity among viral antigen-specific CD4+ T cells and their de novo recruitment during persistent polyomavirus infection

Virus-specific CD4+ T cells optimize antiviral responses by providing help for antiviral humoral responses and CD8+ T cell differentiation. Although CD4+ T cell responses to viral infections that undergo complete clearance have been studied extensively, less is known about virus-specific CD4+ T cell responses to viruses that persistently infect their hosts. Using a mouse polyomavirus (MPyV) infection model, we previously demonstrated that CD4+ T cells are essential for recruiting naive MPyV-specific CD8+ T cells in persistently infected mice. In this study, we defined two dominant MPyV-specific CD4+ T cell populations, one directed toward an epitope derived from the nonstructural large T Ag and the other from the major viral capsid protein of MPyV. These MPyV-specific CD4+ T cells vary in terms of their magnitude, functional profile, and phenotype during acute and persistent phases of infection. Using a minimally myeloablative-mixed bone marrow chimerism approach, we further show that naive virus-specific CD4+ T cells, like anti-MPyV CD8+ T cells, are primed de novo during persistent virus infection. In summary, these findings reveal quantitative and qualitative differences in the CD4+ T cell response to a persistent virus infection and demonstrate that naive antiviral CD4+ T cells are recruited during chronic polyomavirus infection.

CD8 T Cells Recruited Early in Mouse Polyomavirus Infection Undergo Exhaustion

Repetitive Ag encounter, coupled with dynamic changes in Ag density and inflammation, imparts phenotypic and functional heterogeneity to memory virus-specific CD8 T cells in persistently infected hosts. For herpesvirus infections, which cycle between latency and reactivation, recent studies demonstrate that virus-specific T cell memory is predominantly derived from naive precursors recruited during acute infection. Whether functional memory T cells to viruses that persist in a nonlatent, low-level infectious state (smoldering infection) originate from acute infection-recruited naive T cells is not known. Using mouse polyomavirus (MPyV) infection, we previously showed that virus-specific CD8 T cells in persistently infected mice are stably maintained and functionally competent; however, a sizeable fraction of these memory T cells are short-lived. Further, we found that naive anti-MPyV CD8 T cells are primed de novo during persistent infection and contribute to maintenance of the virus-specific CD8 T cell population and its phenotypic heterogeneity. Using a new MPyV-specific TCR-transgenic system, we now demonstrate that virus-specific CD8 T cells recruited during persistent infection possess multicytokine effector function, have strong replication potential, express a phenotype profile indicative of authentic memory capability, and are stably maintained. In contrast, CD8 T cells recruited early in MPyV infection express phenotypic and functional attributes of clonal exhaustion, including attrition from the memory pool. These findings indicate that naive virus-specific CD8 T cells recruited during persistent infection contribute to preservation of functional memory against a smoldering viral infection.

Cutting edge: shift in antigen dependence by an antiviral MHC class Ib-restricted CD8 T cell response during persistent viral infection.

The requirement for Ag in maintaining memory CD8 T cells often differs between infections that are acutely resolved and those that persist. Using the mouse polyoma virus (PyV) persistent infection model, we recently described a novel CD8 T cell response directed to a PyV peptide presented by Q9, an MHC class Ib molecule. This antiviral Q9-restricted CD8 T cell response is characterized by a 3-mo expansion phase followed by a long-term plateau phase. In this study, we demonstrate that viral Ag is required for this protracted inflation phase but is dispensable for the maintenance of this Q9-restricted CD8 T cell response. Moreover, proliferation by memory T cells, not recruitment of naive PyV-specific T cells, is primarily responsible for Q9-restricted, anti-PyV CD8 T cell inflation. These data reveal a dynamic shift in Ag dependence by an MHC class Ib-restricted memory CD8 T cell response during a persistent viral infection.

A highly immunogenic vaccine against A/H7N9 influenza virus.

Since the first case of human infection in March 2013, continued reports of H7N9 cases highlight a potential pandemic threat. Highly immunogenic vaccines to this virus are urgently needed to protect vulnerable populations who lack protective immunity. In this study, an egg- and adjuvant-independent adenoviral vector-based, hemagglutinin H7 subtype influenza vaccine (HAd-H7HA) demonstrated enhanced cell-mediated immunity as well as serum antibody responses in a mouse model. Most importantly, this vaccine provided complete protection against homologous A/H7N9 viral challenge suggesting its potential utility as a pandemic vaccine.

MHC class Ib-restricted CD8 T cells differ in dependence on CD4 T cell help and CD28 costimulation over the course of mouse polyomavirus infection

We recently identified a protective MHC class Ib-restricted CD8 T cell response to infection with mouse polyomavirus. These CD8 T cells recognize a peptide from aa 139–147 of the VP2 viral capsid protein bound to the nonpolymorphic H-2Q9 molecule, a member of the Qa-2 family of β2m-associated MHC class Ib molecules. Q9:VP2.139-specific CD8 T cells exhibit an unusual inflationary response characterized by a gradual expansion over 3 mo followed by a stable maintenance phase. We previously demonstrated that Q9:VP2.139-specific CD8 T cells are dependent on Ag for expansion, but not for long-term maintenance. In this study, we tested the hypothesis that the expansion and maintenance components of the Q9:VP2.139-specific T cell response are differentially dependent on CD4 T cell help and CD28 costimulation. Depletion of CD4+ cells and CD28/CD40L blockade impaired expansion of Q9:VP2.139-specific CD8 T cells, and intrinsic CD28 signaling was sufficient for expansion. In contrast, CD4 T cell insufficiency, but not CD28/CD40L blockade, resulted in a decline in frequency of Q9:VP2.139-specific CD8 T cells during the maintenance phase. These results indicate that the Q9:VP2.139-specific CD8 T cell response to mouse polyomavirus infection depends on CD4 T cell help and CD28 costimulation for inflationary expansion, but only on CD4 T cell help for maintenance.

RIG-I ligand enhances the immunogenicity of recombinant H7HA protein

Avian H7N9 influenza virus infection with fatal outcomes continues to pose a pandemic threat and highly immunogenic vaccines are urgently needed. In this report we show that baculovirus-derived recombinant H7 hemagglutinin protein, when delivered with RIG-I ligand, induced enhanced antibody and T cell responses and conferred protection against lethal challenge with a homologous H7N9 virus. These findings indicate the potential utility of RIG-I ligands as vaccine adjuvants to increase the immunogenicity of recombinant H7 hemagglutinin.

Peptide Immunization Elicits Polyomavirus-Specific MHC Class Ib-Restricted CD8 T Cells in MHC Class Ia Allogeneic Mice

Unlike the polymorphic MHC class Ia molecules, MHC class Ib molecules are oligomorphic or nonpolymorphic. We recently discovered a protective CD8 T cell response to mouse polyomavirus (MPyV) in H-2(b) haplotype mice that is restricted by H2-Q9, a member of the Qa-2 MHC class Ib family. Here, we demonstrate that immunization with a peptide corresponding to a virus capsid-derived peptide presented by Q9 also elicits MHC class Ib-restricted MPyV-specific CD8 T cells in mice of H-2(s) and H-2(g7) strains. These findings support the concept that immunization with a single MHC class Ib-restricted peptide can expand CD8 T cells in MHC class Ia allogeneic hosts.