Antiviral cytotoxic T cells cross-reactively recognize disparate peptide determinants from related viruses but ignore more similar self- and foreign determinants

Immunogenicity and safety of yellow fever vaccine in HIV-1-infected patients

OBJECTIVES

The objective of this study is to investigate immunogenicity and safety of the yellow fever vaccine (YFV) in HIV-infected (HIV+) patients with high CD4 T-cell counts.

DESIGN

In this prospective, comparative study of YFV-naive adults: 40 HIV+ under antiretroviral therapy (ART) with CD4 T-cell count above 350 cells/μl and plasma HIV-RNA less than 50 copies/ml for at least 6 months and 31 HIV-negative (HIV-) received one injection of the YF-17D strain vaccine.

METHODS

Serologic response was assessed by using a plaque reduction neutralizing test and YFV-specific T cells by using an INFγ-Elispot assay.

RESULTS

YFV was well tolerated in both groups. Most participants had asymptomatic YFV viremia at day (D) 7 after vaccination (77% of HIV- and 82% of HIV+, P = 0.58), with higher plasma level of YFV RNA in HIV+ than in HIV- (median 2.46 log10 copies/ml (range: 1.15-4.16) and 1.91 log10 copies/ml (1.15-3.19), respectively, P = 0.011). A significant but transient decrease in CD4 cell counts was seen at D7 in both groups, more pronounced in HIV- than in HIV+ patients (-261.5 versus -111.5 cells/μl, respectively, P = 0.0003), but no HIV breakthrough was observed in plasma. All participants developed protective neutralizing antibody levels from D28 and up to 1 year after injection. At D91, fewer HIV+ than HIV- participants exhibited YFV T-cell response (20 versus 54%, respectively, P = 0.037).

CONCLUSION

At 1 year, YFV was immunogenic and well tolerated in HIV-infected adults under ART with CD4 T-cell counts above 350 cells/μl. However, a lower immunity of YFV T cells in HIV-infected patients was observed as compared with HIV- participants.

CLINICAL TRIALS REGISTRATION

NCT01426243.

West Nile virus T-cell ligand sequences shared with other flaviviruses: a multitude of variant sequences as potential altered peptide ligands

Phylogenetic relatedness and cocirculation of several major human pathogen flaviviruses are recognized as a possible cause of deleterious immune responses to mixed infection or immunization and call for a greater understanding of the inter-Flavivirus protein homologies. This study focused on the identification of human leukocyte antigen (HLA)-restricted West Nile virus (WNV) T-cell ligands and characterization of their distribution in reported sequence data of WNV and other flaviviruses. H-2-deficient mice transgenic for either A2, A24, B7, DR2, DR3, or DR4 HLA alleles were immunized with overlapping peptides of the WNV proteome, and peptide-specific T-cell activation was measured by gamma interferon (IFN-γ) enzyme-linked immunosorbent spot (ELISpot) assays. Approximately 30% (137) of the WNV proteome peptides were identified as HLA-restricted T-cell ligands. The majority of these ligands were conserved in ∼≥88% of analyzed WNV sequences. Notably, only 51 were WNV specific, and the remaining 86, chiefly of E, NS3, and NS5, shared an identity of nine or more consecutive amino acids with sequences of 64 other flaviviruses, including several major human pathogens. Many of the shared ligands had an incidence of >50% in the analyzed sequences of one or more of six major flaviviruses. The multitude of WNV sequences shared with other flaviviruses as interspecies variants highlights the possible hazard of defective T-cell activation by altered peptide ligands in the event of dual exposure to WNV and other flaviviruses, by either infection or immunization. The data suggest the possible preferred use of sequences that are pathogen specific with minimum interspecies sequence homology for the design of Flavivirus vaccines.

Comprehensive analysis of T cell epitope discovery strategies using 17DD yellow fever virus structural proteins and BALB/c (H2d) mice model

Immunomics research uses in silico epitope prediction, as well as in vivo and in vitro approaches. We inoculated BALB/c (H2d) mice with 17DD yellow fever vaccine to investigate the correlations between approaches used for epitope discovery: ELISPOT assays, binding assays, and prediction software. Our results showed a good agreement between ELISPOT and binding assays, which seemed to correlate with the protein immunogenicity. PREDBALB/c prediction software partially agreed with the ELISPOT and binding assay results, but presented low specificity. The use of prediction software to exclude peptides containing no epitopes, followed by high throughput screening of the remaining peptides by ELISPOT, and the use of MHC-biding assays to characterize the MHC restrictions demonstrated to be an efficient strategy. The results allowed the characterization of 2 MHC class I and 17 class II epitopes in the envelope protein of the YF virus in BALB/c (H2d) mice.

Can we really learn from model pathogens?

Cytolytic T (Tc) lymphocytes are the first order response of the adaptive immune system in the recovery from primary viral infections. These effector cells execute their function either by direct cytotoxicity through the Fas or perforin pathway and/or by the release of cytokines that either directly or indirectly exert antiviral activity. Mice respond to infection by closely related viruses with a vigorous Tc response, which is characterized by extensive crossreactivity on target cells infected with these viruses. However, the action of these cells can be beneficial, detrimental or neutral. From our current state of knowledge, no generalizations as to protective or detrimental effects of cytolytic effector functions in recovery from virus infections can be made. Thus, virus-host immune interactions have to be assessed individually and cannot be generalized.

Exocytosis and Fas mediated cytolytic mechanisms exert protection from West Nile virus induced encephalitis in mice

Infection of mice with the flaviviruses West Nile virus (WNV) and Murray Valley encephalitis (MVE) induces cytolytic T-cell responses which are highly cross-reactive on target cells infected with heterologous flaviviruses. Of C57BL/6 mice infected with low doses (10(2)-10(6) PFU) of either virus, 30-40% develop encephalitis and die within 10-12 days. Mice with defects in the Fas or granule exocytosis (perforin and granzymes A and B) pathway of cellular cytotoxicity display reduced mortality and increased survival time when infected with MVE and are protected from encephalitis when deficient in both pathways. This contrasts with infection with WNV where defects in these cytolytic mechanisms increase the percentage of mice that succumb to encephalitis. Thus, no generalizations as to protective or detrimental effects of cytolytic effector functions in recovery from closely related flavivirus infections can be made. Virus-host immune interactions have to be assessed individually and cannot be generalized.

Conserved amino acids 193–324 of non-structural protein 3 are a dominant source of peptide determinants for CD4+ and CD8+ T cells in a healthy Japanese encephalitis virus-endemic cohort

Our earlier identification of the non-structural protein 3 (NS3) of Japanese encephalitis virus (JEV) as a dominant CD4+ as well as CD8+ T cell-eliciting antigen in a healthy JEV-endemic cohort with a wide HLA distribution implied the presence of several epitopes dispersed over the length of the protein. Use of various truncated versions of NS3 in lymphocyte stimulation and interferon (IFN)-γ secretion assays revealed that amino acids (aa) 193–324 of NS3 were comparable with, if not superior to, the full-length protein in evoking Th1 responses. The potential of this 14·4 kDa stretch to stimulate IFN-γ production from both subtypes of T cells in a manner qualitatively and quantitatively similar to the 68 kDa parent protein suggested the presence within it of both class I and II epitopes and demonstrated that the entire immunogenicity of NS3 was focused on aa 193–324. Interestingly, this segment contained five of the eight helicase motifs of NS3. Analysis of variability of the NS3 protein sequence across 16 JEV isolates revealed complete identity of aa 219–318, which is contained within the above segment, suggesting that NS3-specific epitopes tend to cluster in relatively conserved regions that harbour functionally critical domains of the protein.

CD8 T cell responses to viral infections in sequence

Our current understanding of virus-specific T cell responses has been shaped by model systems with mice, where naive animals are infected with a single viral pathogen. Paradigms derived from such models, however, may not always be applicable to a natural setting, where a host is exposed to numerous pathogens over its lifetime. Accumulating data in animal models and with some human diseases indicate that a host's prior history of infections can impact the specificity of future CD8 T cell responses, even to unrelated viruses. This can have both beneficial and detrimental consequences for the host, including altered clearance of virus, distinct forms of immunopathology, and substantial changes in the pool of memory T cells. Here we will describe the characteristics of CD8 T cells and the dynamics of their response to heterologous viral infections in sequence.

CD8+ T cells mediate recovery and immunopathology in West Nile virus encephalitis

C57BL/6J mice infected intravenously with the Sarafend strain of West Nile virus (WNV) develop a characteristic central nervous system (CNS) disease, including an acute inflammatory reaction. Dose response studies indicate two distinct kinetics of mortality. At high doses of infection (10(8) PFU), direct infection of the brain occurred within 24 h, resulting in 100% mortality with a 6-day mean survival time (MST), and there was minimal destruction of neural tissue. A low dose (10(3) PFU) of infection resulted in 27% mortality (MST, 11 days), and virus could be detected in the CNS 7 days postinfection (p.i.). Virus was present in the hypogastric lymph nodes and spleens at days 4 to 7 p.i. Histology of the brains revealed neuronal degeneration and inflammation within leptomeninges and brain parenchyma. Inflammatory cell infiltration was detectable in brains from day 4 p.i. onward in the high-dose group and from day 7 p.i. in the low-dose group, with the severity of infiltration increasing over time. The cellular infiltrates in brain consisted predominantly of CD8(+), but not CD4(+), T cells. CD8(+) T cells in the brain and the spleen expressed the activation markers CD69 early and expressed CD25 at later time points. CD8(+) T-cell-deficient mice infected with 10(3) PFU of WNV showed increased mortalities but prolonged MST and early infection of the CNS compared to wild-type mice. Using high doses of virus in CD8-deficient mice leads to increased survival. These results provide evidence that CD8(+) T cells are involved in both recovery and immunopathology in WNV infection.

Lymphocyte activation and hepatic cellular infiltration in immunocompetent mice infected by dengue virus

Activation and expansion of dengue virus-specific T cells and abnormal liver functions in dengue patients have been documented. However, it remains to be determined whether T cells are involved in the pathogenic mechanism of dengue virus infection. In this study, immunocompetent C57BL/6 mice were employed to study dengue virus-induced T cell activation. Mice were inoculated with 10(8) PFU dengue virus serotype 2 strain 16681 by the intravenous route. Dengue viral core RNA was detected by RT-PCR in mouse serum, liver, spleen, and brain at different time points after infection. Splenic T cells were activated as evidenced by their expression of CD69 and O-glycosylated CD43 at as early as day 3 after infection. Splenic T cell expression of O-glycosylated CD43 and IFN-gamma production coordinately peaked at day 5. Coincided with the peak of splenic T cell activation was hepatic lymphocyte infiltration and elevation of liver enzymes. Flow cytometric analysis revealed the infiltrating CD8(+) T cell to CD4(+) T cell ratio was 5/3. After a second inoculation of dengue virus, hepatic T cell infiltration and liver enzyme levels increased sharply. The infiltrating hepatic CD8(+) T cell to CD4(+) T cell ratio increased to 5.8/1. A strong correlation was found between T cell activation and hepatic cellular infiltration in immunocompetent mice infected with dengue virus. The kinetics of liver enzyme elevation also correlated with that of T cell activation. These data suggest a relationship between T cell infiltration and elevation of liver enzymes.

Direct visualization of cross-reactive effector and memory allo-specific CD8 T cells generated in response to viral infections

CD8 T cell cross-reactivity between heterologous viruses has been shown to provide protective immunity, induce immunopathology, influence the immunodominance of epitope-specific T cell responses, and shape the overall memory population. Virus infections also induce cross-reactive allo-specific CTL responses. In this study, we quantified the allo-specific CD8 T cells elicited by infection of C57BL/6 (B6) mice with lymphocytic choriomeningitis virus (LCMV). Cross-reactive LCMV-specific CD8 T cells were directly visualized using LCMV peptide-charged MHC tetramers to costain T cells that were stimulated to produce intracellular IFN-gamma in response to allogeneic target cells. The cross-reactivity between T cells specific for LCMV and allogeneic Ags was broad-based, in that it involved multiple LCMV-derived peptides, but there were distinctive patterns of reactivity against allogeneic cells with different haplotypes. Experiments indicated that this cross-reactivity was not due to the expression of two TCR per cell, and that the patterns of allo-reactivity changed during sequential infection with heterologous viruses. The allo-specific CD8 T cells generated by LCMV infection were maintained at relatively high frequencies in the memory pool, indicating that memory allo-specific CD8 T cell populations can arise as a consequence of viral infections. Mice previously infected with LCMV and harboring allo-specific memory T cells were refractory to the induction of tolerance to allogeneic skin grafts.

Antigenic Structure of Flavivirus Proteins

The increased activity of Dengue virus in the tropical regions of the world and the recent movement of West Nile virus from the eastern to the western hemisphere emphasize the fact that vector-borne flaviviruses are medically important emerging infectious diseases. These facts warrant continued efforts to decode all facets of flavivirus immunology. This chapter reviews current understanding of the antigenic fine structure of flaviviral structural and nonstructural (NS) proteins and their involvement in B- an T-cell host responses. The virion structural glycoprotein E elicits both virus-neutralizing antibodies and antiviral Th-cell responses. Consistent with the current hypothesis of the MHC class I pathway of protein processing, immunodominant flaviviral Tc-cell epitopes mainly reside on the NS proteins. To prepare effective and inexpensive subunit vaccines, we will need to continue to better understand these structure-function relationships of flavivirus proteins.

Yellow fever virus 17D envelope and NS3 proteins are major targets of the antiviral T cell response in mice

The yellow fever virus 17D vaccine strain is one of the most effective and safe vaccines available. The immune response after immunization is characterized by long-lasting high titers of neutralizing antibodies. Here, we have initiated a characterization of YFV-17D-specific cellular immune responses. This study makes three points. First, we have identified two CD8 T cell epitopes and one CD4 T cell epitope. An H-2Kb-restricted dominant epitope was mapped in the NS3 protein, whereas the viral envelope protein harbored an H-2Db-restricted subdominant epitope and the I-Ab-restricted CD4 T cell epitope. Second, illustrating the concept of immunodomination, we found that after abrogation of the dominant response in H-2Kb knockout mice, the frequencies of T cells recognizing the subdominant Db-restricted epitope increased dramatically. Finally, the H-2Db-restricted epitope lacks the canonical Asn anchor residue at position 5, indicating that epitopes may be missed by strict application of the H-2Db-binding motif. Identification of these T cell epitopes will facilitate studies on the cellular immunity against YFV-based expression or immunization vectors.