Activating CTL precursors to reveal CTL function without skewing the repertoire by in vitro expansion

Peptide-MHC multimer-based monitoring of CD8 T-cells in HIV-1 infection and AIDS vaccine development

The use of MHC multimers allows precise and direct detecting and analyzing of antigen-specific T-cell populations and provides new opportunities to characterize T-cell responses in humans and animals. MHC-multimers enable us to enumerate specific T-cells targeting to viral, tumor and vaccine antigens with exceptional sensitivity and specificity. In the field of HIV/SIV immunology, this technique provides valuable information about the frequencies of HIV- and SIV-specific CD8(+) cytotoxic T lymphocytes (CTLs) in different tissues and sites of infection, AIDS progression, and pathogenesis. Peptide-MHC multimer technology remains a very sensitive tool in detecting virus-specific T -cells for evaluation of the immunogenicity of vaccines against HIV-1 in preclinical trials. Moreover, it helps to understand how immune responses are formed following vaccination in the dynamics from priming point until T-cell memory is matured. Here we review a diversity of peptide-MHC class I multimer applications for fundamental immunological studies in different aspects of HIV/SIV infection and vaccine development.

In vitro expansion of antigen-specific CD8(+) T cells distorts the T-cell repertoire

Short-term in vitro expansion of antigen-specific T cells is an appreciated assay for the analysis of small memory T-cell populations. However, how well short-term expanded T cells represent the direct ex vivo situation remains to be elucidated. In this study we compared the clonality of Epstein-Barr virus (EBV) and cytomegalovirus (CMV)-specific CD8(+) T cells directly ex vivo and after in vitro stimulation with antigen. Our data show that the antigen-specific T cell repertoire significantly alters after in vitro culture. Clear shifts in clonotype hierarchy were observed, with the most dominant ex vivo clonotype decreasing after stimulation at the expense of several previously subdominant clonotypes. Notably, these alterations were more pronounced in polyclonal T-cell populations compared to mono- or oligoclonal repertoires. Furthermore, TCR diversity significantly increased after culture with antigen. These results suggest that the T-cell repertoire is highly subjective to variation after in vitro stimulation with antigen. Hence, although short-term expansion of T cells provides a simple and efficient tool to examine antigen-specific immune responses, caution is required if T-cell populations are expanded prior to detailed, clonotypic analyses or other repertoire-based investigations.

Mucosal immunity and HIV-1 infection: applications for mucosal AIDS vaccine development

Natural transmission of human immunodeficiency virus type 1 (HIV-1) occurs through gastrointestinal and vaginal mucosa. These mucosal tissues are major reservoirs for initial HIV replication and amplification, and the sites of rapid CD4(+) T cell depletion. In both HIV-infected humans and SIV-infected macaques, massive loss of CD4(+) CCR5(+) memory T cells occurs in the gut and vaginal mucosa within the first 10-14 days of infection. Induction of local HIV-specific immune responses by vaccines may facilitate effective control of HIV or SIV replication at these sites. Vaccines that induce mucosal responses, in particular CD8(+) cytotoxic T lymphocytes (CTL), have controlled viral replication at mucosal sites and curtailed systemic dissemination. Thus, there is strong justification for development of next generation vaccines that induce mucosal immune effectors against HIV-1 including CD8(+) CTL, CD4(+) T helper cells and secretory IgA. In addition, further understanding of local innate mechanisms that impact early viral replication will greatly inform future vaccine development. In this review, we examine the current knowledge concerning mucosal AIDS vaccine development. Moreover, we propose immunization strategies that may be able to elicit an effective immune response that can protect against AIDS as well as other mucosal infections.

Narrowed TCR diversity for immunised mice challenged with recombinant influenza A-HIV Env(311-320) virus

Understanding CD8+ T cell responses generated by live virus vectors is critical for the rational design of next generation HIV CTL-based vaccines. We used recombinant influenza viruses expressing the HIV Env(311-320) peptide in the neuraminidase stalk to study response magnitude, cytokine production and repertoire diversity for the elicited CD8+ D(d)Env(311) CTL set. The insertion of the CD8+ D(d)Env(311) epitope into the NA stalk resulted in a decrease in viral fitness that was reflected in lower lung viral titres. While not affecting the magnitude of endogenous primary influenza-specific responses, the introduction of the D(d)Env(311) CD8+ T cell epitope altered the hierarchy of responses following secondary challenge. The CD8+ K(d)NP(147) response increased 9-fold in the spleen following secondary infection whereas the CD8+ D(d)Env(311) response increased 15-fold in the spleen. Moreover, this study is the first to describe narrowing of CD8+ TCR repertoire diversity in the context of an evolving secondary immune response against influenza A virus. Analysis of Vbeta bias for CD8+ D(d)Env(311) T cell responses showed a narrowing of CD8+ Vbeta8.1/8.2 D(d)Env(311) TCR repertoire diversity. This work further emphasizes the importance of understanding vaccine-induced CD8+ T cell responses.

Generation of functionally active HIV-1 specific CD8+ CTL in intestinal mucosa following mucosal, systemic or mixed prime-boost immunization

Gastrointestinal and vaginal mucosa are major sites of entry in natural HIV infection and therefore the preferred sites to elicit high-avidity CD8+ CTL by vaccination. We directly compare systemic and mucosal immunization in mice after DNA priming and boosting with rgp160 env expressed either in MVA or Ad for their ability to induce mucosal as well as systemic HIV-specific CTL. The optimal CTL response in the gut mucosa was observed after priming with the HIV-1 gp160 env DNA vaccine and boosting with rMVA or rAd encoding the same envelope gene all administered intrarectally (IR). Maximum levels of high-avidity CD8+ T cells were seen in intestinal lamina propria following this regimen. When the prime and boost routes were distinct, the delivery site of the boost had a greater impact than the DNA priming. IM DNA prime and IR rMVA boost were more effective than IR DNA prime and IM rMVA boost for eliciting mucosal CD8+ T-cell avidity. A systemic DNA-prime-followed by systemic rMVA boost induced high levels of high-avidity CD8+ T cells systemically, but responses were undetectable in mucosal sites. A single systemic immunization with rMVA was sufficient to induce high-avidity IFN-gamma secreting CD8+ T cells in systemic organs, whereas a single mucosal immunization with rMVA was not sufficient to elicit high-avidity CD8+ T cells in mucosa. Thus, a heterologous mucosal DNA prime-viral vectored boost strategy was needed. The requirement for a heterologous DNA prime-recombinant viral boost strategy for generation of high-avidity CD8+ T cells in mucosal sites in mice may be more stringent than for the induction of high-avidity CD8+ T cells in systemic compartments.

Enhanced cell surface expression, immunogenicity and genetic stability resulting from a spontaneous truncation of HIV Env expressed by a recombinant MVA

During propagation of modified vaccinia virus Ankara (MVA) encoding HIV 89.6 Env, a few viral foci stained very prominently. Virus cloned from such foci replicated to higher titers than the parent and displayed enhanced genetic stability on passage. Sequence analysis showed a single nucleotide deletion in the 89.6 env gene of the mutant that caused a frame shift and truncation of 115 amino acids from the cytoplasmic domain. The truncated Env was more highly expressed on the cell surface, induced higher antibody responses than the full-length Env, reacted with HIV neutralizing monoclonal antibodies and mediated CD4/co-receptor-dependent fusion. Intramuscular (i.m.), intradermal (i.d.) needleless, and intrarectal (i.r.) catheter inoculations gave comparable serum IgG responses. However, intraoral (i.o.) needleless injector route gave the highest IgA in lung washings and i.r. gave the highest IgA and IgG responses in fecal extracts. Induction of CTL responses in the spleens of individual mice as assayed by intracellular cytokine staining was similar with both the full-length and truncated Env constructs. Induction of acute and memory CTL in the spleens of mice immunized with the truncated Env construct by i.d., i.o., and i.r. routes was comparable and higher than by the i.m. route, but only the i.r. route induced CTL in the gut-associated lymphoid tissue. Thus, truncation of Env enhanced genetic stability as well as serum and mucosal antibody responses, suggesting the desirability of a similar modification in MVA-based candidate HIV vaccines.

Role of alpha3 domain of class I MHC molecules in the activation of high- and low-avidity CD8+ CTLs

CD8 can serve as a co-receptor or accessory molecule on the surface of CTL. As a co-receptor, CD8 can bind to the alpha3 domain of the same MHC class I molecules as the TCR to facilitate TCR signaling. To evaluate the role of the MHC class I molecule alpha3 domain in the activation of CD8(+) CTL, we have produced a soluble 227 mutant of H-2D(d), with a point mutation in the alpha3 domain (Glu227 --> Lys). 227 mutant class I-peptide complexes were not able to effectively activate H-2D(d)-restricted CD8 T cells in vitro, as measured by IFN-gamma production by an epitope-specific CD8(+) CTL line. However, the 227 mutant class I-peptide complexes in the presence of another MHC class I molecule (H-2K(b)) (that cannot present the peptide) with a normal alpha3 domain can induce the activation of CD8(+) CTL. Therefore, in order to activate CD8(+) CTL, the alpha3 domain of MHC class I does not have to be located on the same molecule with the alpha1 and alpha2 domains of MHC class I. A low-avidity CD8(+) CTL line was significantly less sensitive to stimulation by the 227 mutant class I-peptide complexes in the presence of the H-2K(b) molecule. Thus, low-avidity CTL may not be able to take advantage of the interaction between CD8 and the alpha3 domain of non-presenting class I MHC molecules, perhaps because of a shorter dwell time for the TCR-MHC interaction.

A novel functional CTL avidity/activity compartmentalization to the site of mucosal immunization contributes to protection of macaques against simian/human immunodeficiency viral depletion of mucosal CD4+ T cells

The presence of high-avidity CTLs in the right compartment can greatly affect clearance of a virus infection (for example, AIDS viral infection of and dissemination from mucosa). Comparing mucosal vs systemic immunization, we observed a novel compartmentalization of CTL avidity and proportion of functionally active Ag-specific CD8(+) T cells to tissues proximal to sites of immunization. Whereas both s.c. and intrarectal routes of immunization induced tetramer(+) cells in the spleen and gut, the mucosal vaccine induced a higher percentage of functioning IFN-gamma(+) Ag-specific CD8(+) T cells in the gut mucosa in mice. Translating to the CD8(+) CTL avidity distribution in rhesus macaques, intrarectal vaccination induced more high-avidity mucosal CTL than s.c. vaccination and protection of mucosal CD4(+) T cells from AIDS viral depletion, whereas systemic immunization induced higher avidity IFN-gamma-secreting cells in the draining lymph nodes but no protection of mucosal CD4(+) T cells, after mucosal challenge with pathogenic simian/human immunodeficiency virus. Mucosal CD4(+) T cell loss is an early critical step in AIDS pathogenesis. The preservation of CD4(+) T cells in colonic lamina propria and the reduction of virus in the intestine correlated better with high-avidity mucosal CTL induced by the mucosal AIDS vaccine. This preferential localization of high-avidity CTL may explain previous differences in vaccination results and may guide future vaccination strategy.

Enhancement of CD8+ T cell immunity in the lung by CpG oligodeoxynucleotides increases protective efficacy of a modified vaccinia Ankara vaccine against lethal poxvirus infection even in a CD4-deficient host

Immunostimulatory CpG oligodeoxynucleotides (ODN) have proven effective as adjuvants for protein-based vaccines, but their impact on immune responses induced by live viral vectors is not known. We found that addition of CpG ODN to modified vaccinia Ankara (MVA) markedly improved the induction of longer-lasting adaptive protective immunity in BALB/c mice against intranasal pathogenic vaccinia virus (Western Reserve; WR). Protection was mediated primarily by CD8(+) T cells in the lung, as determined by CD8-depletion studies, protection in B cell-deficient mice, and greater protection correlating with CD8(+) IFN-gamma-producing cells in the lung but not with those in the spleen. Intranasal immunization was more effective at inducing CD8(+) T cell immunity in the lung, and protection, than i.m. immunization. Addition of CpG ODN increased the CD8(+) response but not the Ab response. Depletion of CD4 T cells before vaccination with MVA significantly diminished protection against pathogenic WR virus. However, CpG ODN delivered with MVA was able to substitute for CD4 help and protected CD4-depleted mice against WR vaccinia challenge. This study demonstrates for the first time a protective adjuvant effect of CpG ODN for a live viral vector vaccine that may overcome CD4 deficiency in the induction of protective CD8(+) T cell-mediated immunity.

The NS2 protein of human respiratory syncytial virus suppresses the cytotoxic T-cell response as a consequence of suppressing the type I interferon response

The NS1 and NS2 proteins of human respiratory syncytial virus (HRSV) have been shown to antagonize the type I interferon (IFN) response, an effect subject to host range constraints. We have now found that the HRSV NS2 protein strongly controls IFN induction in mouse cells in vitro, validating the use of the mouse model to study the consequences of these gene deletions on host immunity. We evaluated the effects of deleting the NS1 and/or NS2 gene on the induction of HRSV-specific pulmonary cytotoxic T lymphocytes (CTL) in BALB/c and 129S6 mice in response to intranasal infection with HRSV lacking the NS1 and/or NS2 gene and subsequent challenge with wild-type (wt) HRSV. In mice infected with HRSV lacking the NS2 gene (DeltaNS2) or lacking the NS2 gene in combination with the NS1 gene (DeltaNS1/2 HRSV), the magnitude of the pulmonary CTL response was substantially elevated compared to that of mice infected with wt HRSV or the DeltaNS1 mutant, whether measured by binding of CD8(+) cells to an HRSV-specific major histocompatibility complex class I tetramer, by measurement of CD8(+) cells secreting gamma interferon (IFN-gamma) in response to specific in vitro stimulation, or by a standard chromium release cell-killing assay. In contrast, in STAT1 knockout mice, which lack responsiveness to type I IFN, the level of IFN-gamma-secreting CD8(+) cells was not significantly different for HRSV lacking the NS2 gene, suggesting that the increase in CTL observed in IFN-responsive mice is type I IFN dependent. Thus, the NS2 protein of HRSV suppresses the CTL component of the adaptive immune response, and this appears to be a consequence of its suppression of type I IFN.

Impact of vaccine-induced mucosal high-avidity CD8+ CTLs in delay of AIDS viral dissemination from mucosa

Natural HIV transmission occurs through mucosa, but it is debated whether mucosal cytotoxic T lymphocytes (CTLs) can prevent or reduce dissemination from the initial mucosal site to the systemic circulation. Also, the role of CTL avidity in mucosal AIDS viral transmission is unknown. To address these questions, we used delay in acute-phase peak viremia after intrarectal challenge as an indicator of systemic dissemination. We found that a peptide-prime/poxviral boost vaccine inducing high levels of high-avidity mucosal CTLs can have an impact on dissemination of intrarectally administered pathogenic SHIV-ku2 in macaques and that such protection correlates better with mucosal than with systemic CTLs and particularly with levels of high-avidity mucosal CTLs.

Maximizing antigen targeting to the proteasome for gene-based vaccines

Wild-type or immunoevasive antigens can drive weak CD8+T-cell responses against both dominant and subdominant epitopes during gene-based vaccination. For many antigens, fusion to ubiquitin (Ub) to target them to the proteasome circumvents this problem. Although this procedure works in most cases, for one subset of antigens, Ub fusion does not improve immune responses. To determine why these failures occur, we have evaluated in detail the 'rules' for proteasome targeting that have been applied in mammalian vaccine studies, but that were actually defined in yeast systems. To do this, we fused a series of engineered Ub genes to green fluorescent protein (GFP) and tested their ability to target GFP to the proteasome for enhanced antigen processing and CD8+ T-cell responses. Here we demonstrate that Ub fusion mediates enhanced CD8+ responses by proteasome targeting rather than by enhancing protein translation. We also show that several of the yeast-defined Ub constructs failed to target the proteasome in mammalian cells and likewise failed to enhance transgene-specific CD8+ T-cell responses in mice. In contrast, when mammalian-optimized constructs were applied to target the influenza virus nucleoprotein, CD8+ responses were enhanced against its refractory subdominant epitope in mice. This work demonstrates that Ub fusion has efficacy to enhance CD8+ responses, especially against subdominant antigen epitopes, provided constructs are optimized for mammalian use.

Expression of interleukin-4 by recombinant respiratory syncytial virus is associated with accelerated inflammation and a nonfunctional cytotoxic T-lymphocyte response following primary infection but not following challenge with wild-type virus

The outcome of a viral infection or of immunization with a vaccine can be influenced by the local cytokine environment. In studies of experimental vaccines against respiratory syncytial virus (RSV), an increased stimulation of Th2 (T helper 2) lymphocytes was associated with increased immunopathology upon subsequent RSV infection. For this study, we investigated the effect of increased local expression of the Th2 cytokine interleukin-4 (IL-4) from the genome of a recombinant RSV following primary infection and after a challenge with wild-type (wt) RSV. Mice infected with RSV/IL-4 exhibited an accelerated pulmonary inflammatory response compared to those infected with wt RSV, although the wt RSV group caught up by day 8. In the first few days postinfection, RSV/IL-4 was associated with a small but significant acceleration in the expansion of pulmonary T lymphocytes specific for an RSV CD8(+) cytotoxic T-lymphocyte (CTL) epitope presented as a major histocompatibility complex class I tetramer. However, by day 7 the response of tetramer-positive T lymphocytes in the wt RSV group caught up and exceeded that of the RSV/IL-4 group. At all times, the CTL response of the RSV/IL-4 group was deficient in the production of gamma interferon and was nonfunctional for in vitro cell killing. The accelerated inflammatory response coincided with an accelerated accumulation and activation of pulmonary dendritic cells early in infection, but thereafter the dendritic cells were deficient in the expression of B7-1, which governs the acquisition of cytolytic activity by CTL. Following a challenge with wt RSV, there was an increase in Th2 cytokines in the animals that had previously been infected with RSV/IL-4 compared to those previously infected with wt RSV, but the CD8(+) CTL response and the amount of pulmonary inflammation were not significantly different. Thus, a strong Th2 environment during primary pulmonary immunization with live RSV resulted in early inflammation and a largely nonfunctional primary CTL response but had a minimal effect on the secondary response.

Interferon-gamma is produced by CD8 T cells in response to HLA-A24-restricted hepatitis C virus epitopes after sustained virus loss

Differences in cytotoxic T lymphocyte activity in hepatitis C virus infection may account for the outcome of interferon monotherapy. To investigate this hypothesis, we analysed the response of peripheral CD8(+) T cells that recognized epitopes presented by HLA-A*2402. We synthesized HLA/beta2-microglobulin/peptide complexes using two epitopes. Production of interferon-gamma by CD8(+) T cells in response to plastic-bound monomeric HLA/peptide complex was observed frequently in sustained virus responders (SVR) (n = 13) against all the peptides, NS31296-1304 (the percentage of responding patients, 61.5%) and core 129-137 (53.8%), while no interferon-gamma production was observed in non-responders (NR) (n = 13) for any of the peptides. Tetramer-staining showed the presence of CD8(+) T cells specific for all the peptides except NS31296-1304 in two SVR at the end of interferon monotherapy, although hardly any such cells were found in four NR. Specific killing was observed against peptides NS31296-1304 (3/4) and core 129-137 (1/4) in sustained responders but none in non-responders. These results suggest that the responses of cytotoxic T lymphocytes (CTLs) were induced during interferon therapy in these patients and that interferon-gamma production by CD8(+) T lymphocytes against HCV NS31296-1304 and core 129-137 are well maintained in patients with SVR compared with those with NR. These findings emphasize the importance of the CD8(+) T cell response in controlling HCV infection.

Protection against lethal vaccinia virus challenge in HLA-A2 transgenic mice by immunization with a single CD8+ T-cell peptide epitope of vaccinia and variola viruses

CD8(+) T lymphocytes have been shown to be involved in controlling poxvirus infection, but no protective cytotoxic T-lymphocyte (CTL) epitopes are defined for variola virus, the causative agent of smallpox, or for vaccinia virus. Of several peptides in vaccinia virus predicted to bind HLA-A2.1, three, VETFsm(498-506), A26L(6-14), and HRP2(74-82), were found to bind HLA-A2.1. Splenocytes from HLA-A2.1 transgenic mice immunized with vaccinia virus responded only to HRP2(74-82) at 1 week and to all three epitopes by ex vivo enzyme-linked immunosorbent spot (ELISPOT) assay at 4 weeks postimmunization. To determine if these epitopes could elicit a protective CD8(+) T-cell response, we challenged peptide-immunized HLA-A2.1 transgenic mice intranasally with a lethal dose of the WR strain of vaccinia virus. HRP2(74-82) peptide-immunized mice recovered from infection, while naïve mice died. Depletion of CD8(+) T cells eliminated protection. Protection of HHD-2 mice, lacking mouse class I major histocompatibility complex molecules, implicates CTLs restricted by human HLA-A2.1 as mediators of protection. These results suggest that HRP2(74-82), which is shared between vaccinia and variola viruses, may be a CD8(+) T-cell epitope of vaccinia virus that will provide cross-protection against smallpox in HLA-A2.1-positive individuals, representing almost half the population.

Transcutaneous immunization induces mucosal CTLs and protective immunity by migration of primed skin dendritic cells

Transcutaneous immunization (TCI), the application of vaccines on the skin, induces robust systemic and mucosal antibodies in animal models and in humans. The means by which mucosal immune responses to vaccine antigens are elicited by TCI has not been well characterized. We examined the effect of TCI with an HIV peptide vaccine on the induction of mucosal and systemic CTL responses and protective immunity against mucosal challenge with live virus in mice. Robust HIV-specific CTL responses in the spleen and in the gut mucosa were detected after TCI. The responses were dependent upon the addition of an adjuvant and resulted in protection against mucosal challenge with recombinant vaccinia virus encoding HIV gp160. Although it is clear that adjuvant-activated DCs migrated mainly to draining lymph nodes, coculture with specific T cells and flow cytometry studies with DCs isolated from Peyer's patches after TCI suggested that activated DCs carrying skin-derived antigen also migrated from the skin to immune-inductive sites in gut mucosa and presented antigen directly to resident lymphocytes. These results and previous clinical trial results support the observation that TCI is a safe and effective strategy for inducing strong mucosal antibody and CTL responses.

IL-10 mediates suppression of the CD8 T cell IFN-gamma response to a novel viral epitope in a primed host

Priming to Ag can inhibit subsequent induction of an immune response to a new epitope incorporated into that Ag, a phenomenon referred to as original antigenic sin. In this study, we show that prior immunity to a virus capsid can inhibit subsequent induction of the IFN-gamma effector T cell response to a novel CD8-restricted antigenic epitope associated with the virus capsid. Inhibition does not involve Ab to the virus capsid, as it is observed in animals lacking B cells. CD8-restricted virus-specific T cell responses are not required, as priming to virus without CTL induction is associated with inhibition. However, IL-10(-/-) mice, in contrast to IL-10(+/+) mice, generate CD8 T cell and Ab responses to novel epitopes incorporated into a virus capsid, even when priming to the capsid has resulted in high titer Ab to the capsid. Furthermore, capsid-primed mice, unable to mount a response to a novel epitope in the capsid protein, are nevertheless able to respond to the same novel epitope delivered independently of the capsid. Thus, inhibition of responsiveness to a novel epitope in a virus-primed animal is a consequence of secretion of IL-10 in response to presented Ag, which inhibits local generation of new CD8 IFN-gamma-secreting effector T cells. Induction of virus- or tumor Ag-specific CD8 effector T cells in the partially Ag-primed host may thus be facilitated by local neutralization of IL-10.

Late effects of the Chernobyl radiation accident on T cell-mediated immunity in cleanup workers

The main goal of this investigation was to evaluate the abnormal T-cell immunity in cleanup workers who took part in the cleanup after the Chernobyl accident in 1986. Peripheral blood mononuclear cells (MNCs) of apparently healthy cleanup workers (n = 134) were used to analyze the phenotype and proliferative response to mitogens in vitro. Evaluation of the MNC phenotype of cleanup workers did not reveal a significant disturbance in the T-cell subpopulation content except for an increase in CD3+CD16+56+ (NKT) cells. Immunophenotyping of phytohemagglutinin (PHA)-activated MNCs demonstrated suppression of CD4+ T-cell propagation and augmentation of CD8+ T-cell propagation in vitro compared to control individuals. DNA synthesis in the MNCs of cleanup workers was markedly inhibited after activation for 3 days with suboptimal concentrations of PHA, pokeweed mitogen and PMA. In contrast to control individuals, the monocytes of cleanup workers were able to stimulate the proliferation of T cells from healthy individuals but inhibited the proliferation of T cells from cleanup workers. This study affords a better understanding of the response of MNCs to stimulation with suboptimal concentrations of PHA and provides an approach to a more accurate analysis of the immunological disorders found after exposure to radiation from Chernobyl-related activities.

Granulocyte-macrophage colony-stimulating factor expressed by recombinant respiratory syncytial virus attenuates viral replication and increases the level of pulmonary antigen-presenting cells

An obstacle to developing a vaccine against human respiratory syncytial virus (RSV) is that natural infection typically does not confer solid immunity to reinfection. To investigate methods to augment the immune response, recombinant RSV (rRSV) was constructed that expresses murine granulocyte-macrophage colony-stimulating factor (mGM-CSF) from a transcription cassette inserted into the G-F intergenic region. Replication of rRSV/mGM-CSF in the upper and lower respiratory tracts of BALB/c mice was reduced 23- to 74- and 5- to 588-fold, respectively, compared to that of the parental rRSV. Despite this strong attenuation of replication, the level of RSV-specific serum antibodies induced by rRSV/mGM-CSF was comparable to, or marginally higher than, that of the parental rRSV. The induction of RSV-specific CD8(+) cytotoxic T cells was moderately reduced during the initial infection, which might be a consequence of reduced antigen expression. Mice infected with rRSV/mGM-CSF had elevated levels of pulmonary mRNA for gamma interferon (IFN-gamma) and interleukin 12 (IL-12) p40 compared to animals infected by wild-type rRSV. Elevated synthesis of IFN-gamma could account for the restriction of RSV replication, as was observed previously with an IFN-gamma-expressing rRSV. The accumulation of total pulmonary mononuclear cells and total CD4(+) T lymphocytes was accelerated in animals infected with rRSV/mGM-CSF compared to that in animals infected with the control virus, and the level of IFN-gamma-positive or IL-4-positive pulmonary CD4(+) cells was elevated approximately twofold. The number of pulmonary lymphoid and myeloid dendritic cells and macrophages was increased up to fourfold in mice infected with rRSV/mGM-CSF compared to those infected with the parental rRSV, and the mean expression of major histocompatibility complex class II molecules, a marker of activation, was significantly increased in the two subsets of dendritic cells. Enhanced antigen presentation likely accounts for the maintenance of a strong antibody response despite reduced viral replication and would be a desirable property for a live attenuated rRSV vaccine.

High-affinity T helper epitope induces complementary helper and APC polarization, increased CTL, and protection against viral infection

Natural viral proteins do not always make optimal vaccines. We have found that sequence modification to increase epitope affinity for class II MHC molecules (epitope enhancement) can improve immunogenicity. Here we show first that a higher-affinity helper epitope-enhanced HIV vaccine not only induces more cytotoxic T lymphocytes (CTLs), but also skews helper cells toward Th1 cytokine production and protects against HIV-1 recombinant vaccinia viral challenge. Furthermore, we elucidate a novel mechanism in which the higher-affinity vaccine induces dramatically more effective helper cells with a higher level of CD40L per helper cell and more positive cells, which in turn more effectively conditions dendritic cells (DCs) for CTL activation in a second culture. The improved helper cells also induce much greater IL-12 production by DCs, accounting for the reciprocal T helper polarization to Th1, and increase costimulatory molecule expression. Thus, increasing affinity for class II MHC results in a complementary interaction in which T helper and antigen-presenting cells polarize each other, as well as increase CTL, and provide greater vaccine efficacy against viral infection.