Cell cooperation in antibody responses to influenza virus. I. priming of helper t cells by internal components of virion

Intrastructural help: improving the HIV-1 envelope antibody response induced by virus-like particle vaccines

PURPOSE OF REVIEW

The importance of IgG Fc-effector functions for the efficacy of HIV vaccines is increasingly recognized. Although different types of vaccines were shown to induce antibodies with different Fc-activities, there is no clear strategy how to raise antibody responses with a desired pattern of Fc-effector functions. Given the central role of T-helper cells in regulating the germinal center reaction and the differentiation of B cells in an antigen-specific manner, the review will discuss whether T-helper cells directed against non-HIV envelope (Env) antigens could be harnessed to improve the HIV-Env antibody response.

RECENT FINDINGS

Comparing CD4 T-cell responses in HIV-infected individuals with and without neutralizing antibody breadth suggests that robust Gag-specific CD4 T cells may provide important T-cell help to Env-specific B cells. In a murine model, GagPol-specific T-helper cells were shown to provide intrastructural help for HIV-Env-specific antibody responses after immunization with a virus-like particle vaccine. GagPol-specific T-helper cells imprinted the IgG subtype ratio observed for Gag onto the HIV-Env antibody response and modulated the glycosylation pattern of the HIV Env-specific antibodies.

SUMMARY

Intrastructural help is a promising strategy to improve overall levels and Fc-effector functions of the HIV-Env antibody response.

GagPol-specific CD4⁺ T-cells increase the antibody response to Env by intrastructural help

Background

Immunization of rhesus macaques against Gag of SIV resulted in a more rapid appearance of Env antibodies after infection with SIV or SHIV challenge viruses although the vaccines lacked an Env component. We therefore explored whether T helper cells specific for internal HIV proteins could provide intrastructural help for Env-specific B cells and thus increase the Env antibody response.

Results

Mice were immunized by adenoviral vector or DNA vaccines against GagPol and then boosted with virus-like particles (VLP) containing GagPol and Env. Env-specific antibody levels after the VLP booster immunizations were significantly higher in GagPol-immunized mice than in mock-vaccinated controls. Adoptive transfer of CD4+ T cells from GagPol-immunized mice also enhanced the Env antibody response to VLP immunization in the recipient mice. Depending on the presence of VLPs, co-cultivation of CD4+ T cells from GagPol-primed mice with BCR transgenic B cells specific for a protein presented on the surface of the VLPs also resulted in the activation of the B and T cells.

Conclusions

Our study indicates that GagPol-specific T helper cells may provide intrastructural help for Env antibody responses. This cross-talk between immune responses directed against different components of the retroviral particle may be relevant for the immunopathogenesis of retroviral infections and allow to improve virus like particle vaccine approaches against HIV.

Coadministration of seasonal influenza vaccine and MVA-NP+M1 simultaneously achieves potent humoral and cell-mediated responses

Current seasonal influenza vaccines have reduced immunogenicity and are of suboptimal efficacy in older adults. We have previously shown that the novel candidate vaccine MVA-NP+M1 is able to boost memory T cell responses in adults aged 50-85 years. Preclinical studies have demonstrated that viral vectored vaccines can act as adjuvants when coadministered with protein-based vaccines. We have conducted a phase I clinical trial to compare the coadministration of seasonal influenza vaccine and MVA-NP+M1 with seasonal influenza vaccine alone in adults aged 50 years and above. This combination of vaccines was safe and well tolerated. T cell responses to internal influenza proteins were boosted to significantly higher levels in the group receiving MVA-NP+M1 compared with the group receiving seasonal influenza vaccine alone. Rates of seroprotection and seroconversion against the three vaccine strains were similar in both groups; however, there was a significant increase in the geometric mean titer ratio for the H3N2 component of seasonal influenza vaccine in the coadministration group. While some vaccine combinations result in immune interference, the coadministration of MVA-NP+M1 alongside seasonal influenza vaccine is shown here to increase some influenza strain-specific antibody responses and boost memory T cells capable of recognizing a range of influenza A subtypes.

Influenza viral neuraminidase: the forgotten antigen

Influenza is the most common cause of vaccine-preventable morbidity and mortality despite the availability of the conventional trivalent inactivated vaccine and the live-attenuated influenza vaccine. These vaccines induce an immunity dominated by the response to hemagglutinin (HA) and are most effective when there is sufficient antigenic relatedness between the vaccine strain and the HA of the circulating wild-type virus. Vaccine strategies against influenza may benefit from inclusion of other viral antigens in addition to HA. Epidemiologic evidence and studies in animals and humans indicate that anti-neuraminidase (NA) immunity will provide protection against severe illness or death in the event of a significant antigenic change in the HA component of the vaccine. However, there is little NA immunity induced by trivalent inactivated vaccine and live-attenuated influenza vaccine. The quantity of NA in influenza vaccines is not standardized and varies significantly among manufacturers, production lots and tested strains. The activity and stability of the NA enzyme is influenced by concentration of divalent cations. If immunity against NA is desirable, a better understanding of how the enzymatic properties affect the immunogenicity is needed.

Immunogenicity of chloroplast-derived HIV-1 p24 and a p24-Nef fusion protein following subcutaneous and oral administration in mice

High-level expression of foreign proteins in chloroplasts of transplastomic plants provides excellent opportunities for the development of oral vaccines against a range of debilitating or fatal diseases. The HIV-1 capsid protein p24 and a fusion of p24 with the negative regulatory protein Nef (p24-Nef) accumulate to ∼4% and ∼40% of the total soluble protein of leaves of transplastomic tobacco (Nicotiana tabacum L.) plants. This study has investigated the immunogenicity in mice of these two HIV-1 proteins, using cholera toxin B subunit as an adjuvant. Subcutaneous immunization with purified chloroplast-derived p24 elicited a strong antigen-specific serum IgG response, comparable to that produced by Escherichia coli-derived p24. Oral administration of a partially purified preparation of chloroplast-derived p24-Nef fusion protein, used as a booster after subcutaneous injection with either p24 or Nef, also elicited strong antigen-specific serum IgG responses. Both IgG1 and IgG2a subtypes, associated with cell-mediated Th1 and humoral Th2 responses, respectively, were found in sera after subcutaneous and oral administration. These results indicate that chloroplast-derived HIV-1 p24-Nef is a promising candidate as a component of a subunit vaccine delivered by oral boosting, after subcutaneous priming by injection of p24 and/or Nef.

Progress towards a universal influenza vaccine

Seasonal influenza is a common and highly transmissible disease, characterized by frequent and unpredictable mutations occurring in the viral envelope glycoproteins. Owing to this high variability, annual reformulation and immunization are required and still, the vaccine is not effective enough when there is an antigenic mismatch with circulating strains. A solution could come from the construction of a universal vaccine that would be based on highly conserved antigens and would be effective against many strains: some universal vaccine developers focus on the Matrix 2 protein, whereas others use additional conserved proteins, such as the nucleoprotein and Matrix 1, or even a range of peptides from these proteins and others to induce cross-strain immunity. This article aims to highlight recent significant advances in the development of a universal vaccine against influenza and focuses mainly on studies using the epitope-based approach that have also entered the clinical trial stage; it includes a brief summary of current vaccines against influenza as well as the ongoing efforts to develop a universal vaccine.

Determinant spreading: lessons from animal models and human disease

Spreading of the immune response is a common theme in organ-specific and systemic autoimmune diseases. We evaluated whether some of the mixed antinuclear antibody patterns characteristic of systemic autoimmunity might be the result of determinant spreading from a single initiating event. Immunisation of healthy mice with individual protein components of the La/Ro ribonucleoprotein (RNP) targeted in systemic lupus erythematosus and primary Sjögren's syndrome induced autoantibodies recognising Ro60 (SS-A), Ro52 (SS-A) and La (SS-B) and in some cases the molecular chaperones calreticulin and Grp78. The endogenous antigen(s) driving determinant spreading might be derived from physiological apoptosis which could explain the involvement of some chaperone proteins in the autoimmune response. Diversified anti-La/Ro antibody responses were initiated by challenge with a single subdominant T epitope of La even though some self epitopes of La were efficiently tolerised. The pattern of autoantibody responses in primary Sjögren's syndrome was strongly influenced by HLA class II phenotype which we speculate controls activation of T cells recognising defined peptides from the La/Ro RNP. In this way, HLA class II alleles may be critical in influencing initiation and spreading of systemic autoimmune reactions. Molecular mimicry of such determinants by exogenous agents might readily initiate spreading of an autoimmune response in genetically susceptible hosts.

T-helper reactivity to simian immunodeficiency virus gag synthetic peptides in human immunodeficiency virus type 2 infected individuals

West African populations are infected with divergent strains of human immunodeficiency virus type 2 (HIV2), some of which are closely related to simian immunodeficiency virus (SIV) and it has been postulated that the HIV2 epidemic might have arisen by cross-species spread of SIV into the human population in West Africa. To gain some insight into the possible basis for cross protection between these two closely related viruses, the T-helper responses to 15 synthetic peptides from SIV gag synthetic peptides were investigated in seven HIV2-infected subjects and in seven healthy controls. Significant reactivity to at least one of the synthetic peptides tested was found in all patients and a statistically significant correlation between CD4+ lymphocyte absolute numbers and the number of reacting peptides was observed. A marginal lymphocyte reactivity was found in two of the healthy controls studied. In conclusion, this preliminary evidence that HIV2-infected patients exhibit T-cell responses to SIV gag peptides suggests that both viruses share t-helper epitopes in the gag viral region and raises the possibility of cross protection between SIV and HIV2 which may be relevant for HIV2 vaccine research based on closely related retroviruses.

Intra- and intermolecular spreading of autoimmunity involving the nuclear self-antigens La (SS-B) and Ro (SS-A)

We have tested the extent of immune self-tolerance to the ubiquitously expressed nuclear/cytoplasmic autoantigens La (SS-B) and Ro (SS-A) in healthy, nonautoimmune mice. Immunization of mice with recombinant mouse La resulted in a specific, isotype-switched autoantibody response, which was initially directed toward the La C subfragment (aa 111-242) but rapidly spread to involve the La A (aa 1-107) and La F (aa 243-345) regions of the La antigen. Intramolecular spreading of the anti-La antibody response was further demonstrated by the appearance of autoantibodies to multiple, nonoverlapping antigenic regions of La, after immunization of mice with the 107-aa La A subfragment. Moreover, immunization of mice with recombinant mouse or human La also elicited specific anti-60-kDa Ro IgG antibodies in all strains tested. Mice immunized with 60-kDa Ro produced a high titer anti-Ro antibody response, which was also associated with intermolecular spreading, resulting in the specific appearance of anti-La autoantibodies. These findings show that the development of autoantibodies to multiple components of the La/Ro ribonucleoprotein complex may follow initiation of immunity to a single component. In addition, the data reveal the incomplete nature of immune tolerance to La and Ro despite their endogenous expression in all nucleated cells. These observations are likely to account for the coexistence of anti-La/Ro antibodies in autoimmune disease and suggest a general explanation for the appearance of mixed autoantibody patterns in systemic autoimmune disorders.

Kinetics and specificities of the T helper-cell response to gp120 in the asymptomatic stage of HIV-1 infection

Peripheral blood mononuclear cells from 36 asymptomatic HIV-1 seropositive individuals were tested longitudinally for in vitro T-cell proliferation and IL-2 production in response to synthetic peptides spanning the entire gp120 of HIV-1. At baseline, significant T-cell proliferative to pooled and individual peptides was observed in 15 of the 36 donors. After 12 months, proliferative responses to peptide pools were lost or decreased significantly in most donors. Responses appeared to fluctuate over time: at 12 months new recognition sites were detected in four of 10 donors showing T-cell proliferation at baseline, as well as in five of 15 donors with no previous proliferative responses. IL-2 production appeared to be a more sensitive and longer preserved parameter of T-helper cell function: at baseline the majority of donors with no T-cell proliferation produced IL-2 in response to pooled peptides. This response was not decreased significantly after 12 months. The overall patterns of response to both pooled and individual peptides were heterogeneous among donors. Multiple recognition sites were detected in both variable and conserved regions of gp120, but no pool or individual peptide was recognized by all responders. Functional T-cell responses were not statistically correlated to CD4+ cell percentile and absolute numbers.

Initiation of autoimmunity to the p53 tumor suppressor protein by complexes of p53 and SV40 large T antigen

Antinuclear antibodies (ANAs) reactive with a limited spectrum of nuclear antigens are characteristic of systemic lupus erythematosus (SLE) and other collagen vascular diseases, and are also associated with certain viral infections. The factors that initiate ANA production and determine ANA specificity are not well understood. In this study, high titer ANAs specific for the p53 tumor suppressor protein were induced in mice immunized with purified complexes of murine p53 and the Simian virus 40 large T antigen (SVT), but not in mice immunized with either protein separately. The autoantibodies to p53 in these mice were primarily of the IgG1 isotype, were not cross-reactive with SVT, and were produced at titers up to 1:25,000, without the appearance of other autoantibodies. The high levels of autoantibodies to p53 in mice immunized with p53/SVT complexes were transient, but low levels of the autoantibodies persisted. The latter may have been maintained by self antigen, since the anti-p53, but not the SVT, response in these mice could be boosted by immunizing with murine p53. Thus, once autoimmunity to p53 was established by immunizing with p53/SVT complexes, it could be maintained without a requirement for SVT. These data may be explained in at least two ways. First, altered antigen processing resulting from the formation of p53/SVT complexes might activate autoreactive T helper cells specific for cryptic epitopes of murine p53, driving anti-p53 autoantibody production. Alternatively, SVT-responsive T cells may provide intermolecular-intrastructural help to B cells specific for murine p53. In a second stage, these activated B cells might themselves process self p53, generating p53-responsive autoreactive T cells. The induction of autoantibodies during the course of an immune response directed against this naturally occurring complex of self and nonself antigens may be relevant to the generation of specific autoantibodies in viral infections, and may also have implications for understanding the pathogenesis of ANAs in SLE. In particular, our results imply that autoimmunity can be initiated by a "hit and run" mechanism in which the binding of a viral antigen to a self protein triggers an immune response that subsequently can be perpetuated by self antigen.

Dissociation of influenza virus hemagglutinin and neuraminidase eliminates their intravirionic antigenic competition

When presented together on the intact influenza virus particle, the external hemagglutinin (HA) and neuraminidase (NA) antigens are competitive, with HA dominant over NA in both T- and B-cell priming (B. E. Johansson, T. M. Moran, and E. D. Kilbourne, Proc. Natl. Acad. Sci. USA 84:6869-6873, 1987). Dissociation and purification of HA and NA from virus and their injection separately or in combination into BALB/c mice eliminates their antigenic competition as measured by antibody response, confirming that it is their structural association that leads to what we have termed intravirionic antigenic competition. We discuss this phenomenon with respect to previously described intermolecular antigenic competition and with regard to its probable mechanism. Our findings are relevant to contemporary interest in viral vaccine vectors and multicomponent vaccines.

Insertion of myoglobin T-cell epitopes into the Escherichia coli alkaline phosphatase

We are interested in antigen processing mechanisms of antigen-presenting cells, and to what extent the susceptibility of protein antigens to degradation contributes to immunogenicity. Understanding the biochemistry of antigen processing may be essential for reliable prediction of T-cell epitopes and for the design of vaccines that are optimized for T-cell priming. To examine possible effects of protein structural context on antigen presentation, we used genetic engineering techniques to insert helper T-cell epitopes derived from sperm whale myoglobin into surface loops of the highly stable Escherichia coli alkaline phosphatase, with the expectation that presentation of the myoglobin guest epitopes might vary with their position in the carrier protein. Levels of recombinant protein expression in E. coli cells and residual enzyme activity depended on the location of the guest peptides in the alkaline phosphatase carrier. Mutants with insertions between residues 189-190 of the carrier were recovered with yields and activities similar to the wild type protein; however, insertion of the same peptides at a second site, between residues 165-166, led to low recoveries and diminished phosphatase activities. Subcutaneous injection of mice with one of the purified recombinant proteins in complete Freund's adjuvant induced T cells that responded to in vitro challenge with myoglobin. The potential use of this system to dissect processing mechanisms is discussed.

Purified influenza virus hemagglutinin and neuraminidase are equivalent in stimulation of antibody response but induce contrasting types of immunity to infection

BALB/c mice immunized with graded doses of chromatographically purified hemagglutinin (HA) and neuraminidase (NA) antigens derived from A/Hong Kong/1/68 (H3N2) influenza virus demonstrated equivalent responses when HA-specific and NA-specific serum antibodies were measured by enzyme-linked immunosorbent assays (ELISAs). Antibody responses measured by hemagglutination inhibition or neuraminidase inhibition titrations showed similar kinetic patterns, except for more rapid decline in hemagglutination inhibition antibody. Injection of mice with either purified HA or NA resulted in immunity manifested by reduction in pulmonary virus following challenge with virus containing homologous antigens. However, the nature of the immunity induced by the two antigens differed markedly. While HA immunization with all but the lowest doses of antigen prevented manifest infection, immunization with NA was infection-permissive at all antigen doses, although reduction in pulmonary virus was proportional to the amount of antigen administered. The immunizing but infection-permissive effect of NA immunization over a wide range of doses is in accord with results of earlier studies with mice in which single doses of NA and antigenically hybrid viruses were used. The demonstrable immunogenicity of highly purified NA as a single glycoprotein without adjuvant offers a novel infection-permissive approach with potentially low toxicity for human immunization against influenza virus.

Functional analysis of influenza-specific helper T cell clones in vivo. T cells specific for internal viral proteins provide cognate help for B cell responses to hemagglutinin

We compared the effects of adoptively transferred Th cell clones specific for the influenza hemagglutinin (HA), matrix (M), or nucleoprotein (NP) on the antibody response of nude mice infected with A/PR/8/34 influenza virus. We show that the production of antibodies to the HA absolutely requires the presence of virus-specific Th cells. Further, transfer of a Th clone specific for the internal proteins, M or NP, was as effective as was transfer of an HA-specific clone in supporting an antibody response to the HA. With each of the clones, the kinetics of the response were accelerated by approximately 3 d compared with the antibody response of normal BALB/c mice. The HA- and M-specific clones supported an isotype switch from IgM to IgG and IgA similar to that which occurs during a normal antibody response. Finally, as shown by coinfection experiments, the response required a cognate T-B interaction whether the determinants recognized by the Th and B cell are located on the same viral protein or on different viral proteins within the same virus particle. The implications of these findings for understanding the T-B interactions that occur during an effective antiviral antibody response are discussed.

Helper T cells induced by an immunopurified herpes simplex virus type I (HSV-I) 115 kilodalton glycoprotein (gB) protect mice against HSV-I infection

Three herpes simplex virus type I (HSV-I) glycoproteins of apparent molecular masses 103, 63, and 115 kD have been purified using virus-specific monoclonal antibodies (mAb) G8D1, C2D2, and T157, respectively. Both G8D1 and C2D2 neutralize HSV-I in vitro and passively protect CBA mice against HSV-I infection in vivo, whereas T157 is neither neutralizing nor passively protective. However, mice given a single subcutaneous injection of 30 micrograms 115 kD glycoprotein in saline were completely protected against lethal challenges of HSV-I administered intraperitoneally or in the footpad 7 d after immunization. In contrast, mice similarly immunized with 103 or 63 kD glycoproteins were only partially protected. The prophylactic immunity was correlated with an early induction of specific antibody, which became even more evident 3 d after virus challenge. There was a remarkable similarity in antibody isotype distribution between the responses to 115 kD glycoprotein and to heat-inactivated intact HSV-I. However, the prechallenge sera from 115 kD glycoprotein hyperimmunized mice were again neither virus-neutralizing nor passively protective. All three glycoproteins induced only low levels of delayed-type hypersensitivity (DTH). Pretreatment of mice with cyclophosphamide significantly enhanced DTH to 115 kD and 103 kD glycoproteins in the absence of antibody, but failed to confer significant immunity, indicating that DTH alone is insufficient for protection. Splenic and lymph node Ig- (B cell-depleted) cells from mice protectively immunized with 115 kD glycoprotein could adoptively transfer effective protection and enhance a virus neutralizing antibody response in normal recipients challenged with a lethal dose of HSV-I. Both the protection and the ability to enhance neutralizing antibody were diminished when the cells were treated with mAb GK 1.5 and complement. These results therefore demonstrate that the 115 kD glycoprotein, though not apparently containing accessible epitopes for the induction of virus-neutralizing antibody, possesses determinants capable of activating helper T cells. These L3T4+ cells confer strong protective immunity by enhancing protective antibody upon challenge infection, probably through associative help.

Fine specificity analysis of human influenza-specific cloned cell lines

Influenza-specific human-T-cell clones, proliferating in the presence of virus-infected cells with restriction by class II molecules and displaying class II-restricted CTL activity or specific helper activity in antibody synthesis, have been analyzed for antigenic specificities. All of them were obtained by in vitro stimulation against influenza A/Texas virus. In all cases the virus specificity appeared identical in cytolytic and proliferative responses. Three of the clones were broadly cross-reactive, recognizing all or almost all type A influenza strains. The three remaining clones were subtype specific when tested with human strains and recognized the surface glycoproteins of influenza virus. One of these lines reacted with an epitope of the neuraminidase N2 while the other two recognized the hemagglutinin H3. By using a large panel of mammalian and avian influenza strains, it can be demonstrated that hemagglutinin-specific human T cells can recognize a cross-reacting determinant shared by H3 and H4 subtypes of hemagglutinin which has never been detected with antibodies.

Unusual restriction of a proliferative line reacting with influenza A and B viruses

A human T-cell line, B3, has been obtained by cloning spleen cells at limiting dilutions in the presence of influenza-A-virus-infected autologous cells. B3 cells were OKT 3+4+8-, E rosetting+, Sig- and were HLA-DR (+) after stimulation and HLA-DR (-) when resting. They proliferate specifically in the presence of influenza-virus-infected cells. Remarkable is that (a) the proliferation was obtained with viruses of both A and B types and (b) only autologous cells seem to be able to present the viral antigen to B3 cells.