Rapid memory CD8+ T-lymphocyte induction through priming with recombinant Mycobacterium smegmatis

A recombinant Mycobacterium smegmatis-based surface display system for developing the T cell-based COVID-19 vaccine

The immune escape mutations of SARS-CoV-2 variants emerged frequently, posing a new challenge to weaken the protective efficacy of current vaccines. Thus, the development of novel SARS-CoV-2 vaccines is of great significance for future epidemic prevention and control. We herein reported constructing the attenuated Mycobacterium smegmatis (M. smegmatis) as a bacterial surface display system to carry the spike (S) and nucleocapsid (N) of SARS-CoV-2. To mimic the native localization on the surface of viral particles, the S or N antigen was fused with truncated PE_PGRS33 protein, which is a transportation component onto the cell wall of Mycobacterium tuberculosis (M.tb). The sub-cellular fraction analysis demonstrated that S or N protein was exactly expressed onto the surface (cell wall) of the recombinant M. smegmatis. After the immunization of the M. smegmatis-based COVID-19 vaccine candidate in mice, S or N antigen-specific T cell immune responses were effectively elicited, and the subsets of central memory CD4+ T cells and CD8+ T cells were significantly induced. Further analysis showed that there were some potential cross-reactive CTL epitopes between SARS-CoV-2 and M.smegmatis. Overall, our data provided insights that M. smegmatis-based bacterial surface display system could be a suitable vector for developing T cell-based vaccines against SARS-CoV-2 and other infectious diseases.

Chimeric Human Papillomavirus-16 Virus-like Particles Presenting P18I10 and T20 Peptides from HIV-1 Envelope Induce HPV16 and HIV-1-Specific Humoral and T Cell-Mediated Immunity in BALB/c Mice

In this study, the HIV-1 P18I10 CTL peptide derived from the V3 loop of HIV-1 gp120 and the T20 anti-fusion peptide of HIV-1 gp41 were inserted into the HPV16 L1 capsid protein to construct chimeric HPV:HIV (L1:P18I10 and L1:T20) VLPs by using the mammalian cell expression system. The HPV:HIV VLPs were purified by chromatography. We demonstrated that the insertion of P18I10 or T20 peptides into the DE loop of HPV16 L1 capsid proteins did not affect in vitro stability, self-assembly and morphology of chimeric HPV:HIV VLPs. Importantly, it did not interfere either with the HIV-1 antibody reactivity targeting sequential and conformational P18I10 and T20 peptides presented on chimeric HPV:HIV VLPs or with the induction of HPV16 L1-specific antibodies in vivo. We observed that chimeric L1:P18I10/L1:T20 VLPs vaccines could induce HPV16- but weak HIV-1-specific antibody responses and elicited HPV16- and HIV-1-specific T-cell responses in BALB/c mice. Moreover, could be a potential booster to increase HIV-specific cellular responses in the heterologous immunization after priming with rBCG.HIVA vaccine. This research work would contribute a step towards the development of the novel chimeric HPV:HIV VLP-based vaccine platform for controlling HPV16 and HIV-1 infection, which is urgently needed in developing and industrialized countries.

Priming With Recombinant BCG Expressing Novel HIV-1 Conserved Mosaic Immunogens and Boosting With Recombinant ChAdOx1 Is Safe, Stable, and Elicits HIV-1-Specific T-Cell Responses in BALB/c Mice

BCG is currently the only licensed vaccine against tuberculosis (TB) and confers protection against meningitis and miliary tuberculosis in infants, although pulmonary disease protection in adults is inconsistent. Recently, promising HIV-1 immunogens were developed, such as the T-cell immunogens "tHIVconsvX," designed using functionally conserved protein regions across group M strains, with mosaic immunogens to improve HIV-1 variant match and response breadth. In this study, we constructed an integrative E. coli-mycobacterial shuttle plasmid, p2auxo.HIVconsvXint, expressing the immunogens HIVconsv1&2. This expression vector used an antibiotic resistance-free mechanism for plasmid selection and maintenance. It was first transformed into a glycine auxotrophic E. coli strain and subsequently transformed into a lysine auxotrophic Mycobacterium bovis BCG strain to generate vaccines BCG.HIVconsv12auxo.int and BCG.HIVconsv22auxo.int. The DNA sequence coding for the HIVconsv1&2 immunogens and protein expression were confirmed and working vaccine stocks were genetically and phenotypically characterized. We demonstrated that BCG.HIVconsv1&22auxo.int in combination with ChAdOx1.tHIVconsv5&6 were well tolerated and induced HIV-1-specific T-cell responses in adult BALB/c mice. In addition, we showed that the BCG.HIVconsv1&22auxo.int vaccine strains were stable in vitro after 35 bacterial generations and in vivo 7 weeks after inoculation. The use of integrative expression vectors and novel HIV-1 immunogens are likely to have improved the mycobacterial vaccine stability and specific immunogenicity and may enable the development of a useful vaccine platform for priming protective responses against HIV-1/TB and other prevalent pediatric pathogens shortly following birth.

Recombinant Mycobacterium smegmatis with a pMyong2 vector expressing Human Immunodeficiency Virus Type I Gag can induce enhanced virus-specific immune responses

Recently, we have developed a novel Mycobacterium-Escherichia coli shuttle vector system using pMyong2, which can provide an enhanced expression of heterologous genes in recombinant Mycobacterium smegmatis (rSmeg). To investigate the usefulness of rSmeg using pMyong2 in vaccine application, we vaccinated M. smegmatis with pMyong2 system expressing Human Immunodeficiency Virus Type I (HIV-1) Gag p24 antigen (rSmeg-pMyong2-p24) into mice and examined its cellular and humoral immune responses against HIV gag protein. We found that rSmeg-pMyong2-p24 expressed higher levels of Gag protein in bacteria, macrophage cell line (J774A.1) and mouse bone marrow derived dendritic cells (BMDCs) compared to rSmeg strains using two other vector systems, pAL5000 derived vector (rSmeg-pAL-p24) and the integrative plasmid, pMV306 (rSmeg-pMV306-p24). Inoculation of mice with rSmeg-pMyong2-p24 elicited more effective immunity compared to the other two rSmeg strains, as evidenced by higher levels of HIV-1 Gag-specific CD4 and CD8 T lymphocyte proliferation, interferon gamma ELISPOT cell induction, and antibody production. Furthermore, rSmeg-pMyong2-p24 showed a higher level of cytotoxic T cell response against target cells expressing Gag p24 proteins. Our data suggest that Mycobacterium-Escherichia coli shuttle vector system with pMyong2 may provide an advantage in vaccine application of rSmeg over other vector systems.

Oral administration of recombinant Mycobacterium smegmatis expressing a tripeptide construct derived from endogenous and microbial antigens prevents atherosclerosis in ApoE(-/-) mice

INTRODUCTION

Immunotherapy by inducing oral tolerance to atherogenic self-antigens is gaining importance as an alternative treatment modality for atherosclerosis. The use of live bacterial vectors to express the recombinant antigen in vivo will obviate the need for large-scale purification of recombinant protein and may also augment the efficacy of oral tolerance induction.

AIM

The objective of the study was to explore the use of recombinant Mycobacterium smegmatis as a live vector for oral delivery of antigens to induce immune tolerance.

METHOD AND RESULTS

We developed a M. smegmatis vector to secrete a recombinant tripeptide construct (AHC; peptides from Apolipoprotein B, Heat-shock protein 60 and Chlamydia pneumoniae outer membrane protein) expressed in a dendroaspin protein scaffold in pJH154 background. Immune response and oral tolerance to the cloned peptides were studied in C57/BL6 mice. The efficacy of this live vaccine to control atherosclerosis was studied in ApoE(-/-) knockout mice in C57/BL6 background. Oral administration of M. smegmatis secreting the cloned AHC antigen was found to induce tolerance to cloned protein and reduce the development of atherosclerosis by 24.0% compared to control. Protection against atherosclerosis was associated with increase in expression of regulatory T cell-associated markers including CTLA4 (1.8-fold), Foxp3 (2.6-fold), TGF-β (2.8-fold), IL10 (2.9-fold), and reduction in lipids, macrophage infiltration, and expression of inflammatory mediators in aorta.

CONCLUSIONS

Our results suggest that M. smegmatis can be developed as an oral carrier of recombinant proteins to treat inflammatory autoimmune diseases.

The use of directed evolution to create a stable and immunogenic recombinant BCG expressing a modified HIV-1 Gag antigen

Numerous features make Mycobacterium bovis BCG an attractive vaccine vector for HIV. It has a good safety profile, it elicits long-lasting cellular immune responses and in addition manufacturing costs are affordable. Despite these advantages it is often difficult to express viral antigens in BCG, which results in genetic instability and low immunogenicity. The aim of this study was to generate stable recombinant BCG (rBCG) that express high levels of HIV antigens, by modification of the HIV genes. A directed evolution process was applied to recombinant mycobacteria that expressed HIV-1 Gag fused to the green fluorescent protein (GFP). Higher growth rates and increased GFP expression were selected for. Through this process a modified Gag antigen was selected. Recombinant BCG that expressed the modified Gag (BCG[pWB106] and BCG[pWB206]) were more stable, produced higher levels of antigen and grew faster than those that expressed the unmodified Gag (BCG[pWB105]). The recombinant BCG that expressed the modified HIV-1 Gag induced 2 to 3 fold higher levels of Gag-specific CD4 T cells than those expressing the unmodified Gag (BCG[pWB105]). Mice primed with 10⁷ CFU BCG[pWB206] and then boosted with MVA-Gag developed Gag-specific CD8 T cells with a frequency of 1343±17 SFU/10⁶ splenocytes, 16 fold greater than the response induced with MVA-Gag alone. Levels of Gag-specific CD4 T cells were approximately 5 fold higher in mice primed with BCG[pWB206] and boosted with MVA-Gag than in those receiving the MVA-Gag boost alone. In addition mice vaccinated with BCG[pWB206] were protected from a surrogate vaccinia virus challenge.

Immunotherapeutic efficacy of recombinant Mycobacterium smegmatis expressing Ag85B-ESAT6 fusion protein against persistent tuberculosis infection in mice

The application of immunotherapy in combination with chemotherapy is considered an effective treatment strategy against persistent Mycobacterium tuberculosis (Mtb) infection. In this study, we constructed a novel recombinant Mycobacterium smegmatis (rMS) strain that expresses Ag85B and ESAT6 fusion protein (AE-rMS). Immunization of C57BL/6 mice with AE-rMS generated mainly Th1-type immune responses by strongly stimulating IFN-γ- and IL-2-producing splenocytes and increasing antigen-specific cytotoxic T lymphocyte (CTL) activity. To test the immunotherapeutic efficacy of AE-rMS, a persistent tuberculosis infection (PTBI) model was established via tail-vein injection of C57BL/6 mice with 1×10(4) colony forming units (CFU) of Mtb strain H37Rv in combination with concurrent chemotherapy drugs isoniazid (INH) and pyrazinamide (PZA). PTBI mice immunized with AE-rMS showed high levels of IFN-γ secreted by splenocytes and decreased bacteria loads in lung. Treatment with only the anti-tuberculosis (anti-TB) drugs RFP and INH (RI), decreased bacteria loads to low levels, with the Th1-type immune response further attenuated. Moreover, AE-rMS, when combined with RI treatment, further reduced the bacteria load as well as the pathological tissue damage in lung. Together, these results demonstrated the essential roles of AE-rMS-induced Th1-type responses, providing an effective treatment strategy by combining AE-rMS and RI for persistent TB.

Current and novel approaches to vaccine development against tuberculosis

Antibiotics and vaccines are the two most successful medical countermeasures that humans have created against a number of pathogens. However a select few e.g., Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB) have evaded eradication by vaccines and therapeutic approaches. TB is a global public health problem that kills 1.4 million people per year. The past decade has seen significant progress in developing new vaccine candidates, but the most fundamental questions in understanding disease progression and protective host responses that are responsible for controlling Mtb infection still remain poorly resolved. Current TB treatment requires intense chemotherapy with several antimicrobials, while the only approved vaccine is the classical viable whole-cell based Bacille-Calmette-Guerin (BCG) that protects children from severe forms of TB, but fails to protect adults. Taken together, there is a growing need to conduct basic and applied research to develop novel vaccine strategies against TB. This review is focused on the discussion surrounding current strategies and innovations being explored to discover new protective antigens, adjuvants, and delivery systems in the hopes of creating an efficacious TB vaccine.

An oncolytic adenovirus enhanced for toll-like receptor 9 stimulation increases antitumor immune responses and tumor clearance

Oncolytic viruses represent a multifaceted tool for cancer treatment. In addition to specific killing of cancer cells (oncolysis), these agents also provide danger signals prompting the immune system to stimulate an antitumor immune response. To increase adenovirus adjuvancy, we engineered the genome of Ad5D24 by inserting 18 immunostimulatory islands (Ad5D24-CpG). The toxicity and immunogenicity profile of Ad5D24-CpG showed that the safety of the maternal virus was retained. The efficacy of the CpG-enriched virus was assessed in a xenograft model of lung cancer where a significant increase in antitumor effect was seen in comparison with controls. When the experiment was repeated in animal depleted of natural killer (NK) cells, Ad5D24-CpG lost its advantage. The same was seen when Toll-like receptor (TLR)9 was blocked systemically. In a syngeneic model of melanoma (B16-OVA), we observed a significant increase of OVA-specific T cells and a decrease of activation of myeloid-derived suppressor cells in Ad5D24-CpG-treated mice. In conclusion, we have generated the first genetically modified oncolytic adenovirus backbone able to enhance TLR9-stimulation for increased antitumor activity.

Protective and therapeutic efficacy of Mycobacterium smegmatis expressing HBHA-hIL12 fusion protein against Mycobacterium tuberculosis in mice

Tuberculosis (TB) remains a major worldwide health problem. The only vaccine against TB, Mycobacterium bovis Bacille Calmette-Guerin (BCG), has demonstrated relatively low efficacy and does not provide satisfactory protection against the disease. More efficient vaccines and improved therapies are urgently needed to decrease the worldwide spread and burden of TB, and use of a viable, metabolizing mycobacteria vaccine may be a promising strategy against the disease. Here, we constructed a recombinant Mycobacterium smegmatis (rMS) strain expressing a fusion protein of heparin-binding hemagglutinin (HBHA) and human interleukin 12 (hIL-12). Immune responses induced by the rMS in mice and protection against Mycobacterium tuberculosis (MTB) were investigated. Administration of this novel rMS enhanced Th1-type cellular responses (IFN-γ and IL-2) in mice and reduced bacterial burden in lungs as well as that achieved by BCG vaccination. Meanwhile, the bacteria load in M. tuberculosis infected mice treated with the rMS vaccine also was significantly reduced. In conclusion, the rMS strain expressing the HBHA and human IL-12 fusion protein enhanced immunogencity by improving the Th1-type response against TB, and the protective effect was equivalent to that of the conventional BCG vaccine in mice. Furthermore, it could decrease bacterial load and alleviate histopathological damage in lungs of M. tuberculosis infected mice.

Anti-tuberculosis immunity induced in mice by vaccination with Mycobacterium smegmatis over-expressing Antigen 85B is due to the increased influx of IFNgamma-positive CD4 T cells into the lungs

BCG vaccine is unsafe for use in patients with AIDS. Mycobacterium smegmatis (Msm), an avirulent species unlike virulent Mycobacterium tuberculosis (H37Rv, Mtb) has been used as a carrier vaccine with ambiguous results due to the elicitation of poor immune responses to antigens in mice. In this study, we over-expressed the immunodominant antigen 85B in M. smegmatis (Msm-OEAg85B) and compared the immunogenicity of Msm-OEAg85B with that of wild-type Msm. Mice which were vaccinated with either Msm or Msm-OEAg85B and challenged 2 weeks later with Mtb. Vaccine-induced protection and lung T cell responses were evaluated post vaccination and post challenge. Unlike wild-type Msm that elicited minimal T cell responses in mice, MsmOE-Ag85B induced enhanced CD4+IFNgamma+ T cell responses that leveled off over 2 weeks. After virulent challenge at 2 weeks, Mtb grew progressively in the lungs of naive mice and mice vaccinated with wild-type Msm, but showed reduced growth (<0.6 log(10)) and therefore protection in Msm-OEAg85B-vaccinated mice. Lungs of Msm-OEAg85B-vaccinated mice showed increased numbers of CD4+IFNgamma+ T cells suggesting that the reduced bacterial growth was likely due to the enhanced T cell response in lungs. Since wild-type Msm was unable to protect but Msm-OEAg85B was, we suggest that Msm can be genetically manipulated to over-express selected Mtb antigens, thereby paving the way for safer vaccines that can be used in immunodeficient patients.

Recombinant Mycobacterium smegmatis vaccine candidates

Mycobacterium smegmatis is a species of rapidly growing saprophytes with a number of properties that make it an effective vaccine vector. Recombinant M. smegmatis expressing protective antigens of different pathogens and molecules modulating the immune responses offers some potential for reduction of the burden of tuberculosis, HIV and hepatitis B infections. This paper discusses the molecular methods used to generate recombinant M. smegmatis and the results obtained with some of these recombinants.

Vaccination with Human Induced Pluripotent Stem Cells Creates an Antigen-Specific Immune Response Against HIV-1 gp160

Induced pluripotent stem cells (iPSCs) are artificially derived from somatic cells that have been transduced with defined reprogramming factors. A previous report has indicated the possibility of using iPSCs as an immune stimulator to generate antigen-specific immunity. In our current study, we have investigated whether human iPSCs (hiPSCs) have the ability to enhance specific immune response against a human immunodeficiency virus type 1 (HIV-1) antigen in a xenogenic mouse model. Our results show that BALB/c mice immunized with hiPSCs transduced with an adenoviral vector encoding HIV-1 gp160 exhibited prominent antigen-specific cellular immune responses. We further found that pre-treatment of hiPSCs with ionizing radiation promotes the secretion of pro-inflammatory cytokines such as interleukin-1 alpha (IL-1α), IL-12, and IL-18. These cytokines might promote the activation of antigen-presenting cells and the effective induction of cellular immunity. Our present findings thus demonstrate that a hiPSCs-based vaccine has the potential to generate cellular immunity against viral antigens such as HIV-1 gp160 in a xenogenic condition.

Novel recombinant Mycobacterium bovis BCG, ovine atadenovirus, and modified vaccinia virus Ankara vaccines combine to induce robust human immunodeficiency virus-specific CD4 and CD8 T-cell responses in rhesus macaques

Mycobacterium bovis bacillus Calmette-Guérin (BCG), which elicits a degree of protective immunity against tuberculosis, is the most widely used vaccine in the world. Due to its persistence and immunogenicity, BCG has been proposed as a vector for vaccines against other infections, including HIV-1. BCG has a very good safety record, although it can cause disseminated disease in immunocompromised individuals. Here, we constructed a recombinant BCG vector expressing HIV-1 clade A-derived immunogen HIVA using the recently described safer and more immunogenic BCG strain AERAS-401 as the parental mycobacterium. Using routine ex vivo T-cell assays, BCG.HIVA(401) as a stand-alone vaccine induced undetectable and weak CD8 T-cell responses in BALB/c mice and rhesus macaques, respectively. However, when BCG.HIVA(401) was used as a priming component in heterologous vaccination regimens together with recombinant modified vaccinia virus Ankara-vectored MVA.HIVA and ovine atadenovirus-vectored OAdV.HIVA vaccines, robust HIV-1-specific T-cell responses were elicited. These high-frequency T-cell responses were broadly directed and capable of proliferation in response to recall antigen. Furthermore, multiple antigen-specific T-cell clonotypes were efficiently recruited into the memory pool. These desirable features are thought to be associated with good control of HIV-1 infection. In addition, strong and persistent T-cell responses specific for the BCG-derived purified protein derivative (PPD) antigen were induced. This work is the first demonstration of immunogenicity for two novel vaccine vectors and the corresponding candidate HIV-1 vaccines BCG.HIVA(401) and OAdV.HIVA in nonhuman primates. These results strongly support their further exploration.

Non-classical natural killer T cells modulate plasmid DNA vaccine antigen expression and vaccine-elicited immune responses by MCP-1 secretion after interaction with a beta2-microglobulin-independent CD1d

The magnitude and durability of a plasmid DNA vaccine-induced immune response is shaped by immune effector molecules at the site of vaccination. In the present study, we show that antigen expression is modified by type II NKT cells, after interaction with a beta2-microglobulin-independent CD1d receptor. After activation, during the first days following plasmid DNA vaccination, NKT cells release IL-5 and MCP-1, leading to a T helper 0 (T(H)0) cytokine/chemokine profile and a stronger CD8(+)/CD4(+) T cell immune response. Our data indicate that this phenomenon was induced through the strong T(H)1 chemokine MCP-1 during the early phases of plasmid DNA vaccination because injecting the type II NKT cell-associated MCP-1 during the first 5 days led to 2-3-fold increases in vaccine-elicited T cell responses. This study demonstrates a critical role for NKT cells in plasmid DNA vaccine-induced immune responses. Manipulation of NKT cell function or co-administration of MCP-1 may represent novel methods for enhancing immune responses to plasmid DNA vaccines.

Obstacles to the successful development of an efficacious T cell-inducing HIV-1 vaccine

An efficacious vaccine to HIV-1 is direly needed to stem the global pandemic. Immunogens that elicit broadly cross-neutralizing antibodies to HIV-1 remain elusive, and thus, most HIV-1 vaccine efforts are focusing on induction of T cells. The notion that T cells can mediate protection against HIV-1 has been called into question by the failure of the STEP trial, which was designed to test this concept by the use of an E1-deleted Ad vaccine carrier. Lack of efficacy of the STEP trial vaccine underscores our limited knowledge about correlates of immune protection against HIV-1 and stresses the need for an enhanced commitment to basic research, including preclinical and clinical vaccine studies. In this review, we discuss known correlates of protection against HIV-1 and different vaccine strategies that have been or are being explored to induce such correlates, focusing on T cell-inducing vaccines and particularly on Ad vectors.

A prime-boost immunisation regimen using recombinant BCG and Pr55(gag) virus-like particle vaccines based on HIV type 1 subtype C successfully elicits Gag-specific responses in baboons

Mycobacterium bovis BCG is considered an attractive live bacterial vaccine vector. In this study, we investigated the immune response of baboons to a primary vaccination with recombinant BCG (rBCG) constructs expressing the gag gene from a South African HIV-1 subtype C isolate, and a boost with HIV-1 subtype C Pr55(gag) virus-like particles (Gag VLPs). Using an interferon enzyme-linked immunospot assay, we show that although these rBCG induced only a weak or an undetectable HIV-1 Gag-specific response on their own, they efficiently primed for a Gag VLP boost, which strengthened and broadened the immune responses. These responses were predominantly CD8+ T cell-mediated and recognised similar epitopes as those targeted by humans with early HIV-1 subtype C infection. In addition, a Gag-specific humoral response was elicited. These data support the development of HIV-1 vaccines based on rBCG and Pr55(gag) VLPs.

Recombinant Mycobacterium bovis BCG prime-recombinant adenovirus boost vaccination in rhesus monkeys elicits robust polyfunctional simian immunodeficiency virus-specific T-cell responses

While mycobacteria have been proposed as vaccine vectors because of their persistence and safety, little has been done systematically to optimize their immunogenicity in nonhuman primates. We successfully generated recombinant Mycobacterium bovis BCG (rBCG) expressing simian immunodeficiency virus (SIV) Gag and Pol as multigenic, nonintegrating vectors, but rBCG-expressing SIV Env was unstable. A dose and route determination study in rhesus monkeys revealed that intramuscular administration of rBCG was associated with local reactogenicity, whereas intravenous and intradermal administration of 10(6) to 10(8) CFU of rBCG was well tolerated. After single or repeat rBCG inoculations, monkeys developed high-frequency gamma interferon enzyme-linked immunospot responses against BCG purified protein derivative. However, the same animals developed only modest SIV-specific CD8(+) T-cell responses. Nevertheless, high-frequency SIV-specific cellular responses were observed in the rBCG-primed monkeys after boosting with recombinant adenovirus 5 (rAd5) expressing the SIV antigens. These cellular responses were of greater magnitude and more persistent than those generated after vaccination with rAd5 alone. The vaccine-elicited cellular responses were predominantly polyfunctional CD8(+) T cells. These findings support the further exploration of mycobacteria as priming vaccine vectors.

CD4+ T lymphocytes mediate in vivo clearance of plasmid DNA vaccine antigen expression and potentiate CD8+ T-cell immune responses

There is evidence that the limited immunogenicity of plasmid DNA vaccines is the result, at least in part, of the rapid clearance of vaccine antigen expression by antigen-specific immune responses. However, the cell types responsible for the clearance of plasmid DNA vaccine antigens are not known. Here we demonstrate that macrophages, NK cells, and CD8(+) T cells did not significantly contribute to the DNA antigen clearance but CD4(+) T cells played the crucial role in attenuating plasmid DNA vaccine antigen expression. Adoptive transfer experiments demonstrate that CD4(+) T cells facilitated DNA vaccine antigen clearance in a Fas/FasL-dependent manner. Furthermore, we show that depletion of CD4(+) T cells prevented the clearance of vaccine antigen and the appearance of a CD8(+) T-cell immune response. Inoculation of major histocompatibility complex class II KO mice with the plasmid DNA led to persistent antigen expression and abolition of a CD8(+) T-cell immune response. Importantly, the prolongation of antigen expression by disrupting the CD4(+) T-cell Fas/FasL myocytes signaling led to a 3- to 5-fold increase of antigen-specific CD8(+) T-cell responses. These data demonstrate a dominant role of CD4(+) T cell-mediated cytotoxicity in plasmid DNA vaccine antigen clearance.

Memory CD8+ T cell responses expand when antigen presentation overcomes T cell self-regulation

Antimicrobial memory CD8+ T cell responses are not readily expanded by either repeated infections or immunizations. This is a major obstacle to the development of T cell vaccines. Prime-boost immunization with heterologous microbes sharing the same CD8+ epitope can induce a large expansion of the CD8+ response; however, different vectors vary greatly in their ability to boost for reasons that are poorly understood. To investigate how efficient memory T cell expansion can occur, we evaluated immune regulatory events and Ag presentation after secondary immunization with strong and weak boosting vectors. We found that dendritic cells were essential for T cell boosting and that Ag presentation by these cells was regulated by cognate memory CD8+ T cells. When weak boosting vectors were used for secondary immunization, pre-established CD8+ T cells were able to effectively curtail Ag presentation, resulting in limited CD8+ T cell expansion. In contrast, a strong boosting vector, vaccinia virus, induced highly efficient Ag presentation that overcame regulation by cognate T cells and induced large numbers of memory CD8+ T cells to expand. Thus, efficient targeting of Ag to dendritic cells in the face of cognate immunity is an important requirement for T cell expansion.

A multigene HIV type 1 subtype C modified vaccinia Ankara (MVA) vaccine efficiently boosts immune responses to a DNA vaccine in mice

Heterologous prime-boost vaccine strategies have generated high frequencies of antigen-specific T cells in preclinical and clinical trials of candidate HIV vaccines. We have developed a DNA (SAAVI DNA-C) and MVA (SAAVI MVA-C) vaccine based on HIV-1 subtype C for testing in clinical trials. Both vaccines contain five subtype C genes: gag, reverse transcriptase, tat, and nef, expressed as a polyprotein, and a truncated env (gp150). The individual vaccines induced CD8(+) and CD4(+) T cells specific for the vaccine-expressed antigens in BALB/c mice. Combining the vaccines in a DNA prime and MVA boost regimen increased the cumulative peptide response compared to the DNA vaccine alone 10-fold, to over 6000 SFU/10(6) splenocytes in the IFN-gamma ELISPOT assay. Th1 cytokine IFN-gamma and TNF-alpha levels from HIV-specific CD8(+) and CD4(+) T cells increased 20- and 8-fold, respectively, with a SAAVI MVA-C boost. Effector and effector memory RT- and Env-specific memory CD8(+) T cell subsets were boosted after MVA immunization, and over time the cells returned to an intermediate memory phenotype similar to that prior to the boost. Immunization of guinea pigs with the DNA-MVA combination induced high titers of antibodies to gp120, although neutralizing activity was weak or absent. The demonstration that these vaccines induce potent cellular immune responses merits their testing in clinical trials.

Genetic alteration of Mycobacterium smegmatis to improve mycobacterium-mediated transfer of plasmid DNA into mammalian cells and DNA immunization

Mycobacteria target and persist within phagocytic monocytes and are strong adjuvants, making them attractive candidate vectors for DNA vaccines. We characterized the ability of mycobacteria to deliver transgenes to mammalian cells and the effects of various bacterial chromosomal mutations on the efficiency of transfer in vivo and in vitro. First, we observed green fluorescent protein expression via microscopy and fluorescence-activated cell sorting analysis after infection of phagocytic and nonphagocytic cell lines by Mycobacterium smegmatis or M. bovis BCG harboring a plasmid encoding the fluorescence gene under the control of a eukaryotic promoter. Next, we compared the efficiencies of gene transfer using M. smegmatis or BCG containing chromosomal insertions or deletions that cause early lysis, hyperconjugation, or an increased plasmid copy number. We observed a significant-albeit only 1.7-fold-increase in the level of plasmid transfer to eukaryotic cells infected with M. smegmatis hyperconjugation mutants. M. smegmatis strains that overexpressed replication proteins (Rep) of pAL5000, a plasmid whose replicon is incorporated in many mycobacterial constructs, generated a 10-fold increase in plasmid copy number and 3.5-fold and 3-fold increases in gene transfer efficiency to HeLa cells and J774 cells, respectively. Although BCG strains overexpressing Rep could not be recovered, BCG harboring a plasmid with a copy-up mutation in oriM resulted in a threefold increase in gene transfer to J774 cells. Moreover, M. smegmatis strains overexpressing Rep enhanced gene transfer in vivo compared with a wild-type control. Immunization of mice with mycobacteria harboring a plasmid (pgp120(h)(E)) encoding human immunodeficiency virus gp120 elicited gp120-specific CD8 T-cell responses among splenocytes and peripheral blood mononuclear cells that were up to twofold (P < 0.05) and threefold (P < 0.001) higher, respectively, in strains supporting higher copy numbers. The magnitude of these responses was approximately one-half of that observed after intramuscular immunization with pgp120(h)(E). M. smegmatis and other nonpathogenic mycobacteria are promising candidate vectors for DNA vaccine delivery.

Plasmid DNA vaccine-elicited cellular immune responses limit in vivo vaccine antigen expression through Fas-mediated apoptosis

Particularly potent cellular or humoral immune responses are needed to confer protection in animal models against such pathogens as HIV/SIV, Mycobacterium tuberculosis, and malarial parasites. Persistent, high-level vaccine Ag expression may be required for eliciting such potent and durable immune responses. Although plasmid DNA immunogens are being explored as potential vaccines for protection against these pathogens, little is known about host factors that restrict long-term plasmid DNA vaccine Ag expression in vivo. We observed rapid damping of transgene expression from a plasmid DNA immunogen in wild-type, but not in T cell-deficient mice. This damping of Ag expression was temporally associated with the emergence of Ag-specific cellular immune responses. A requirement for Fas and the appearance of apoptotic nuclei at the site of vaccine inoculation suggest that T cells induce Fas-mediated apoptosis of plasmid DNA vaccine Ag-expressing cells. These studies demonstrate that high levels of in vivo Ag expression are associated with high-frequency cellular immune responses that in turn rapidly down-regulate vaccine Ag expression in vivo. These findings argue that it may not be possible to maintain persistent, high-level production of vaccine Ag in vivo to drive persistent immune responses as long as vaccine Ag production can be limited by host immune responses.