Improved Vaccination Strategies Research Articles

Evaluation of the Safety and Immunogenicity of a Candidate Tuberculosis Vaccine, MVA85A, Delivered by Aerosol to the Lungs of Macaques

Tuberculosis (TB) is a reemerging disease. The only available vaccine, Mycobacterium bovis BCG, is delivered intradermally and confers highly variable efficacy against pulmonary disease. There is an urgent need for improved vaccination strategies. Murine studies suggest that immunizations delivered directly to the respiratory mucosa might be a more effective route of vaccination. This study compared the immunogenicity of a leading candidate tuberculosis (TB) vaccine, modified vaccinia virus Ankara expressing antigen 85A (MVA85A), in rhesus macaques, delivered either as an aerosol or as an intradermal boost immunization 12 weeks after an intradermal BCG prime vaccine. Aerosol vaccination was well tolerated. MVA85A delivered by aerosol or by intradermal injection induced antigen-specific immune responses in the periphery and the lung, with a trend toward the highest response when the compartment and route of delivery were matched. The ability of poxvirus-vectored vaccines delivered by the systemic route to induce responses in the mucosal immune compartment in macaques is in contrast to the independent compartmentalization of mucosal and systemic immune systems described in mice. Unlike intradermal vaccination, aerosol vaccination did not induce a detectable serum anti-vector antibody response. The delivery of vaccines to the lungs might provide an immunization strategy that limits the induction of systemic anti-vector immunity, which would be extremely useful in the development of improved vaccine strategies. This is the first study to show a recombinant MVA-vectored vaccine to be highly immunogenic when delivered by the aerosol route to nonhuman primates. These results provide important safety and proof-of-concept data for further evaluation of this route of immunization for use in human clinical trials.

Longitudinal Changes in CD4+ T-Cell Memory Responses Induced by BCG Vaccination of Newborns

Improved vaccination strategies against tuberculosis are needed, such as approaches to boost immunity induced by the current vaccine, BCG. Design of these strategies has been hampered by a lack of knowledge of the kinetics of the human host response induced by neonatal BCG vaccination. Furthermore, the functional and phenotypic attributes of BCG-induced long-lived memory T-cell responses remain unclear. We assessed the longitudinal CD4 T-cell response following BCG vaccination of human newborns. The kinetics, function, and phenotype of these cells were measured using flow cytometric whole-blood assays. We showed that the BCG-specific CD4 T-cell response peaked 6-10 weeks after vaccination and gradually waned over the first year of life. Highly activated T-helper 1 cells, predominantly expressing interferon γ, tumor necrosis factor α, and/or interleukin 2, were present at the peak response. Following contraction, BCG-specific CD4 T cells expressed high levels of Bcl-2 and displayed a predominant CD45RACCR7 central memory phenotype. However, cytokine and cytotoxic marker expression by these cells was more characteristic of effector memory cells. Our findings suggest that boosting of BCG-primed CD4 T cells with heterologous tuberculosis vaccines may be best after 14 weeks of age, once an established memory response has developed.

H9N2 avian influenza virus in Korea: evolution and vaccination

Low pathogenic avian influenza (LPAI) H9N2 viruses have been circulating in the Eurasian poultry industry resulting in great economic losses due to declined egg production and moderate to high mortality. In Korea, H9N2 LPAI was first documented in 1996 and it caused serious economic loss in the Korean poultry industry, including layer and broiler breeder farms. Since then, the H9N2 viruses that belong to the Korea group have been prevalent in chickens and have continuously evolved through reassortment in live bird markets. To control LPAI outbreaks, since 2007, the Korean veterinary authority has permitted the use of the inactivated oil adjuvant H9N2 LPAI vaccine. Although only oil-based inactivated vaccine using the egg-passaged vaccine virus strain (A/chicken/Korea/01310/2001) is permitted and used, several new technology vaccines have been recently suggested for the development of cost-effective and highly immunogenic vaccines. In addition, several different differentiation of infected from vaccinated animals (DIVA) strategies have been suggested using appropriate vaccines and companion serologic tests for discriminating between naturally infected and vaccinated animals. Recent reports demonstrated that the Korean LPAI H9N2 virus underwent antigenic drift and evolved into distinct antigenic groups and thus could escape from vaccine protection. Therefore, improved vaccination strategies including periodic updates of vaccine seed strains are required to achieve efficient control and eradication of LPAI H9N2 in Korea. Further, vaccination should be part of an overall integrated strategy to control the disease, including continued nation-wide surveillance, farm biosecurity, and DIVA strategy.

Host Responses to Persistent Mycobacterium avium Subspecies paratuberculosis Infection in Surgically Isolated Bovine Ileal Segments

A lack of appropriate disease models has limited our understanding of the pathogenesis of persistent enteric infections with Mycobacterium avium subsp. paratuberculosis. A model was developed for the controlled delivery of a defined dose of M. avium subsp. paratuberculosis to surgically isolated ileal segments in newborn calves. The stable intestinal segments enabled the characterization of host responses to persistent M. avium subsp. paratuberculosis infections after a 9-month period, including an analysis of local mucosal immune responses relative to an adjacent uninfected intestinal compartment. M. avium subsp. paratuberculosis remained localized at the initial site of intestinal infection and was not detected by PCR in the mesenteric lymph node. M. avium subsp. paratuberculosis-specific T cell proliferative responses included both CD4 and γδ T cell receptor (γδTcR) T cell responses in the draining mesenteric lymph node. The levels of CD8(+) and γδTcR(+) T cells increased significantly (P < 0.05) in the lamina propria, and M. avium subsp. paratuberculosis-specific tumor necrosis factor alpha (TNF-α) and gamma interferon secretion by lamina propria leukocytes was also significantly (P < 0.05) increased. There was a significant (P < 0.05) accumulation of macrophages and dendritic cells (DCs) in the lamina propria, but the expression of mucosal toll-like receptors 1 through 10 was not significantly changed by M. avium subsp. paratuberculosis infection. In conclusion, surgically isolated ileal segments provided a model system for the establishment of a persistent and localized enteric M. avium subsp. paratuberculosis infection in cattle and facilitated the analysis of M. avium subsp. paratuberculosis-specific changes in mucosal leukocyte phenotype and function. The accumulation of DC subpopulations in the lamina propria suggests that further investigation of mucosal DCs may provide insight into host responses to M. avium subsp. paratuberculosis infection and improve vaccine strategies to prevent M. avium subsp. paratuberculosis infection.

Wilms Tumor Protein-1-Derived 9-Mer Peptide Induces CD4 T-Cell Responses in an HLA-DR Restricted Manner

AbstractAbstract 4351Wilms‘ tumor protein-1 (WT1) is one of the most investigated tumor-associated antigens (TAA) in hematological malignancies. CD8 T-cell responses against several WT1-derived peptides have been characterized and are known to contribute to disease control after allogeneic hematopoietic stem cell transplantation (HSCT). Also the identification of human leukocyte antigen (HLA) class II-restricted CD4 T-cell epitopes from WT1 is a challenging task of T-cell-based cancer immunotherapy to improve the effectiveness of WT1 peptide vaccination.We found a highly immunogenic WT1 peptide composed of only 9 amino acids having the ability to induce IFN-γ secretion in CD4 T-cells in an HLA DR-restricted manner. This finding is of great interest as it was generally accepted that HLA class II binding peptides are composed of at least 12 amino acids being recognized by CD4 T-cells, whereas HLA class I binding peptides are composed of 8–11 amino acids being recognized by CD8 T-cells (Wang et al Mol. Immunol. 2002). However, both HLA class I and class II molecules bind to primary and secondary peptide anchor motifs covering the central 9–10 amino acids. Thus, considering this common structural basis for peptide binding there is a possibility that the WT1 9-mer peptide binds to HLA class II molecules, and induces CD4 T-cell responses.IFN-γ induction in response to several WT1 9-mer peptides was screened in 24 HLA-A*02:01 positive patients with acute myeloid leukemia or myelodysplastic syndrome after allogeneic HSCT. Responses to one WT1 9-mer peptide were exclusively detected in CD3+CD4+ T-cells of 2 patients after allogeneic HSCT, but not in CD3+CD4+ T-cells of their corresponding HSC donors. CD4+ T-cell responses to this WT1 9-mer peptide exhibited high levels of functional avidity, as IFN-γ induction was detected after stimulation with 100 ng peptide per mL. Peptide-induced IFN-γ production was confirmed with IFN-γ ELISPOT assays and the HLA restriction of the T-cell response was determined by HLA blocking antibodies. The reaction was significantly blocked by anti-pan HLA class II antibody (85 % reduction), but neither by pan-HLA class I nor by anti-HLA A2 antibody. To identify the subtype of HLA class II molecule, blocking assays with antibodies against HLA-DP, HLA-DR and HLA-DQ were performed. IFN-γ induction was completely abrogated by anti-HLA-DR antibody (99 % reduction) (fig 1, p value of unpaired student‘s t-test <0.0001 for the medium control vs anti-pan HLA class II antibody or anti-HLA-DR antibody, respectively). To test whether IFN-γ was exclusively induced in CD4 T cells, CD4 or CD8 T-cells were depleted from PBMC. Whereas CD8 T-cell depletion did not affect IFN-γ induction, CD4 T-cell depletion completely abrogated the WT1 9-mer peptide induced response (fig 2). CD4 T-cells responding to the WT1 9-mer peptide were indicated to be functional cytotoxic T-cells with an effector CD4 T-cell phenotype. Longitudinal analyses demonstrated the persistence and functionality of WT1 9-mer specific CD4 T-cells in PBMC of patients even at day 1368 after allogeneic HSCT.These data indicate for the first time that a TAA-derived 9-mer peptide can induce HLA class II-restricted CD4 T-cell responses. Vaccination with the characterized WT1 9-mer peptide can enhance the induction and maintenance of not only CD4 but also indirect CD8 T-cell responses. Considering that CD4 T-cells play an important role in tumor rejection, the possibility that other TAA-derived 9-mer peptides having the potential to induce CD4 T-cell responses should be explored in other settings of tumor immunology as well to improve vaccination strategies. [Display omitted] Disclosures:No relevant conflicts of interest to declare.

Candida albicans Infection Affords Protection against Reinfection via Functional Reprogramming of Monocytes

Immunological memory in vertebrates is often exclusively attributed to T and B cell function. Recently it was proposed that the enhanced and sustained innate immune responses following initial infectious exposure may also afford protection against reinfection. Testing this concept of "trained immunity," we show that mice lacking functional T and B lymphocytes are protected against reinfection with Candida albicans in a monocyte-dependent manner. C. albicans and fungal cell wall β-glucans induced functional reprogramming of monocytes, leading to enhanced cytokine production in vivo and in vitro. The training required the β-glucan receptor dectin-1 and the noncanonical Raf-1 pathway. Monocyte training by β-glucans was associated with stable changes in histone trimethylation at H3K4, which suggests the involvement of epigenetic mechanisms in this phenomenon. The functional reprogramming of monocytes, reminiscent of similar NK cell properties, supports the concept of "trained immunity" and may be employed for the design of improved vaccination strategies.

HIV-1 gp120 Vaccine Induces Affinity Maturation in both New and Persistent Antibody Clonal Lineages

Most antibodies that broadly neutralize HIV-1 are highly somatically mutated in antibody clonal lineages that persist over time. Here, we describe the analysis of human antibodies induced during an HIV-1 vaccine trial (GSK PRO HIV-002) that used the clade B envelope (Env) gp120 of clone W6.1D (gp120(W6.1D)). Using dual-color antigen-specific sorting, we isolated Env-specific human monoclonal antibodies (MAbs) and studied the clonal persistence of antibodies in the setting of HIV-1 Env vaccination. We found evidence of V(H) somatic mutation induced by the vaccine but only to a modest level (3.8% ± 0.5%; range 0 to 8.2%). Analysis of 34 HIV-1-reactive MAbs recovered over four immunizations revealed evidence of both sequential recruitment of naïve B cells and restimulation of previously recruited memory B cells. These recombinant antibodies recapitulated the anti-HIV-1 activity of participant serum including pseudovirus neutralization and antibody-dependent cell-mediated cytotoxicity (ADCC). One antibody (3491) demonstrated a change in specificity following somatic mutation with binding of the inferred unmutated ancestor to a linear C2 peptide while the mutated antibody reacted only with a conformational epitope in gp120 Env. Thus, gp120(W6.1D) was strongly immunogenic but over four immunizations induced levels of affinity maturation below that of broadly neutralizing MAbs. Improved vaccination strategies will be needed to drive persistent stimulation of antibody clonal lineages to induce affinity maturation that results in highly mutated HIV-1 Env-reactive antibodies.

Th17 cells mediate clade specific mucosal immunity (49.26)

Abstract The IL-17 family of cytokines phylogenetically pre-dates the evolution of T-cells in jawed vertebrates, suggesting that the ontogeny of the Th17 lineage must have arisen to confer an evolutionary advantage to the host over innate sources of IL-17. Using a model of mucosal immunization with the encapsulated bacteria, Klebsiella pneumoniae, we show that intranasal immunization strongly induces IL-17 producing cells in the lung. Further analysis reveals that these cells are conventional Th17 cells and are antigen specific as they proliferate and produce IL-17 upon re-stimulation with heat-killed K. pneumoniae but not heat-killed Escherichia coli or Staphylococcus aureus. In vivo, immunization induced Th17 cells but not Th1 cells undergo activation/expansion and contraction overtime and the Th17 responses together with anit-K. pneumoniae antibody responses lead to the protection of immunized mice from live K. pneumoniae infection. However, B-cell/antibody mediated protection is limited only to the vaccine strain. In contrast memory Th17 cells proliferate in response to conserved outer membrane proteins and confer protection against subsequent bacterial challenge with several serotypes of K. pneumoniae, including the recently described multi-drug resistant New Delhi metallolactamase strain. These data suggest that Th17 adjuvants can improve vaccination strategies against drug resistant encapsulated bacteria.

Adult Immunization 2012: Politics, Process, and Progress

In this issue, the ACIP presents the recommended Adult Immunization Schedule for 2012. The editorialist discusses the rationale behind key changes between this and previous ACIP recommendations.

CD40-Activated B Cells Can Efficiently Prime Antigen-Specific Naïve CD8+ T Cells to Generate Effector but Not Memory T cells

BackgroundThe identification of the signals that should be provided by antigen-presenting cells (APCs) to induce a CD8+ T cell response in vivo is essential to improve vaccination strategies using antigen-loaded APCs. Although dendritic cells have been extensively studied, the ability of other APC types, such as B cells, to induce a CD8+ T cell response have not been thoroughly evaluated.Methodology/Principal FindingsIn this manuscript, we have characterized the ability of CD40-activated B cells, stimulated or not with Toll-like receptor (TLR) agonists (CpG or lipopolysaccharide) to induce the response of mouse naïve CD8+ T cells in vivo. Our results show that CD40-activated B cells can directly present antigen to naïve CD8+ T cells to induce the generation of potent effectors able to secrete cytokines, kill target cells and control a Listeria monocytogenes infection. However, CD40-activated B cell immunization did not lead to the proper formation of CD8+ memory T cells and further maturation of CD40-activated B cells with TLR agonists did not promote the development of CD8+ memory T cells. Our results also suggest that inefficient generation of CD8+ memory T cells with CD40-activated B cell immunization is a consequence of reduced Bcl-6 expression by effectors and enhanced contraction of the CD8+ T cell response.ConclusionsUnderstanding why CD40-activated B cell immunization is defective for the generation of memory T cells and gaining new insights about signals that should be provided by APCs are key steps before translating the use of CD40-B cell for therapeutic vaccination.

In this issue

In this issue

Immunization of patients with autoimmune inflammatory rheumatic diseases (the EULAR recommendations)

The European League Against Rheumatism (EULAR) recommendations for vaccination in adult patients with autoimmune inflammatory rheumatic diseases (AIIRD) have been recently published. These evidence-based recommendations were based on existing literature in combination with expert opinion. Although patients with AIIRD are at increased risk of suffering from (complicated) infectious diseases--and vaccination seems a tool to reduce this risk--still many questions and controversies remain for the individual patient. In this overview, taking influenza as an example, the background of the recommendations, their clinical implications, and the direction of future research are discussed. The increase in knowledge on vaccine-preventable infections will allow us to further improve vaccination strategies.

Host Defense Pathways Against Fungi: The Basis for Vaccines and Immunotherapy

Fungal vaccines have long been a goal in the fields of immunology and microbiology to counter the high mortality and morbidity rates owing to fungal diseases, particularly in immunocompromised patients. However, the design of effective vaccination formulations for durable protection to the different fungi has lagged behind due to the important differences among fungi and their biology and our limited understanding of the complex host–pathogen interactions and immune responses. Overcoming these challenges is expected to contribute to improved vaccination strategies aimed at personalized efficacy across distinct target patient populations. This likely requires the integration of multifaceted approaches encompassing advanced immunology, systems biology, immunogenetics, and bioinformatics in the fields of fungal and host biology and their reciprocal interactions.

Recombinant BCG ΔureC hly+ Induces Superior Protection Over Parental BCG by Stimulating a Balanced Combination of Type 1 and Type 17 Cytokine Responses

Background. New vaccines against tuberculosis (TB) are urgently needed because the only available vaccine, Mycobacterium bovis bacillus Calmette-Guérin (BCG), fails to protect against pulmonary TB in adults. The recombinant ΔureC hly+ BCG (rBCG) is more efficient than parental BCG (pBCG) against pulmonary TB in preclinical studies and has proven safe and immunogenic in phase I clinical trials.Methods. In an attempt to identify the mechanisms underlying the superior protection of rBCG, we compared the immune responses elicited after vaccination and subsequent aerosol infection with Mycobacterium tuberculosis (MTB) in mice.Results. We demonstrate that both rBCG and pBCG induce marked type 1 cytokine responses, whereas only rBCG elicits a profound type 17 cytokine response in addition. We observed earlier recruitment of antigen-specific T lymphocytes to the lung upon MTB infection of rBCG-vaccinated mice. These T cells produced abundant type 1 cytokines after restimulation, resulting in 10-fold reduced bacterial burden 90 days after infection.Conclusions. Our findings identify a general immunologic pathway for improved vaccination strategies against TB that can also be harnessed by other vaccine candidates.

Loss of anti-mycobacterial efficacy in mice over time following vaccination with Mycobacterium bovis bacillus Calmette-Guérin

Mycobacterium bovis bacillus Calmette-Guérin (BCG) is the most often used vaccine worldwide and sole vaccine against tuberculosis. BCG is protective against severe form of childhood tuberculosis but less or not protective to adult pulmonary tuberculosis. Therefore, improved vaccination strategies and development of new tuberculosis vaccines are urgent demands. For those purposes, appropriate animal models that reflect human are critically useful. However, in animal models, BCG vaccination protects well against subsequent challenge of Mycobacterium tuberculosis. In this study we evaluated the duration of protective efficacy of the BCG vaccination in mice over time and found that efficacy was diminished 40 weeks after vaccination. The aged mice older than 45 weeks are protected sufficiently after the vaccination with BCG, suggesting that loss of its efficacy is not dependent on the age of mice but rather depends on the period from vaccination. The loss of protection occurred in TH1 polarized STAT6 deficient mice despite the maintenance of interferon (IFN)-gamma production activity of lymph node cells and splenic CD4 + T cells against M. tuberculosis antigens. Our data suggest that the duration from vaccination may explain the variation in BCG efficacy against adult pulmonary tuberculosis.

Tuberculosis vaccines in clinical trials

Effective prophylactic and/or therapeutic vaccination is a key strategy for controlling the global TB epidemic. The partial effectiveness of the existing TB vaccine, bacille Calmette–Guérin (BCG), suggests effective vaccination is possible and highlights the need for an improved vaccination strategy. Clinical trials are evaluating both modifications to the existing BCG immunization methods and also novel TB vaccines, designed to replace or boost BCG. Candidate vaccines in clinical development include live mycobacterial vaccines designed to replace BCG, subunit vaccines designed to boost BCG and therapeutic vaccines designed as an adjunct to chemotherapy. There is a great need for validated animal models, identification of immunological biomarkers of protection and field sites with the capacity for large-scale efficacy testing in order to develop and license a novel TB vaccine or regimen.

Telomerase-specific GV1001 peptide vaccination fails to induce objective tumor response in patients with cutaneous T cell lymphoma

There is currently no curative therapy for cutaneous T cell lymphoma (CTCL). New therapies are therefore needed. Telomerase, the enzyme that allows for unrestricted cell divisions of cancer cells, is a promising target for cancer therapy. The telomerase-specific peptide vaccination GV1001 has shown promising results in previous studies. Since telomerase is expressed in malignant cells of CTCL, GV1001 vaccination in CTCL is a promising new therapeutic approach. We sought to investigate the efficacy of GV1001 vaccination in CTCL patients and characterize the induced immune response. Six CTCL patients were vaccinated with the GV-peptide using granulocyte/macrophage colony-stimulating factor as adjuvant. Objective clinical response and the T cell response were assessed. None of the patients demonstrated objective clinical response to the vaccination whereas one patient showed disease progression. 1/6 patients acquired a GV1001-specifc T cell response with a Th1 cytokine profile and expression of skin-homing receptors. This hTERT-specific T cell response was not associated with beneficial modulation of the tumor-infiltrating leukocytes. Furthermore, removal of regulatory T cells did not enhance responsiveness to GV1001 in vitro in any of the patients analyzed. Our results suggest that the GV1001 vaccination is not effective in CTCL patients and disease progression in 1/6 patients raises concerns about its safety. By analyzing skin-homing properties of GV1001-specific T cells and the involvement of regulatory T cells we nevertheless provide insight into vaccine-induced immune responses which may help to improve vaccine strategies in CTCL.

Increased Neutralizing Antibody Response after Simultaneous Immunization with Leucogen and the Feline Leukemia Virus Transmembrane Protein

To develop improved vaccination strategies against feline leukemia virus (FeLV), rats were immunized with the transmembrane envelope protein p15E of FeLV alone or in combination with the commercial vaccine Leucogen® comprising the nonglycosylated FeLV surface envelope protein. Binding and neutralizing antibodies were induced in both groups and in the group immunized with Leucogen alone. Higher titers of antibodies neutralizing FeLV were induced by simultaneous immunization with Leucogen and p15E compared to the responses using Leucogen or p15E alone, suggesting that combination vaccines should be used in the future. Epitope mapping of p15E-specific antibodies induced by simultaneous immunization with Leucogen and p15E revealed the same pattern of response as obtained after immunization with p15E alone: one epitope was localized in the membrane-proximal external region (MPER) and the other in the fusion peptide-proximal region, and they are related to the epitopes detected after immunization with p15E of the porcine endogenous retrovirus and the koala retrovirus. The data indicate that these epitopes in the MPER are an effective target for neutralization and that antigens containing them may therefore prove to be a useful component of vaccines against retroviruses, including HIV-1.

Essential role of the Wnt pathway effector Tcf-1 for the establishment of functional CD8 T cell memory

Immune protection from intracellular pathogens depends on the generation of terminally differentiated effector and of multipotent memory precursor CD8 T cells, which rapidly regenerate effector and memory cells during recurrent infection. The identification of factors and pathways involved in CD8 T cell differentiation is of obvious importance to improve vaccination strategies. Here, we show that mice lacking T cell factor 1 (Tcf-1), a nuclear effector of the canonical Wingless/Integration 1 (Wnt) signaling pathway, mount normal effector and effector memory CD8 T cell responses to infection with lymphocytic choriomeningitis virus (LCMV). However, Tcf-1-deficient CD8 T cells are selectively impaired in their ability to expand upon secondary challenge and to protect from recurrent virus infection. Tcf-1-deficient mice essentially lack CD8 memory precursor T cells, which is evident already at the peak of the primary response, suggesting that Tcf-1 programs CD8 memory cell fate. The function of Tcf-1 to establish CD8 T cell memory is dependent on the catenin-binding domain in Tcf-1 and requires the Tcf-1 coactivators and Wnt signaling intermediates beta-catenin and gamma-catenin. These findings demonstrate that the canonical Wnt signaling pathway plays an essential role for CD8 central memory T cell differentiation under physiological conditions in vivo. They raise the possibility that modulation of Wnt signaling may be exploited to improve the generation of CD8 memory T cells during vaccination or for therapies designed to promote sustained cytotoxic CD8 T cell responses against tumors.

Evaluation of Streptococcus iniae killed bacterin and live attenuated vaccines in hybrid striped bass through injection and bath immersion

Streptococcus iniae poses a serious threat to finfish aquaculture operations worldwide. Stringent regulatory standards limit the use of antibiotics to treat S. iniae infections; improved vaccination strategies are thus of great interest. We investigated the potential for efficient, non-injectable batch vaccination via the use of live attenuated vaccines. Three attenuated S. iniae strains with genetic mutations eliminating the production of virulence factors--capsular polysaccharide (delta cpsD), M-like protein (delta simA), and phosphoglucomutase (delta pgmA)--were evaluated in parallel with an adjuvanted, formalin-killed, whole-cell S. iniae bacterin. Juvenile hybrid striped bass (HSB; Morone chrysops x M. saxatilis) were vaccinated through intraperitoneal (i.p.) injection or bath immersion and held for 800 degree-days prior to challenge with a lethal dose of the virulent wild-type (WT) S. iniae parent strain. The delta cpsD, delta pgmA, and bacterin vaccines provided the highest level of vaccination safety (0% mortality), whereas the delta simA mutant, although it caused 12 to 16% vaccination-related mortality, was the only vaccine candidate to provide 100% protection in both i.p. and immersion delivery models. Our studies demonstrate the efficacy of live attenuated vaccines for prevention of S. iniae infection, and identify immersion delivery of live vaccines as an attractive option for use in commercial aquaculture settings.