An anthrax subunit vaccine candidate based on protective regions of Bacillus anthracis protective antigen and lethal factor

Studies have confirmed the key role of Bacillus anthracis protective antigen (PA) in the US and UK human anthrax vaccines. However, given the tripartite nature of the toxin, other components, including lethal factor (LF), are also likely to contribute to protection. We examined the antibody and T cell responses to PA and LF in human volunteers immunized with the UK anthrax vaccine (AVP). Individual LF domains were assessed for immunogenicity in mice when given alone or with PA. Based on the results obtained, a novel fusion protein comprising D1 of LF and the host cell-binding domain of PA (D4) was assessed for protective efficacy. Murine protection studies demonstrated that both full-length LF and D1 of LF conferred complete protection against a lethal intraperitoneal challenge with B. anthracis STI spores. Subsequent studies with the LFD1-PAD4 fusion protein showed a similar level of protection. LF is immunogenic in humans and is likely to contribute to the protection stimulated by AVP. A single vaccine comprising protective regions from LF and PA would simplify production and confer a broader spectrum of protection than that seen with PA alone.

Repertoire of HLA-DR1-restricted CD4 T cell responses to capsular Caf1 antigen of Y. pestis HLA-transgenic mice

Yersinia pestis is the causative agent of plague, a rapidly fatal infectious disease that has not been eradicated worldwide. The capsular Caf1 protein of Y. pestis is a protective antigen under development as a recombinant vaccine. However, little is known about the specificity of human T cell responses for Caf1. We characterized CD4 T cell epitopes of Caf1 in 'humanized'-HLA-DR1 transgenic mice lacking endogenous MHC class II molecules. Mice were immunized with Caf1 or each of a complete set of overlapping synthetic peptides, and CD4 T cell immunity was measured with respect to proliferative and IFNgamma T cell responses and recognition by a panel of T cell hybridomas, as well as direct determination of binding affinities of Caf1 peptides to purified HLA-DR molecules. Although a number of DR1-restricted epitopes were identified following Caf1 immunization, the response was biased towards a single immunodominant epitope near the C-terminus of Caf1. In addition, potential promiscuous epitopes, including the immunodominant epitope, were identified by their ability to bind multiple common HLA alleles, with implications for the generation of multivalent vaccines against plague for use in humans.

Anthrax lethal toxin and the induction of CD4 T cell immunity

Bacillus anthracis secretes exotoxins which act through several mechanisms including those that can subvert adaptive immunity with respect both to antigen presenting cell and T cell function. The combination of Protective Antigen (PA) and Lethal Factor (LF) forming Lethal Toxin (LT), acts within host cells to down-regulate the mitogen activated protein kinase (MAPK) signaling cascade. Until recently the MAPK kinases were the only known substrate for LT; over the past few years it has become evident that LT also cleaves Nlrp1, leading to inflammasome activation and macrophage death. The predicted downstream consequences of subverting these important cellular pathways are impaired antigen presentation and adaptive immunity. In contrast to this, recent work has indicated that robust memory T cell responses to B. anthracis antigens can be identified following natural anthrax infection. We discuss how LT affects the adaptive immune response and specifically the identification of B. anthracis epitopes that are both immunogenic and protective with the potential for inclusion in protein sub-unit based vaccines.

Anthrax Lethal Factor as an Immune Target in Humans and Transgenic Mice and the Impact of HLA Polymorphism on CD4+ T Cell Immunity

Bacillus anthracis produces a binary toxin composed of protective antigen (PA) and one of two subunits, lethal factor (LF) or edema factor (EF). Most studies have concentrated on induction of toxin-specific antibodies as the correlate of protective immunity, in contrast to which understanding of cellular immunity to these toxins and its impact on infection is limited. We characterized CD4+ T cell immunity to LF in a panel of humanized HLA-DR and DQ transgenic mice and in naturally exposed patients. As the variation in antigen presentation governed by HLA polymorphism has a major impact on protective immunity to specific epitopes, we examined relative binding affinities of LF peptides to purified HLA class II molecules, identifying those regions likely to be of broad applicability to human immune studies through their ability to bind multiple alleles. Transgenics differing only in their expression of human HLA class II alleles showed a marked hierarchy of immunity to LF. Immunogenicity in HLA transgenics was primarily restricted to epitopes from domains II and IV of LF and promiscuous, dominant epitopes, common to all HLA types, were identified in domain II. The relevance of this model was further demonstrated by the fact that a number of the immunodominant epitopes identified in mice were recognized by T cells from humans previously infected with cutaneous anthrax and from vaccinated individuals. The ability of the identified epitopes to confer protective immunity was demonstrated by lethal anthrax challenge of HLA transgenic mice immunized with a peptide subunit vaccine comprising the immunodominant epitopes that we identified.

Exposure to anthrax toxin alters human leucocyte expression of anthrax toxin receptor 1

Anthrax is a toxin-mediated disease, the lethal effects of which are initiated by the binding of protective antigen (PA) with one of three reported cell surface toxin receptors (ANTXR). Receptor binding has been shown to influence host susceptibility to the toxins. Despite this crucial role for ANTXR in the outcome of disease, and the reported immunomodulatory consequence of the anthrax toxins during infection, little is known about ANTXR expression on human leucocytes. We characterized the expression levels of ANTXR1 (TEM8) on human leucocytes using flow cytometry. In order to assess the effect of prior toxin exposure on ANTXR1 expression levels, leucocytes from individuals with no known exposure, those exposed to toxin through vaccination and convalescent individuals were analysed. Donors could be defined as either 'low' or 'high' expressers based on the percentage of ANTXR1-positive monocytes detected. Previous exposure to toxins appears to modulate ANTXR1 expression, exposure through active infection being associated with lower receptor expression. A significant correlation between low receptor expression and high anthrax toxin-specific interferon (IFN)-γ responses was observed in previously infected individuals. We propose that there is an attenuation of ANTXR1 expression post-infection which may be a protective mechanism that has evolved to prevent reinfection.

Innate immunogenicity and in vitro protective potential of Schistosoma mansoni lung schistosomula excretory-secretory candidate vaccine antigens

Excretory secretory products (ESP) of Schistosoma mansoni developing larvae are ideal potential vaccines as such molecules may readily induce host primary immune responses, and local memory immune response effectors that would target, surround, and pursue the larvae while negotiating the lung blood capillaries. We herein characterized the cytokines response ESP, e.g., SG3PDH, 14-3-3-like protein, TPX, and calpain induce in the natural context of infection, and defined the global cytokine profile conducive to effective schistosome larvae killing. Accordingly, spleen cells (SC) taken from naive, and 7-, or 9-day S. mansoni-infected mice were stimulated in vitro with the selected ESP, in a recombinant or multiple antigen peptide (MAP) form, and examined for production of T helper type (Th) 1, Th2, and Th17 cytokines, and the ability to mediate in vitro attrition of lung-stage schistosomula. The study indicated that larval ESP principally elicit Th1 and Th17 type cytokines. Recombinant SG3PDH was the only test ESP to additionally activate SC from S. mansoni-infected BALB/c mice to release higher IL-4 levels than unstimulated SC and mediate significant (P

Dissolving Microneedle Delivery of Nanoparticle Encapsulated Antigen Elicits Efficient Cross-Priming and Th1 Immune Responses by Murine Langerhans Cells

Dendritic cells (DCs) of the skin play an important role in skin-mediated immunity capable of promoting potent immune responses. We availed of polymeric dissolving microneedle (MN) arrays laden with nano-encapsulated antigen to specifically target skin DC networks. This modality of immunization represents an economic, efficient and potent means of antigen delivery directly to skin DCs, which are inefficiently targeted by more conventional immunization routes. Following MN immunization, Langerhans cells (LCs) constituted the major skin DC subset capable of cross-priming antigen-specific CD8(+) T cells ex-vivo. While all DC subsets were equally efficient in priming CD4(+) T cells, LCs were largely responsible for orchestrating the differentiation of CD4(+) IFN-γ and IL-17 producing effectors. Importantly, depletion of LCs prior to immunization had a profound effect on CD8(+) CTL responses in vivo, and vaccinated animals displayed reduced protective anti-tumour and viral immunity. Interestingly, this cross-priming bias was lost following MN immunization with soluble antigen, suggesting that processing and cross-presentation of nano-particulate antigen is favoured by LCs. Therefore, these studies highlight the importance of LCs in skin immunization strategies and that targeting of nano-particulate immunogens through dissolvable polymeric MNs potentially provides a promising technological platform for improved vaccination strategies.Journal of Investigative Dermatology accepted article preview online, 22 September 2014. doi:10.1038/jid.2014.415.