Bacterial ghosts as adjuvants: mechanisms and potential


Abstract

Bacterial ghosts (BG) are empty cell envelopes derived from Gram-negative bacteria. They contain many innate immunostimulatory agonists, and are potent activators of a broad range of cell types involved in innate and adaptive immunity. Several considerable studies have demonstrated the effectiveness of BG as adjuvants as well as their ability to induce proinflammatory cytokine production by a range of immune and non-immune cell types. These proinflammatory cytokines trigger a generalized recruitment of T and B lymphocytes to lymph nodes that maximize the chances of encounter with their cognate antigen, and subsequent elicitation of potent immune responses. The plasticity of BG has allowed for the generation of envelope-bound foreign antigens in immunologically active forms that have proven to be effective vaccines in animal models. Besides their adjuvant property, BG also effectively deliver DNA-encoded antigens to dendritic cells, thereby leading to high transfection efficiencies, which subsequently result in higher gene expressions and improved immunogenicity of DNA-based vaccines. In this review, we summarize our understanding of BG interactions with the host immune system, their exploitation as an adjuvant and a delivery system, and address important areas of future research interest.

Figures

Figure?1
Figure?1
Scanning (S) and transmission (T) electron microscopies (EM) of BG. (I) SEM of a BG showing intact cellular morphology except for the presence of a transmembrane tunnel structure as indicated by an arrow. (II) TEM of a BG showing loss of cytoplasmic and nuclear contents.
Figure?2
Figure?2
Expression plasmids used for the synthesis of BG. (I) Gene E expression under the chemical inducer T7-lactose (lac) promoter operator (PO) system with the lac repressor (lacI) regulatory element. In this system, bacteria are allowed to grow until 0.3 OD600nm and then gene E is induced by the addition of a chemical inducer, IPTG (II) Gene E expression under the temperature sensitive lambda promoter (λpR) with the thermo-sensitive repressor c1857 regulatory element. In this system, bacteria are allowed to grow until 0.3 OD600nm and then gene E is induced by thermal shift to 42?°C. (III) Gene E expression under the λpR with dual c1857 and arabinose-inducible araC protein regulatory elements. The λpR promoter with the thermolabile repressor cI857 suppresses the lysis gene transcription under 28?°C for the normal growth of the bacterial cells. However, the λpR promoter system may be leaky leading to undesired expression of the lysis gene. In this system, the leaky expression of gene E at 28?°C is avoided by the anti-sense RNA of the lysis gene produced by the ParaBAD promoter in the presence of L-arabinose that binds to its complementary sense RNA of the lysis gene caused by the leaky λpR promoter.
Figure?3
Figure?3
Signal transduction by BG. BG activate immune and non-immune cells through TLR2 and TLR4 pathways, culminating in the production of a variety of proinflammatory cytokines, chemokines, and host defence genes via MyD88 and MyD88 independent signaling pathways. Signaling through TLR2 or TLR4 is the MyD88 dependent adapter molecule that passes the signal to MAPK and IkB cascades. These signaling cascades result in the production of NF-kB and AP-1 transcription factors which subsequently induce a variety of genes involved in innate and adaptive immunity. Signaling through TLR4 is also MyD88 independent and occurs via IRF3 pathway, which results in the production of NO and anti-viral cytokines including interferon-α and interferon-β.
Figure?4
Figure?4
How BG induce effective humoral and cell mediated immune (CMI) responses. BG activate immune (DC, macrophages, B and T cells) and non-immune cells (epithelial cells, fibroblasts, and keratinocytes) either through TLR2 or TLR4 pathways. The cumulative effect of the stimulation of these cell types results in the enhanced activation of T and B cells, which subsequently lead to the induction of efficient humoral and CMI responses. The presence of LPS in BG improves the antigen cross presenting ability of DC to CD8+ T cells, and thus helps in the elicitation of potent cytotoxic T cell responses. Moreover, the ability of BG to induce cytokine and chemokine production in a number of lymphoid and non-lymphoid cells results in the generalized recruitment of T, B and DC to lymph nodes that maximize the chances of encounter with their cognate antigen and development of effective immune responses subsequently.
Figure?5
Figure?5
BG have wide applications both in human and in veterinary fields.
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