New directions in nicotine vaccine design and use

Abstract

Clinical trials of nicotine vaccines suggest that they can enhance smoking cessation rates but do not reliably produce the consistently high serum antibody concentrations required. A wide array of next-generation strategies are being evaluated to enhance vaccine efficacy or provide antibody through other mechanisms. Protein conjugate vaccines may be improved by modifications of hapten or linker design or by optimizing hapten density. Conjugating hapten to viruslike particles or disrupted virus may allow exploitation of naturally occurring viral features associated with high immunogenicity. Conjugates that utilize different linker positions on nicotine can function as independent immunogens, so that using them in combination generates higher antibody concentrations than can be produced by a single immunogen. Nanoparticle vaccines, consisting of hapten, T cell help peptides, and adjuvants attached to a liposome or synthetic scaffold, are in the early stages of development. Nanoparticle vaccines offer the possibility of obtaining precise and consistent control of vaccine component stoichiometry and spacing and immunogen size and shape. Passive transfer of nicotine-specific monoclonal antibodies offers a greater control of antibody dose, the ability to give very high doses, and an immediate onset of action but is expensive and has a shorter duration of action than vaccines. Viral vector-mediated transfer of genes for antibody production can elicit high levels of antibody expression in animals and may present an alternative to vaccination or passive immunization if the long-term safety of this approach is confirmed. Next-generation immunotherapies are likely to be substantially more effective than first-generation vaccines.

Keywords: Addiction; Immunogen; Immunotherapy; Nicotine; Vaccine.

Figure 14.1

Key elements of conjugate vaccine interaction with immune cells that are relevant to nicotine vaccine design. For a more detailed account of humoral immunity, see McHeyzer-Williams and McHeyzer-Williams (2005). Humans have >10⁸ naive B cells bearing surface antibody (B cell receptor) with different specificities that are capable of binding a wide range of chemical structures, including small molecules such as nicotine. When a nicotine vaccine is administered, (A) the nicotine component (B cell epitope) of the conjugate vaccine binds to those naive B cells that have appropriate surface antibody, and the entire conjugate is internalized. Internalization is enhanced by having multiple nicotine molecules attached to each carrier protein so that many surface antibody interactions take place. The carrier protein is digested within the B cell and peptide fragments (T cell epitopes) are displayed on its surface in association with MHC class II molecules. It is the display of these T cell epitopes that will subsequently allow helper T cells that have encountered the same T cell epitopes to recognize and interact with nicotine-specific B cells. (B) Immunogen is phagocytized by antigen-presenting cells (APCs) such as dendritic cells. This is a nonspecific process (does not involve immunogen receptors) that can be facilitated by the presence of an adjuvant (not shown). Particle uptake is enhanced if it is an appropriate size. APCs digest the carrier protein and display T cell epitope peptides on their surface in association with MHC class II molecules. Only peptides (not nicotine) can serve as T cell epitopes. Some T cells bear surface receptors capable of binding the T cell epitopes presented by APCs. (C) T cell interaction with APCs allows them differentiate into T helper (Th) cells that can provide stimulatory signals to B cells. (D) Those Th cells bearing receptors that are specific for the particular T cell epitope derived from the nicotine immunogen recognize and interact with the correct subset of B cells (those capable of binding nicotine) because those B cells now bear the immunogen’s T cell epitope on their surface. It is this specific recognition of T cell epitopes displayed by nicotine-specific B cells that allows Th cells to interact selectively with these B cells. (E) Activated B cells undergo maturation into antibody-secreting cells and memory cells (not shown) that can be activated by subsequent booster doses of vaccine. Adapted from materials provided by Y. Chang.

Figure 14.2

Immunogen structures, not drawn to scale. (A) Nicotine, with positions commonly used for attachment of linkers indicated as R_1–4. (B) Conjugate vaccine. Nicotine is covalently attached to a carrier protein through a short linker that allows the nicotine to be accessible for binding to B cells. Some linkers are illustrated in Fig. 14.3A. A high density of nicotine hapten on the carrier protein facilitates its binding and uptake. Covalently linking nicotine to carrier assures that nicotine hapten and carrier protein will be taken into the same B cell. Conjugate immunogens are generally mixed with adjuvant to enhance antibody generation. (C) Nanoparticle vaccine: Synthetic scaffold (illustration is a DNA tetrahedron) (Liu et al., 2012). Nanoparticle scaffolds allow vaccine components to be covalently linked with readily controlled density and stoichiometry. T cell help can be provided by attachment of either whole protein or shorter peptide sequences. Some molecularly defined adjuvants, such as CpG oligonucleotides, can also be covalently linked to the nanoparticle. (D) Nanoparticle vaccine: Liposome or synthetic vesicle. Vaccine components such as hapten that require interactions with cell surface receptors can be attached to or embedded within the vesicle surface, while components intended for delivery to the cell interior can be encapsulated within.

Figure 14.3

Potential options for combining nicotine vaccines or using them in combination with medications. (A) Trivalent nicotine vaccine (de Villiers et al., 2013). Rats were vaccinated with three structurally distinct nicotine immunogens (linkers attached at different positions, carrier proteins; rEPA, recombinant exoprotein A; KLH, keyhole limpet hemocyanin; malKLH, maleimide activated KLH) or a dose-matched monovalent immunogen s.c. with alum adjuvant. After the last of three vaccine doses, rats received a single i.v. dose of 0.03 mg/kg nicotine. The trivalent immunogen elicited higher nicotine-specific antibody titers and concentrations than the monovalent vaccine (left) and reduced nicotine distribution to the brain to a greater extent (right). ^*p <0.05. (B) Combined use of the nicotine-specific mAb Nic311 and mec-amylamine to block the subjective effects of nicotine in rats trained in a two-lever nicotine discrimination assay. Each point is the mean (±SEM)% responding on the nicotine lever during consecutive daily test sessions with the 0.4 mg/kg nicotine training dose in each treatment group following administration of control antibody (IgG) and saline (open circles), Nic311 alone (open squares), ascending doses of MEC alone (solid circles), or both (solid squares). Dashed horizontal lines indicate criterion levels of performance for discrimination of the 0.4 mg/kg nicotine training dose. Significantly different from control IgG +saline, ^*p <0.05, ^***p <0.001. Significantly different from Nic311+saline, ##p <0.01, ###p <0.001 (LeSage et al., 2012).