Intestinal immunization of mice with antigen conjugated to anti-MHC class II antibodies

Targeted Delivery of Cyclotides via Conjugation to a Nanobody

Many naturally occurring peptides have poor proteolytic stability, which limits their therapeutic applications. Cyclotides are plant-derived cyclic peptides that resist proteolysis due to their highly constrained structure, comprising a head-to-tail cyclic backbone and three disulfide bonds that form a cystine-knotted core. This structure makes them useful as scaffolds onto which peptide sequences (epitopes) can be grafted. In this study, VHH7, an alpaca-derived nanobody that targets murine class II MHC molecules, was used for the targeted delivery of cyclotides to antigen-presenting cells (APCs). The cyclotides MCoTI-I, and MCoTI-I with a HA-tag (YPYDVPDYA) grafted into loop 6 (MCoTI-HA), were tested for immunogenic properties. To produce the requisite VHH7-peptide conjugates, a site-specific sortase A-catalyzed reaction in combination with a copper-free strain-promoted cycloaddition reaction was used. MCoTI-I alone did not display any obvious antibody response, thus showing the capacity of cyclotides as immunologically silent scaffolds. By contrast, MCoTI-I conjugated to VHH7 elicited antibodies against cyclic or linear MCoTI-I, thus suggesting a simple and robust approach for targeting cyclotides to APCs, and potentially to other cell types. A similar antibody response was observed when MCoTI-HA was conjugated to VHH7, but there was no reactivity toward a linear HA-tag itself, suggesting differences in conformational constraint between cyclotide-presented and linear epitopes. Studies of commercially available HA antibodies applied to MCoTI-HA confirmed that the conformation of peptide immunogens affects their reactivity. Thus, the production of antibodies that recognize constrained epitopes may benefit from engraftment onto scaffolds such as cyclotides. More broadly, this study validates that a prototypic cyclotide, a member of a peptide family that has proven to be useful as drug design scaffolds in many other studies, can efficiently reach a specific target in vivo.

Antigen Targeting to Human HLA Class II Molecules Increases Efficacy of DNA Vaccination

It has been difficult to translate promising results from DNA vaccination in mice to larger animals and humans. Previously, DNA vaccines encoding proteins that target Ag to MHC class II (MHC-II) molecules on APCs have been shown to induce rapid, enhanced, and long-lasting Ag-specific Ab titers in mice. In this study, we describe two novel DNA vaccines that as proteins target HLA class II (HLA-II) molecules. These vaccine proteins cross-react with MHC-II molecules in several species of larger mammals. When tested in ferrets and pigs, a single DNA delivery with low doses of the HLA-II-targeted vaccines resulted in rapid and increased Ab responses. Importantly, painless intradermal jet delivery of DNA was as effective as delivery by needle injection followed by electroporation. As an indication that the vaccines could also be useful for human application, HLA-II-targeted vaccine proteins were found to increase human CD4⁺ T cell responses by a factor of ×10³ in vitro. Thus, targeting of Ag to MHC-II molecules may represent an attractive strategy for increasing efficacy of DNA vaccines in larger animals and humans.

Efficient vaccine against pandemic influenza: combining DNA vaccination and targeted delivery to MHC class II molecules

There are two major limitations to vaccine preparedness in the event of devastating influenza pandemics: the time needed to generate a vaccine and rapid generation of sufficient amounts. DNA vaccination could represent a solution to these problems, but efficacy needs to be enhanced. In a separate line of research, it has been established that targeting of vaccine molecules to antigen-presenting cells enhances immune responses. We have combined the two principles by constructing DNA vaccines that encode bivalent fusion proteins; these target hemagglutinin to MHC class II molecules on antigen-presenting cells. Such DNA vaccines rapidly induce hemagglutinin-specific antibodies and T cell responses in immunized mice. Responses are long-lasting and protect mice against challenge with influenza virus. In a pandemic situation, targeted DNA vaccines could be produced and tested within a month. The novel DNA vaccines could represent a solution to pandemic preparedness in the advent of novel influenza pandemics.

DNA vaccine that targets hemagglutinin to MHC class II molecules rapidly induces antibody-mediated protection against influenza

New influenza A viruses with pandemic potential periodically emerge due to viral genomic reassortment. In the face of pandemic threats, production of conventional egg-based vaccines is time consuming and of limited capacity. We have developed in this study a novel DNA vaccine in which viral hemagglutinin (HA) is bivalently targeted to MHC class II (MHC II) molecules on APCs. Following DNA vaccination, transfected cells secreted vaccine proteins that bound MHC II on APCs and initiated adaptive immune responses. A single DNA immunization induced within 8 d protective levels of strain-specific Abs and also cross-reactive T cells. During the Mexican flu pandemic, a targeted DNA vaccine (HA from A/California/07/2009) was generated within 3 wk after the HA sequences were published online. These results suggest that MHC II-targeted DNA vaccines could play a role in situations of pandemic threats. The vaccine principle should be extendable to other infectious diseases.

Molecular basis of the mucosal immune system: from fundamental concepts to advances in liposome-based vaccines

The mucosal immune system, the primary portal for entry of most prevalent and devastating pathogens, is guarded by the special lymphoid tissues (mucosally associated lymphoid tissues) for immunity. Mucosal immune infection results in induction of IgA-manifested humoral immunity. Cell-mediated immunity may also be generated, marked by the presence of CD4+ Th1 and CD8+ cells. Furthermore, the immunity generated at the mucosal site is transported to the distal mucosal site as well as to systemic tissues. An understanding of the molecular basis of the mucosal immune system provides a unique platform for designing a mucosal vaccine. Coadministration of immunostimulatory molecules further accelerates functioning of the immune system. Mimicking receptor-mediated binding of the pathogen may be achieved by direct conjugation of antigen with an immunostimulatory molecule or encapsulation in a carrier followed by anchoring of a ligand having affinity to the cells of the mucosal immune system. Nanotechnology has played a significant role in mucosal vaccine development and among the available options liposomes are the most promising. Liposomes are phospholipid bilayered vesicles that can encapsulate protein as well as DNA-based vaccines and offer coencapsulation of adjuvant along with the antigen. At the same, time ligand-conjugated liposomes augment interaction of antigen with the cells of the mucosal immune system and thereby serve as suitable candidates for the mucosal delivery of vaccines. This article exhaustively explores strategies involved in the generation of mucosal immunity and also provides an insight to the progress that has been made in the development of liposome-based mucosal vaccine.

Efficient generation of a monoclonal antibody against the human C-type lectin receptor DCIR by targeting murine dendritic cells

Dendritic cells (DCs) are very important for the generation of long lasting immune responses against pathogens or the induction of anti-tumor responses. Targeting antigen to dendritic cells via monoclonal antibodies specific for DC cell surface receptors such as DEC205 was shown to elicit potent cellular and humoral immune responses in vivo. Therefore, we investigated whether this novel strategy might also be useful for the generation of new monoclonal antibodies against molecules of choice. We show, that by targeting the extracellular domain of the human C-type lectin receptor ClecSF6/DCIR/LLIR (hDCIR) to DEC205 on DCs in vivo, we were able to generate highly specific monoclonal antibodies against hDCIR.

Approaches to enhance the efficacy of DNA vaccines

DNA vaccines consist of antigen-encoding bacterial plasmids that are capable of inducing antigen-specific immune responses upon inoculation into a host. This method of immunization is advantageous in terms of simplicity, adaptability, and cost of vaccine production. However, the entry of DNA vaccines and expression of antigen are subjected to physical and biochemical barriers imposed by the host. In small animals such as mice, the host-imposed impediments have not prevented DNA vaccines from inducing long-lasting, protective humoral, and cellular immune responses. In contrast, these barriers appear to be more difficult to overcome in large animals and humans. The focus of this article is to summarize the limitations of DNA vaccines and to provide a comprehensive review on the different strategies developed to enhance the efficacy of DNA vaccines. Several of these strategies, such as altering codon bias of the encoded gene, changing the cellular localization of the expressed antigen, and optimizing delivery and formulation of the plasmid, have led to improvements in DNA vaccine efficacy in large animals. However, solutions for increasing the amount of plasmid that eventually enters the nucleus and is available for transcription of the transgene still need to be found. The overall conclusions from these studies suggest that, provided these critical improvements are made, DNA vaccines may find important clinical and practical applications in the field of vaccination.

Anti-class II monoclonal antibody-targeted Vibrio cholerae TcpA pilin: modulation of serologic response, epitope specificity, and isotype

Toxin-coregulated pilus (TCP) is a colonization factor required for cholera infection. It is not a strong immunogen when delivered in the context of whole cells, yet pilus subunits or TcpA derivative synthetic peptides induce protective responses. We examined the efficacy of immunizing mice with TCP conjugated to anti-class II monoclonal antibodies (MAb) with or without the addition of cholera toxin (CT) or anti-CD40 MAb to determine if the serologic response to TcpA could be manipulated. Anti-class II MAb-targeted TCP influenced the anti-TCP peptide serologic response with respect to titer and isotype. Responses to TcpA peptide 4 were induced with class II MAb-targeted TCP and not with nontargeted TCP. Class II MAb-targeting TcpA reduced the response to peptide 6 compared to the nontargeted TCP response. Class II MAb-targeted TcpA, if delivered with CT, enhanced the serologic response to TcpA peptides. The effectiveness of the combination of targeted TCP and CT was reduced if anti-CD40 MAb were included in the primary immunization. These data establish the need to understand the role of TCP presentation in the generation of B-cell epitopes in order to optimize TcpA-based cholera vaccines.

Class II-targeted antigen is superior to CD40-targeted antigen at stimulating humoral responses in vivo

We examined the efficacy of using monoclonal antibodies to target antigen (avidin) to different surface molecules expressed on antigen presenting cells (APC). In particular, we targeted CD40 to test whether the "adjuvant" properties of CD40 signaling combined with targeted antigen would result in enhanced serologic responses. We targeted avidin to class II as a positive control and to CD11c as a negative control. These surface proteins represent an ensemble of surface molecules that signal upon ligation and that are expressed on professional APC, in particular dendritic cells (DC). We observed that targeting class II molecules on APC was superior to targeting CD40, or CD11c. However, CD40 and CD11c could function as targets for antigen bound monoclonal antibodies under certain conditions. Interestingly, inclusion of anti-CD40 mAb with the targeting anti-class II-targeted antigens negatively affects humoral response, suggesting that CD40 signaling under certain conditions may suppress processing and/or presentation of targeted antigen.

Characterization of mucoadhesive microspheres for the induction of mucosal and systemic immune responses

In the present study, mucoadhesive polymer-dispersed microspheres (MS) were examined as a potential mucosal vaccine carrier. A major focus of the study was aimed at directly assessing the influence of antigen release and persistence in the mouse small intestine for the induction of mucosal and systemic immune responses. BALB/c mice were immunized with various forms of MS containing chicken egg ovalbumin (OVA) by administration into the duodenum. No detectable anti-OVA immune responses were observed following the administration of OVA alone or that of MS without mucoadhesive polymer (MS-0). MS-10 containing 10% mucoadhesive polymer rapidly released OVA and hardly induced anti-OVA antibody responses in either serum or fecal extracts. In contrast, MS-8 and MS-6 (with 8 and 6% mucoadhesive polymer) showed controlled release of OVA, which elicited strong OVA-specific IgG and IgA responses in serum and fecal extracts, respectively. Additionally, the strongest immune responses were induced in mice immunized with MS-8, which had both the optimal release-profile of OVA and the longest persistence in the small intestine. These findings indicate that antigen movement in the small intestine is an important factor and that appropriate microsphere forms with mucoadhesive polymers might be useful candidates as mucosal vaccine carriers.

Secretory immunoglobulin A: from mucosal protection to vaccine development

Immune responses taking place in mucosal tissues are typified by secretory immunoglobulin A (S-IgA) molecules, which are assembled from proteins expressed in two cell lineages. The heavy and light chains as well as the J chain are produced in plasma cells, whereas the secretory component (SC) is associated to the immunoglobulin complex during transcytosis across the epithelial layer. S-IgA antibodies represent the predominant immunoglobulin class in external secretions, and the best defined entity providing specific immune protection for mucosal surfaces by blocking attachment of bacteria and viruses. S-IgA constitutes greater than 80% of all antibodies produced in mucosa-associated lymphoid tissues in humans. The existence of a common mucosal immune system permits immunization on one mucosal surface to induce secretion of antigen-specific S-IgA at distant sites. In addition, S-IgA antibodies not only function in external secretions, but also exert their antimicrobial properties within the epithelial cell during transport across the epithelium. Passive mucosal delivery of monoclonal IgA molecules neutralizes pathogens responsible for gastrointestinal and respiratory infections. Mucosal and systemic immunity can be achieved by orally administered recombinant S-IgA molecules carrying a protective bacterial epitope within the SC polypeptide primary sequence.

IgA production in MHC class II-deficient mice is primarily a function of B-1a cells

Mice deficient in MHC class II expression (C2d mice) do not make antibody to protein antigens administered systemically, but their ability to produce IgA antibody to antigen administered at mucosal sites has not been described. We investigated IgA production by C2d mice and their IgA antibody response to antigen given orally. Young C2d mice had normal amounts of serum IgA, intestinal-secreted IgA and normal numbers of intestinal IgA plasma cells, compared to control C57BL/6 mice. IgA production by C2d mice increased with age. Following oral immunization with cholera toxin, C57BL/6 mice responded with IgA and IgG antibody, and had increased numbers of IgA plasma cells, but C2d mice gave no response. The Peyer's patch and mesenteric lymph node tissues of C2d mice contained very few CD4-expressing T cells. Thus, C2d mice have no typical mucosal CD4 Th cells and cannot respond to a strong oral immunogen, yet they still produced and secreted IgA. We hypothesized that B-1 lymphocytes could provide a source of IgA independent of antigen-specific T cell help. Young C2d mice had normal numbers of peritoneal B-1a cells and their frequency increased with age. To test the role of these B-1a cells, we bred C2d mice to obtain mice that had no MHC class II expression and expressed the Xid gene that confers deficiency in B-1a cells. These double-deficient mice had 10-fold less serum and secreted IgA than all other F2 littermates. We conclude that B-1a cells are essential for the majority of IgA production in C2d mice. Thus, the C2d mouse may provide a useful tool for analysis of the role of intestinal IgA provided by B-1a cells.

Oral immunization with recombinant Norwalk virus-like particles induces a systemic and mucosal immune response in mice

Recombinant Norwalk virus-like particles (rNV VLPs) produced in insect cells were evaluated as an oral immunogen in CD1 and BALB/c mice by monitoring rNV-specific serum total and subclass immunoglobulin G (IgG) and intestinal IgA responses. Dose and kinetics of response were evaluated in the presence and absence of the mucosal adjuvant cholera toxin (CT). rNV-specific serum IgG and intestinal IgA were detected in the absence of CT, and the number of responders was not significantly different from that of mice administered VLPs with CT at most doses. The use of CT was associated with induction of higher levels of IgG in serum; this effect was greater at higher doses of VLPs. IgG in serum was detected in the majority of animals by 9 days postimmunization (dpi), and intestinal IgA responses were detected by 24 dpi. In the absence of CT, IgG2b was the dominant IgG subclass response in both mouse strains. Thus, nonreplicating rNV VLPs are immunogenic when administered orally in the absence of any delivery system or mucosal adjuvant. These studies demonstrate that rNV VLPs are an excellent model to study the oral delivery of antigen, and they are a potential mucosal vaccine for NV infections.

Protective immunity against HSV-2 in the mouse vagina

Herpes simplex virus type 2 (HSV-2) is a sexually transmitted pathogen that infects the genital tract. The high prevalence of HSV-2 in humans underscores the need to develop an effective vaccine. Efforts to develop vaccines to protect women against this and other sexually transmitted pathogens would be facilitated by a better understanding of the immune mechanisms that protect the female reproductive tract against infections in animal models. Such information would be invaluable in developing vaccine strategies to promote the type and magnitude of immune responses in the genital tract that would effectively protect against infection. This review focuses on recent studies using a progestin-treated adult mouse model to explore mucosal immunity to HSV-2 in the vagina. Evidence indicating a major role for both humoral and T cell immunity is presented.

Immunoglobulin G is the main protective antibody in mouse vaginal secretions after vaginal immunization with attenuated herpes simplex virus type 2

We investigated the protective role of antibodies in vaginal secretions of mice that were immune to vaginal challenge with herpes simplex virus type 2 (HSV-2). Unfractionated vaginal immunoglobulins from immune and nonimmune mice and affinity-purified immunoglobulin G (IgG) and secretory IgA (S-IgA) from immune secretions were adjusted to their concentrations in vivo. Wild-type HSV-2 was incubated in the immunoglobulin preparations for 15 min in vitro, followed by inoculation into vaginae of nonimmune mice. HSV-2 was neutralized by unfractionated antibody and purified IgG from immune secretions but not by unfractionated nonimmune antibody or by purified immune S-IgA. The protective effect of IgG in vivo was investigated by passively transferring purified serum IgG from immune and nonimmune donors to nonimmune recipients before vaginal challenge infection. Immune IgG significantly reduced the percentage of vaginal epithelium infected, concentrations of shed virus protein in the vaginal lumen, and illness scores, even though the viral antibody titers in serum and vaginal secretions of recipient mice at the time of challenge were only 29 and 8%, respectively, of those in actively immunized mice. Additionally, removal of vaginal secretions from immune mice 10 min before vaginal challenge with HSV-2 significantly increased the concentration of shed virus protein in the vaginal lumen after challenge. Collectively, the data indicate that IgG antibody in vaginal secretions of immune mice provides early protection against vaginal challenge infection, probably by neutralizing virus in the vaginal lumen. In contrast, S-IgA antibody contributed relatively little to immune protection of the vagina.

Intranasal antigen targeting to MHC class II molecules primes local IgA and serum IgG antibody responses in mice

Covalent conjugates of hen egg lysozyme (HEL) and anti-major histocompatibility complex (MHC) class II monoclonal antibodies (mAb) were used to immunize mice intranasally. Three weeks after intranasal (IN) priming, mice responded rapidly to IN challenge with a mixture of HEL and cholera toxin (CT), by producing large titres of anti-HEL IgA and IgG1 antibody in serum, and IgA antibody in nasal secretions. No secondary response to HEL plus CT occurred in mice that received no priming or mice primed with HEL alone. The secondary serum IgG antibody response was dominated by the IgG1 subclass. HEL combined with CT adjuvant worked much better than HEL alone in producing the secondary response. Control conjugates, containing an IgG isotype-matched mAb without specificity for mouse tissues, provided poor priming. Mice expressing MHC class II molecules, to which the anti-MHC II mAb could not bind, produced a weak antibody response compared with those that expressed the appropriate. MHC class II molecule. Our results demonstrate that immunotargeting to MHC class II molecules via the IN route allows priming of the local IgA and circulating IgG antibody, and indicate that this technique is a feasible approach for delivery of subunit vaccines in the upper respiratory tract.