Conventional vaccine strategies have been highly efficacious for several decades in reducing mortality and morbidity due to infectious diseases. The bane of conventional vaccines, such as those that include whole organisms or large proteins, appear to be the inclusion of unnecessary antigenic load that, not only contributes little to the protective immune response, but complicates the situation by inducing allergenic and/or reactogenic responses. Peptide vaccines are an attractive alternative strategy that relies on usage of short peptide fragments to engineer the induction of highly targeted immune responses, consequently avoiding allergenic and/or reactogenic sequences. Conversely, peptide vaccines used in isolation are often weakly immunogenic and require particulate carriers for delivery and adjuvanting. In this article, we discuss the specific advantages and considerations in targeted induction of immune responses by peptide vaccines and progresses in the development of such vaccines against various diseases. Additionally, we also discuss the development of particulate carrier strategies and the inherent challenges with regard to safety when combining such technologies with peptide vaccines.

There is currently no licensed vaccine against Chlamydia trachomatis, the leading cause of sexually transmitted bacterial disease worldwide. Conventional vaccination attempts using surface-exposed chlamydial antigens have achieved only partial success. We have employed a novel vaccination strategy using a secreted protein, chlamydial protease-like activity factor (CPAF), which has been shown to degrade host major histocompatibility complex transcription factors and keratin-8 and therefore may allow immune evasion and establishment of a productive infection. Intranasal immunization using recombinant CPAF (rCPAF) plus interleukin-12 (IL-12) (rCPAF؉IL-12 immunization) was used to assess the protective immunity against genital Chlamydia muridarum infection in BALB/c mice. rCPAF؉IL-12 immunization induced robust gamma interferon (IFN-␥) production and minimal IL-4 production by splenocytes upon in vitro recall with rCPAF. The total and immunoglobulin G2a (IgG2a) anti-rCPAF antibody levels in serum were significantly elevated after rCPAF؉IL-12 vaccination, as were the total antibody, IgG2a, and IgA levels in bronchoalveolar lavage and vaginal fluids when the animals were compared to animals that received rCPAF alone. rCPAF؉IL-12-vaccinated mice displayed significantly reduced bacterial shedding upon chlamydial challenge and accelerated resolution of infection compared to mock-immunized (phosphate-buffered saline) animals. Moreover, rCPAF؉IL-12-immunized animals exhibited protection against pathological consequences of chlamydial infection, including the development of hydrosalpinx and oviduct dilatation. This vaccination regimen also reduced the development of fibrosis and the influx of neutrophils into the upper genital tract when the animals were compared to mock-immunized (phosphate-buffered saline) animals after bacterial challenge. rCPAF؉IL-12-mediated resolution of the bacterial infection and protection against Chlamydia-induced inflammatory disease were highly dependent on endogenous IFN-␥ production. Together, these results demonstrate that secreted chlamydial antigens may be novel vaccine candidates to induce protective immunity.

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Chlamydia has been shown to evade host-specific IFN-γ-mediated bacterial killing; however, IFN-γ-deficient mice exhibit suboptimal late phase vaginal Chlamydia muridarum clearance, greater dissemination, and oviduct pathology. These findings introduce constraints in understanding results from murine chlamydial vaccination studies in context of potential implications to humans. In this study, we used mice deficient in either IFN-γ or the IFN-γ receptor for intranasal vaccination with a defined secreted chlamydial Ag, chlamydial protease-like activity factor (CPAF), plus CpG and examined the role of IFN-γ derived from adoptively transferred Ag-specific CD4+ T cells in protective immunity against genital C. muridarum infection. We found that early Ag-specific IFN-γ induction and CD4+ T cell infiltration correlates with the onset of genital chlamydial clearance. Adoptively transferred IFN-γ competent CPAF-specific CD4+ T cells failed to enhance the resolution of genital chlamydial infection within recipient IFN-γ receptor-deficient mice. Conversely, IFN-γ production from adoptively transferred CPAF-specific CD4+ T cells was sufficient in IFN-γ-deficient mice to induce early resolution of infection and reduction of subsequent pathology. These results provide the first direct evidence that enhanced anti-C. muridarum protective immunity induced by Ag-specific CD4+ T cells is dependent upon IFN-γ signaling and that such cells produce sufficient IFN-γ to mediate the protective effects. Additionally, MHC class II pathway was sufficient for induction of robust protective anti-C. muridarum immunity. Thus, targeting soluble candidate Ags via MHC class II to CD4+ T cells may be a viable vaccine strategy to induce optimal IFN-γ production for effective protective immunity against human genital chlamydial infection.

Francisella tularensis is an intracellular, Gram-negative bacterium that is the causative agent of pulmonary tularemia. The pathogenesis and mechanisms related to innate resistance against F. tularensis are not completely understood. Mast cells are strategically positioned within mucosal tissues, the major interface with the external environment, to initiate innate responses at the site of infection. Mast cell numbers in the cervical lymph nodes and the lungs progressively increased as early as 48 h after intranasal F. tularensis live vaccine strain (LVS) challenge. We established a primary bone marrow-derived mast cell-macrophage coculture system and found that mast cells significantly inhibit F. tularensis LVS uptake and growth within macrophages. Importantly, mice deficient in either mast cells or IL-4 receptor displayed greater susceptibility to the infection when compared with corresponding wild-type animals. Contact-dependent events and secreted products including IL-4 from mast cells, and IL-4 production from other cellular sources, appear to mediate the observed protective effects. These results demonstrate a previously unrecognized role for mast cells and IL-4 and provide a new dimension to our understanding of the innate immune mechanisms involved in controlling intramacrophage Francisella replication.innate ͉ LVS ͉ macrophages ͉ mucosal ͉ pulmonary