Key antigens of Leishmania species identified in the context of host responses in Leishmania-exposed individuals from disease-endemic areas were prioritized for the development of a subunit vaccine against visceral leishmaniasis (VL), the most deadly form of leishmaniasis. Two Leishmania proteins—nucleoside hydrolase and a sterol 24-c-methyltransferase, each of which are protective in animal models of VL when properly adjuvanted— were produced as a single recombinant fusion protein NS (LEISH-F3) for ease of antigen production and broad coverage of a heterogeneous major histocompatibility complex population. When formulated with glucopyranosyl lipid A-stable oil-in-water nanoemulsion (GLA-SE), a Toll-like receptor 4 TH1 (T helper 1) promoting nanoemulsion adjuvant, the LEISH-F3 polyprotein induced potent protection against both L. donovani and L. infantum in mice, measured as significant reductions in liver parasite burdens. A robust immune response to each component of the vaccine with polyfunctional CD4 TH1 cell responses characterized by production of antigen-specific interferon-γ, tumor necrosis factor and interleukin-2 (IL-2), and low levels of IL-5 and IL-10 was induced in immunized mice. We also demonstrate that CD4 T cells, but not CD8 T cells, are sufficient for protection against L. donovani infection in immunized mice. Based on the sum of preclinical data, we prepared GMP materials and performed a phase 1 clinical study with LEISH-F3+GLA-SE in healthy, uninfected adults in the United States. The vaccine candidate was shown to be safe and induced a strong antigen-specific immune response, as evidenced by cytokine and immunoglobulin subclass data. These data provide a strong rationale for additional trials in Leishmania-endemic countries in populations vulnerable to VL.

+T-cell induction via GLA-SE. Thus, we demonstrate that IL-18 and caspase-1/11 are components of the response to immunization with the TLR4 agonist/squalene oil-inwater based adjuvant, GLA-SE, providing implications for other adjuvants that combine oils with TLR agonists. Additional supporting information may be found in the online version of this article at the publisher's web-site Keywords: Adjuvants IntroductionVaccines rely on triggering the innate immune system to generate protective immunity. Many empirically established vaccines serendipitously contain molecules known as pathogen-associated molecular patterns (PAMPs) that engage host-germline encoded receptors, e.g. TLRs, which contribute to their immunogenicity Correspondence: Dr. Anthony L. Desbien e-mail: Anthony.Desbien@IDRI.org [1]. In the case where antigen preparations alone fail to be immunogenic, extrinsic adjuvants, such as alum or oil emulsions, are added to make vaccines effective. Such extrinsic adjuvants were empirically developed and despite their use in billions of vaccine doses, their mechanisms of action are not completely understood [2,3]. Knowledge of the salient attributes of empirical adjuvants would facilitate the design of the next generation of vaccines.In order to function as adjuvants, deliberately incorporated PAMPs require proper formulation [4,5]. For unknown reasons, squalene oil-in-water emulsions (SEs), such as AS02 and MF59, C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu 408Anthony L. Desbien et al. Eur. J. Immunol. 2015. 45: 407-417 are excellent formulations for TLR agonists, greatly enhancing the magnitude and quality of the immune response [6][7][8][9][10][11]. While it is clear that squalene emulsions by themselves are potent adjuvants, exactly how they engage the immune system or cooperate with TLR agonists is unclear. The commercial product MF59 is the prototypical squalene adjuvant. MF59 has been shown to activate the innate immune system, alter antigen presentation, and enhance humoral responses [3,[12][13][14]. While these attributes are ostensibly useful for generation of immunity, the specific components of the immune system triggered by squalene emulsions remain to be elucidated. The inflammasome is a pathogen recognition system required for protection against many diseases [15]. Engagement of the inflammasome occurs by diverse stimuli including pathogenand host-derived molecules. Known triggers include cytoplasmic dsDNA as well as aberrantly localized host-derived molecules, referred to as danger-associated molecular patterns [16]. In particular, extracellular ATP acts as a danger-associated molecular pattern, and has been recently demonstrated to contribute to activity of MF59, suggesting that MF59 acts via the inflammasome [17]. Further, the antibody response to immunization with MF59 has been linked to the adapter protein ASC, a component of the inflammasome, but not caspase-1 or NLRP3 (where NLR is nod-like receptor), indicating that adjuvant activity of MF59 is mediated by stil...

Vaccination with an isolated antigen is frequently not sufficient to elicit a protective immune response. The addition of adjuvants to the antigen can increase the magnitude and breadth of the response generated, but quantification of this increase as a function of adjuvant has been intractable. We have directly determined the variation of the immunoglobulin G variable-chain repertoire of an entire organism as a function of vaccination. Using the well-established Plasmodium vivax antigen, PvRII, and massively parallel sequencing, we showed that the use of a Toll-like receptor (TLR) agonist in the vaccine formulation increased the diversity of the variable region sequences in comparison to the use of an oil-in-water emulsion adjuvant alone. Moreover, increased variable domain diversity in response to the use of TLR agonist-based adjuvants correlated with improved antigen neutralization. The use of TLR agonists also broadened the range of polymorphic variants against which these antibodies could be effective. In addition, a peptide microarray demonstrated that inclusion of adjuvants changed the profile of linear epitopes from PvRII that were recognized by serum from immunized animals. The results of these studies have broad implications for vaccine design--they may enable tailored adjuvants that elicit the broad spectrum of antibodies required to neutralize drifted and polymorphic pathogen strains as well as provide a method for rapid determination of correlates of adjuvant-induced humoral immunity.

Therapy of intracellular pathogens can be complicated by drug toxicity, drug resistance and the need for prolonged treatment regimens. One approach which has shown promise is immunotherapy. Leishmaniasis, a vector-borne disease ranked among the six most important tropical infectious diseases by the WHO, has been treated clinically with crude or defined vaccine preparations or cytokines such as IFN-γ and GM-CSF in combination with chemotherapy. We have attempted to develop an improved and defined immunotherapeutic using a mouse model of cutaneous leishmaniasis. We hypothesized that immunotherapy may be improved by using TLR synergy to enhance the parasite-specific immune response. We formulated L110f, a well-established Leishmania poly-protein vaccine candidate, in conjunction with either monophosphoryl lipid A (MPL), a TLR 4 agonist, or CpG, a TLR 9 agonist, or a combination of these and evaluated anti-leishmania immune responses in absence or presence of active disease. Only mice treated with L110f + MPL-CpG were able to induce a strong effective T cell response during disease and subsequently cured lesions and reduced parasite burden when compared to mice treated with L110f and either single adjuvant. Our data help to define a correlate of protection during active infection and indicate TLR synergy to be a potentially valuable tool in treating intracellular infections.

Designing modern vaccine adjuvants depends on understanding the cellular and molecular events that connect innate and adaptive immune responses. The synthetic TLR4 agonist GLA formulated in a stable emulsion (GLA-SE) augments both cellular and humoral immune responses to vaccine antigens. This adjuvant is currently included in several vaccines undergoing clinical evaluation including those for tuberculosis, leishmaniasis, and influenza. Delineation of the mechanisms of adjuvant activity will enable more informative evaluation of clinical trials. Early after injection, GLA-SE induces substantially more antigen-specific B cells, higher serum antibody titers and greater numbers of T follicular helper (TFH) and TH1 cells than alum, the squalene-in-water emulsion (SE) alone, or GLA without SE. GLA-SE augments antigen-specific B cell differentiation into germinal center and memory precursor B cells as well as pre-plasmablasts that rapidly secrete antibodies. CD169+ SIGNR1+ subcapsular medullary macrophages are the primary cells to take up GLA-SE after immunization and are critical for the innate immune responses, including rapid IL-18 production, induced by GLA-SE. Depletion of subcapsular macrophages (SCMϕ) or abrogation of IL-18 signaling dramatically impairs the antigen-specific B cell and antibody responses augmented by GLA-SE. Depletion of SCMϕ also drastically reduces the TH1 but not TFH response. Thus the GLA-SE adjuvant operates through interaction with IL-18-producing SCMϕ for the rapid induction of B cell expansion and differentiation, antibody secretion and TH1 responses, whereas augmentation of TFH numbers by GLA-SE is independent of SCMϕ.