Associations with proliferating T cells on day 28 post-second vaccination did not reach a sufficient cross-validation score (score<0.5). agent of plague, is a category A potential bioterrorism pathogen that poses a high risk to both national security and public health. Plague has three major clinical forms, bubonic, pneumonic, and septicemic, but the highest concern is for the pneumonic form which allows for direct person-to-person transmission via infectious respiratory droplets and therefore remains a serious public health threat. Thus, due attention should be given to public health preparedness including prevention and early detection. Vaccines are effective in preventing and controlling infectious diseases. However, there is no approved plague vaccine and vaccine development efforts are hampered by a lack of reliable markers of protection. Plague is currently a rare disease worldwide and is associated with high mortality; therefore, measuring vaccine efficacy based on protection from natural infection is impractical. This means, as with other category A pathogens, plague vaccine development efforts need to rely on inferred correlates of protection, which requires a good understanding of immunity against Yp. Animal studies have demonstrated that both antibody and cell-mediated immunity (CMI) are essential for protection against challenge with Yp.110Different forms of plague vaccines including killed Yp, live attenuated Yp, and subunit vaccines have been studied. Subunit vaccines containing F1 capsular and virulence (V) antigens show the most promising results. A vaccine that had F1 and V antigens mixed with alhydrogel adjuvant was shown to elicit antibody responses in humans, but without measurable CMI.11Interestingly, the post-vaccination sera from this clinical trial protected mice from lethal Yp challenge. Similarly, a recent dose titration clinical trial with a new F1/V subunit vaccine containing flagellin as an adjuvant conducted by the Vaccine and Treatment Evaluation Unit (VTEU) network showed good antibody responses at 6 and 10 g, again in the absence of significant CMI.12This vaccine was shown to induce excellent antibody responses in Rabbit polyclonal to Caspase 3 mice and non-human primates (NHP), and protect mice against respiratory challenge with Yp.13The protective capacity of antibody responses induced by flagellin-adjuvanted F1/V plague vaccine in humans remains to be studied. The lack of CMI from both clinical trials with subunit vaccines was unexpected because these same subunit vaccines have been Mivebresib (ABBV-075) shown to elicit protective CMI in animal models.4,5,10One possible explanation for the lack of measurable vaccine-specific CMI found in subunit plague vaccine trials is the limitation of the in vitro assays used (e.g., antigen concentration and duration of in vitro restimulation of T cells). In the first trial, the T-cell activation markers and gross changes in T-cell counts were measured ex vivo without antigenic restimulation.11In the recently completed VTEU clinical trial,12only 24 h stimulation with F1/V antigens was used before collection of culture supernatants for cytokine quantification. Vaccine-specific T cells are generally of low frequency and can be measured reliably only after optimal in vitro stimulation.14This study was carried Mivebresib (ABBV-075) out with the objectives of evaluating the protective function of antibodies elicited by flagellin adjuvanted F1/V vaccine, reevaluating vaccine-induced T-cell responses using optimal in vitro restimulation conditions, and identifying gene expression markers of good vaccine-induced immune responses. == Results == == Mivebresib (ABBV-075) Antibody responses induced by F1/V vaccine prevent macrophage lytic effects of a recombinant Yptb == We used the caspase-3 assay to determine the ability of vaccine-induced antibodies to protect macrophages from lytic effect of recombinantYersinia pseudotuberculosis(Yptb) expressing V antigen. Caspase 3 release is a hallmark of apoptosis.15Figure1shows the inverse anti-V caspase-3 levels by study visit day and treatment group. Tabular results for per-visit and fold change results are provided in Supplementary Table3. Combined results for samples from volunteers vaccinated with 6 and 10 g of F1/V vaccine showed that median inverse caspase-3 levels increased by 29% on day 14 (median fold change of 1 1.29 andP= 0.0076) and 75% on day 28 (median fold change of 1 1.75 andP< 0.0001) post-second vaccination (Fig.1a, Supplementary Table3). == Fig. 1. F1/V vaccine increased protective levels of antibody, CD4+ T-cell, and IFN- responses. == aMacrophage-protective levels of Mivebresib (ABBV-075) antibodies were statistically significantly increased from pre-vaccination on days 14 and 28 with highest response on day 28 post-second vaccination (Wilcoxon signed-rank testP-values are shown).bdCD4+ that expressed IL-10, IL-4, or co-expressed TNF- and IFN- were statistically increased from pre-vaccination on day 28 but not day 14.eIFN- cytokine concentrations measured in culture supernatants were statistically significantly increased from pre-vaccination on day 14 but not day 28. Statistical.