A lipid A-based TLR4 mimetic effectively adjuvants a Yersinia pestis rF-V1 subunit vaccine in a murine challenge model.

Vaccination can significantly reduce worldwide morbidity and mortality to infectious diseases, thereby reducing the health burden as a result of microbial infections. Effective vaccines contain three components: a delivery system, an antigenic component of the pathogen, and an adjuvant. With the growing use of purely recombinant or synthetic antigens, there is a need to develop novel adjuvants that enhance the protective efficacy of a vaccine against infection. Using a structure-activity relationship (SAR) model, we describe here the synthesis of a novel TLR4 ligand adjuvant compound, BECC438, by bacterial enzymatic combinatorial chemistry (BECC). This compound was identified using an in vitro screening pipeline consisting of (i) NFκB activation and cytokine production by immortalized cell lines, (ii) cytokine production by primary human PBMCs, and (iii) upregulation of surface costimulatory markers by primary human monocyte-derived dendritic cells. Using this SAR screening regimen, BECC438 was shown to produce an innate immune activation profile comparable to the well-characterized TLR4 agonist adjuvant compound, phosphorylated hexa-acyl disaccharide (PHAD). To evaluate the in vivo adjuvant activity of BECC438, we used the known protective Yersinia pestis (Yp) antigen, rF1-V, in a murine prime-boost vaccination schedule followed by lethal challenge. In addition to providing protection from lethal challenge, BECC438 stimulated production of higher levels of rF1-V-specific total IgG as compared to PHAD after both prime and boost vaccinations. Similar to PHAD, BECC438 elicited a balanced IgG1/IgG2c response, indicative of active TH2/TH1-driven immunity. These data demonstrate that the novel BECC-derived TLR4L adjuvant, BECC438, elicits cytokine profiles in vitro similar to PHAD, induces high antigen-specific immune titers and a TH1-associated IgG2c immune titer skew, and protects mice against a lethal Yp challenge.