Immunization with the Haemophilus ducreyi Hemoglobin Receptor HgbA with Adjuvant Monophosphoryl Lipid A Protects Swine from a Homologous but Not a Heterologous Challenge

Abstract

ABSTRACT Haemophilus ducreyi , the etiological agent of chancroid, has a strict requirement for heme, which it acquires from its only natural host, humans. Previously, we showed that a vaccine preparation containing the native hemoglobin receptor HgbA purified from H. ducreyi class I strain 35000HP (nHgbA I ) and administered with Freund's adjuvant provided complete protection against a homologous challenge. In the current study, we investigated whether nHgbA I dispensed with monophosphoryl lipid A (MPL), an adjuvant approved for use in humans, offered protection against a challenge with H. ducreyi strain 35000HP expressing either class I or class II HgbA (35000HP hgbA I and 35000HP hgbA II , respectively). Pigs immunized with the nHgbA I /MPL vaccine were protected against a challenge from homologous H. ducreyi strain 35000HP hgbA I but not heterologous strain 35000HP hgbA II , as evidenced by the isolation of only strain 35000HP hgbA II from nHgbA I -immunized pigs. Furthermore, histological analysis of the lesions showed striking differences between mock-immunized and nHgbA I -immunized animals challenged with strains 35000HP hgbA I but not those challenged with strain 35000HP hgbA II . Mock-immunized pigs were not protected from a challenge by either strain. The enzyme-linked immunosorbent assay (ELISA) activity of the nHgbA I /MPL antiserum was lower than the activity of antiserum from animals immunized with the nHgbA I /Freund's vaccine; however, anti-nHgbA I from both studies bound whole cells of 35000HP hgbA I better than 35000HP hgbA II and partially blocked hemoglobin binding to nHgbA I . In conclusion, despite eliciting lower antibody ELISA activity than the nHgbA I /Freund's, the nHgbA I /MPL vaccine provided protection against a challenge with homologous but not heterologous H. ducreyi , suggesting that a bivalent HgbA vaccine may be needed.