Defining Correlates of Humoral Immunity Against Inhalational Melioidosis

Prof Paul Brett, University of Nevada, USA

Burkholderia pseudomallei, the etiologic agent of melioidosis, causes severe disease in humans and animals. Diagnosis and treatment of melioidosis is challenging and no licensed vaccines currently exist. The 6-deoxyheptan capsular polysaccharide (CPS) expressed by this bacterial pathogen is both a virulence factor and a protective antigen. In this study, we investigated the protective capacity of CPSspecific polyclonal antibodies and monoclonal antibodies (mAbs) in the presence or absence of trimethoprim/sulfamethoxazole (TMP/SMX). Purified CPS from Burkholderia thailandensis E555 was chemically activated and covalently linked to recombinant CRM197 diphtheria toxin mutant (CRM197) to produce the glycoconjugate, CPS-CRM197. 10x Genomics single cell RNA-sequencing was used to produce recombinant CPS-specific mAbs following the isolation of activated B cells from CPS-CRM197 immunized mice. C57BL/6 mice were then immunized with CPS-CRM197 or CPS-specific mAbs in the presence or absence of TMP/SMX. Following lethal inhalational challenges of B. pseudomallei K96243, 70% of the actively immunized animals were still alive upon termination of the study at day 70. In addition, the majority of tissues from the survivors were found to be sterile when assessed for bacterial loads. In contrast, only 10% of the animals passively immunized with the mAbs survived the same timeframe. While administration of TMP/SMX alone as a control in these studies resulted in low levels of protection, high survival rates (90-100%) were observed for actively and passively immunized mice that also received the antibiotic. Collectively, these studies demonstrate important role(s) for CPS-specific antibodies in combatting disease caused by B. pseudomallei and provide valuable insights towards the development of novel combination therapies for melioidosis.