Professor of Pediatrics and Immunology Duke Human Vaccine Institute, Duke Univerity, Durham, North Carolina, USA Durham, North Carolina, United States
Background: Persons with syphilis produce treponemal and anti-lipid antibodies during infection with T. pallidum subsp. pallidum (TPA), but those antibodies are often insufficient for spirochete clearance. Deconvoluting antibody responses during syphilitic infection may provide insights to guide antigen selection for vaccine development and also illuminate facets of TPA pathogenesis.
Methods: We enrolled patients with early syphilis from clinics in the United States, Colombia, and Malawi between November 2019-May 2022. This interim analysis included a subset of the data matrix from 76 participants and 65 antigenic targets. Serum samples were assayed for reactivity to predicted extracellular loops (ECLs) of TPA Nichols outer membrane proteins (OMPs) displayed on Pyrococcus furiosus thioredoxin [PfTrx]) scaffolds. Serum reactivity to OMP ECLs and native PfTrx were normalized using a human monoclonal antibody (AT00D) reactive with the PfTrx scaffold. Comparison of the area under the curve to loop containing PfTrx and native scaffold was used to identify serum samples that recognize TPA OMP ECLs.
Results: Most predicted ECLs of TPA OMPs were not recognized by sera from persons with syphilis. Some ECLs were recognized from 1-2 participants (e.g., TP0126 ECL4) or by a minority of participants (e.g., TP0479 ECL4 [11/73, 15%], TP0733 ECL1 [11/73, 15%]). Other ECLs were recognized by sera from more participants (e.g., TP0858 ECL4 [25/73, 34%]), but no loop or combination of loops from a single protein was recognized by all participants. Of 8 participants that had detectable activity against BamA ECL4, an opsonic target known to contain a polymorphism between the Nichols and Mexico A strains, 4 participants recognized both variants equally and 4 participants reacted more strongly with the Nichols variant. Reactivity patterns to certain ECLs between our clinical sites appear to be similar (e.g., TP0858 ECL4: Colombia 12/39 [31%], Malawi 14/34 [41%]).
Conclusion: This is the first large scale screening of human reactivity to predicted TPA OMPs ECLs. These data suggest a possible mechanism for incomplete TPA clearance in syphilis patients and that vaccines that elicit robust antibody responses to immunogenic ECLs may be protective. Future analyses will focus on correlation of these data with clinical, geographic, and spirochete sequence data.
