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Advisory Committee Chair

Moon H Nahm

Advisory Committee Members

William H Benjamin

Rakesh P Patel

Jamil S Saad

Janet L Yother

Document Type

Dissertation

Date of Award

2017

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

Streptococcus pneumoniae (pneumococcus) is an important human pathogen that asymptomatically colonizes the nasopharynx but can disseminate opportunistically to cause diseases such as otitis media, pneumonia, septicemia, or meningitis. The pneumo-coccal capsular polysaccharide is necessary to prevent opsonophagocytosis as nonencap-sulated pneumococci are largely avirulent. Currently, 98 antigenically distinct capsule types (serotypes) have been identified, which vary in virulence. O-acetylation, a subtle capsular modification, contributes to pneumococcal capsular diversity and modulates capsular physicochemical properties and interaction with innate or vaccine immunity. Replacement serotypes (and future vaccine types) 33F and 15B have O-acetylated capsules due to membrane-bound O-acetyltransferases (MOATs). We hypothesize that future vaccine pressure targeting capsular O-acetylation may result in microevolution to de-O-acetylated types. Herein, we create and characterize MOAT-defective variants and determine the effect of O-acetylation loss on capsular shielding, colonization-like param-eters, and resistance to opsonophagocytosis. Loss of WcjE-mediated O-acetylation in serotype 33F enhanced survival after drying and may therefore enhance pneumococcal spread. Loss of WciG-mediated O-acetylation in serotypes 33A and 33F resulted in sero-logically and biochemically distinct capsule types, 33X1 and 33X2, which functioned as nonencapsulated pneumococci as indicated by increased biofilm formation, adhesion to nasopharyngeal cells, and susceptibility to opsonophagocytosis. Accordingly, serotypes 33X1 and 33x2 will not likely arise in invasive disease, but may be found in carriage or otitis media. In serogroup 15, using sensitive serological tools, we discovered that putatively de-O-acetylated serotype 15C actually retains 0.1-2% capsular O-acetylation and is dis-tinct from an experimental WciZ-null serotype 15X. However, loss of WciZ-mediated O-acetylation from serotype 15B capsule did not affect the biological functions of capsule. Antibodies elicited by the 23-valent pneumococcal polysaccharide vaccine (PPV23) tar-geted serotype 15B significantly but only slightly more than serotype 15C; therefore, in-clusion of serotype 15B in future conjugate vaccines may not expand serotype 15C. This thesis work primarily suggests that the biological and immunological conse-quences of capsular O-acetylation loss are polysaccharide-context dependent. Since con-tinual addition of emerging replacement serotypes to the conjugate vaccine increases vaccine valency and production costs, biological and immunological properties of possible variants and replacement serotypes should be considered in effort to choose vaccine candidate capsular types offering optimal cross-protection.

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