LAUSR

Abstract Background Streptococcus pneumoniae is the main bacterial cause of pneumonia in the United States and globally. Although pneumococcal conjugate vaccines are highly effective against invasive pneumococcal disease, they are less effective against pneumonia, particularly in the elderly and those with immune deficiency. Given the additional challenge of antibiotic resistance, immunotherapy holds promise for treatment of pneumococcal pneumonia. The current PCV13 vaccine is less effective against serotype (ST) 3, which carries a higher risk of mortality than other vaccine-included STs. Our group has previously identified murine monoclonal antibodies (mAb) to ST3 capsular polysaccharide (PPS3) that are protective in experimental models of sepsis and pneumonia. The aim of the present study is to isolate and develop PPS3-specific human monoclonal antibodies (humAbs) as adjunctive immunotherapy for pneumonia. Methods We sorted individual PPS3-specific memory B cells from PBMCs isolated on days 0 and 7 post-vaccination from pneumococcal polysaccharide (PPS)-based vaccine (Pneumovax or Prevnar13) recipients using fluorescently labeled PPS3. Immunoglobulin heavy (Igh) and light (Igl) chains were sequenced, cloned into IgG1 and κ or λ vectors, and expressed in HEK-293 cells. PPS3 specificity was confirmed using ELISA. Results Here, we report the first 7 PPS3-specific humAbs isolated: 5 used lambda light chains and two used kappa light chains. Six of these humAbs used variable heavy 3 (VH3) Igh gene elements. Kappa humAbs used VH3-30 or VH3-7, whilst lambda humAbs used VH3-9, VH3-72 or VH3-23. Sequence analysis revealed somatic mutations in complementary determining as well as framework regions. Initial studies show that some humAbs agglutinate ST3 in vitro. Structure-function relationship studies are ongoing to identify specific determinants of PPS3 binding and biological efficacy against ST3 in vitro and in vivo. Conclusion The results of this study provide further understanding of the biology of PPS3 antibodies and may facilitate design of adjunctive immunotherapy to treat and prevent ST3 disease. Disclosures All authors: No reported disclosures.