Despite the benefits of existing vaccines, Streptococcus pneumoniae is still responsible for the greatest proportion of respiratory tract infections around the globe, thereby substantially contributing to morbidity and mortality in humans. B-1 cells are key players of bacterial clearance during pneumococcal infection and even provide long-lasting immunity towards S. pneumoniae. Previous reports strongly suggest an essential role of the immunoinhibitory adapter Src homology domain 3 lymphocyte protein 2 (SLy2) for B-1 cell-mediated antibody production. The objective of this study is to evaluate S. pneumoniae-directed B cell responses in the context of SLy2 deficiency.
B-1 cell populations were analyzed via flow cytometry before and after pneumococcal immunization of SLy2-deficient and wild-type control mice. Global and vaccine-specific immunoglobulin M (IgM) and IgG antibody titers were assessed by enzyme-linked immunosorbent assay. To investigate survival rates during acute pneumococcal lung infection, mice were intranasally challenged with S. pneumoniae (serotype 3). Complementary isolated splenic B cells were stimulated in vitro and their proliferative response was assessed by fluorescent staining. In vitro antibody secretion was quantified by LEGENDplex.
We demonstrate increased frequencies of B-1 cells and elevated titers of preantigenic IgM in SLy2-deficient mice. In addition, these mice produce significantly more amounts of IgM and IgG upon pneumococcal vaccination. Knocking out SLy2 did not induce survival advantages in our murine model of acute pneumonia, indicating the presence of compensatory mechanisms.
Our results reveal reinforced specific antibody responses towards pneumococcal polysaccharides and enhanced IgG secretion as a consequence of SLy2 deficiency, which could be relevant to the development of more efficient vaccines.

© 2020 The Authors. Immunity, Inflammation and Disease published by John Wiley & Sons Ltd.