SLy2‐deficiency promotes B‐1 cell immunity and triggers enhanced production of IgM and IgG₂ antibodies against pneumococcal vaccine

Abstract

Background 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. Methods 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. Results 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 IgG2 upon pneumococcal vaccination. Knocking out SLy2 did not induce survival advantages in our murine model of acute pneumonia, indicating the presence of compensatory mechanisms. Conclusion Our results reveal reinforced specific antibody responses towards pneumococcal polysaccharides and enhanced IgG2 secretion as a consequence of SLy2 deficiency, which could be relevant to the development of more efficient vaccines.

SLy2‐overexpression impairs B‐cell development in the bone marrow and the IgG response towards pneumococcal conjugate‐vaccine

“…Surprisingly, the IgM serum concentrations were fully comparable within the genotypes, indicating that SLy2 controls IgM induction under TI, but not TD conditions (Figure 3). This is in accordance with recent findings regarding SLy2‐deficient mice 13 . By contrast, SLy2‐Tg mice produced significantly lower amounts of IgG 1 and IgG 3 than their Wt counterparts.…”

“…The investigation of SLy2‐deficient mice revealed an enlarged compartment of B‐1 cells and enhanced proliferative B‐cell responses in the absence of the adapter. Moreover, natural serum IgM levels are increased in these mice, accompanied by improved antibody responses to pPS 13,19 . On the contrary, SLy2‐transgenic (Tg) mice, specifically overexpressing the protein in T and B cells, harbor decreased proportions of B‐1 cells and significantly reduced titers of homeostatic IgM 12,17 .…”

The emerging and diverse roles of the SLy/SASH1‐protein family in health and disease—Overview of three multifunctional proteins

“…30,58,59 In line with that, we found natural IgM levels being significantly increased in the serum of SLy2 Ko mice at steady state. 57 Complementary, the antibody responses toward immunization with TI and TD antigens are enhanced in these mice, as seen for TNP-Ficoll, TNP-KLH, and pneumococcal vaccine (Pneumovax23 and Prevenar13). Elevated levels of Pneumococcus-specific IgM and IgG 2 antibodies after immunization point to a role of SLy2 for immune responses toward Streptococcus pneumoniae.…”

“…46 When it comes to bacterial infection, increased levels of S. pneumoniae-specific antibodies in SLy2 Ko mice were not sufficient to provide survival advantages in the course of acute pneumococcal pneumonia. 57 However, humoral immunity is especially important in the context of septicemia, while nasopharyngeal clearance of S. pneumoniae has been shown to require CD4 + T helper cells. 60 As a consequence, the role of SLy2 should be additionally investigated in the context of pneumococcal sepsis, where the disease outcome mainly depends on antibody-mediated mechanisms.…”

“…Of importance, two studies independently reported alterations within the natural B‐1 cell compartment of SLy2‐deficient mice. These include a significant increase in peritoneal IgM + CD5 + or B220 + CD5 + B‐1a cells and increased rates of BM‐resident CD19 + CD43 + CD5 ‐ B‐1b cells 46,57 . B‐1 cells are a unique subset of innate‐like B cells that constitutively produce protective antibody and rapidly respond toward invading pathogens such as Streptococcus pneumoniae 30,58,59 .…”