B-Cell Mediated Humoral Responses Are Impaired in Sepsis Survivors

Sepsis survivors exhibit a sustained compromise in innate immunity, and high mortality rates due to increased vulnerability to infections. Previously we demonstrated in a cecal ligation and puncture (CLP) model, that sepsis survivors had a reduced TNFα response to LPS-challenge in vivo, which is accompanied by an increase in CD4⁺ChAT⁺ T cells. These data together with clinical observations of low endogenous levels of IgG in sepsis survivors led us to hypothesize that T-cell dependent (T-D) B cell responses might also be altered after CLP. In order to assess alterations in the adaptive humoral response in sepsis survivors, we immunized C57BL/6J sham surgery and CLP mice with a single intraperitoneal (I.P.) injection of NP (29)-KLH (T-D antigen) in alum 4 weeks post-CLP (or sham surgery). We observed sepsis survivors showed reduced serum NP-specific IgG and IgG1 antibodies compared to sham controls. Furthermore, to confirm whether this reduced immune response is mediated through a T-D or T cell-independent (T-I) type II mechanism, we used other carrier molecules CGG (T-D) and Ficoll (T-I), respectively. To evaluate time dependent kinetics, we immunized mice with single I.P. injection of either NP (20)-CGG or NP (55)-Ficoll. We analyzed both low-affinity and high-affinity anti-NP antibodies through NP (23)- and NP (7)- BSA ELISAs, respectively. We found a significant reduction in anti-NP IgG and IgG1 antibodies in CLP mice post-NP-CGG immunization (p<0.01 and p<0.001, respectively) while the response was not diminished after NP-Ficoll immunization at day 7; suggesting that compromised antibody response is mediated through a T-cell dependent pathway. Flow cytometry data also revealed that the splenic frequency of NP-specific CD138⁺ intracellular IgG1 plasma cells was decreased in CLP mice immunized with NP-CGG as compared to sham mice at day 7, 14 and 21 (p<0.001, p<0.05 and p<0.05, respectively). Since follicular dendritic cells (FDCs) play a key role in B-cell activation and affinity maturation of antibodies, we performed immunofluorescence staining of FDCs in spleens from sham surgery and CLP mice. Interestingly, we found a decreased number of FDC clusters in CLP mice compared to sham controls. In summary, our study is clinically relevant as it may explain the hypogammaglobulinemia seen in sepsis surviving patients.