Scrub typhus weakens humoral immune responses through suppression of germinal center and B cell activation

Abstract

Abstract Scrub typhus is a poorly studied, but life-threatening, disease due to uncontrolled growth of Orientia tsutsugamushi bacteria within endothelial cells and phagocytes, in addition to dysregulated Th1 immune responses and inflammation. Cellular and humoral immunity in infected patients and experimental animals are short-lived, and wane within 1–2 years; however, the underlying mechanism of this transient immunity is unclear. No reports have examined splenic germinal center (GC) or B cell activation during this infection. The study objective is to fill this knowledge gap by using endothelium-targeted C57BL/6 mouse models that mimic pathological features of human scrub typhus. Through NanoString, qRT-PCR, and flow cytometry, we found significant repression of key markers crucial for B cell development (CCR7, BAFFR, PAX5) and GC formation (SLAMF1, ICOS-L, CXCR5, BCL-6) in both mild and severe disease outcomes. The widespread impairments in B and T follicular helper (Tfh) cell activation correlated with excessive Th1 cell activation during disease progression. Immunofluorescence staining (B220, CD3, GL7) revealed evidence of GC formation with sublethal infection, but the near absence of organized GCs at late stages of lethal infection, which was consistent with results from our splenic multi-color flow cytometry and serum antigen-specific ELISA assays for IgG isotypes. Ongoing RNA-seq studies for splenic B cells isolated from sub-lethally or lethally infected mice will provide in-depth, B cell-specific insight during infection. In conclusion, this is the first study to show profound alterations in B cell activation and GC formation in murine models, which helps understand “poor” humoral immunity seen with human scrub typhus. Supported by grants from NIH (R01 EY028773 to JS, R01 AI132674 to LS, R21 AI156536 to LS)