Alterations in Germinal Center Formation and B Cell Activation during Severe Orientia tsutsugamushi Infection in Mice

Scrub typhus is a poorly studied but life-threatening disease caused by the intracellular bacterium Orientia tsutsugamushi (Ot). Cellular and humoral immunity in Ot-infected patients is not long-lasting, waning as early as one-year post-infection; however, its underlying mechanisms remain unclear. To date, no studies have examined germinal center (GC) or B cell responses in Ot-infected humans or experimental animals. This study was aimed at evaluating humoral immune responses at acute stages of severe Ot infection and possible mechanisms underlying B cell dysfunction. Following inoculation with Ot Karp, a clinically dominant strain known to cause lethal infection in C57BL/6 mice, we measured antigen-specific antibody titers, revealing IgG2c as the dominant isotype induced by infection. Splenic GC responses were evaluated by immunohistology, co-staining for B cells (B220), T cells (CD3), and GCs (GL-7). Organized GCs were evident at day 4 post-infection (D4), but they were nearly absent at D8, accompanied by scattered T cells throughout splenic tissues. Flow cytometry revealed comparable numbers of GC B cells and T follicular helper (Tfh) cells at D4 and D8, indicating that GC collapse was not due to excessive death of these cell subtypes at D8. B cell RNAseq analysis revealed significant differences in expression of genes associated with B cell adhesion and co-stimulation at D8 versus D4. The significant downregulation of S1PR2 (a GC-specific adhesion gene) was most evident at D8, correlating with disrupted GC formation. Signaling pathway analysis uncovered downregulation of 71% of B cell activation genes at D8, suggesting attenuation of B cell activation during severe infection. This is the first study showing the disruption of B/T cell microenvironment and dysregulation of B cell responses during Ot infection, which may help understand the transient immunity associated with scrub typhus. Author SummaryScrub typhus is an understudied disease caused by the intracellular bacterium O. tsutsugamushi. A hallmark of scrub typhus is the unexplained, non-durable immunity after infection. While cellular immune responses are known to be important for controlling this infection, our understanding of B cell and GC responses remains limited. In this study, we examined B cell activation and GC responses using our recently established murine model of severe scrub typhus. We measured bacterial antigen-specific antibody titers and identified IgG2c, an IFN-{gamma}-driven isotype, as the major IgG subtype. We also found that O. tsutsugamushi infection disrupted spleen morphology, exemplified by randomly dispersed T cells and lost GC structures. Transcriptomic analysis of purified splenic B cells demonstrated reduced expression of genes encoding critical adhesion and co-stimulation molecules, including GC-adhesion gene S1PR2, at severe stages of infection. Several humoral immune response pathways relevant to B cell receptor signaling, B cell activation and differentiation were significantly downregulated during infection. This study is the first report describing humoral immunity in a scrub typhus mouse model and provides detailed evidence that B cell and GC responses are impaired during acute infection.