Differential spleen immune signatures and germinal center responses during acute infection with Orientia tsutsugamushi Karp versus Gilliam strains.

INTRODUCTION

Scrub typhus is an emerging and neglected tropical disease caused by Orientia tsutsugamushi (Ot). Immunity in scrub typhus patients is known to be short-lived; however, its underlying mechanisms remain unclear. No reports have examined humoral immune signatures to clinically prevalent Ot strains.

METHODS

We compared two clinically relevant Ot strains, Karp and Gilliam, in a C57BL/6 murine model. Using equivalent infectious doses, we assessed splenic B cell and germinal center (GC) responses during acute infection through flow cytometry, immunohistology, serological analyses, and RNA sequencing.

RESULTS

Karp infection resulted in high tissue bacterium burdens and 50% mortality rates, whereas Gilliam infection was self-healing with limited bacterium dissemination and growth. Yet, Gilliam induced strong splenic B cell responses, as judged by total numbers of B cells, follicular B cells, and marginal zone B cells, which correlated with serum IgG and IgM levels. Given that Karp- but not Gilliam-infected spleens displayed GC disorganization/loss and MZ abrogation, we compared splenic RNAseq profiles. On day 4 post-infection, Karp induced significant elevation of transcriptional inflammatory signatures (Ccl2, Il33, Ifng) and inflammatory gene pathways (Il1, Il6, Tnf, Ifng), the characteristics of severe scrub typhus. Moreover, IPA analysis revealed Karp-induced neutrophil activation/degranulation and defense response pathways, which were in sharp contrast to Gilliam-induced upregulation of phagocytosis signaling pathways. Splenocyte flow cytometry confirmed significantly higher influx of activated phagocyte subsets (neutrophils, M1 macrophages, and inflammatory monocytes) during Karp infection than Gilliam infection, indicating greater infiltration of innate immune cells in severe scrub typhus.

DISCUSSION: Collectively, this study provides the first lines of evidence for Ot strain-related, cellular and humoral immune signatures in the spleens, which help understand differential host immune responses during acute scrub typhus.