Study of Type I interferon responses during Orientia tsutsugamushi infection

Abstract

Type I Interferons (TI IFNs) induced by microbial infections play a pivotal role in protective immune responses. Recently, various bacterial pathogens have been shown to cause TI IFN production. However, the effect of TI IFNs induced by bacteria on immune responses is quite controversial. Orientia tsutsugamushi, an obligate intracellular bacterium that causes scrub typhus, can also induce TI IFNs during mammalian infection, but the effect of TI IFNs on immune responses against O. tsutsugamushi remains elusive. Here, I studied the molecular details of the induction of TI IFNs during O. tsutsugamushi infection in vitro and the role of TI IFNs in generating adaptive immune responses against the bacterial pathogen in vivo. Induction of TI IFNs during O. tsutsugamushi infection was consistently observed in mouse embryonic fibroblasts (MEF) and bone marrow-derived macrophages (BMDM). Using various MEF and BMDM derived from specific transgenic knockout (KO) mice, I searched for signaling molecules required for induction of TI IFNs by O. tsutsugamushi infection. Studies showed that IKK-, RIG-I, cGAS, MAVS, and STING are required for the induction of TI IFNs, but signaling adaptors involved in TLR and NOD signaling are dispensable. In vitro replication of O. tsutsugamushi in MEF or BMDM deficient in a gene encoding the TI IFN receptor (IFNAR) did not significantly change when compared to wild type cells. In addition, the survival rate of TI IFNAR KO mice lethally challenged with O. tsutsugamushi was similar to that of wild type mice, indicating that T1 IFN responses do not play a significant role in protective immunity during primary infection of O. tsutsugamushi. Nevertheless, it is interesting to note that early antibody responses against 56 kD type specific antigen (TSA56), a major outer membrane protein, were significantly delayed in IFNAR KO mice when compared to wild type. These findings correlate with a delayed alteration of follicular architecture in the spleen as well as a delay in active differentiation of follicular helper T (TFH) cells in the spleen of IFNAR KO mice infected with O. tsutsugamushi, suggesting that T1 IFNs may affect the generation of specific antibody responses via promoting the differentiation of naïve CD4+ T cells into TFH cells in secondary lymphoid organs. Also, CD8+ memory T cell responses against O. tsutsugamushi antigen are significantly reduced in IFNAR KO mice whereas CD4+ memory T cell responses remain intact. Therefore, T1 IFNs produced by O. tsutsugamushi infection do not significantly affect protective immunity during primary infection, but they might promote the generation of specific adaptive immune responses by supporting the differentiation of TFH cells as well as enhancing the generation of CD8+ memory T cell responses.