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SUMMARYMemory B cells are key providers of long-lived immunity against infectious disease, yet in chronic viral infection they do not produce effective protection. How chronic viral infection disrupts memory B cell development, and whether such changes are reversible, remains unknown. Here, we uncover type-I interferon (IFN-I) dynamics as a key determinant in shaping chronic memory B cell development. Through single-cell (sc)ATAC-sequencing and scRNA-sequencing, we identified a unique memory subset enriched for IFN-stimulated genes (ISGs) during chronic lymphocytic choriomeningitis virus infection. Blockade of IFNAR-1 early in infection transformed the chromatin landscape of chronic memory B cells, decreasing accessibility at ISG-inducing transcription factor binding motifs and inducing a phenotypic change in the dominating memory B cell subset. However, timing was critical, with memory B cells resistant to intervention after 4 weeks post-infection. Together, our research identifies a key mechanism to instruct memory B cell identity during viral infection.One Sentence Summary:IFN dynamics in chronic versus acute viral infection determines memory B cell development.

TVM cells are antigen-naïve, semi-differentiated CD8 T cells that develop in response to common γ (γc) chain cytokines, like IL-15 and IL-4. TVM cells respond more rapidly than conventional naïve CD8 T cells to antigen stimulation, thereby providing early control of viral and bacterial infections. TVM cells comprise 20% of the naive CD8 T cell pool in young mice but are selectively retained and make up to 50% of all antigenically naïve CD8 T cells in aged mice. In addition, aged TVM cells exhibit markers of senescence and lose proliferative capacity compared to young TVM cells. Recently, it was shown that IL-4 generated during helminth infection drove TVM cell expansion and improved pathogen clearance in subsequent infections. However, it is not known if the expanded subset is maintained with age or whether helminth infections similarly expand or improve the function of aged TVM cells. To evaluate this, we infected young mice with the helminth, Heligmosomoides polygyrus, and aged them to over 18 months. There was no difference in number or function of TVM cells in previously infected as compared to naïve aged mice. We then infected aged mice with helminths and observed no increase in TVM cell numbers, but there was a reduction in IFN-γ production by aged TVM cells. When naïve TVM cells were transferred to a young helminth infected host, proliferation of aged TVM cells was markedly reduced as compared to young TVM cells. Collectively these data suggest that, the protective effect of helminth infection in young mice, mediated by TVM cell expansion, is firstly not maintained into advanced age. Secondly, this effect cannot be generated in aged mice due to aged TVM cells being intrinsically less responsive to the helminth-induced cytokine environment.