Summary Cellular lipid requirements are achieved through a combination of biosynthesis and import programs. Using isotope tracer analysis, we show that type I interferon (IFN) signaling shifts the balance of these programs by decreasing synthesis and increasing import of cholesterol and long chain fatty acids. Genetically enforcing this metabolic shift in macrophages is sufficient to render mice resistant to viral challenge, demonstrating the importance of reprogramming the balance of these two metabolic pathways in vivo. Unexpectedly, mechanistic studies reveal that limiting flux through the cholesterol biosynthetic pathway spontaneously engages a type I IFN response in a STING-dependent manner. The upregulation of type I IFNs was traced to a decrease in the pool size of synthesized cholesterol, and could be inhibited by replenishing cells with free cholesterol. Taken together, these studies delineate a metabolic-inflammatory circuit that links perturbations in cholesterol biosynthesis with activation of innate immunity.

Significance Potent antiviral CD4 Th1 responses generated at the onset of persistent infection are lost as infection progresses. However, it is unknown how CD4 T cell responses are mounted in the midst of an established persistent infection to restore the diminishing Th1 response. We report that an established persistent virus infection suppresses the induction and distribution of new virus-specific CD4 Th1 cells. The failure to generate new Th1 responses is mediated by chronic type I interferon (IFN-I) signaling, and its blockade effectively restored de novo Th1 development. Our study identifies a mechanism of immunosuppression and a method to restore Th1 generation during persistent infection.

Persistent viral infections are simultaneously associated with chronic inflammation and highly potent immunosuppressive programs mediated by IL-10 and PDL1 that attenuate antiviral T cell responses. Inhibiting these suppressive signals enhances T cell function to control persistent infection; yet, the underlying signals and mechanisms that program immunosuppressive cell fates and functions are not well understood. Herein, we use lymphocytic choriomeningitis virus infection (LCMV) to demonstrate that the induction and functional programming of immunosuppressive dendritic cells (DCs) during viral persistence are separable mechanisms programmed by factors primarily considered pro-inflammatory. IFNγ first induces the de novo development of naive monocytes into DCs with immunosuppressive potential. Type I interferon (IFN-I) then directly targets these newly generated DCs to program their potent T cell immunosuppressive functions while simultaneously inhibiting conventional DCs with T cell stimulating capacity. These mechanisms of monocyte conversion are constant throughout persistent infection, establishing a system to continuously interpret and shape the immunologic environment. MyD88 signaling was required for the differentiation of suppressive DCs, whereas inhibition of stimulatory DCs was dependent on MAVS signaling, demonstrating a bifurcation in the pathogen recognition pathways that promote distinct elements of IFN-I mediated immunosuppression. Further, a similar suppressive DC origin and differentiation was also observed in Mycobacterium tuberculosis infection, HIV infection and cancer. Ultimately, targeting the underlying mechanisms that induce immunosuppression could simultaneously prevent multiple suppressive signals to further restore T cell function and control persistent infections.

T-cell-dependent antigenic stimulation drives the differentiation of B cells into antibody-secreting plasma cells and memory B cells, but how B cells regulate this process is unclear. We show that LKB1 expression in B cells maintains B-cell quiescence and prevents the premature formation of germinal centers (GCs). Lkb1-deficient B cells (BKO) undergo spontaneous B-cell activation and secretion of multiple inflammatory cytokines, which leads to splenomegaly caused by an unexpected expansion of T cells. Within this cytokine response, increased IL-6 production results from heightened activation of NF-jB, which is suppressed by active LKB1. Secreted IL-6 drives T-cell activation and IL-21 production, promoting T follicular helper (T FH ) cell differentiation and expansion to support a~100-fold increase in steady-state GC B cells. Blockade of IL-6 secretion by BKO B cells inhibits IL-21 expression, a known inducer of T FH -cell differentiation and expansion. Together, these data reveal cell intrinsic and surprising cell extrinsic roles for LKB1 in B cells that control T FH -cell differentiation and GC formation, and place LKB1 as a central regulator of T-celldependent humoral immunity.