Studies have established a role for T cells in resolving persistent viral infections yet emerging evidence indicates that both T and B cells are required to control some viruses. This marked improvement in antiviral humoral immunity did not rely on the cessation of IFN-I signaling in B cells but on alterations in the virus-specific CD8+ T cell response. Using two-photon microscopy and Gimeracil in vivo calcium imaging we observed that cytotoxic T lymphocytes (CTLs) productively engaged and killed LCMV-specific B cells in a perforin-dependent manner within the first few days of infection. Blockade of IFN-I signaling protected LCMV-specific B cells by promoting CTL dysfunction. Therapeutic manipulation of this pathway may facilitate efforts to promote humoral immunity during persistent viral infection in humans. Our findings illustrate how events that occur early after infection can disturb the resultant adaptive response and contribute to Gimeracil viral persistence. INTRODUCTION Humoral responses depend in part on B cells engaging cognate antigens and interacting with CD4+ helper T cells. This is usually followed by the subsequent coordination of antibody-secreting cell (ASC) differentiation germinal RHOH12 center (GC) development to facilitate antibody affinity maturation and memory B cell generation (1 2 Because humoral responses are simultaneously susceptible to shifts in direct costimulatory and inhibitory signals to B cells as well as those that affect the differentiation and activation of their partner CD4+ helper T cells driving a successful anti-pathogen humoral immune response is highly dependent upon the pathogen in question (1 3 In particular viral infections exhibit different patterns with some being cleared Gimeracil rapidly and others establishing long-term persistence (4 5 In animals disruptions in humoral immunity due to disturbances in the B cell compartment or CD4+ helper T cell functions compromise antiviral immunity to numerous viral pathogens (6-9). Chronic noncytopathic viral infections such as hepatitis C virus (HCV) hepatitis B virus (HBV) HIV and lymphocytic choriomeningitis virus (LCMV) elicit poor neutralizing antibody responses even after the acute phase of viral replication has passed (10-13). In mice intravenous inoculation with the Armstrong strain of LCMV results in an acute infection that is typically cleared within 1 week (14). By contrast infection with persistence-prone strains such as clone 13 (CL13) results in prolonged viremia and viral reservoir occupancy similar to that observed in chronic human infections (5 14 Study of the LCMV model has generated numerous insights into T and B cell biology (13 17 18 Chronic viral infections are known to perturb B cell responses typically resulting in excessive proliferation and differentiation as well as ectopic follicle formation (19-21). The development Gimeracil of neutralizing antibodies to CL13 is substantially delayed and serum viral titers often decline to undetectable levels before neutralizing antibodies to the virus emerge (22). Consequently B cells and humoral immunity in general were postulated to play no role in viral clearance even though LCMV infection elicits a very large humoral response that is almost entirely directed toward nonneutralizing epitopes (13). Although nonneutralizing specificities have been shown to be beneficial (22 23 the failure to generate neutralizing antibodies to LCMV has been a source of much investigation over the past several decades. Many factors were shown to negatively affect neutralizing antibody development after LCMV infection. Several of these factors stem from the magnitude of the CD8+ and CD4+ T cell responses elicited by LCMV (24 25 Specifically earlier studies reported that cytotoxic T lymphocyte (CTL)-mediated disruptions in normal splenic architecture and chemokine guidance create antigen nonspecific immunosuppression (26 27 Similarly the massive CD4+ T cell expansion and T follicular helper lineage commitment bias after infection was also shown to impair humoral immunity because partial removal of CD4+ T helper cell function resulted in accelerated neutralizing antibody responses (28). Another hypothesis set forth to Gimeracil explain the delayed development of LCMV-specific neutralizing antibodies is that the endogenous B cell repertoire lacks reactivity against the LCMV glycoprotein (GP) or that B cells with this reactivity simply fail to expand after infection (29). A sophisticated series.