Immune Training Enhances Anti-Viral Responses and Improves Outcomes in <i>Pax5</i>-/+ Mice Susceptible to Chronic Infection

Key words: Trained immunity, Pax5-/+ mice, B cell precursor acute lymphoblastic leukemia (B-ALL), lymphocytic choriomeningitis virus (LCMV), bone marrow microenvironment (BME), chronic infection Background: Pax5 heterozygosity (Pax5-/+), resulting in reduced PAX5 levels in mice, mimics germline or somatic PAX5 dysregulation, contributing to diseases including B-ALL. In contrast to the well-characterized roles of PAX5 during early B cell development, little is known about how Pax5 heterozygosity impacts anti-viral responses. Viral infections pose a significant global burden. Host susceptibility to chronic pathogens is determined by many factors including genetic variation leading to immunodeficient or dysregulated anti-viral immune responses. Aims: We aimed to characterize the effects of a chronic viral infection in the BME of the Pax5-/+ mouse model using a chronic strain of the lymphocytic choriomeningitis virus (LCMV). LCMV is a non-cytopathic virus that has been extensively utilized to investigate virus-induced immunopathology, effector responses and immune tolerance. Methods: Pax5-/+ mice backcrossed to the C57BL/6J background were infected with LCMV-Docile (1*106 PFU). Using flow cytometry, multiplex ELISA, plaque and antibody neutralizing assays, innate and late adaptive immune responses in infected Pax5-/+ and WT (Pax5+/+) were assessed followed by β-glucan pre-administration as a prophylactic prevention. Results: We found that infection with the LCMV chronic Docile strain resulted in decreased survival of Pax5-/+ mice (n of 12-21, P&amp;lt;0.05). Early (day 15) viral titers were not different between Pax5-/+ and WT mice in multiple organs and early adaptive CD8+ T cell (CTL) immunity was also robust in Pax5-/+ mice. However, LCMV-specific neutralizing antibody production was severely compromised in Pax5-/+ mice following chronic infection leading to impaired long-term viral clearance and a pro-inflammatory milieu in the BM or serum with CXCL9, CXCL10, CCL4 and CCL3 being elevated at day 120 post infection in Pax5-/+ mice. In addition, increased CTL exhaustion as well as reduced plasma cells were observed in the BM of Pax5-/+ mice (n of 3-5, P&amp;lt;0.05). Importantly, survival outcomes were improved upon prophylactic treatment with the β-glucan immune trainer through induction of heterologous protection against chronic infection (n of 6-10, P&amp;lt;0.05). β-glucan enhanced viral clearance, CTL immunity and reduced PD-L1 expression on monocytes in multiple LCMV-resident host organs. Conclusion: Our study showed that Pax5-/+ mice succumbed to LCMV chronic Docile strain due to impaired LCMV-specific neutralizing antibody production and long-term viral clearance, as well as increased inflammation. Prophylactic treatment with β-glucan improved survival by enhancing viral clearance, CTL immunity, and reducing monocyte immunosuppression. New insight from this study will help design effective prophylactic strategies against chronic viral infections particularly in genetically-predisposed susceptible hosts.