P337: LONG TERM INFECTION-DRIVES CHANGES IN THE BONE MARROW MICRO-ENVIRONMENT OF PAX5+/- MICE PREDISPOSED TO B CELL PRECURSOR ACUTE LYMPHOBLASTIC LEUKEMIA

Abstract

Background: Pax5 heterozygosity (Pax5+/-) in mice mimics germline or somatic Pax5 dysregulation (resulting in reduced Pax5 levels) observed in childhood B-cell precursor acute lymphoblastic leukemia (BCP-ALL). While a link between general non-specific infectious exposure and BCP-ALL has been demonstrated in the Pax5+/- mouse model, the effects of a specifically tractable infection are poorly understood. In particular, little is known about potential interactions of pre-leukemic early progenitor B cell populations and immune cells present in the bone marrow microenvironment (BME) during an infection. Aims: We aimed to characterize the short-term and long-term effects of a viral infection in the BME of the Pax5+/- mouse model employing 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 (N10) were infected with LCMV. Using flow cytometry, ELISA and plaque assay, innate and late adaptive immune responses in infected Pax5+/- and WT (Pax5+/+) were assessed. Pre-leukemic early progenitor B cell populations, antigen-specific CD8+ T cells and regulatory T cells (Treg) were evaluated in the BME. This was complemented by short-term ex vivo assays where B cells were stimulated with differing immune stimuli. Results: When we investigated immune responses in a short-term ex vivo culture system using the bone marrow of Pax5-/+ and WT mice, we found differences in the induction of the interleukin 7 receptor (IL-7r) on immature Pro-B cells as well as the production of cytokines (IL-6 and TNF-▫▫) in response to the toll-like receptor (TLR7) agonist R848 and the lymphocytic choriomeningitis virus (LCMV) Docile strain. When we infected Pax5-/+ and WT mice with LCMV-Docile, we found that innate (early interferon production) and adaptive immune responses (tetramer positive CD8+ T cells and early viral titers) were not intrinsically impaired in Pax5-/+ mice. However, at day 90 post-infection, LCMV-specific T cells were present within the BME and tetramer specific CD8+ T cells raised against the LCMV nucleocapsid protein (np 396) were increased in the BM of Pax5-/+ infected mice (Figure 1A). Furthermore, when we evaluated Major Histocompatibility Complex Class II (MHC-II) expression on different B cell subsets in the BME of infected and uninfected mice, we found that there was a significant upregulation in all immature B-cell subsets (Pro-B, PreBI-II) in response to infection in the WT but not in the Pax5-/+ infected mice. Pax5-/+ mice were susceptible to LCMV infection resulting in shorter long-term survival compared to infected WT mice (Figure 1B). Image:Summary/Conclusion: This study demonstrates that the BME environment as well as survival of pre-leukemic Pax5-/+ mice is differently affected by LCMV infection supporting the observations that immune dysregulation could contribute to the emergence of the leukemic clone1. 1Supported by the Carreras foundation project no. DJCLS07R/2019