Ectopic B cell response to Ehrlichia muris in the liver

Abstract

Abstract The development of high-affinity, isotype-switched Abs and memory cells is generally facilitated by Germinal Center reactions (GCs) that form in the secondary lymphoid organs within 4–5 days of antigen encounter. However, in the case of Ehrlichia muris (E. Muris), an obligate intracellular tick borne pathogen, GCs are not detectable up to day 35. Instead a robust B cell response dominated by IgM antibody forming cells (AFCs) originates from extrafollicular (EF) sites in the spleen, with somewhat lower, but still large frequencies of IgG AFCs. Interestingly, E. Muris is known to infect the liver, and we found both AFCs as well as histologic collections of plasmablasts in the liver parenchyma. In both cases the number of total AFCs was far larger than that which bound to lysates that contain E. Muris antigens, suggesting either a non-specific or very low affinity response. We hypothesize that these EF sites to be locations where B cells undergo somatic hypermutation (SHM) and acquire mutations that increase affinity to E. muris. Such mutations would allow what appear to be low affinity non-specific responses to gain detectable affinity for E. Muris. To test this, we performed laser microdissection of these plasmablast patches followed by sequencing of BCR variable regions. These studies revealed that somatic hypermutation occurred in the spleen as well as liver and thus demonstrated that liver is a novel locus for generating B cell immune responses that include V region mutation and affinity-based selection.