# Polyclonal B cells acquire LCMV antigens in a GP1-dependent manner

## TL;DR

This study reveals how B cells acquire LCMV antigens for presentation to T cells, identifying a potential GP1 receptor on B cells.

## Contribution

The first evidence of a GP1 receptor on polyclonal B cells facilitating LCMV antigen acquisition is presented.

## Key findings

- Polyclonal B cells can acquire LCMV antigens without productive infection.
- LCMV-GP bearing particles of various sizes are accessible to B cells via pinocytosis.
- A GP1 receptor on B cells is suggested based on antibody blocking experiments.

## Abstract

The polyclonal, T-dependent nature of hypergammaglobulinemia during murine infection with LCMV is well defined, however the mechanism by which polyclonal B cells acquire antigens for presentation remains unknown. Here we use LCMV-specific CD4 + transgenic T cells to explore several hypotheses for B cell antigen uptake. We found that antigens produced by cells infected with LCMV in vitro are available to polyclonal B cells and their presentation to CD4+ T cells enabled robust co-activation. The in vitro nature of our model demonstrates that in vivo factors such as cytokine milieu and antigen release by NK/CD8+ are not required. We show that non-replicative UVC-irradiated LCMV enables antigen access to B cells, thus productive infection of B cells is not required for antigen acquisition. B cells loaded with LCMV antigens in vitro can efficiently present these antigens to CD4+ T cells in LCMV-infected mice, thereby validating our model in vivo. Using transmission electron microscopy we identified LCMV-GP bearing particles of various sizes including <50nm, a size accessible to B cells via pinocytosis. The particles morphologically resemble exosomes or viral particles (infective or defective interfering). We show that polyclonal B cell access to Ag is maintained when exosome release or viral defective interfering particle release is inhibited. Finally, we used a monovalent Fab derived from the LCMV-neutralizing antibody KL25 to show that access to the viral GP1 protein is important in order for polyclonal B cells to efficiently acquire LCMV antigen for presentation, suggesting the presence of a GP1 receptor on B cells.

Murine lymphocytic choriomeningitis virus (LCMV) infection is a powerful model that has enabled researchers to make countless important immunological discoveries. LCMV offers a convenient and well-characterized model to study acute versus persistent viral infections as well as many of the same immune dysregulations seen in human chronic viral infections. Specifically, LCMV causes T-dependent hypergammaglobulinemia which is more pronounced during chronic infections. In this paper, we use LCMV to study how polyclonal B cells acquire antigens to present to cognate helper T cells, a necessary event in establishing hypergammaglobulinemia. Our aim is that by solving this unanswered question with the LCMV model we may glean valuable insights that could lead to advances in combatting viral infections in humans. In this study we use various techniques including the direct assaying of antigen presentation between polyclonal B cells and LCMV specific CD4 T cells to exclude many possible hypotheses for B cell antigen acquisition. Collectively, the results of our study provides the first evidence for a receptor on polyclonal B cells that recognizes LCMV GP1.