Title: Class II MHC trafficking and XBP-1 in immune cells
Description: This dissertation describes two separate lines of inquiry into cell biological processes that are required for proper immune function. These two areas of inquiry are: the role of class II MHC localization within B cells and dendritic cells, and the role of XBP-1 in B cell differentiation into plasma cells. In order to understand whether localization of class II MHC to multivesicular bodies (MVBs) was functionally important we generated a mouse that expressed a dominant negative version of the protein Skd1 (Skd1-DN). Skd1-DN has previously been shown to prevent the formation of MVBs. In the mouse model that we generated the Skd1-DN protein was expressed at levels that were too low to interfere with MVB formation in B cells, and thus the importance of class II MHC localization to MVBs could not be studied. Class II MHC molecules were found to be modified by ubiquitin on the cytoplasmic tail of the β-chain. Mutant class II MHC molecules that could not be modified with ubiquitin assembled normally, but accumulated at the cell surface. The mislocalization of these mutant class II MHC molecules implies the existence of a pathway that controls class II MHC localization in a ubiquitin-dependent manner.

A mouse model was generated in which the endogenous β-chain locus was replaced with a mutant β-chain that cannot be modified with ubiquitin. We expect that this mouse model will be useful in determining the importance of class II MHC ubiquitination in the context of an immune response. The differentiation of B cells into plasma cells requires the unfolded protein response transcription factor XBP-1, since B cells that lack XBP-1 fail to fully differentiate into plasma cells. We found no evidence for an accumulation of unfolded proteins in B cells that lacked XBP-1, indicating that XBP-1 is not required by B cells to ensure the correct folding of proteins. We did find that XBP-1 deficiency compromised signaling via the BCR, as shown by decreased phosphorylation of downstream signaling components upon activation with antigen. Additionally XBP-1-deficient B cells misregulated the plasma cell specific transcription factors IRF4 and BLIMP1. We propose that the decreased efficiency of BCR signaling and the misregulation of transcription factor levels in XBP-1-deficient B cells contribute to their failure to differentiate into plasma cells.