Abstract
A breakdown in B cell self-tolerance can lead to antibody-mediated autoimmune
disease. This thesis aims to explore how B cell tolerance can be broken in two distinct,
but complementary projects within the context of the Goodnow somatic mutation
hypothesis. In both scenarios, B cells that escape self-tolerance and generate
autoantibodies are referred to as “rogue” B cells.
First, this thesis aimed to elucidate the precise steps undertaken by expanded rogue
B cell clones in patients with chronic Hepatitis C virus (HCV)-associated
cryoglobulinemic vasculitis, an autoimmune disease characterised by the production
of a rheumatoid factor cryoglobulin autoantibody. The rogue B cell clones in the HCV
cryoglobulinemic vasculitis patients were confirmed to be the source of the
autoantibody. The rogue B cell clone precursor antibodies failed to bind the HCV
envelope glycoprotein E2, yet bound multimerised self-antigen IgG relative to
membrane IgM density. These findings disfavour a molecular mimicry hypothesis, and
instead indicate IgG immune complexes may be sufficient to drive recruitment of the
rogue B cell clone precursors. Finally, the rogue B cells clones were found to carry
somatic lymphoma-associated, non-immunoglobulin gene mutations and
chromosomal aberrations, predicted to cause hyperactivation of the NF-kB signalling
pathway and escape of B cell tolerance. This finding provides additional evidence in
support of the Goodnow somatic mutation hypothesis.
Second, this thesis examined rogue germinal centre (GC) B cells that arise in the
absence of the receptor FAS. Rogue GC B cells loose specificity for the foreign antigen
and incidentally generate autoantibodies. However, the accumulation of rogue GC B
cells cannot be explained by our current understanding of affinity-based selection in
the GC. This work revealed rogue GC B cells, unlike “conventional” GC B cells
undergoing affinity maturation to the foreign antigen, can be identified by low
expression of CD21 and high expression of B220 (CD21loB220hi). Moreover, rogue
GC B cells were found to be rapidly entering cell cycle, enriched for a dark zone
phenotype and T-cell dependent, reminiscent of positively selected GC B cells. Thus,
rogue GC B cells typically removed by FAS, likely persist in the competitive GC
microenvironment despite their loss of BCR specificity to foreign antigen, because they
retain the capacity to undergo T-cell dependent positive GC selection.