Investigation into immune tolerance

Abstract

The immune system faces a unique conundrum of balancing the need to defend the body against pathogens and concurrently avoiding producing cells that binding self-tissues. This thesis highlights the complex nature of control of immune tolerance. In this thesis we have explored the physiological control mechanism applied to self-reactive B cells in the periphery. Utilising a mouse model, we generated B cells displaying an antibody that cross-reacts with two related protein antigens expressed on self versus foreign cells. These self-reactive peripheral B cells previously thought of as permanently silenced by clonal anergy, could become “reawakened” and enter the germinal centre. This thesis describes a mechanism by which these anergic cells actively acquire the capacity to distinguish between structurally distinguish foreign from self. This mechanism has broad implications for generating neutralising antibodies against pathogens that mimic self-antigens. In addition to exploring these normal tolerance mechanisms this thesis has also explored the mechanisms and consequences of failures of a specific failure of immune tolerance in lipopolysaccharide-responsive beige-like anchor (LRBA) deficiency. To understand the pathogenesis of the syndrome, C57BL/6 mice carrying a homozygous truncating mutation in Lrba were produced using CRISPR/Cas9-mediated gene targeting. These mice revealed that LRBA has a critical, cell-autonomous role in promoting cytotoxic T-lymphocyte antigen-4 (CTLA-4) accumulation within CD4 effector T cells and FOXP3+ T-regulatory cells. Low CTLA-4 did not translate into increased CD86 on B cells unless the LRBA-deficient mice were immunised, and neither immunisation nor chronic lymphocytic choriomeningitis virus infection precipitated immune dysregulation. LRBA deficiency did not alter antigen-specific B-cell activation, germinal centre formation, isotype switching or affinity maturation. Paradoxically, CD86 was decreased on germinal centre B cells in LRBA-deficient mice, pointing to compensatory mechanisms for controlling CD86 in the face of low CTLA-4. These results add to the experimental rationale for treating LRBA deficiency with the CTLA4-Ig fusion protein, Abatacept, and pose questions about the limitations of laboratory experiments in mice to reproduce human disease in natura. These two areas of research within immune tolerance are distinct yet complementary, and have provided broad insights into the complexities of the immune system.