The role of β₂ integrins on dendritic cells in tolerogenic and autoimmune environments

Abstract

The human immune system reacts swiftly to pathogens while remaining oblivious to self, an important concept termed self-tolerance. Dendritic cells (DCs) are pivotal in this process, as they mediate both initiation of adaptive immune responses as well as induction of tolerance. Recently, evidence has been accumulating that β₂ integrins, heterodimeric adhesion receptors that can assume an active conformation in response to stimulation through various receptors, may have both pro-inflammatory and immunoregulatory roles on DCs. In this thesis, I investigated the hypothesis that dysregulation of β₂ integrin signalling on DCs can contribute to aberrant inflammatory responses, such as in the autoimmune disease rheumatoid arthritis (RA). To test this hypothesis, I first developed and optimised a multi-colour flow cytometry panel to measure both expression (‘total’ β₂ integrin) and expression of the active conformation (‘active’ β₂ integrin) of three β₂ integrin subunits: CD11a, CD11b and the pairing subunit CD18. As conformational state of integrins is highly important for the functionality, this provided important proof of concept that simultaneous staining for both ‘total’ and activation-specific epitopes is possible. Then I utilised mature and tolerogenic monocyte-derived DCs (Mo-DCs) as a model system to explore the role of β₂ integrins in circumstances of immune activation and tolerance. I found that tolerogenic Mo-DCs expressed significantly more active CD11a, while expressing significantly less active CD11b compared to mature MoDCs. Furthermore, treating mature Mo-DCs with a CD11b-blocking antibody significantly decreased their T cell stimulatory ability, suggesting that CD11b might have pro-inflammatory roles on DCs. Lastly, I compared total and active β₂ integrin expression on DCs and monocyte populations in RA patients and healthy controls. While no differences in β₂ integrin expression were observed in peripheral blood (PB), comparing synovial fluid (SF) to PB of active RA patients showed that expression of total and active CD11a was significantly reduced in SF, while CD11b was significantly increased. To conclude, I found that CD11a and CD11b might play opposing roles on DCs in both tolerogenic and autoimmune environments. From my findings, it could be concluded that CD11a has immunoregulatory roles on DCs, while CD11b is proinflammatory.