The role of immunoproteasome subunit LMP7 in modulating T-helper cell differentiation and progression of autoimmune diseases

Abstract

The Proteasome is the central proteolytic machinery in cells, which plays an important role in antigen processing. The processing of the antigens is done by the catalytic active sites present in the inner -ring of the 20S core subunit of the proteasome. These active sites are responsible for the generation of ligands that can be loaded on MHC class I molecule and presented to T-cells for mounting an immune response. The inner -ring harbours six catalytic active sites-two copies each of 1, 2, and 5, which are responsible for caspase-like, trypsin-like, and chymotrypsin-like activity.In the cells of the haematopoietic origin like lymphocytes and monocytes, these constitutive proteasome subunits are replaced by the immunoproteasome subunits designated as 1i (LMP2), 2i (MECL) and 5i (LMP7) which causes a marked change in their cleavage preferance and efficient processing of MHC-I ligands. This thesis identified a novel role of the immunoproteasome subunit LMP7 in the progression of autoimmune diseases like rheumatoid arthritis and colitis. We also looked into the molecular details of this novel finding focussing on transcription factors and regulatory proteins especially with respect to different T-helper cell differentiation pathways like Th1, Th17, and Tregs, which are involved in the progression of autoimmune diseases. We also studied the role of the non-cytokine immunomodulator prostaglandin E2 in the production of key cytokines involved in different T-helper cell differentiation pathways.In chapter I, we characterized PR-957 which is a selective inhibitor of the immunoproteasome subunit LMP7, the subunit responsible for the chymotrypsin-like activity of the immunoproteasome. We could show that treatment with PR-957 resulted in an attenuation of disease progression in mouse models of rheumatoid arthritis with reduction in cytokine production and inflammation.In chapter II, we looked into the molecular mechanism as how LMP7 is involved in the progression of autoimmune diseases. We could show both in-vitro and in-vivo that LMP7 specific inhibition or deficiency results in reduced Th1 and Th17 differentiation but enhances regulatory T-cell differentiation. This blocking of Th1 and Th17 differentiation by LMP7-specific inhibition was due to reduced phosphorylation of STAT1 and STAT3, respectively. The enhancement of regulatory T-cells was attributed to increased phosphorylation of SMAD proteins. These findings were confirmed in mouse models of colitis in-vivo.In chapter III, we looked at the effect of the non-cytokine immunomodulator prostaglandin E2 on IL-23 production from human monocytes. We could show that PGE2 blocks the production of both IL-12 and IL-23 from human monocytes in a cAMP-dependent manner by inhibiting the transcription of the common p40 subunit.Taken together, the data in this thesis supports a novel role of the immunoproteasome subunit LMP7 in modulating different T-helper cell differentiation pathways and provide a therapeutic rationale for targeting LMP7 in autoimmune disorders.