Immune related adverse events linked to severe inflammatory arthritis have been identified in 10-43% of patients receiving immune checkpoint inhibitors, ?CTLA4, ?PD-1 or ?PD-L1, indicating the importance of immune inhibitory pathways in maintaining homeostasis in the joints. We obtained human synovial fluid and found a high frequency of PD-L1 expression by synovial macrophages. We propose to examine the role of PD-L1 expression in synovial macrophages and T cells during murine collagen induced arthritis (CIA). We found that immune checkpoint inhibitor, PD-L1, is primarily expressed in the joint compared to other immune checkpoints. Additionally, we found synovial macrophages and CD3+ T cells are the main PD-L1 expressing cells in the joint, and PD-L1 expression is increased during CIA. We extensively profiled the synovial macrophages and found three populations of PD-L1 expressing cells: F4/80intCD115+CCR2-MHCII-CD73-Tim4-PD-L1hi (CD115+ macrophages), F4/80hiCD115-CCR2intMHCIIhiCD73intTim4intPD-L1int (MHCII+ macrophages), and F4/80intCD115- CCR2lowMHCII-CD73loTim4intPD-L1int (F4/80+ macrophages). We have also found that PD-L1 has the highest frequency of expression in CD4+ and CD8+ T cells. Furthermore, we identified a novel subset of joint tissue resident memory T cells in the nave joint that are phenotypically CD8+CD69+ and CD8+CD69+CD103+. We hypothesize that the expression of PD-L1 by macrophages and/or T cells are essential for synovial homeostasis and regulation of joint inflammation.
In Aim 1, we will examine the importance of PD-L1 expression by synovial macrophages through the selective deletion of PD-L1 using the cre/flox system. We will generate two macrophages specific PD-L1 KO mice, LyzcrePD-L1fl/fl and CD115crePD-L1fl/fl. We will induce CIA in LyzcrePD-L1fl/fl, CD115crePD-L1fl/fl, Lyzcre and CD115cre mice and harvest the joint to determine the protective role of synovial macrophage PD-L1 expression in CIA (Subaim 1.1). We will also perform parabiosis experiments by surgically joining CD45.1 C57BL6 and CD45.2 CD115crePD-L1fl/fl to determine if PD-L1+ synovial macrophages are resident or bone marrow derived during CIA (Subaim 1.2). We have found that PD-L1 is highly expressed in synovial T cells.
In Aim 2, we will examine the protective function of synovial T cells following CIA. We will use LckcreP D-L1fl/fl mice, which lack PD-L1 expression in CD3+ T cells to examine the effects of PD-L1 specific deletion in T cells during CIA (Subaim 2.1). To test if tissue resident memory T cells play a protective role during CIA, we will utilize a tissue resident chimera mouse model to selectively delete peripheral T cells in LckcrePD- L1fl/fl mice. Donor T cells from LckcrePD-L1fl/fl or Lckcre mice will replace peripheral T cell populations to confirm the protective function of tissue resident memory T cells in the joint (Subaim 2.2). Our project has high translational potential since joint specific PD-L1 expression could be modulated with therapeutic purposes in humans for rheumatoid arthritis or check point inhibitors induced arthritis.
This project will determine the importance of PD-L1 expression on macrophages and T cells within the joint during synovial inflammation during collagen induced arthritis. This will allow for more informed study into the treatment of immune related adverse events due to checkpoint inhibitor treatment and rheumatoid arthritis.
