The Role of Checkpoint inhibitors in CIA

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Abstract Immune related adverse events linked to severe inflammatory arthritis have been identified in 10-43% of patients receiving immune checkpoint inhibitors, αCTLA4, αPD-1 or αPD-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 naïve 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.