Programmed Death-1 (PD-1)-Expressing T-Cells are Pathogenic in Early Rheumatoid Arthritis and Correlate with Response to Treatment

Aim: Programmed death protein 1 (PD-1) expressing T cells are expanded in the circulation of individuals with rheumatoid arthritis (RA). However, little is known about the functional role of these cells in disease pathogenesis, particularly those of the CD8+ lineage. To address this, we sought to investigate the genetic profiles of CD4+ and CD8+ PD-1-expressing lymphocytes of the synovial infiltrate, and to characterise these cells in disease.

Method: Isolated peripheral blood mononuclear cells (PBMCs) from patients with established RA (n=5) were sorted into CD4+/CD8+ PD-1+/- populations by fluorescence activated cell sorting. mRNA was then isolated, converted into cDNA libraries, before subjection to total RNA sequencing using a NextSeq500. In conjunction with this, quantitative reverse-transcription PCR was used to support our findings. Finally, we assessed for alterations in CD4+PD-1+ and CD8+PD-1+ cell gene expression in synovial tissue (ST) biopsies (n=19) before and after six-month triple disease modifying anti-rheumatic drug (tDMARD) treatment.

Results: Comparisons of the gene signatures between CD4+PD-1+ vs. PD-1- cells, and CD8+PD-1+ vs. PD-1- cells from early RA PBMCs identified significant upregulation of genes involved in the inflammatory response including IL21, CXCL13, and c-MAF, and in pathways including antigen presentation, T helper cell differentiation, and T cell exhaustion. Analysis of the gene signatures from early RA ST before and after six-month tDMARD treatment revealed down-regulation of the CD4+PD-1+ and CD8+PD-1+ gene signatures (including genes such as PDCD1, CXCL13, and CTLA4) following treatment.

Conclusions: Our study reveals the pathogenic nature of CD4+ and CD8+ PD-1+ cells in RA, and also suggests a mechanism through which tDMARDs exert their effect through influencing T cell populations in the synovial compartment. Furthermore, we identify factors associated with B cell help that are enhanced in the RA ST compared with PBMCs, highlighting the importance of these molecules in driving inflammation within the synovium.