TNFSF4 and rheumatoid arthritis: To better understand the association between the expression of OX40L by SMs and RA-related disease progression, we next analyzed three different CD4+ T cell subsets that are closely linked to OX40/OX40L signaling activity (Th1, Tregs, and Tfh) [19,23,24], using a flow cytometry-based approach to assess the relative frequencies of these three different subsets as a percentage of the overall CD4+ T cell population in RA patient synovial tissue samples.