CD4 and rheumatoid arthritis: When Leijtenet al. 50 compared PsA patients’ synovial fluid to rheumatoid arthritis synovial fluid, they saw that CD4+CD8+ lymphocytes were increased and that CD4+ Th17 and type 3 lymphocytes were also increased in the PsA population49, the same cells that produce IL-17A and IL-22.