CD4 and rheumatoid arthritis: Among the many environmental factors described as being able to affect the generation of Mo-DC, four of them appear to be particularly relevant in the context of RA: (i) the dietary metabolites that are agonists of the aryl hydrocarbon receptor (AhR), (ii) the extracellular acidosis, (iii) the GM-CSF produced by synovial CD4+ T-cells, and (iv) the synoviocytes and the synovial fluid.