Lactate produced by tumor cells, apoptotic cells and in inflamed tissues has been implicated in pathological conditions such as tumor growth, rheumatoid arthritis, viral infection and atherosclerotic plaque by modulating tumor associated- and muscle-macrophage polarization, CD4+ T cell mobility, type I interferon production and efferocytosis by apoptotic cells11–17. Here, CD4 is linked to rheumatoid arthritis.