CCL4 and COVID-19: Blocking the cytokine–receptor axes, such as CCL2-CCR2, CCL3-CCR1, CCL3-CCR5, CCL4-CCR5, CCL3L1-VSIR, and CCL3L1-CCR1, may be a novel strategy to reverse TEX and furhter to treat severe COVID-19 with cytokine storm-related immune exhaustion.