CCL4 and treatment resistant depression: The present study revealed that, compared with healthy controls, TRD patients with or without pain both had elevated levels of both pro-inflammatory (e.g., IL-17α, IL-2, and IL-6) and anti-inflammatory (e.g., IL-10, IL-13) cytokines, as well as other cytokines (e.g., GM-CSF, fractalkine, MIP-3α, and MIP-1β), while anti-inflammatory IL-4 levels decreased in both the pain and without pain groups.