CCL13 and major depressive disorder: We hypothesized that plausible explanations for this negative correlation between MCP-4 levels and disease severity might be due to either the complexity of chemokine role in MDD in which other biological factors might downregulate MCP-4 levels in severely depressed patients or this might be similar to MCP-1, MCP-4 might display some neuroprotective and neuromodulatory effects.