CD4 and major depressive disorder: We noted an enhanced immune response not only in CD4+ naive T cells but also in other lymphocytes, which might be one of the main causes of depression.[3, 37] As mentioned previously, there is substantial evidence for an inflammatory protein signature in MDD.[1, 37, 38] Furthermore, the DEGs analysis indicated that this might be due to impaired proliferation and differentiation of CD4+ naive T cells.