CD4 and major depressive disorder: Given that PTSD [373] and depression [230] are associated with decreased CD4+CD25+FoxP3+ Treg cell numbers, one hypothesis is that a failure of immunoregulation promotes an over-reacting of the inflammatory response to trauma or stressful life events, and thus, predisposes those individuals to the development of stress-related disorders in general, and PTSD and depression in particular.