SPI1 and major depressive disorder: Recently, we reported that levels of colony stimulating factor 1 receptor (CSF1R) in the spleen, but not postmortem brain, from patients with depression were significantly lower than those of control groups, and that there was a negative correlation between CSF1R and interacting protein SPI1 in the spleen57, suggesting a brain–spleen axis in psychiatric disorders such as depression26,57,58.