Inflammatory Mediators Both Directly and Indirectly Promote Microglial Proliferation

Microglia-the predominant immune cell of the central nervous system (CNS)-possess an astounding capacity for proliferation. In development, this proliferation ensures that microglia are present at a sufficient density to perform their vital functions throughout development and into adulthood. During diseases or following CNS injuries, microglial proliferation similarly promotes an increase in microglial density to respond to damage. However, the governing mechanisms for microglial proliferation remain unknown. While many factors have been suggested to promote microglial proliferation-known as mitogens-or to increase microglial densities both in vitro and in vivo, there has been no standardized comparison of these factors. Here, we screened 22 of these factors in serum-free microglial cultures which more faithfully recapitulate in vivo microglial biology. We confirmed three cytokines-colony stimulating factor-2, interleukin-3 and tumor necrosis factor-ɑ-promote microglia proliferation. We similarly tested the remaining non-mitogenic factors for an indirect ability to regulate microglial proliferation by conditioning media from other CNS cell lineages and measuring the capacity for conditioned media to promote microglial proliferation. Of the tested factors and lineages, only interleukin-1ɑ and interleukin-1β promoted the release of a microglial mitogen from astrocytes, which we confirmed to be CSF2. Together, we demonstrate that in standardized conditions, very few factors that were previously reported to promote microglial proliferation or increase microglial densities, are directly, or indirectly, mitogenic.