The present study explored whether human umbilical cord blood mononuclear cells (UCBMC) could improve the neurogenic niche of the aged brain and stimulate the endogenous stem/progenitor cells to generate new neurons. As determined by stereological analysis of both DCX and BrdU, a single peripherally administered injection of UCBMC appeared to stimulate neurogenesis. The finding that the administration of UCBMC also increased the number of proliferative cells generated within 24 hours following the treatment, suggests that the increase in neurogenesis observed in this study may be a consequence of an increase in proliferation rather than changes in differentiation or survival of newly generated cells. To support this hypothesis, it will be important to allow more time for the cells to fully mature and then determine if there is still no change in the phenotype of the BrdU+ cells. It will also be important to determine what effect UCBMC have on the survival of the BrdU+ cells.
In addition, it was determined that UCBMC were able to increase cell proliferation for at least fifteen days in the aged rats. This suggests that the UCBMC may have a beneficial effect on the microenvironment of the aged brain. In support of this hypothesis we show that coinciding with an increase in neurogenesis in the aged treated rats, there was a decrease in the number of activated microglia in the dentate gyrus. A negative correlation between the degree of inflammation as indicated by the activation of microglia and the number of newborn neurons has been previously described . Consistent with previous studies showing that UCBMC have the potential to reduce neuroinflammation [20, 23, 27, 30] in the aged brain, we did find that neurogenesis correlated with the number of activated microglia, suggesting that UCBMC were stimulating neurogenesis by decreasing microglia activation. Although other possibilities cannot be excluded, since UCBMC could be having multiple effects including increasing trophic support as previously published [20, 28, 29].
UCBMC have been shown to reduce neuroinflammation [20, 23, 27, 30] and, consistent with previous studies, we show here that the peripherally administered UCBMC do have anti-inflammatory properties. It appears that one of the factors that leads to the negative regulation of neural stem cells is inflammation [32–34]. A primary source of inflammation in the CNS is from the macrophages/microglia which can produce a wide array of cytotoxic factors, including proinflammatory cytokines such as tumor necrosis factor (TNF), IL-1, IL-6 and IL-12 [35, 36]. With age, microglia shift from a quiescent state into an active proinflammatory state. It is not clear if this change in activation state is in response to injury, infection, or debris or if it is due to dysregulated cytokine levels. Another possibility recently proposed, is that microglia becoming senescent and this leads to them becoming dysfunctional [37, 38]. It has previously been demonstrated in models of induced inflammation through the use of LPS or radiation, a dramatic decrease in proliferation and neurogenesis occur, and when the inflammation is alleviated the replicative potential of the stem cells returns [32, 34]. This effect is likely a protective mechanism so that the DNA is not exposed to the noxious inflammatory environment which could damage the replicating DNA. This correlation also imparts support to the hypothesis that UCBMC stimulate neurogenesis by decreasing inflammation, particularly the activation state of microglia. However, it does not rule out the possibility that UCBMC may be acting on multiple targets, with microglia only representing one part of the total mechanism.
While UCBMC do seem to have an effect on microglia, it is not clear how this occurs. A number of studies have shown that T-cells appear to act on macrophages/microglia to cause them to adopt a phenotype that is 'pro-repair' (i.e. the macrophages/microglia: clear debris, buffer toxic compounds, and produce growth factors), without being pro-inflammatory (i.e. producing TNF-α, NO, or COX-2) and this effect can promote neurogenesis and be neuroprotective [39–43]. As T-cells are a major fraction of UCBMC, it is possible that the naïve T-cells in the UCBMC are able to induce a protective T-cell mediated response in the aged rats, since adult PBMC did not have an effect. Alternatively, the CD34+ stem cells in the UCBMC may be involved. Taguchi et al.  has shown that CD34+ stem cells can increase both angiogenesis and neurogenesis as part of the protective mechanism against stroke. From the results of the current study it can not be determined if the effects of the UCBMC are a result of direct action on the brain or though peripheral effect. However, the fact that we did not detect any immunoreactivity for human nuclei in the brains of the UCBMC-treated rats raises the possibility that the UCBMC may be acting through a peripheral mechanism. Moreover, the observation that the adult PBMC did not alter hippocampal neurogenesis ruled out the possibility of a non-specific effect due to an influx of cells, supporting our belief that the increase in neurogenesis, which occurred following treatment with UCBMC was not due to an influx of cells but was specific to UCBMC.
The present study did not attempt to determine if decreasing senescence of the neural stem cells could reverse the cognitive decline with age. There is still much debate surrounding the role of neurogenesis in learning and memory [44–49] and whether cellular senescence of the stem cell pool with age leads to an aging phenotype. While not a goal of the current study, it will be important to determine if the rejuvenation of the aged stem/progenitor cell pool can reverse the age-related cognitive decline.
In summary, this study demonstrates that a single peripheral injection of UCBMC could stimulate the endogenous neural stem/progenitor cells to increase proliferation. We also determined that the UCBMC were able to improve the microenvironment of the aged brain by reducing the number of activated microglia, and this reduction is correlated with an increase in neurogenesis. Further work will be important to determine the mechanism of action of UCBMC in the aged rats, including the possible role of the immune system in a T-cell mediated response, as well as the affects of angiogenesis via the CD34+ stem cells. It will also be important in future experiments to determine the duration that a single injection of UCBMC will elevated proliferation in aged rats. Not only do the results of this study provide novel insight into the state of the aged stem cell niche, the ability of the UCBMC to exert their effects while being administered minimally invasively may make translation to the clinical setting more likely. For this reason it will be important in future studies to determine the most efficacious dose and dosing regimen. Nevertheless, this is the first time that a systemic injection of hematopoietic cells has been shown to restore the regenerative potential of the aged brain, providing a novel insight into how the regenerative potential of the aged stem cell niches could be restored.