Neurodegenerative diseases may be associated with altered immune-brain interaction. The senescence-accelerated mouse prone 10 (SAMP10) is a model of brain aging that undergoes an early onset cerebral neurodegeneration following immune senescence. We hypothesized that the brain-immune interaction is perturbed in SAMP10 mice. We created 4 groups of radiation chimeras by intra-bone marrow bone marrow transplantation using young and aged SAMP10 and B6 mice as recipients with 5-week-old GFP transgenic B6 mice as donors and analyzed chimeras immunohistochemically 4 months later. Donor's marrow-derived cells of the myeloid lineage entered discrete brain regions through the attachments of choroid plexus. In chimeric mice with aged SAMP10 mice being used as recipients, larger numbers of marrow cells entered more brain regions than the other groups, especially in the diencephalon. We performed multiplex cytokine assays to determine tissue concentration of 10 cytokines in the diencephalon prepared from young and aged SAMP10 and B6 mice. Aged SAMP10 mice exhibited higher tissue concentrations of IL-6, G-CSF, CCL11, CXCL1 and CXCL10 than the other groups of chimera. Immunohistochemistry revealed that these cytokines were expressed in astrocytic processes of the attachments of choroid plexus, periventricular astrocytes, tanycytes, and hypothalamic neurons. Therefore, the enhanced recruitment of bone marrow-derived cells into the brain may be associated with region-specific changes in profiles of tissue cytokine microenvironment, which represents a manifestation of perturbed brain-immune interaction in SAMP10 mice.

Thymus transplantation, in conjunction with bone marrow transplantation (BMT), has been attracting attention for the treatment of various diseases. Recently, donor lymphocyte infusion (DLI) has been used as a helpful tool for establishing donor chimerism and preventing a relapse of leukemia/lymphoma. However, the effects of DLI on transplanted and recipient thymuses have not been explored. We therefore performed DLI in the intrabone marrow-BMT + thymus transplantation setting. We have found that DLI leads to derangements in both recipient thymuses and transplanted thymuses; by 2 wk after BMT, we saw a decrease in total cell number, a lower percentage of CD4(+)CD8(+) cells, and the obliteration of the thymic corticomedullary junction. Four weeks later, the thymic impairment became more serious. However, when we depleted the CD4(+) T cells (CD4(-)-DLI), the recipient thymic recovery and transplanted thymic development were significantly restored by the treatment. In addition, there were much greater levels of TNF-α and Fas ligand, and a lower percentage of regulatory T cells in the DLI group than in the CD4(-)-DLI group. These findings indicate that inflammation induced by DLI, especially by CD4(+) T cells, plays a crucial role in the thymic impairment.

Although the immune system modulates higher functions of the brain under non-inflammatory conditions, how immune cells interact with brain parenchymal cells remains to be determined. Using bone marrow chimeric mice in which the recipients' immune system was reconstituted by marrow cells derived from GFP-transgenic mice by syngeneic intra-bone marrow-bone marrow transplantation (IBM-BMT) and by intravenous (IV)-BMT, we examined the distribution, density and differentiation of donor-derived marrow cells in the brain parenchyma 2 weeks and 1, 4 and 8 months after BMT. Marrow-derived cells started to populate discrete brain regions from 1 to 4 months after BMT, exhibited ramified morphology and expressed Iba-1. The ramified marrow-derived cells were distributed in more brain regions and for a longer time after IBM-BMT than IV-BMT. Most of these discrete regions were adjacent to the attachments of choroid plexus that comprised thinned brain parenchyma consisting of astroglial processes in the narrow channel between the ependyma and pia. These specific portions of astroglial processes expressed fractalkine. In the choroid plexus stroma, not only Iba-1+ myeloid cells but also non-myeloid CXCL12-expressing cells were of bone marrow-origin. Transcripts of fractalkine, CXCL12 and their related molecules such as CX3CR1, ADAM10 and CXCR4 were detected in the tissue consisting of the choroid plexus, the attachments and adjacent brain parenchyma. Thus, bone marrow cells selectively enter the discrete brain regions adjacent to the attachments of choroid plexus and differentiate into ramified myeloid cells. Fractalkine in the attachments of choroid plexus and CXCL12 in the choroid plexus stroma may be involved in these brain-immune interactions.

Perturbation in the brain–immune interaction may play a role in the pathogenesis of neurodegenerative diseases. The senescence-accelerated mouse prone 10 (SAMP10) mice undergo early onset of age-related neurodegenerative changes and impaired cognition. Given the elevated levels of brain pro-inflammatory cytokines, dystrophic microglia and defects in cytokine-mediated neuroprotective glial responses, as well as an early involution of the thymus and impaired T cell functions, the brain–immune interaction could be perturbed in SAMP10 mice. We created radiation chimeras in which bone marrow cells of young and old SAMP10 and C57BL/6 (B6) mice were reconstituted by bone marrow cells derived from GFP transgenic B6 mice by intra-bone marrow–bone marrow transplantation (IBM–BMT) and analyzed these chimeras 4 months after BMT. In chimeras in which B6 mice were recipients, donor-derived cells entered several discrete regions of the brain parenchyma mostly adjacent to the tenia of the choroid plexus but not the remaining major parts of the brain parenchyma. On the other hand, in chimeras in which old SAMP10 mice were recipients, a larger number of donor-derived cells entered more regions, such as hypothalamus, white matter and brain stem, than in chimeras with B6 mice being recipients. The enhanced recruitment of bone marrow-derived cells into the brain parenchyma in old SAMP10 mice may be a manifestation of disturbances in the brain–immune interaction that might be related to early onset neurodegenerative changes.

Although the immune system modulates the brain function under steady-state conditions, it has not been clear where and how the peripheral immune cells interact with brain parenchymal cells. We created radiation chimera mice in which recipients' immune system was reconstituted by bone marrow cells derived from donor GFP transgenic mice by a novel intra-bone marrow–bone marrow transplantation (IBM–BMT) method and by conventional intravenous (IV)–BMT. We examined the distribution, differentiation and density of donor-derived bone marrow (BM) cells in the brain parenchyma 2 weeks and 1, 4 and 8 months after the BMT. We found BM-derived cells in several discrete brain regions mostly adjacent to the tenia of the choroid plexus from 1 to 4 months after BMT, but not in the remaining major parts until 8 months after BMT. BM-derived cells exhibited ramified morphology and expressed Iba-1 but not GFAP, CNPase or NeuN, indicative of the myeloid lineage. The densities of BM-derived cells in the brain parenchyma increased in a time dependent manner after IBM–BMT but not after IV–BMT, therefore the BM-derived cell densities 8 months after IBM–BMT were significantly higher than those after IV–BMT. The choroid plexus stroma contained more BM-derived cells after IBM–BMT than after IV–BMT. These results suggested that the tenia of the choroid plexus represents a novel route of entry for peripheral immune cells into the brain parenchyma under steady-state conditions.