Microglial cell responses to fetal ventral mesencephalic tissue grafting and to active and adoptive immunizations

Microglia express cytokines, major histocompatibility (MHC) loci, and several other immunologically important constituents. The aim of this study was to detect immunological responses of microglial cells following allogeneic dopaminergic transplantation using active and adoptive immunizations. Adult inbred Fisher 344 (F344 RT1) rats were unilaterally dopamine (DA) depleted in striatum by injection of 6-hydroxydopamine. The degree of degeneration was assessed by recording the rotational response to apomorphine. Fetal ventral mesencephalic tissue containing DA neuroblasts from Wistar–Furth (WF, RT1u) rat donors (9–12 mm CRL) were later implanted in striatum on the lesioned side. Lymph nodes and spleen cells were collected aseptically, resuspended, and diluted for isovolumetric injections. Animals selected for active immunization were injected intraperitoneally with varying amounts of WF lymphocytes. Animals selected for adoptive immunization (transferred immunity) were intraperitoneally injected with 108F344 lymphocytes prepared from animals actively immunized 3 weeks previously. Monoclonal antibodies against CD4 (OX38), CD8 (OX8), CD11b (OX42), MHC class I (OX18), monomorphic MHC class II (OX-6), and ED1 and polyclonal antibodies against tyrosine hydroxylase (TH) were used for immunohistochemistry. We found that the degree of ED1-positive cell proliferation was well correlated to the immunization patterns. Groups that were actively immunized with or without prior adoptive immunization had a larger amount of reactive microglial proliferation. ED1 immunohistochemistry revealed patterns of immunolabeling of engrafted areas: 8–12 weeks after grafting in nonimmunized and adoptively immunized groups reactive microglial proliferation occurred only at the graft periphery. Active and adoptive + active immunization led to ED1-IR within the grafts themselves. At early stages nonimmunized groups had an ED1 pattern which was partially inside the grafts. At early time points nonimmunized groups contained ameboid microglial cells within the grafts which disappeared at later stages and were absent in the immunized groups. ED1-positive ameboid microglial cells within the grafts may be of graft origin and constitute a part of a continued normal development of the fetal tissue.