Acquired tolerance for minor histocompatibility antigens after HLA identical bone marrow transplantation.

Skin tissue of a healthy chimera 7 years after HLA identical bone marrow transplantation was found to express a minor histocompatibility (mH) antigen against which cytotoxic T lymphocytes (CTLs) had been discovered at a time of acute graft-versus-host disease (aGVHD). We were prompted to investigate the apparent tolerance to this persistent mH antigen and used limiting dilution analysis to monitor in vitro anti-host CTL responses in time after bone marrow transplantation. A high anti-host CTL precursor frequency was found during acute GVHD, declining in time until beyond detection level in the healthy chimera 7 years after transplantation. In this case report (i) CTL precursor frequencies are used for the first time to monitor in vitro tolerance induction to persistent host mH antigens after HLA identical BMT in man; and (ii) it is shown that LDA may be a potential tool for quantification and specificity analysis of CTL responses to mH antigens.     

Sequential expression of adhesion and costimulatory molecules in graft-versus-host disease target organs after murine bone marrow transplantation across minor histocompatibility antigen barriers.

Graft-versus-host disease (GVHD) is a potentially fatal complication after allogeneic bone marrow transplantation. However, few data exist thus far on the molecular signals governing leukocyte trafficking during the disease. We therefore investigated the sequential pattern of distinct adhesion, costimulatory, and apoptosis-related molecules in GVHD organs (ileum, colon, skin, and liver) after transplantation across minor histocompatibility barriers (B10.D2 --> BALB/c, both H-2d). To distinguish changes induced by the conditioning regimen from effects achieved by allogeneic cell transfer, syngeneic transplant recipients (BALB/c --> BALB/c) and irradiated nontransplanted mice were added as controls. Irradiation upregulated the expression of vascular cell adhesion molecule (VCAM)-1, intercellular adhesion molecule (ICAM)-l, and B7-2 in ileum, as well as VCAM-1 and B7-2 in colon, on day 3 in all animals. Whereas in syngeneic mice these effects were reversed from day 9 on, allogeneic recipients showed further upregulation of VCAM-1, ICAM-1, B7-1, and B7-2 in these organs on day 22, when GVHD became clinically evident. Infiltration of CD4+ and CD8+ donor T cells was noted on day 9 in skin and liver and on day 22 in ileum and colon. Surprisingly, the expression of several other adhesion molecules, such as ICAM-2, platelet-endothelial cell adhesion molecule 1, E-selectin, and mucosal addressin cell adhesion molecule 1, did not change. Proapoptotic and antiapoptotic markers were balanced in GVHD organs with the exception of spleen, in which a preferential expression of the proapoptotic Bax could be noted. Our results indicate that irradiation-induced upregulation of VCAM-1, ICAM-1, and B7-2 provides early costimulatory signals to incoming donor T cells in the intestine, followed by a cascade of proinflammatory signals in other organs once the alloresponse is established.     

Long-term survival after autologous bone marrow transplantation for follicular lymphoma in first remission.

The role of autologous stem cell transplantation (ASCT) in the treatment of follicular lymphoma is still being defined in the era of antibody therapy. Here we report the long-term 12-year clinical outcomes of patients treated with autologous bone marrow transplantation (ABMT) for follicular non-Hodgkin's lymphoma (NHL) in first remission. Between 1988 and 1993, advanced-stage follicular NHL patients in need of initial therapy were enrolled in 2 consecutive prospective treatment trials of either standard-dose CHOP induction (83 patients) or high-dose CHOP plus granulocyte-colony stimulating factor (G-CSF) (20 patients). Patients who achieved an adequate remission with induction therapy underwent conditioning with cyclophosphamide and total body irradiation (TBI) followed by ABMT in first remission using bone marrow (BM) purged in vitro with anti-B cell monoclonal antibodies and rabbit complement (96 patients). At 12-year follow-up, 61% of the patients are alive and 43% remain in continuing complete remission. The only predictors of decreased progression-free survival proved to be histologic BM involvement at time of harvest (hazard ratio [HR] 2.27, 95% confidence interval [CI] 1.3-3.9, P<.004) and PCR detectable disease in the BM product after purging (HR 4.18, 95% CI 1.99-8.8, P=.0002). No significant predictors of overall survival were identified. These results at 12-year follow-up suggest that a subset of follicular lymphoma patients can experience prolonged survival with ABMT in first remission.     

Long-term observation of immunologic reconstitution after allogeneic bone marrow transplantation: differences in recovery among functional T cell subsets

A 27-year-old male with acute lymphoblastic leukemia in the first relapse received allogeneic marrow graft from an HLA-identical sister on April 28, 1978. We studied in vitro immunologic parameters such as T cell surface phenotypes, proliferative response to PHA and Con A; T cell suppressor and helper function for in vitro immunoglobulin production stimulated with pokeweed mitogen serially after the transplantation. Despite a low OKT4/OKT8 ratio for more than 5 yrs after marrow grafting, proliferative response to PHA recovered to normal 4 yrs and 9 months post-transplant. In contrast, proliferative response to Con A did not recover. T cell suppressor function induced by Con A had recovered at 3 yrs 6 months; however helper T cell function took longer to recover. These findings suggest differences in functional recovery of T cell subsets following allogeneic marrow transplantation.     

The intragraft vascularized bone marrow component plays a critical role in tolerance induction after reconstructive transplantation

The role of the vascularized bone marrow component as a continuous source of donor-derived hematopoietic stem cells that facilitate tolerance induction of vascularized composite allografts is not completely understood. In this study, vascularized composite tissue allograft transplantation outcomes between recipients receiving either conventional bone marrow transplantation (CBMT) or vascularized bone marrow (VBM) transplantation from Balb/c (H2d) to C57BL/6 (H2b) mice were compared. Either high- or low-dose CBMT (1.5 × 10⁸ or 3 × 10⁷ bone marrow cells, respectively) was applied. In addition, recipients were treated with costimulation blockade (1 mg anti-CD154 and 0.5 mg CTLA4Ig on postoperative days 0 and 2, respectively) and short-term rapamycin (3 mg/kg/day for the first posttransplant week and then every other day for another 3 weeks). Similar to high-dose conventional bone marrow transplantation, 5/6 animals in the vascularized bone marrow group demonstrated long-term allograft survival (>120 days). In contrast, significantly shorter median survival was noted in the low-dose CBMT group (~64 days). Consistently high chimerism levels were observed in the VBM transplantation group. Notably, low levels of circulating CD4⁺ and CD8⁺ T cells and a higher ratio of Treg to Teff cells were maintained in VBM transplantation and high-dose CBMT recipients (>30 days) but not in low-dose VBM transplant recipients. Donor-specific hyporesponsiveness was shown in tolerant recipients in vitro. Removal of the vascularized bone marrow component after secondary donor-specific skin transplantation did not affect either primary allograft or secondary skin graft survival.     

Restitution of the thymus in lethally irradiated mice after transplantation of syngeneic or allogeneic bone marrow

The early restitution of the thymus of bone marrow chimeras was investigated by the immunoperoxidase technique using monoclonal antibodies against Thy-1 and Lyt-1, Lyt-2, Lyt-3. Within two weeks, normal thymus histology was restored in mice which received untreated syngeneic BM or syngeneic or allogeneic BM pretreated with SAL (specificed antilymphocytic serum). Irradiation depleted the thymic cortex of small Thy-1+, Lyt-1+2+3+ cells but did not affect a medullary population of medium sized weakly stained Thy-1+, strongly stained Lyt-1+ cells. Preceded by the appearance of an increasing number of large Thy-1+, Lyt-1- blasts (days 2 and 4), the thymic cortex was repopulated (beginning on day 6) by smaller Thy-1+ cells which acquired Lyt-1, Lyt-2 and Lyt-3 though, obviously not in a strictly sequential manner. Simultaneously, the medullary radioresistant cells disappeared, nd the medulla was subsequently repopulated (beginning on day 8) by thymocytes of a mature phenotype. Early restitution of the thymus in radiation control mice was similar to the bone marrow chimeras. The results indicate that the histological restitution of the thymus originates substantially from radioresistant precursors of host origin. Graft-versus-host reaction induced by untreated allogeneic bone marrow cells prevented normal thymic restitution. A delayed localized cortical repopulation with small Thy-1+, Lyt-1+2+3+ cells, progressive destruction of thymic architecture and almost no restoration of the medullary immunocompetent thymocytes were noted. T cell differentiation obviously was seriously affected by the injuries to the thymic microenvironment due to alloreactive T cells.     

Cytokines transduced bone marrow stromal cell lines promote immunohematopoietic reconstitution in mice after allogeneic bone marrow transplantation

Impaired immune reconstitution following allogeneic T-cell depleted bone marrow transplantation (allo-TCD-BMT) is a major obstacle to its clinical application. Stromal cell line QXMSC1, established from bone marrow cells of BALB/c(H-2d), was transfected with murine IL-3 and/ or IL-2 gene, and injected into lethally irradiated C57BL/6(H2b) mice. We evaluated its effects on immunologic and hematopoietic reconstitution after allo-TCD-BMT. The results showed that QXMSC1-IL-3 + IL-2 could significantly increase the numbers of hematopoietic primitive progenitors (CFU-S), committed progenitors (CFU-GM, and BFU-E), and lymphocytes (CD8+ cells, CD4+ cells, and B cells). Similarly, immune functions of recipient mice were significantly enhanced in the QXMSC1-IL-3 + IL-2 group. In addition, QXMSC1-IL-3 or QXMSC1-IL-2 also exerted apparent effects on accelerating immune reconstitution, but these effects were far less than that of QXMSC1-IL-3 + IL-2. Our results demonstrated that stromal cell-mediated IL-3 and IL-2 gene therapy may be a potent approach in promoting immunologic and hematopoietic reconstitution after allo-TCD-BMT.     

Magnetic resonance imaging of the bone marrow after bone marrow transplantation or immunosuppressive therapy in aplastic anemia.

To compare magnetic resonance (MR) images of the bone marrow (BM) after bone marrow transplantation or immunosuppressive therapy in patients with aplastic anemia (AA), MR imaging of BM was reviewed retrospectively in 16 patients (13 males and 3 females, mean age 26 yr) with AA who completely responded clinically after transplantation or immunosuppressive therapy. The signal intensity (SI) of BM was classified into four patterns according to the increasing amount of cellular marrow, i.e., pattern I to IV. SI of MR imaging of BM exhibited an increase of cellular marrows following both transplantation and immunosuppressive therapy. Of the eight patients on transplantation, the SI of the lumbar spinal BM was pattern III in two patients and IV in six on T1-weighted and short tau inversion recovery (STIR) images. In the eight patients with immunosuppressive therapy, the SI of the lumbar spinal BM was pattern II in one, III in five, and IV in two on T1-weighted images and pattern II in one, III in four, and IV in three on STIR images. SI on MR imaging of the lumbar spinal BM showed a more cellular pattern in patients on transplantation than in those on immunosuppressive therapy.     

Long-term haematopoietic reconstitution and survival without interleukin-7 in a murine syngeneic bone marrow transplantation model

We created a syngeneic mouse bone marrow transplantation (BMT) model to examine the effect of endogenous interleukin-7 (IL-7) on long-term (>or=140 days) haematopoietic reconstitution and survival after BMT. Wild-type (WT) IL-7(+/+) and knockout (KO) IL-7(-/-) mice were lethally irradiated and transplanted with bone marrow. Survival is best (85.7%) in the group WT grafts transplanted into WT recipients (WT-->WT) with a trend towards poorer survival in the other groups (WT-->KO: 60%, KO-->WT: 50%, KO-->KO: 45.5%, differences statistically not significant). If the recipient is deficient for IL-7-producing cells, T- and B-cell reconstitution remain incomplete. If the graft lacks IL-7-producing cells there is a significant delay in T- and NK-cell reconstitution. Interestingly, in the absence of IL-7, T-cell reconstitution is neither delayed nor incomplete because of an expansion of TCRalphabeta(+)/CD4(-)/CD8(-) double negative T cells. Long-term survival and lymphocyte reconstitution after syngeneic BMT can occur despite the absence of IL-7.     

Persistence of recipient lymphocytes in NOD mice after irradiation and bone marrow transplantation

The non-obese diabetic (NOD) mouse is a unique and invaluable model of autoimmune disease, in particular type 1 diabetes. Bone marrow transplantation as a therapy for type 1 diabetes has been explored in NOD mice. NOD mice require higher doses of conditioning irradiation for successful allogeneic bone marrow transplantation, suggesting that NOD hematopoietic cells are radioresistant compared to those of other mouse strains. However, studies of hematopoietic reconstitution in NOD mice are hampered by the lack of mice bearing a suitable cell-surface marker that would allow transferred cells or their progeny to be distinguished. In order to monitor hematopoietic reconstitution in NOD mice we generated congenic NOD mice that carry the alternative allelic form of the pan-leukocyte alloantigen CD45. Following irradiation and congenic bone marrow transplantation, we found that the myeloid lineage was rapidly reconstituted by cells of donor origin but substantial numbers of recipient T lymphocytes persisted even after supra-lethal irradiation. This indicates that radiation resistance in the NOD hematopoietic compartment is a property primarily of mature T lymphocytes.     

骨髓干细胞移植后mdx鼠腓肠肌病理变化

目的 研究骨髓干细胞移植后mdx鼠腓肠肌组织病理变化. 方法 7～9周龄mdx鼠20只平均分为4组,放射处理后移植1.2×107细胞/只同种异基因全骨髓干细胞,于移植后4、8、12及16周用HE染色观察腓肠肌组织细胞形态及核中心移位纤维(CNF).C57鼠和未治疗mdx鼠各5只作阳性和阴性对照. 结果 CS7鼠腓肠肌横切面可见肌细胞大小形态基本一致,无核中心移位现象.各细胞移植治疗组和阴性对照组mdx鼠均有大量的炎细胞浸润,核中心移位明显.未治疗mdx鼠CNF最高,约达70%;移植后4、12和16周,CNF分别为55%、50%和44%. 结论 骨髓干细胞移植后mdx鼠腓肠肌CNF随移植时间延长逐渐减少,提示骨髓干细胞移植后长久持续参与受损骨骼肌的修复与再生.     

混合骨髓移植后过继免疫治疗小鼠白血病

背景：急性白血病自体造血干细胞移植后复发率高,异基因造血干细胞移植后移植相关病死率高,混合造血干细胞移植及移植后过继免疫治疗有可能取长补短,提高疗效。 目的：观察自体骨髓混合H-2半相合异体骨髓移植后供体淋巴细胞输注+白细胞介素2治疗对小鼠白血病的疗效。 方法：将Balb/c小鼠经直线加速器照射3 Gy后分为白血病模型组、白血病模型照射组、混合移植组、自体骨髓移植组,均尾静脉注射5×10⁵ K562(GFP+/NeoR+)或K562(GFP-/NeoR-)细胞。7 d后6 Gy照射,自体骨髓移植组移植自体骨髓细胞或联合白细胞介素2治疗;混合移植组移植小鼠自体骨髓细胞混合1/10的H-2半相合异体骨髓细胞后应用白细胞介素2或联合供体淋巴细胞输注治疗。4周后行小鼠外周血及骨髓细胞形态检查,外周血细胞亚群、GFP及NeoR基因测定,肝、脾匀浆细胞GFP和NeoR基因测定。 结果与结论：白血病模型组小鼠因骨髓造血功能衰竭于20 d内全部死亡,白血病模型照射组小鼠因造血功能衰竭于14 d内全部死亡;自体骨髓移植组、混合移植组均有多少不等小鼠无白血病存活超过28 d,且混合骨髓移植后及自体骨髓移植后应用白细胞介素2治疗可提高白血病小鼠长期无病生存率,在此基础上联合供体淋巴细胞输注可更进一步提高白血病小鼠长期无病生存率。     

Long-term survival after blood and marrow transplantation: comparison with an age- and gender-matched normative population.

A plateau in long-term survival patterns of patients undergoing blood and marrow transplantation (BMT) from allogeneic donors is apparent, but whether their expected survival ever parallels that of the normative population is unclear. This study attempts to identify a cutoff time for classifying BMT patients as long-term survivors and compares their actual survival with the expected survival of an age- and gender-matched "normal" population. In this study, the records of 1386 patients who underwent allogeneic BMT at Princess Margaret Hospital between 1970 and 2002 were reviewed. Hazard rates (HRs), Kaplan-Meier survival estimates, and loss curves were used to propose a cutoff time classifying patients as long-term survivors. Factors predictive of overall survival and survival for long-term survivors were investigated. Actual survival for these patients was compared with the expected survival of the Canadian "normal" population. A cutoff time of 6 years post-BMT was proposed to define long-term survivors based on loss curves of hazard ratios and yearly survival statistics. The only statistically significant predictor of survival among long-term survivors was having a male donor (HR = 0.39; 95% confidence interval [CI] = 0.17-0.88). Although only 62% of patients survived the first year post-BMT, 98.5% of patients alive after 6 years survived at least another year. Almost 1/3 (31%) of the deaths in long-term survivors resulted from causes unrelated to transplantation or relapse. The observed number of deaths among BMT patients exceeded the expected number from the Canadian population; however, the difference in life expectancy decreased the longer that a patient survived. The 95% CIs for the observed/expected number of deaths cover 1, indicative of no difference, after the tenth year post-BMT. A cutoff of 6 years is proposed to define long-term survivorship after BMT. Life expectancy remained reduced compared with that of the "normal" population; however, this difference decreased the longer that a patient survived. Known risk factors of short-term survival disappeared, with only donor gender predictive of survival among long-term survivors.     

转染IL-6基因骨髓基质细胞系对骨髓移植后免疫功能重建的促进作用

目的　探讨转染IL 6基因的骨髓基质细胞系对同基因骨髓移植 (BMT)后小鼠免疫功能重建的促进作用。方法　将IL 6cDNA片段连接到pcDNA3真核表达载体上。用脂质体将pcD NA3IL 6转入骨髓基质细胞系QXMSC1,ELISA法测定转染IL 6基因骨髓基质细胞QXMSC1IL 6培养上清中IL 6的含量 ,有限稀释挑选多个细胞克隆 ,选择表达量最高的转基因细胞系QXMSC1IL 6用于动物实验。BABL c小鼠经γ射线致死量照射后 ,输入同基因骨髓细胞 (10 7 只 )同时输入骨髓基质细胞QXMSC1IL 6 (5× 10 5 只 )。在骨髓移植后 30d、6 0d检测BMT小鼠淋巴细胞对LPS ,ConA增殖反应 ,T辅助细胞前体 (helpTlymphocyteprecursor,HTLp) ,杀伤性T细胞前体 (cytotoxicTlymphocyteprecursor,CTLp) ,迟发型超敏反应 (delayed typehypersensitivity ,DTH)及空斑形成细胞数 (plaqueformingcell,PFC) ,反映骨髓移植后小鼠免疫功能。结果　成功构建pcDNA3IL 6重组体。该细胞体外培养 2 4h分泌IL 6的含量为 11.15 (± 2 .4 1) μg 10 6 。QXMSC1IL 6细胞系能明显增强BMT后淋巴细胞对LPS、ConA反应性 ,小鼠对异基因小鼠脾细胞DTH反应增强 ,脾脏中HTLp ,CTLp及PFC数明显增加。转入外源IL 6cDNA基因的骨髓基质细胞系QXMSC1IL 6在体内能明显促进BMT后小鼠T、B淋巴     

抗TCRαβ单克隆抗体联合大剂量供体骨髓细胞输注诱导小鼠皮肤移植耐受的研究

目的　探讨抗TCRαβ单克隆抗体联合大剂量供体骨髓细胞输注方法在同种异基因小鼠皮肤移植耐受诱导中的作用。方法　第 0天 ,C5 7BL 6 (H 2 b,B6 )小鼠尾静脉注入 2× 10 8BALB c(H 2 d,B c)来源的骨髓细胞 ,同时腹腔注射抗TCRαβ单克隆抗体 5 0 0 μg。第 6天进行皮肤移植。在不同的时间对B6小鼠进行迟发型超敏反应测定 ,混合淋巴细胞反应分析 ,并进行MLR的IL 2逆转实验和过继转移实验 ,初步探讨耐受形成机制。结果　抗TCRαβ单克隆抗体联合大剂量供体骨髓细胞输注处理的B6小鼠中供体皮肤移植物平均存活为 5 0 .4d ,显著长于其它各组 (P <0 .0 0 1)。相对于正常对照组小鼠 ,耐受B6小鼠在迟发型超敏反应和混合淋巴细胞反应中均表现出显著的低反应性 (P <0 .0 0 1)。IL 2逆转实验结果表明 ,克隆不应答 (anergy)可能参与了移植耐受的形成。体内、外过继转移实验均显示耐受B6小鼠脾细胞中存在抑制细胞活性。结论　抗TCRαβ单克隆抗体联合大剂量供体骨髓细胞输注可以在组织相容性抗原完全不相同的成年小鼠间成功地诱导出皮肤移植物的长期耐受。多种耐受机制 ,包括克隆不应答、抑制细胞 ,都参与了耐受的形成。     

Long-term survival and late deaths after allogeneic bone marrow transplantation. Late Effects Working Committee of the International Bone Marrow Transplant Registry.

BACKGROUND AND METHODS: It is uncertain whether mortality rates among patients who have undergone bone marrow transplantation return to the level of the mortality rates of the general population. We analyzed the characteristics of 6691 patients listed in the International Bone Marrow Transplant Registry. All the patients were free of their original disease two years after allogeneic bone marrow transplantation. Mortality rates in this cohort were compared with those of an age-, sex-, and nationality-matched general population. Cox proportional-hazards regression was used to identify risk factors for death more than two years after transplantation (late death). RESULTS: Among patients who were free of disease two years after transplantation, the probability of living for five more years was 89 percent (95 percent confidence interval, 88 to 90 percent). Among patients who underwent transplantation for aplastic anemia, the risk of death by the sixth year after transplantation did not differ significantly from that of a normal population. Mortality remained significantly higher than normal throughout the study among patients who underwent transplantation for acute lymphoblastic leukemia or chronic myelogenous leukemia and through the ninth year among those who underwent transplantation for acute myelogenous leukemia. Recurrent leukemia was the chief cause of death among patients who received a transplant for leukemia, whereas chronic graft-versus-host disease was the chief cause among those who received a transplant for aplastic anemia. Advanced, long-standing disease before transplantation and active chronic graft-versus-host disease were important risk factors for late death. CONCLUSIONS: In patients who receive an allogeneic bone marrow transplant as treatment for acute myelogenous or lymphoblastic leukemia, chronic myelogenous leukemia, or aplastic anemia and who are free of their original disease two years later, the disease is probably cured. However, for many years after transplantation, the mortality among these patients is higher than that in a normal population.     

Dendritic cell depletion and repopulation in the lung after irradiation and bone marrow transplantation in mice

Dendritic cells (DCs) are essential for innate and adaptive immunity, but are purported to exhibit variable radiosensitivity in response to irradiation in various bone marrow transplantation (BMT) protocols. To address this controversy, we analyzed the magnitude of depletion and repopulation of both lung CD11b(pos) DC and CD103(pos) DC subsets in response to irradiation and BMT in a murine model. In our study, CD45.2(pos) donor bone marrow cells were transplanted into irradiated CD45.1(pos) recipient mice to examine the depletion of recipient DC subsets and the repopulation of donor DC subsets. We observed an apoptosis-mediated and necrosis-mediated depletion (> 90%) of the recipient CD103(pos) DC subset, and only a 50-60% depletion of recipient CD11b(pos) DCs from lung parenchymal tissue on Days 3 and 5, whereas recipient alveolar and lung macrophages were much less radiosensitive, showing an approximately 50% depletion by Days 14-21 after treatment. A repopulation of lung tissue with donor DC subsets had occurred by Days 10 and 28 for CD11b(pos) DCs and CD103(pos) DCs, whereas alveolar and lung macrophages were repopulated by 6 and 10 weeks after treatment. Furthermore, the infection of mice with Streptococcus pneumoniae further accelerated the turnover of lung DCs and lung macrophage subsets. Our data illustrate the vulnerability of lung CD103(pos) DCs and CD11b(pos) DCs to irradiation, and indicate that an accelerated turnover of lung DC subsets occurs, relative to pulmonary and lung macrophages. Our findings may have important implications in the development of adjuvant immune-stimulatory protocols that could reduce the risk of opportunistic infections in patients undergoing BMT.     

Distribution and characterization of GFP(+) donor hematogenous cells in Twitcher mice after bone marrow transplantation

The twitcher mouse is a murine model of globoid cell leukodystropy, a genetic demyelinating disease caused by a mutation of the galactosylceramidase gene. Demyelination of the central nervous system commences around 20 postnatal days. Using GFP-transgenic mice as donors, the distribution of hematogenous cells after bone marrow transplantation was investigated in the twitcher mice. Bone marrow transplantation was carried out at 8 postnatal days. In twitcher chimeric mice examined before 30 postnatal days, numerous GFP(+) cells were detected in spleen and peripheral nerve but only a few were detected in the liver, lung, and spinal white matter. In contrast, at 35 to 40 postnatal days when demyelination is evident, many GFP(+) cells with ameboid form were detected in the white matter of the spinal cord, brainstem, and cerebrum. Approximately half of these GFP(+) cells were co-labeled with Mac-1. In twitcher chimeric mice examined after 100 postnatal days, the majority of GFP/Mac-1 double-positive cells displayed the morphological features of ramified microglia with fine delicate processes and was distributed diffusely in both gray and white matter. These results suggest that a significant number of donor hematogenous cells are able to infiltrate into the brain parenchyma, repositioning themselves into areas previously occupied by microglia, and to ameliorate lethality.