The immune response: the afferent arm

Redox Signaling, Neuroinflammation, and Neurodegeneration

Production of pro-inflammatory and anti-inflammatory cytokines is part of the defense system that mostly microglia and macrophages display to induce normal signaling to counteract the deleterious actions of invading pathogens in the brain. Also, redox activity in the central nervous system (CNS) constitutes an integral part of the metabolic processes needed by cells to exert their normal molecular and biochemical functions. Under normal conditions, the formation of reactive oxygen and nitrogen species, and the following oxidative activity encounter a healthy balance with immunological responses to preserve cell functions in the brain. However, under different pathological conditions, inflammatory responses recruit pro-oxidant signals and vice versa. The aim of this article is to review the basic concepts about the triggering of inflammatory and oxidative responses in the CNS. Recent Advances: Diverse concurrent toxic pathways are described to provide a solid mechanistic scope for considering intervention at the experimental and clinical levels that are aimed at diminishing the harmful actions of these two contributing factors to nerve cell damage. Critical Issues and Future Directions: The main conclusion supports the existence of a narrow cross-talk between pro-inflammatory and oxidative signals that can lead to neuronal damage and subsequent neurodegeneration. Further investigation about critical pathways crosslinking oxidative stress and inflammation will strength our knowlegde on this topic. Antioxid. Redox Signal. 28, 1626-1651.

Maintenance of phenotype and function of cryopreserved bone-derived cells

The emerging fields of tissue engineering and regenerative medicine require large numbers of cells for therapy. Although the properties of cells obtained from a variety of fresh tissues have been delineated, the knowledge regarding cryopreserved grafts-derived cells remains elusive. Previous studies have shown that living cells could be isolated from cryopreserved bone grafts. However, whether cryopreserved bone-derived cells can be applied in regenerative medicine is largely unknown. The present study was to evaluate the potential application of cryopreserved grafts-derived cells for tissue regeneration. We showed that cells derived from cryopreserved bone grafts could maintain good proliferation activity and osteogenic phenotype. The biological phenotype of these cells could be well preserved. The transplantation of cryopreserved bone-derived cells on scaffold could promote new bone formation in nude mice and enhance the osteointegration for dental implants in canine, which confirmed their osteogenic capacity, and showed that cells derived from cryopreserved bone were comparable to that of fresh bone in terms of the ability to promote osteogenesis in vivo. This work demonstrates that cryopreserved bone grafts may represent a novel, accessible source of cells for tissue regeneration therapy, and the results of our study may also stimulate the development of other cryopreservation techniques in basic and clinical studies.

Biological therapy of bone defects: the immunology of bone allo-transplantation

IMPORTANCE OF THE FIELD

Bone is one of the most transplanted tissues worldwide. Autograft is the ideal bone graft but is not widely used because of donor site morbidity and restricted availability. Allograft is easily accessible but can transmit infections and elicit an immune response.

AREAS COVERED IN THIS REVIEW

This review identifies all in vitro and in vivo evidence of immune responses following bone transplantation and highlights methods of improving host tolerance to bone allotransplantation.

WHAT THE READER WILL GAIN

In humans, the presence of anti-HLA specific antibodies against freeze-dried and fresh-frozen bone allografts has been demonstrated. Fresh-frozen bone allograft can still generate immune reactions whilst freeze-dried bone allografts present with less immunogenicity but have less structural integrity. This immune response can have an adverse effect on the graft's incorporation and increase the incidence of rejection. Decreasing the immune reaction against the allograft by lowering the immunogenic load of the graft or lowering the host immune response, would result in improved bone incorporation.

TAKE HOME MESSAGE

It is essential that the complex biological processes related to bone immunogenicity are understood, since this may allow the development of safer and more successful ways of controlling the outcome of bone allografting.

HLA sensitization and allograft bone graft incorporation

Achieving union between host bone and massive structural allografts can be difficult. Donor and recipient human leukocyte antigen (HLA) mismatches and recipient antibody response to donor HLA antigens might affect union. In a prospective multiinstitutional study, we enrolled a consecutive series of patients receiving cortex-replacing, massive structural bone allografts to determine the rate of donor-specific HLA antibody sensitization and to investigate the potential effect of such HLA alloantibody sensitization on allograft incorporation. HLA typing of patients and donors was determined by molecular typing methods. Donor-specific HLA sensitization occurred in 57% of the patients but had no demonstrable effect on graft incorporation or union. The type of host-allograft junction did have a major effect on graft incorporation. Cortical-to-cortical allograft-to-host junctions healed more slowly (mean, 542 days) than corticocancellous to corticocancellous allograft-to-host junctions (mean, 243 days). Although HLA sensitization does not appear to delay structural allograft bone incorporation, further followup is required to determine if there is an association between HLA sensitization and long-term graft survival. Based on these preliminary data, measures to further minimize or modulate HLA sensitization or response are not indicated at present for the purposes of improving structural bone allograft union.

LEVEL OF EVIDENCE

Level II, prognostic study.

Human leukocyte antigen sensitization after structural cortical allograft implantations

The incidence and significance of a donor-specific human leukocyte antigen antibody response to massive fresh-frozen human bone allograft implantation is not established. This study was a prospective, multicenter study of a cohort of consecutive patients who self-randomized themselves into two groups based on their alloantibody response to allograft bone transplant. The study hypothesis was that donor-directed antibodies are an independent risk factor influencing incorporation of massive frozen bone allografts. Pretransplant and posttransplant human leukocyte antigen alloantibody analysis was performed and correlated to determine pre-existing and graft-induced antibodies. The surgical outcomes of the two groups of patients were compared to determine the relationship between alloantibody response and bone graft incorporation. Preliminary results revealed that donor-specific human leukocyte antigen sensitization occurred in 17 of 32 (53%) of previously nonsensitized patients. A survival analysis of time to healing based on human leukocyte antigen status showed no evidence of an association between human leukocyte antigen status and time to healing. Longer followup in additional patients will be required to determine if this sensitization is correlated with an alteration in the time to union or with the quality or type of bone graft incorporation.

LEVEL OF EVIDENCE

Therapeutic study, Level II. See the Guidelines for Authors for a complete description of levels of evidence.

Cryopreservation with dimethyl sulfoxide sustains partially the biological function of osteochondral tissue

The clinical routine use of bone allograft transplants dates back to the discovery that grafts devitalized by freezing bear a reduced antigenicity. Graft failures, caused by a host versus graft reaction, however, remain a clinical problem. Previous investigations on pancreatic islet allografts revealed improved survival and biological function when fast cryopreservation (-70 degrees C/min) was performed in the presence of dimethyl sulfoxide (DMSO). The aim of this study was to determine the effect of fast freezing using DMSO on the biological function of osteochondral tissues. Organ culture was performed with neonatal femora of mice, untreated, rapidly frozen (-70 degrees C/min) with DMSO, or frozen without DMSO. After the culture, tissue morphology, cellular proliferation, osteoblast function, osteoclasts, and the presence of antigen-presenting cells were investigated. In untreated control femora histology appeared normal and proliferating and collagen-synthesizing osteoblasts, osteoclasts, and B-cells and macrophages were present. In frozen femora (with and without DMSO) a disintegration of the periosteum and the epiphyseal growth plate were observed and no active osteoblasts could be detected. Osteoclasts were partially detached from the bone surface. Cell proliferation was fully blocked in femora frozen in the absence of DMSO, while freezing in the presence of DMSO preserved cell proliferation in the medullary canal. The proliferating cells do not express epitopes present on the cells of the B-cell or macrophage lineages. Although the biological function of osteoblasts and osteoclasts was lost upon freezing of osteochondral tissue, DMSO included in freezing protocols preserves some residual cell viability which may be of importance for early graft revascularization as has been previously demonstrated by our group.

Quantitative assessment of angiogenesis and osteogenesis after transplantation of bone: comparison of isograft and allograft bone in mice

We performed a vital microscopic study in mice bearing dorsal skinfold chambers to characterize microvascular perfusion and leukocyte/endothelium interaction and their effects on elongation and mineralization of neonatal isograft and allograft bone. Isograft (C57/BL to C57/BL) and allograft bone (C57/ BL to BALB/C) revascularized simultaneously. However, vascular perfusion and density were lower in allograft bone than in isograft bone. Leukocyte/endothelium interaction was the same in isograft and allograft bones. Revascularization was not detected in allograft bone transplanted to presensitized recipients. Moreover, in preexisting vessels at the transplantation site, leukocyte/endothelium interaction was altered in allograft bone of presensitized recipients, despite a normal systemic leukocyte count. Femoral growth resulting from thickening of both epiphyses did not differ between experimental groups, however, mineralization occurred in isograft bone only. Isograft bone was histologically intact, allograft bone hypovital and allograft bone in presensitized recipients necrotic 12 days after implantation. Our findings suggest that graft incorporation or rejection is mediated by the microvasculature and that presensitizing of recipients accelerates rejection of allograft bone.

Human T Lymphocyte Proliferative Response to Resting Porcine Endothelial Cells Results from an HLA-Restricted, IL-10-Sensitive, Indirect Presentation Pathway But Also Depends on Endothelial-Specific Costimulatory Factors

To investigate the mechanisms of cellular rejection in pig-to-human xenotransplantation, the proliferation of different human purified lymphocyte subpopulations in response to swine leukocyte Ag class II-negative porcine aortic endothelial cells (PAEC) was measured in the presence or absence of human autologous adherent cells (huAPC). CD8+ lymphocytes proliferated moderately in the absence of huAPC, and the immune response was slightly increased when huAPC were added. CD56+ lymphocytes failed to proliferate in response to PAEC whether huAPC were present or not. CD4+ lymphocytes alone did not proliferate in response to PAEC, but a strong proliferative response was observed in the presence of metabolically active huAPC. This response was totally abolished by mAbs directed against HLA class II molecules or by pretreatment of huAPC by human IL-10. Even in the presence of huAPC, CD4+ lymphocytes failed to respond to fixed PAEC or to PAEC-lysates, suggesting that PAEC must be viable to support lymphocyte proliferation. Finally, none of the nonendothelial porcine adherent cells tested was able to induce human lymphocyte proliferation, despite the fact that they also provided a large set of xenogeneic peptides. Our results show that the indirect presentation pathway of xenoantigens by huAPC to CD4+ lymphocytes is crucial in the response to porcine endothelial cells, and that IL-10 could be of therapeutic interest to prevent human lymphocyte activation by this pathway. Furthermore, we demonstrated that stimulatory signals specifically provided by endothelial cells are also necessary for this huAPC-restricted proliferative response.